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Soil Carbon Sequestration Book · January 2012
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Preface Developing technologies to reduce the rate of increase of atmospheric concentration of carbon dioxide (CO2) from annual emissions of 8.6 Pg C yr-1 from energy, process industry, land-use conversion and soil cultivation is an important issue of the twenty-first century. The increase in atmospheric concentration of CO2 by 31% since 1750 from fossil fuel combustion and land use change necessitates identification of strategies for mitigating the threat of the attendant global warming. Since the industrial revolution, global emissions of carbon (C) are estimated at 270±30 Pg (Pg = petagram = 1015 g = 1 billion ton) due to fossil fuel combustion and 136±55 Pg due to land use change and soil cultivation. Emissions due to land use change include those by deforestation, biomass burning, conversion of natural to agricultural ecosystems, drainage of wetlands and soil cultivation. Depletion of soil organic C (SOC) pool have contributed 78±12 Pg of C to the atmosphere. Some cultivated soils have lost one-half to two-thirds of the original SOC pool with a cumulative loss of 30–40 Mg C/ha (Mg = megagram = 106 g = 1 ton). The depletion of soil C is accentuated by soil degradation and exacerbated by land misuse and soil mismanagement. Thus, adoption of a restorative land use and recommended management practices (RMPs) on agricultural soils can reduce the rate of enrichment of atmospheric CO2 while having positive impacts on food security,
agro-industries, water quality and the environment. A considerable part of the depleted SOC pool can be restored through conversion of marginal lands into restorative land uses, adoption of conservation tillage with cover crops and crop residue mulch, nutrient cycling including the use of compost and manure, and other systems of sustainable management of soil and water resources. Measured rates of soil C sequestration through adoption of RMPs range from 50 to 1000 kg/ha/year. The global potential of SOC sequestration through these practices is 0.9±0.3 Pg C/year, which may offset one-fourth to one-third of the annual increase in atmospheric CO2 estimated at 3.3 Pg C/year. The cumulative potential of soil C sequestration over 25–50 years is 30–60 Pg. The soil C sequestration is a truly win–win strategy. It restores degraded soils, enhances biomass production, purifies surface and ground waters, and reduces the rate of enrichment of atmospheric CO2 by offsetting emissions due to fossil fuel.
(D. KALAIVANAN)
Contents
S.No
Particulars
Page No
1
Introduction
6
2
Global warming and climate change
8
3
Green house effect
16
4
Effects of green house gases on global warming
16
5
The Global Carbon Cycle
21
6
Soil carbon pool
26
7
Depletion of soil organic carbon/ causes for evolution of Green House Gases
31
8
Carbon sequestration
51
9
What is soil carbon sequestration
53
10
Strategies of C sequestration/soil restoration for mitigating Greenhouse effect
61
11
Conclusions
89
12
The way forward
94
13
References
95
1
List of Tables S.No
Title
Page No
1
Composition and changes to concentration of greenhouse gases in the atmosphere
18
2
Carbon flow from various sources to sink
27
3
Soil C pool of world soils (Billion tons)
29
4
Estimates of soil carbon pool in various ecosystems
29
5
SOC density in various climatic regions
30
6
SOC changes with shifting cultivation in India
36
7
SOC loss due to different erosions
47
8
Soil loss due to wind erosion in Rajasthan
47
9
SOC depletion due to wind erosion
48
10
Productivity loss due to soil erosion in different soil orders
48
11
Types of degradation and causative agents
49
12
Area of degraded lands in India
49
13
SOC accumulation due to conservation tillage
68
14
SOC gain in different countries by conservation tillage
69
15
SOC gain in different countries by crop rotation
71
16
SOC gain due to mulching
73
2
List of Tables (contd…) S.No
Title
Page No
17
Estimation of C balance (Mg ha-1) in plots with and without fertilizer addition in the rice-wheat-jute cropping system in long-term fertilizer experiment at Barrackpore (based on 29 years data)
74
18
Organic C content (%) in soil as influenced by treatments and cropping system (Finger milletMaize- Cowpea) under LTFE over past 20 years
76
19
SOC gain in different countries by manuring or fertilizer use
78
20
SOC (%) gain due to pastures (Cenchrus ciliaris)
81
21
SOC gain in various agro-forestry systems
83
22
SOC gain with raising different tree species
87
3
List of figures S.No
Title
Page No
1
Principal global C pools and fluxes between them
22
2
The atmospheric C pool is increasing at the rate of 3.5 Pg C yr-1. The terrestrial C pool contributes approximately 1.6 Pg C yr-1 through deforestation, biomass burning, draining of wetlands, soil cultivation including those of organic soils, accelerated erosion and hidden C costs of input (e.g. fertilizers, tillage, pesticides, irrigation). Terrestrial C pools are presently sink of 2–4 Pg C yr-1. Conversion to a judicious land use and adoption of recommended practices in managed ecosystems can make these important sinks especially due to CO2 fertilization effects.
24
3
Gaseous emissions through various sources
33
4
Deforestation
35
5
Biomass burning, such as that of the cut forest in Brazil, causes large emissions of greenhouse gases and soot or black carbon into the atmosphere.
39
6
Soil tillage
41
7
Soil erosion leads to preferential removal of topsoil rich in soil organic carbon
45
8
A wide range of processes and technological options for C sequestration in agricultural, industrial and natural ecosystems.
62
9
Management strategies for soil carbon sequestration through restoration of degraded soils and adoption of RMPs on agricultural soils.
63
4
List of Figures (contd…)
S.No
Title
10
No till farming and other conservation tillage practices eliminate drastic soil disturbance, and enhance soil organic matter in the surface layers. Conversion from plow till to no-till with residue mulch is a viable option for SOC sequestration.
11
Mucuna utilis (velvet bean), a suitable cover crop for the humid tropics of West Africa, and other cover crops enhance SOC pool
5
Page No 68
71
Introduction The global population of 7 billion is increasing at the rate of 1.3% (or) it will be 7.3 billion by 2020 or 9.4 billion by 2050. With projected increase in world population and change in food habits, future food demands for cereals will increase drastically. Concomitant with mankind's growing numbers and the progression of the Industrial Revolution, there has been a significant increase in the burning of fossil fuels (coal, gas, and oil) over the past 200 years, the carbon dioxide emissions from which have led to ever-increasing concentrations of atmospheric CO2. This “large-scale geophysical experiment,” to borrow the words of two of the phenomenon's early investigators is still ongoing and expected to continue throughout the current century. Furthermore, this enriching of the air with CO2 is looked upon with great concern, because CO2 is an important greenhouse gas, the augmentation of which is believed by many to have the potential to produce significant global warming. Therefore, and because of perceived serious consequences, such as the melting of polar ice, rising sea levels, coastal flooding, and more frequent and intense droughts, floods, and storms, a concerted effort is underway to slow the rate at which CO2 accumulates in the atmosphere, with the goal of stabilizing its concentration at a level that would prevent dangerous anthropogenic interference with the planet's climate system.
6
One of the more promising ways of reducing the rate of rise of the air's CO2 content is to encourage land management policies that promote plant growth, which removes CO2 from the atmosphere and sequesters its carbon, first in vegetative tissues and ultimately in soils. Some of these policies deal with managed forests and agro-ecosystems, while others apply to natural ecosystems, such as unmanaged forests and grasslands. In all instances, however, questions abound. Can carbon inputs to soils really be enhanced or carbon losses reduced? Can carbon storage in recalcitrant fractions of soil organic matter be increased, making it possible to successfully maintain new stores of sequestered carbon for long periods? Moreover, what if global warming runs wild? Will the ensuing rise in temperature stimulate plant and microbial respiration rates, returning even more CO2 to the air than is removed by photosynthesis and leading to a negative net ecosystem exchange of carbon? These important questions rank high on the priority lists of many research organizations concerned about the planet's future climate and the sustainability of the biosphere. The development of agriculture during the past centuries and particularly in last decades has entailed depletion of soil carbon stocks created through longterm evolution. Agricultural soils are among the planet's largest reservoirs of carbon and hold potential for expanded carbon sequestration (CS), and thus provide a prospective way of mitigating the increasing atmospheric concentration
7
of CO2. Soils can sequester around 20 Pg C in 25 years, more than 10 % of the anthropogenic emissions. At the same time, this process provides other important benefits for soil, crop and environment quality, and prevention of erosion and desertification and for the enhancement of bio-diversity Global warming and climate change Global warming refers to an average increase in the Earth's temperature, which in turn causes changes in climate. A warmer Earth may lead to changes in rainfall patterns, a rise in sea level, and a wide range of impacts on plants, wildlife, and humans. When scientists talk about the issue of climate change, their concern is about global warming caused by human activities. The Global Warming Debate There is a growing concern that increasing levels of carbon dioxide in the atmosphere will change the climate, making Earth warmer and increasing the frequency of extreme weather events. Most climate models predict that if global warming occurs, it will not produce globally uniform effects. Most places in higher latitudes will become warmer, but some will actually cool down (for example, Northwestern Europe). Global precipitation will increase due to increased evaporation from the oceans, but some areas will receive substantially less rainfall than today. It is expected that temperatures will rise more near the
8
poles and less in tropical regions. Nights and winters are expected to warm more than daytime and summer temperatures. Thus in general, warming will tend to occur at the lower ends of current temperature ranges. This has led some to argue that global warming will be generally beneficial to mankind, potentially opening new areas in the upper temperate zones to agricultural enterprises that are not practical today due to the cold climate. Also, increased concentrations of carbon dioxide would have a fertilizing effect on some crops and vegetation, stimulating growth. For Ohio, most climate models predict substantially warmer winters and slightly hotter summers. Some indicate that summers would also be drier. Thus, agriculture would be faced with a longer and drier growing season. The soil organic matter content may decline, increasing risks of soil compaction and erosion, and decreasing plant available water capacity. Adaptations might include the introduction of crops (such as cotton or sorghum) that won’t grow in our current climate. Reducing the number and severity of winter storms might benefit farmers, but the increasing likelihood of summer droughts would present a real challenge. In contrast with rising sea levels, the level of the Great Lakes, including Lake Erie, could drop by 7 to 8 feet, primarily because of reduced rainfall and increased use of irrigation.
9
Global warming a real and underway The mainstream scientific consensus on global warming is becoming clearer every day: changes in our climate are real and they are underway. Now we can do something about it. The evidence that human-induced global warming is real is increasingly clear and compelling. •
Since the beginning of the 20th century, the mean surface temperature of the earth has increased by about 1.1º F (0.6°Celsius).
•
Over the last 40 years, which is the period with most reliable data, the temperature increased by about 0.5 º F (0.2-0.3°Celsius).
•
Warming in the 20th century is greater than at any time during the past 400-600 years. Seven of the ten warmest years in the 20th century occurred in the 1990s. 1998, with global temperatures spiking due to one of the strongest El Niños on record, was the hottest year since reliable instrumental temperature measurements began. Global mean surface temperatures have increased 0.5-1.0°F since the late
19th century. The 20th century's 10 warmest years all occurred in the last 15 years of the century. Of these, 1998 was the warmest year on record. The snow cover in the Northern Hemisphere and floating ice in the Arctic Ocean has decreased. Globally, sea level has risen 4-8 inches over the past century.
10
Worldwide precipitation over land has increased by about one percent. The frequency of extreme rainfall events has increased throughout much of the United States. Increasing concentrations of greenhouse gases are likely to accelerate the rate of climate change. Scientists expect that the average global surface temperature could rise 1-4.5°F (0.6-2.5°C) in the next fifty years, and 2.2-10°F (1.4-5.8°C) in the next century, with significant regional variation. Evaporation will increase as the climate warms, which will increase average global precipitation. Soil moisture is likely to decline in many regions, and intense rainstorms are likely to become more frequent. Sea level is likely to rise two feet along most of the U.S coast. In addition, changes in the natural environment support the evidence from temperature records: •
Melting (and possible disappearance) of glaciers and mountain snow caps that feed the world's rivers and supply a large portion of the fresh water used for drinking and irrigation. The Arctic ice pack has lost about 40% of its thickness over the past four decades;
•
A rise in sea levels due to the melting of the land-based ice sheets in Greenland and Antarctica, with many islands and coastal areas ending up more exposed to storm damage or even underwater. The global sea level is
11
rising about three times faster over the past 100 years compared to the previous 3,000 years; and •
There are a growing number of studies that show plants and animals changing their range and behavior in response to shifts in climate.
•
Increasingly costly "bad weather" events such as heat waves, droughts, floods, and severe storms.
•
Lowered agricultural productivity due to less favorable weather conditions, less available irrigation water, increased heat stress to plants, and an increase in pest activity due to warmer temperatures.
•
Increases in vector-borne infectious diseases like malaria and Lyme disease.
•
Large numbers of extinctions of higher-level species due to their inability to adapt to rapidly changing climate and habitat conditions.
Climate Change-Beyond Withering Weather Climate change is about much more than how warm or cool our temperatures are. Whereas "global warming" refers to increasing global temperatures, "climate change" refers to regional conditions. Climate is defined by a number of factors, including:
12
•
Average regional temperature as well as day/night temperature patterns and seasonal temperature patterns.
•
Humidity.
•
Precipitation (average amounts and seasonal patterns).
•
Average amount of sunshine and level of cloudiness.
•
Air pressure and winds.
•
Storm events (type, average number per year, and seasonal patterns).
To a great extent, this is what we think of as "weather." Indeed, weather patterns are predicted to change in response to global warming: •
some areas will become drier, some will become wetter;
•
many areas will experience an increase in severe weather events like killer heat waves, hurricanes, flood-level rains, and hail storms. It's tempting to think that all of these changes to the world's climate
regions will average out over time and geography and things will be fine. In fact, colder climates like Canada may even see improved agricultural yields as their seasonal temperatures rise. But overall, humanity has made a huge investment in "things as they are now, where they are now." Gone are the days of millennia ago when an unfavorable change in climate might cause a village to pack up their
13
relatively few belongings and move to a better area. We have massive societal and industrial infrastructure in place, and it cannot be easily moved. Climatechange effects will generally not be geographically escapable in the timeframe over which they happen, at least not for the majority of humans and species. Causing serious disruptions to our environment and lives. . . As the Earth continues to warm, there is a growing risk that the climate will change in ways that will seriously disrupt our lives. While on average the globe will get warmer and receive more precipitation, individual regions will experience different climatic changes and environmental impacts. Among the most severe consequences of global warming are: •
A faster rise in sea level,
•
More heat waves and droughts, resulting in more and more conflicts for water resources;
•
More extreme weather events, producing floods and property destruction; and
•
A greater potential for heat-related illnesses and deaths as well as the wider spread of infectious diseases carried by insects and rodents into areas previously free from them.
14
If climatic trends continue unabated, global warming will threaten our health, our cities, our farms and forests, beaches and wetlands, and other natural habitats. We can take action to reduce the threat Fortunately, we can take action to slow global warming. Global warming originated mainly though human related activities which release heat-trapping gases and particles into the air. The most important causes include the burning of fossil fuels such as coal, gas, and oil, and deforestation. To reduce the emission of heat-trapping gases like carbon dioxide, methane, and nitrous oxides, we can curb our consumption of fossil fuels, use technologies that reduce the amount of emissions wherever possible, and protect the world’s forests. We can also do things to mitigate the impacts of global warming and adapt to those most likely to occur, e.g., through careful long-term planning and other strategies that reduce our vulnerability to global warming. Be part of the solution Clearly, global warming is a huge problem. It will take everyone -governments, industry, communities and individuals working together to make a real difference. At UCS, we are working to bring sound scientific information to policymakers and the public to educate them about global warming, its impacts,
15
and about available practical solutions. We are raising awareness of the need for action and working to create Congressional support for sound solutions. However, we do not stop there. We are also advocating policies that will combat global warming over the long term. Things like clean cars that run on alternative fuels, environmentally responsible renewable energy technologies, and stopping the clear-cutting of valuable forests. These are solutions that will help to reduce global warming, and you can be part of them. Green house effect The greenhouse effect is a naturally occurring process that aids in heating the Earth's surface and atmosphere. It results from the fact that certain atmospheric gases, such as carbon dioxide, water vapor, and methane, are able to change the energy balance of the planet by being able to absorb long wave radiation from the Earth's surface. Without the greenhouse effect, life on this planet would probably not exist, as the average temperature of the Earth would be a chilly -18 degrees Celsius, rather than the present 15 degrees Celsius. Effects of green house gases on global warming A number of gases are involved in the greenhouse effect and global warming these gases include: carbon dioxide (CO2); methane (CH4); nitrous oxide (N2O); chlorofluorocarbons (CFC); and tropospheric ozone (O3). Of these
16
gases, the single most important gas is carbon dioxide, which accounts for about 55 % of the change in the intensity of the Earth's greenhouse effect. The contributions of the other gases are 25 % for chlorofluorocarbons, 15 % for methane, and 5 % for nitrous oxide (Table 1). Ozone's contribution to the enhancement of greenhouse effect is still yet to be quantified Carbon dioxide Carbon dioxide, being emitted in large quantity, is mainly responsible for an increase in atmospheric temperature. One of the major concerns is the impact of possible global warming shift in carbon now sequestered in soils. A possible increase of 3oC in temperature over the next 50 years may decrease SOC by 10 to 15% in the top 30cm soils leading to emission of 50 to 100 Pg of C into the atmosphere. Deforestation for expansion of cultivated areas in the tropical regions may lead to an overall reduction in the global soil carbon pool by 20% and a total emission of 150 Pg of carbon. This magnitude of carbon loss from world soils would exacerable the effects of global warming. CO2 is fundamental to sustain life on earth for two reasons i. It is a source of C for photosynthesis ii. Its acts like blanket in atmosphere that keeps the earth warm.
17
Molecular CO2 absorbs short wave solar radiation and releases it as long wave infrared radiation. An atmospheric concentration of 300 ppmv is adequate to support life on the earth but it has reached 370 ppmv and is increase at rate of 1.5 μL L-1 yr -1 (Lee and Dodson, 1996). With increase in CO2 concentration the amount of solar radiation absorbed by atmosphere would increase make earth warmer. This can alter the global hydrological cycle and alter the distribution and productivity of terrestrial biota, which is termed as Global Warming Table 1. Composition and changes to concentration of greenhouse gases in the atmosphere
Annual increase (%)
Contribution to climate change (%)
388 ppm
0.4
40-50
CH4
1774 ppb
0.4
20-25
N2 O
318 ppb
0.25
5-10
CFCs
533 ppt
decreasing
15-20
Gas
Current concentration
CO2
(IPCC, 2007)
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Methane Methane has strong absorption bands in the infrared region of the electromagnetic spectrum leading to an increase in atmospheric temperature. Although CH4 has a relatively short atmospheric lifetime (8-12 years), one molecule of CH4 traps about 32 times more heat than a molecule of CO2. Although the troposphere methane concentration is only 1.78 ppmv as compared to 37 ppmv of CO2, methane is responsible for approximately 25% of the anticipated warming (Aulakh, 1992). Unlike CO2, methane is chemically reactive and is involved in many chemical processes in the atmosphere. Only a small fraction of atmospheric CH4 diffuses to the ozone chemistry in the stratosphere. In the troposphere, OH radicals representing the major sink mechanism of atmospheric CH4 initiate the oxidation of CH4. The global CH4 cycle is closely linked to the CO cycle, 10-35 % of the global annual production of CO is believed to result from the CH4 oxidation. Nitrous oxide Per molecule, N2O is approximately 150 times more effective than CO2. The atmospheric concentration of N2O is increasing at a current rate of amount 0.25% (IPCC, 1994). An increase of 0.2% to 0.3% in N2O concentration in the atmosphere contributes about 5% to the supposed greenhouse warming. Nitrous oxide evolved from soil diffuses into the upper atmosphere. The stratosphere
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contains a few parts per million of ozone which is the only atmospheric gas to protect the biosphere from harmful ultraviolet radiation. Nitrous oxide contributes to the destruction of the protective ozone layer in the stratosphere. According to one estimate, a doubling of the atmospheric N2O would cause a 10% decrease in the ozone layer, which would increase the ultraviolet radiation reaching the earth surface by about 20%. This could result in increased incidence of skin cancer and other health hazards. The estimates of the effects of ozone depletion suggest a 4% to 6% increase in case of skin cancer with each 1% drop in ozone (IPCC, 1994). A number of laboratory and field studies indicate that the mole fraction of N2O produced during terrestrial denitrification can vary from 0 to 1 depending on soil conditions (Aulakh et al., 1992). Chlorofluorocarbons Artificially created chlorofluorocarbons are the strongest greenhouse gas per molecule. However, low concentrations in the atmosphere reduce their overall importance in the enhancement of the greenhouse effect. Current measurements in the atmosphere indicate that the concentration of these chemicals may soon begin declining because of reduced emissions. Reports of the development of ozone holes over the North and South Poles and a general decline in global stratospheric ozone levels over the last two decades has caused many nations to cutback on their production and use of these chemicals. In 1987,
20
the signing of the Montreal Protocol agreement by 46 nations established an immediate timetable for the global reduction of chlorofluorocarbons production and use. The increase in global temperature because of radiative forcing of Green House Gases (GHG’s) in the atmosphere has been estimated at 0.6oC during 20th century and is projected to be 1.4 to 5.8oC by 2100 relative to 1990 (IPCC, 2001). This magnitude of increase may vary spatially and temporally and may influence the precipitation, which may increase globally. The Global Carbon Cycle The importance of atmospheric concentration of CO2 on global temperature was recognized by Arrhenius (1896) towards the end of the nineteenth century, whereas anthropogenic perturbation of the global C cycle during the twentieth century has been an historically unprecedented phenomenon. Understanding the global C cycle and its perturbation by anthropogenic activities is important for developing viable strategies for mitigating climate change. The rate of future increase in atmospheric CO2 concentration will depend on the anthropogenic activities, the interaction of biogeochemical and climate processes on the global C cycle and interaction among principal C pools. There are five global C pools, of which the largest oceanic pool is estimated at38 000 Pg and is increasing at the rate of 2.3 Pg C yr-
21
1
(figure 1). The geological C pool, comprising fossil fuels, is estimated at 4130
Pg, of which 85% is coal, 5.5% is oil and 3.3% is gas. Proven reserves of fossil fuel include 678 Pg of coal (3.2 Pg yr-1 production), 146 Pg of oil (3.6 Pg yr-1of production) and 98 Pg of natural gas (1.5 Pg yr-1 of production; Schrag 2007). Presently, coal and oil each account for approximately 40% of global CO2 emissions (Schrag 2007).
Fig 1. Principal global C pools and fluxes between them Thus, the geological pool is depleting, through fossil fuel combustion, at the rate of 7.0 Pg C yr-1. The third largest pool is pedologic, estimated at 2500 Pg
22
to 1 m depth. It consists of two distinct components: soil organic carbon (SOC) pool estimated at 1550 Pg and soil inorganic carbon (SIC) pool at 950 Pg (Batjes 1996). The SOC pool includes highly active humus and relatively inert charcoal C. It comprises a mixture of: (i) plant and animal residues at various stages of decomposition; (ii) substances synthesized microbiologically and/or chemically from the breakdown products; and (iii) the bodies of live micro-organisms and small animals and their decomposing products (Schnitzer 1991). The SIC pool includes elemental C and carbonate minerals such as calcite, dolomite and gypsum, and comprises primary and secondary carbonates. The primary carbonates are derived from the weathering of parent material. In contrast, the secondary carbonates are formed through the reaction of atmospheric CO2 with CaC2and MgC2 brought in from outside the local ecosystem (e.g. calcareous dust, irrigation water, fertilizers, and manures). The SIC is an important constituent in soils of arid and semi-arid regions. The fourth largest pool is the atmospheric pool comprising 760 Pg of CO2-C, and increasing at the rate of 3.5 Pg C yr-1 or 0.46% yr-1. The smallest among the global C pools is the biotic pool estimated at 560 Pg. The pedologic and biotic C pools together are called the terrestrial C pool estimated at approximately 2860 Pg. The atmospheric pool is also connected to the oceanic pool which absorbs 92.3 Pg yr-1 and releases 90 Pg yr-1 with a net positive balance of 2.3 Pg C yr-1. The oceanic pool will absorb
23
approximately 5 Pg C yr-1 by 2100 (Orr et al. 2001). The total dissolved inorganic C in the oceans is approximately 59 times that of the atmosphere. On the scales of millennia, the oceans determine the atmospheric CO2 concentration, not vice versa (Falkowski et al. 2000).
Fig 2. The atmospheric C pool is increasing at the rate of 3.5 Pg C yr-1. The terrestrial C pool contributes approximately 1.6 Pg C yr-1 through deforestation,
24
biomass burning, draining of wetlands, soil cultivation including those of organic soils, accelerated erosion and hidden C costs of input (e.g. fertilizers, tillage, pesticides, irrigation). Terrestrial C pools are presently sink of 2–4 Pg C yr-1. Conversion to a judicious land use and adoption of recommended practices in managed ecosystems can make these important sinks especially due to CO2 fertilization effects. The link between geological (fossil fuel) and atmospheric pool is in one direction: transfer of approximately 7.0 Pg C yr-1 from fossil fuel consumption to the atmosphere. The rate of fossil fuel consumption may peak by about 2025.The terrestrial and atmospheric C pools are strongly interacting with one another (figure 2). The annual rate of photosynthesis is 120 Pg C, most of which is returned back to the atmosphere through plant and soil respiration. The terrestrial C pool is depleted by conversion from natural to managed ecosystems, extractive farming practices based on low external input and soil degrading land use. The pedologic pool loses 0.4–0.8 Pg C yr-1 to the ocean through erosion induced transportation to aquatic ecosystems. The terrestrial sink is presently increasing at a net rate of1.4G0.7 Pg C yr-1. Thus, the terrestrial sink absorbs approximately 2–4 Pg C yr-1 and its capacity may increase to approximately 5 Pg C yr-1 by 2050 (Cramer et al. 2001; Scholes & Noble 2001). Increase in the terrestrial sink capacity may be due to CO2 fertilization effect and change in land use and
25
management. The biotic pool also contributes to increase in atmospheric CO2 concentration through deforestation and biomass burning. Soil carbon pool World soils contain about 3.2 trillion tons of carbon within the top six feet. An estimated 2.5 trillion tons is in the form of soil organic carbon. This is the organic matter in the soil that makes it fertile. The remaining 0.7 trillion tons is soil inorganic carbon. These are very large numbers. In fact the soil carbon pool is 4.2 times the entire atmospheric pool, and 5.7 times the biotic pool. Thus, even a relatively small increase in soil carbon, if it is taken from the air, could provide a significant reduction in atmospheric carbon. Moreover, because plants feed on carbon dioxide (CO2) in the air, the primary way to store carbon in soil is to grow plants. Improved agriculture is the key to soil storage of carbon. Soil organic matter is concentrated in the upper 12 inches of the soil. So it is readily depleted by anthropogenic (human-induced) disturbances such as land use changes and cultivation. The magnitude of soil carbon depletion is increased by soil degradation, especially due to erosion. There is a consensus among scientists that the burning of fossil fuel, cement production, tropical deforestation, changes in land use together contribute 9.1 Pg C yr-1 towards the atmosphere with almost two-third of it coming from fossil fuel. The contribution of fossil fuel may
26
increase to 8.0 Pg C yr-1 by the year 2010 if present trends of its utilization continue. A total of 5.7 Pg C yr-1 is reabsorbed in terrestrial ecosystem, oceans and in some other unknown sinks leaving a net enrichment of 3.4 Pg C in atmosphere (Table 2). A large part of this enrichment comes directly or indirectly from soils. Table 2. Carbon flow from various sources to sink Pg C yr-1
Carbon flow Sources Fossil fuel combustion & Cement Production
6.4
Land use change
1.1
Tropical deforestation
1.6
Total sources
9.1
Sinks Atmospheric increase in CO2 Terrestrial
3.4
Oceans
2.0
Unknown sink
1.7
Total sink
9.1
2.0
(IPCC, 2001) The pedologic or soil C pool comprises two components: SOC and the soil inorganic carbon (SIC) pool. The SIC pool is especially important in soils of
27
the dry regions. The SOC concentration ranges from a low in soils of the arid regions to high in soils of the temperate regions, and extremely high in organic or peat soils (Table 3). Amongst different soil orders Histosols contain maximum and Vertisols minimum carbon. The SOC pool also varies widely among ecoregions, being higher in cool and moist than warm and dry regions (Table 4). Therefore, the total soil C pool is four times the biotic (trees, etc.) pool and about three times the atmospheric pool. Soils are a major C pool and are estimated to contain 1220 to 1550 Pg C in organic form (soil organic carbon, SOC) and almost half in inorganic form. (Batjes, 1996). Density of SOC in different eco-regions varies from as low as 3.7 in arid to 24 kg m-2 in boreal regions (Table 5). Though the C content in Aridisols is lowest they rank fifth in terms of total quantity of C stored due to vast area under arid regions. Inorganic C in soil is generally very stable but SOC is through changes in land use especially from deforestation, ploughing and erosion. Climate is an important factor of soil formation (Jenny, 1941 & 1980)
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Table 3. Soil C pool of world soils (Billion tons) Soil order
Area (Mha)
SOC
SIC
Alfisols Andisols Aridisols Entisols Gelisols Histosols Inceptisols Mollisols Oxisols Rocky land Shifting sand Spodosols Ultisols Vertisols Total
1262 91 1570 2114 1126 153 1286 901 981 1308 532 335 1105 316 13083
158 20 59 90 316 179 190 121 126 22 2 64 137 42 1526
43 0 456 263 7 0 34 116 0 0 5 0 0 21 945
(Eswaran et al., 2000) Table 4. Estimates of soil carbon pool in various ecosystems Area (109 ha)
SOC pool (Pg C)
• Tropical
1.76
213 – 216
• Temperate
1.04
100 – 153
• Boreal
1.37
338 – 471
Tropical savannas and grasslands
2.25
247 – 264
Temperate grassland &scrub land
1.25
176 – 295
Tundra
0.95
115 – 121
Desert and semi – desert
4.55
159 – 191
Ecosystem Forests
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Cropland
1.60
128 – 165
Wetlands
0.35
225 (IPCC, 2001)
Table 5. SOC density in various climatic regions SOC density (kg m-2)
Biome Tropics
10.0 – 10.4
Warm humid Warm seasonally dry
7.0 – 7.3
Cool
4.4 – 4.7
Arid
3.7 – 4.1
Sub tropics Summer rains
8.6 – 9.1
Winter rains
7.2 – 8.0
Temperate Oceanic
11.7 – 12.9
Continental
5.6 – 5.9
Boreal
23.1 – 24.0
Polar and Alpine
20.6 – 23.2 (Lal,2001)
Consequently, soil properties such as SOC are in dynamic equilibrium with
climate,
particularly
precipitation
and
temperature.
The
strong
interdependence between climate and soil quality makes it necessary to predict
30
the effect of climate changes on soil properties on climate change and vice versa. This seminar is meant to study the importance of land use and management options on C sequestration in soil and estimate the potential of soil C sequestration to mitigate the climate change. Depletion of soil organic carbon / causes for evolution of Green House Gases Carbon sequestration implies not only increasing the amount of C entering in soil but also decreasing the amount leaving through decomposition and erosion. The rate of decomposition of organic matter primarily depends on soil temperature and moisture regimes (Jenny, 1980), and soil texture. In fact, macroclimate has large impact on the portion of the soil organic matter that is potentially active. Consequently, mean annual temperature has a strong impact on SOC pool and its turnover. Depletion of the SOC pool has major adverse economic and ecological consequences, because the SOC pool serves numerous on-site and off-site functions of value to human society and well being. Principal on-site functions of the SOC pool are: • Source and sink of principal plant nutrients (e.g., N, P, S, Zn, Mo); • Source of charge density and responsible for ion exchange; • Absorbent of water at low moisture potentials leading to increase in plant available water capacity;
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• Promoter of soil aggregation that improves soil tilth; • Cause of high water infiltration capacity and low losses due to surface runoff; • Substrate for energy for soil biota leading to increase in soil biodiversity; • Source of strength for soil aggregates leading to reduction in susceptibility to erosion; • Cause of high nutrient and water use efficiency because of reduction in losses by drainage, evaporation and volatilization; • Buffer against sudden fluctuations in soil reaction (pH) due to application of agricultural chemicals; and • Moderator of soil temperature through its effect on soil color and albedo. In addition, there are also off-site functions of SOC pool, which have both economic and environmental significance. Important among these are: • Reduces sediment load in streams and rivers, • Filters pollutants of agricultural chemicals, • Reactors for biodegradation of contaminants, and • Buffers the emissions of GHGs from soil to the atmosphere.
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Factors affecting GHG Emissions from soil
Biomass burning (CO2, CH4, N2O)
Plowing and cultivation (CO2)
Drainage of wetlands (CO2)
Inappropriat e fertilizer and manuring (N2O)
Excessive grazing (CO2, N2O)
Encroachment of grassland by woody species in sub-humid regions
Decline in soil quality • Reduction in aggregation • Compaction • emission
Fig 3. Gaseous emissions through various sources It is because of these multifareous functions that led Albrecht (1938) to observe that ‘‘soil organic matter (SOM) is one of our most important national resources; its unwise exploitation has been devastating; and it must be given its proper rank in any conservation policy.’’ Indeed, the unwise exploitation of this precious resource is due to human greed and short-sightedness causing land misuse and soil mismanagement. Anthropogenic perturbations exacerbate the emission of CO2 from soil caused by decomposition of SOM or soil respiration (Schlesinger, 2000b). The emissions are accentuated by agricultural activities (Fig.3) including tropical
33
deforestation (Fig. 4) and biomass burning (Fig. 5), plowing (Reicosky, 2002), drainage of wetlands and low-input farming or shifting cultivation (Fig. 4; Tiessen et al., 2001). In addition to its impact on decomposition of SOM (Trumbore et al., 1996), macroclimate has a large impact on a fraction of the SOC pool which is active (Franzluebbers et al., 2001a). I.
Deforestation Conversion of natural to agricultural ecosystems increases the maximum
soil temperature and decreases the soil moisture storage in the root zone, especially in drained agricultural soils. Thus, land use history strongly impacts the SOC pool. Northeast of Para’ state in the eastern Amazon of Brazil, Sommer et al. (2000) reported significant changes in SOC pool to 6-m depth because of deforestation and land use change over 100 years of settlement. The SOC pool in soil was 196 Mg C/ha under a primary forest, 185 Mg C/ha under slash and-burn agriculture, and only 146 to 167 Mg C/ha under
(semi-) permanent annual cropping culture. Thus, conversion to agricultural ecosystem depleted SOC pool by 30 to 50 Mg C/ha over 100 years. Forests are major reservoir of terrestrial above ground C and contain an estimated 66 per cent of terrestrial above ground C. Most soil C i.e. 44 per cent is present in forest and 12 per cent in agro-ecosystem (Dixon, 1999).
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Fig 4: Deforestation
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In addition global forest ecosystem accounts for approximately 90 per cent of annual C flux between atmosphere and terrestrial soil C. Deforestation can contribute to large volume of C to atmosphere either by reducing the amount stored in above-ground biomass or increasing the oxidation of SOC. Currently, global deforestation is 15 x 106 ha annually, releasing 1.6 x 1015 g C annually to atmosphere (Dixon et al. 1999). Indian forest scenario seems doing a bit better where the FSI, 1999 report shows an increase of 3897 km2 under forest which makes 19.39% geographical area under forest which is a slight increase when compared to 1997. Shifting cultivation, practiced by about 250 million people world over in about 350 Mha is a major cause of declining forest cover (Datta et al., 2001). In India, shifting cultivation is practiced in North Eastern Hill Regions. Shifting cultivation accentuates the C losses to atmosphere and results in huge losses of soil and soil fertility particularly in short cycles. Table 6. SOC changes with shifting cultivation in India Soil property
West Tripura
South Tripura
1 yr
3 yr
1yr
3yr
SOC (%)
7.73
6.7
8.3
7.0
Available N
589
522
567
511
(Kg ha-1)
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Water holding capacity (kg kg-1)
0.34
0.32
0.40
0.35
CEC [cmol(p+)kg-1]
0 .22
3.38
4.29
3.1
Exchangeable cations
0.84
0.75
1.10
1.06
(cmol(p+)kg-1] (Datta et al.,2001) Datta et al., (2001) have reported a sharp fall in SOC, maximum water holding capacity, exchangeable cations, base saturation and status of available nutrients in soil within first three years of shifting cultivation (Table 6). As forest cover in India is only 2/3 of recommended 33%, hence it in extremely important to take care of the severe deforestation by any alternative land use system for NE region to dissuade people from practicing shifting cultivation. More than 38 million hectares of our land, which though cultivable, has been left uncultivated, classified as “cultivable wasteland”. This area of 38 million hectares is more than the total cultivated area of four countries viz. Pakistan, Nepal, Bangladesh and Japan (The Hindu dt 04 – 05 – 2004). Most of the good lands in tropics are already under intensive agriculture and with increasing population; new lands that are being cleared for agriculture are increasingly the marginal soils. Productivity on marginal soils is in any case low and at the same time these soils are even more susceptible for degradation.
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This situation highlights another cause of deforestation. Agricultural lands become so much degraded that they become useless for production forcing further deforestation and thus the vicious cycle continues. Much of the land being cleared of forest presently is replacing land that was degraded so that increase in agriculture land area does not equal the decrease in forestland. This situation aggravates the loss of C from soil. II.
Biomass burning Biomass burning, an important management tool in several natural and
agro-ecosystems, emits numerous gases immediately but also leaves charcoal as a residual material resistant to further decomposition. Charcoal is an important form of C produced by incomplete combustion. Being a passive component, charcoal does not contribute to SOC pool or soil biological activity. As SOC pool declines due to cultivation, the more resistant charcoal fraction increases as a portion of the total C pool and may constitute up to 35% of the total SOC in fireprone ecosystems (Skjemstad et al., 2002). As the SOC pool declines due to cultivation and soil degradation, the more resistant charcoal fraction increases as a portion of the total C pool. Therefore, biomass burning has important negative and positive impacts on C dynamics. Carbon dating of charcoal has shown some to have been around for over 1500 years, is fairly stable, and a permanent form of C sequestration.
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Fig. 5. Biomass burning, such as that of the cut forest in Brazil, causes large emissions of greenhouse gases and soot or black carbon into the atmosphere. Deforestation and biomass burning are obvious factors leading to the release of CO2, CH4, N2O, and NOX into the atmosphere (Houghton et al., 1999). III.
Soil tillage Similar to deforestation and biomass burning, cultivation of soil, by
plowing and other tillage methods, also enhances mineralization of SOC and releases CO2 into the atmosphere (Reicosky et al., 1999), and is not an obvious source of CO2. Tillage increases SOC mineralization by bringing crop residue closer to microbes where soil moisture conditions favor mineralization (Gregorich et al., 1998), physically disrupts aggregates and exposes hitherto
39
encapsulated C to decomposition (Fig. 6). Both activities decrease soil moisture, increase maximum soil temperature and exacerbate rate of SOC mineralization. Thus, a better understanding of tillage effects on SOC dynamics is crucial to developing and identifying sustainable systems of soil management for C sequestration. There is a strong interaction between tillage and drainage. Both activities decrease soil moisture, increase maximum soil temperature and exacerbate rate of SOC mineralization."Tillage is one of the worst things you can do to the soil," said Dr. Rice at Kansas State. "Spraying the soil with a chemical is often less harmful." By not tilling the soil, and allowing plant life and natural debris to decompose, agricultural experts say, the soil will strengthen and more readily absorb carbon from the atmosphere through plant photosynthesis. By some estimates, about 50 percent of the carbon stored under agricultural lands has been lost over the last 200 years because of plowing and turning over the soils. Pulling that carbon back into the soil, and keeping it trapped there by simply planting in the dead plant debris after winter will not only improve the soils, experts say, it will help solve a host of other environmental problems as well, like soil erosion.
40
Fig. 6: Soil tillage IV.Soil degradation Soil degradation is caused by drastic perturbation of the delicate soilenvironment equilibrium, especially in harsh climates and ecologically sensitive
41
eco-regions. Soil degradation is indicative of the degree of the societal case for the land. As Lowdermilk (1939) wrote, “Individuals, nations, and civilizations write their records on the land – a record that is easy to read by those who understand the simple language of the land”. That record, regrettably, is often in the form of extensive and severe soil degradation. As Adlai E. Stevenson wrote, “Nature is neutral. Man has wrested from nature the power to make the world a desert or to make the desert bloom”. Both soil degradation and the greenhouse effect are creations of human misadventure with nature. Commoner. B (1966, 1972) clearly stated four laws of ecology that govern effects of human intervention in nature: i. Everything is connected to everything else, ii. Everything must go somewhere, iii. Nature knows best (nature shows no mercy for mistakes made by humans or any other species), and iv. There is no such thing as a free lunch (nothing comes form nothing). Human mistakes lead to soil degradation and emission of pollutants into the air and water. With agricultural expansion came soil degradation; with increase in use of agricultural chemicals came environmental pollution; and with increase in irrigation came salinization. Depletion of the SOC pool on
42
agricultural soils is exacerbated by and in turn also exacerbates soil degradation. It comprises physical degradation (i.e., reduction in aggregation, decline in soil structure, crusting, compaction, reduction in water infiltration capacity and water/air imbalance leading to anaerobiosis) and erosion, chemical degradation (i.e., nutrient depletion, decline in pH and acidification, build up of salts in the root zone, nutrient/elemental imbalance and disruption in elemental cycles), and biological degradation (i.e., reduction in activity and species diversity of soil fauna, decline in biomass C and depletion of SOC pool). Soil degradation decreases biomass productivity, reduces the quantity (and quality) of biomass returned to the soil, and as a consequence decreases the SOC pool. Among all soil degradative processes, accelerated soil erosion has the most severe impact on the SOC pool. Several experiments have shown on-site depletion of the SOC pool by accelerated erosion. The most important of soil degradation is soil erosion.
43
Soil Erosion and Desertification Introduction of plowing and turning under of crop residue and weed biomass accentuated risks of accelerated erosion by water on sloping land and wind on flat terrain (Fig.7). Whereas the natural soil erosion created the most fertile soil in river valleys that were the cradle of modern civilization, onset of the accelerated erosion by plowing toppled many of the same civilization by washing/blowing away the mere foundation on which they developed. Accelerated erosion is analogous to “cancer” of the land that causes some of the thriving civilizations to vanish (Olson, 1981). Soil erosion, a quiet crisis, has
44
been a challenge to farmers since the time they began to use the land for settled and intensive agriculture. Because SOC is concentrated on soil surface, even relatively small amount of erosion can cause severe reduction in SOC pool. In India, about 90 Mha land suffers from water erosion and about 50Mha from wind erosion.
Fig. 7: Soil erosion leads to preferential removal of topsoil rich in soil organic carbon 45
Erosion disrupts soil aggregates and exposes previously protected SOC and renders it easily mineralizable. An estimate of water erosion reveals that 5.334 billion tonnes of soil in the country (16. t ha-1 yr-1) is lost annually of which about 29 per cent is lost to sea and 69 per cent is dislocated from one to other place. The amount of C in eroded sediments comes to be 5.34 x 1012 g C assuming a modest average value of 00.1 per cent. Not the entire eroded C adds to atmospheric CO2, however, it has perceptible bearing on C cycle. A similar scene would appear of C losses of wind eroded sediments are computed. (Lal, 1999)
Soil erosional processes
Detachment
Transport and redistribution
Breakdown
Exposure of encapsulated C to microbial process and mineralization
A possible increase in mineralization
Emission of GHGs into the atmosphere
46
Deposition
• Methanogenesis • Denitrification
•Reaggregation •Deep burial
SOC sequestration
Table 7. SOC loss due to different erosions Severity of erosion
SOC loss (Pg C) Water erosion
Wind erosion
Total
Light
1.4 – 2.1
0.5 – 0.8
1.9 – 2.9
Moderate
7.9 – 13.2
2.0 – 3.0
9.9 – 16.2
Strong and extreme
6.7 – 11.2
0.4 – 0.7
7.1 – 11.9
16 – 27
3–5
19 – 32
Total
(Lal, 2000) Lal (2000) has reported the historic loss of carbon because of land area affected by water and wind erosion (Table 7). Table 8. Soil loss due to wind erosion in Rajasthan Soil loss (kg ha-1 d-1)
Mean wind Speed
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯
(kmph)
Bikaner
Jodhpur
Jaisalmer
5
0.5
0.3
1.0
10
120.8
1.4
8.0
20
273.7
15.6
76.7
40
1605.2
NA
1276.0
(Narain et al., 2000) Narain et al. (2000) have reported a loss of 0.3 to 1605.2 kg soil ha-1 d-1 from different locations in Rajasthan (Table 8).
47
Table 9. SOC depletion due to wind erosion Sites
Non-degraded
Degraded
Cultivated
0.19
0.09
Culturable waste
0.20
0.12
Permanent pasture
0.31
0.10 (Narain et al., 2000)
Sharp decline in SOC as a result of wind erosion has also been reported by Narain et al. (2000) (Table 9). Carbon is not the only element lost from soil due to erosion but as most of the nutrients are bound in C skeleton easy mineralization of C from sediments after erosion also promotes the losses of N, P, S and other nutrients. Emission of 10 x 109 g C ha-1 is likely to result in mineralization and loss of 833 kg N, 200 kg P and 143 kg S assuming C/N of 12:1, C/P of 50:1 and C/S of 70:1 (Himes, 1998). As a consequence soil erosion decreases soil productivity. Table 10. Productivity loss due to soil erosion in different soil orders Soil erosion Class
Soil loss (t ha-1)
Loss of productivity (%) Inceptisols
Vertisols
Alfisols
Nil to very slight
0–5
Nil
40
(Velayutham and Bhattacharyya, 2000)
48
Velayutham and Bhattacharyya (2000) have reported heavy losses in soil productivity with increasing soil erosion (Table 10). Table 11. Types of degradation and causative agents Causative
Type of degradation
agents Water
Sheet erosion; rill erosion; gullied land; water logging salinity; sodicity; marshy land
Wind
Sand dunes, sand bars; coastal sand
Man
Shifting cultivation; mine spoils; industrial wasteland; degraded forests
Others
Shallow lands; land slides
Soil degradation is the result of various causative agents – some anthropogenic and some natural (Table 11). Table 12. Area of degraded lands in India Type of land degradation
Area (M ha)
Water erosion
57.16
Wind erosion
10.46
Salt-affected and waterlogged
9.52
Shifting cultivation
2.38
Degraded forests
24.90
Ravines
2.68
Others (mines, querry, land slides and acid sulphate etc.)
0.34
Total
107.43 (MAC, 1994)
49
Recent conservative estimates of degraded area by Ministry of Agriculture show at least 107 Mha degraded area in the country Soil degradation depletes soil organic C pool (Table 12). Global emissions of C from sediments displaced by erosion are estimated to be 1.14 Pg C yr-1 (Lal, 1995). The loss of C by desertification in dry lands may also be very high. Lal has also reported that the soil erosion from dry lands causes annual emission of 0.23 – 0.29 Pg C yr-1 which could be an underestimate because of difficulty in computing loss of soil inorganic C caused by wind or water erosion (Nordt et al, 1999). Slowing worldwide soil degradation, especially desertification could conserve 0.5 – 1.5 Pg terrestrial C annually, an amount relatively significant to 3 Pg C which accumulates in atmosphere every year (Lal, 1999). Lal (2000) estimated that restoring 2 billion ha of degraded lands that were once biologically productive could effectively mitigate the accelerated greenhouse effect. Soil degradation is a biophysical process fueled by socioeconomic and political factors. In socioeconomic terms, “when people are hungry and poverty stricken, they pass on their suffering to the land”. Aldo Leopold in the book “The Quiet Crisis”, sates that “we abuse land because we regard it as a commodity belonging to us. When we see land as a community to which we belong, we may begin to use it with love and respect.
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Carbon sequestration Carbon sequestration refers to the storage of carbon in a stable solid form. It occurs through direct and indirect fixation of atmospheric CO2. Direct soil carbon sequestration occurs by inorganic chemical reactions that convert CO2 into soil inorganic carbon compounds such as calcium and magnesium carbonates. Direct plant carbon sequestration occurs as plants photosynthesize atmospheric CO2 into plant biomass. Subsequently, some of this plant biomass is indirectly sequestered as soil organic carbon (SOC) during decomposition processes. The amount of carbon sequestered at a site reflects the long-term balance between carbon uptake and release mechanisms. Many agronomic, forestry, and conservation practices, including best management practices, leads to a beneficial net gain in carbon fixation in soil. Carbon sequestration is the facilitated redistribution of carbon from the air to the oceans, the terrestrial biomass, soils, and geologic formations. For semiarid and sub-humid regions, carbon sequestration in soils represents the most promising option. This approach is based on the assumption that fluxes or movements of carbon from the air to the soil can be increased while the release of soil carbon back to the atmosphere is decreased. Instead of being a carbon source, soils could be transformed into carbon sinks, absorbing carbon instead of
51
emitting it. This will reduce the rate of atmospheric CO2 increase, thereby mitigating global warming. Catching and holding carbon is called sequestration. It keeps the carbon from being incorporated into carbon dioxide or other greenhouse gases that contribute to global warming. "It's simple: carbon in the air - bad; carbon in the soil - good," Mr. Roberts said recently in a telephone interview. "We've got to get more out of the air and into the soil." Carbon enters the soil as roots, litter, harvest residues, and animal manure. It is stored primarily as soil organic matter (SOM). The density (w/v) of carbon is highest near the surface, but much of the recent SOM decomposes rapidly, releasing CO2 to the atmosphere. Some carbon becomes stabilized, especially in the lower part of the profile. Balanced rates of input and decomposition determine steady state fluxes. However, in many areas, agricultural and other land use activities have upset the natural balance in the soil carbon cycle, contributing to an alarming increase in carbon release. Carbon sequestration in soils has direct advantages for the people who manage this carbon pool. Improved land and soil management practices can
52
yield economic, environmental, and social benefits for local populations and, at the same time, contribute to climate change mitigation. What is soil carbon sequestration? Terrestrial carbon sequestration is defined as either the net removal of CO2 from the atmosphere or the prevention of CO2 net emissions from the terrestrial ecosystems into the atmosphere. Soil carbon sequestration is the process of transferring carbon dioxide from the atmosphere into the soil through crop residues and other organic solids, and in a form that is not immediately remitted. This transfer or “sequestering” of carbon helps off-set missions from fossil fuel combustion and other carbon-emitting activities while enhancing soil quality and long-term agronomic productivity. Soil carbon sequestration can be accomplished by management systems that add high amounts of biomass to the soil, cause minimal soil disturbance, conserve soil and water, improve soil structure, and enhance soil fauna activity. Increased long term (20-50 year) sequestration of carbon in soils, plants and plant products will benefit the environment and agriculture. Land use changes and forest/ soil degradation affect emissions of green house gases (GHGs) rather strongly, thereby causing global warming. These processes have led to a deep concern among the policy makers, scientists and public alike. Vegetation and soils are widely recognized as carbon storage sinks. The global biosphere absorbs
53
roughly 2 billion tonnes of carbon annually, an amount equal to roughly one third of all global carbon emissions from human activity. Significant amounts of this carbon remains stored in the roots of certain plants and in the soil. In fact, the inventory of carbon stored in the global ecosystem equals roughly 1,000 years worth of annual absorption, or 2 trillion tons of carbon. Vegetation and soils are widely recognized as carbon storage sinks. The global biosphere absorbs roughly 2 billion tons of carbon annually, an amount equal to roughly one third of all global carbon emissions from human activity. Significant amounts of this carbon remain stored in the roots of certain plants and in the soil. In fact, the inventory of carbon stored in the global ecosystem equals roughly 1,000 years worth of annual absorption, or 2 trillion tons of carbon. www.earthscience.Com How is Carbon Sequestered in Soils? Through the process of photosynthesis, plants assimilate carbon and return some of it to the atmosphere through respiration. The carbon that remains as plant tissue is then consumed by animals or added to the soil as litter when plants die and decompose. The primary way that carbon is stored in the soil is as soil organic matter (SOM). SOM is a complex mixture of carbon compounds, consisting of decomposing plant and animal tissue, microbes (protozoa,
54
nematodes, fungi, and bacteria), and carbon associated with soil minerals. Carbon can remain stored in soils for millennia, or be quickly released back into the atmosphere. Climatic conditions, natural vegetation, soil texture, and drainage all affect the amount and length of time carbon is stored. Soils in carbon sequestration Worldwide, SOC in the top 1 meter of soil comprises about 3/4 of the earth's terrestrial carbon; nevertheless, there is tremendous potential to sequester additional carbon in soil. For example, many cropland soils of the United States have lost as much as 50% of their original SOC due to the effects of land clearing and tillage. Such conventional farming practices "burn" SOC just as we burn fossil fuels today. However, in the case of SOC this historical decline can be reversed, which is not the case for fossil fuel reserves. With proper management the US can put back much of the SOC depleted over the past two centuries. Improved management of crop, grazing, and forest lands is estimated to potentially offset 30,000-60,000 million metric tons of the carbon released by fossil fuel combustion over the next 50 years. • Soil plays a strategic role in the global carbon balance. It is the biogeochemical interface between the atmosphere, biosphere and hydrosphere.
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• Soil plays a key role in the global carbon balance because it supports all terrestrial ecosystems that cycle most of the atmospheric and terrestrial carbon. • Soil contains more inorganic carbon than the atmosphere and more organic carbon than the biosphere. • Soil is considered to be an active and significant component in global carbon emission and sequestration potential. Carbon and soil organic matter Carbon is a key ingredient in soil organic matter (57% by weight). Plants produce organic compounds by using sunlight energy and combining carbon dioxide from the atmosphere with water from the soil. Soil organic matter is created by the cycling of these organic compounds in plants, animals, and microorganisms into the soil. Well decomposed organic matter forms humus, a dark brown, porous, spongy material that provides a carbon and energy source for soil microbes and plants. When soil is tilled, organic matter previously protected from microbial action is decomposed rapidly because of changes in water, air, and temperature conditions, and the breakdown of soil aggregates accelerates erosion. A soil with high organic matter is more productive than the same soil where much of the organic matter has been “burned” through tillage
56
and poor management practices and transported by surface runoff and erosion. However, organic matter can be restored to about 60 to 70% of natural levels with best farming practices. Relationship between carbon sequestration, land management and other environmental factors The implementation of effective land management practices, especially through stewardship activities such as the Conservation Reserve Program, Wetland Reserve Program, Forestry Incentive Program and conservation tillage, lead to both increase above ground carbon sequestration and to increase SOC. Soils gaining SOC are also generally gaining in other attributes that enhance plant productivity and environmental quality. Increases in SOC generally improve soil structure, increase soil porosity and water holding capacity, as well as improve biological health for a myriad of life forms in soil. In general there is a favorable interplay between carbon sequestration and various recommended land management practices related to soil fertility (e.g., adding mineral fertilizers, manures, sludges and biosolids), tillage, grazing, and forestry. Recommended agronomic, grazing land and forestry practices also enhance land sustainability, wildlife habitat and water quality. In most locations, especially environmentally sensitive settings, these practices also result in decreased water and wind erosion that degrade soil carbon stocks. The same positive relationship
57
that exists between carbon sequestration and recommended land management can, in some settings, improve water quality and aid wildlife habitat restoration. What still needs to be known? Improvement in monitoring and verification protocols for carbon sequestration in soil plant ecosystems is needed for quantitative economic and policy analyses. These protocols need to be quantitatively defensible and readily applicable to fields and watersheds with differing land uses and weather conditions. Such protocols must be acceptable, both domestically and internationally, to scientists, policy makers, landowners, and business groups. These protocols must be suitable for use by employees of government agencies and licensed professionals. Practical techniques to quantify the overall net beneficial impact of agricultural and silvicultural practices on all greenhouse gases, including methane (CH4) and nitrous oxide (N2O) are needed. Other beneficial services derived from improved land practices, such as changes in soil quality, productivity, water and air quality, and erosion must also be recognized and evaluated. Recommended carbon sequestration practices must show benefit for the total environment from a whole ecosystem accounting perspective. Plant breeders and physiologists must be involved to insure that cultivars developed for economic and biological yields also improve soil. The biological yield of
58
above and below ground biomass should have properties, such as elevated lignin and suberin contents that promote accumulation of stable carbon forms in soil and in plant products. The potential impact of climate variability on the stability of sequestered carbon in soils and long-lived plants must be evaluated. Determination of suitable land management practices is needed to minimize risk of carbon release from soil in response to changes in regional weather patterns such as El Niño and La Niña. Longterm studies are needed to insure that currently effective carbon sequestration practices result in stable carbon forms for the long term (at least 20-50 years). A soil carbon economy: A timely opportunity for all Nations The economic value of soil carbon needs to be assessed with consideration for both onsite and offsite effects. Procedures are needed for a defensible soil carbon accounting system, and policies need to be established that provide incentives for net soil carbon sequestration at the global scale. Such policies can provide financial incentives for the restoration of degraded and impoverished soils, while achieving reductions in the rate of buildup of atmospheric CO2 levels. Enhancing the SOC pool through global and national policy incentives may be especially beneficial to improving agronomic productivity, enhancing food security and reducing poverty in countries with degraded soils, thus providing splendid benefits to humankind. The dynamics of
59
carbon sequestration processes must be evaluated in the context of local soil and crop attributes including biogeochemical cycles and soil spatial variability. Accounting for variability of soil and plant processes is needed at a range of scales from fields to watersheds and from regional to continental scales. Spatial and temporal variability of soil C is known to be high in many areas. Any soil C commodity price structure will need to be conservatively discounted for soil C variability. Incentives and plans for adoption of sound technical practices are needed for farmers, ranchers, foresters and other land managers. Implementation of such plans will require better understanding of rural society and the market incentives needed for adopting changes in farming, grazing, and forestry practices that benefit all sectors of society. Adding organic matter to farmland is good for soil quality and crop yields, both short-term and long-term. Continuous no-till is an efficient way of doing this. Cover crops and manure also help raise carbon levels. If you want to sequester carbon to reduce global warming (and possibly receive a small annual payment) think of it as a bonus for being a good farmer. Soil carbon sequestration is a natural, cost-effective, and environmentally- friendly process. Once sequestered, carbon remains in the soil as long as restorative land use, continuous no-till, and other Best Management Practices are followed. It is a win-win option. While mitigating climate change by off-setting fossil fuel
60
emissions, it also improves quality of soil and water resources, enhances agronomic productivity, and buys us time to identify and implement viable alternatives to fossil fuel. Strategies of C sequestration/soil restoration for mitigating Greenhouse effect It is our moral to restore degraded soils and ecosystems for use by future generations. Theodore Roosevelt in his message to congress said, “To waste, to destroy our natural resources, to skin and exhaust the land instead of using it so as to increase its usefulness will result in undermining in the days of our children, the very prosperity which we sought to hand down to them amplified and developed”. Low C content is a common character of most degraded lands, but these soils still have great potential for C accumulation. Carbon sequestration implies not only increasing the amount of C entering in soil but also decreasing the amount leaving through decomposition and erosion. Attempts to sequester C can therefore, be divided in two distinct but interdependent phases. First phase involves conservation of on-site C by controlling deforestation and erosion and in second phase reclamation and rehabilitation of the degraded lands is carried out to improve growing conditions of plants using modern methods for enrichment of C in soil. There are several technological options for sequestration of atmospheric CO2 into one of the other global pools (figure 8 & 9). The choice
61
of one or a combination of several technologies is important for formulating energy policies for future economic growth and development at national and global scales.
Fig. 8: A wide range of processes and technological options for C sequestration in agricultural, industrial and natural ecosystems.
62
Fig: 9: Management strategies for soil carbon sequestration through restoration
of degraded soils and adoption of RMPs on agricultural soils. Conservation of Carbon On-site Soil C can be conserved by physically modifying land configuration or by creating vegetative barriers to control the movement of soil. Soil conservation
63
practices such as bench terracing, contour bunding, graded trenches etc. are effective ways to reduce water erosion. In the areas affected by gully erosion, stabilization of ridges and construction of check dams are essential features of physical modification. Narain et al. (2000) reported that practicing of graded bunds in red and lateritic soils reduced run-off from 20 to 13 per cent and soil loss from 4.0 to 0.1 t ha-1. Vegetation is used to provide strength to the physical barriers to control water erosion but they are often the only means to restrict wind erosion. Narain et al. (2000) have reported strip cropping with alternate strip of crops and grasses, shelterbelt plantations with three to five rows of trees in pyramidal shape as effective barriers to control wind erosion. Enrichment of Carbon in Soil Increasing plant biomass is the only option to increase C in soils. Degraded soils cannot support high plant biomass, they must initially be rehabilitated to sustain higher biomass and then productivity of biomass be increased through modern techniques. Degraded lands are formed due to various causative agents. Therefore, controlling the activity of causative agents is the first step of rehabilitation, which is to be followed by site-specific corrective measure to
64
arrest land degradation. Conversion to an appropriate land use and adopting RMPs lead to SOC sequestration through the following processes: Aggregation: increase in stable micro-aggregates through formation of organo-mineral complexes encapsulates C and protects it against microbial activities. Humification: an increase in chemically recalcitrant humic compounds enhances the relative proportion of passive fraction of SOC. A high clay content and relatively higher proportion of high activity clays (HACs) enhance the retention of recalcitrant SOC fraction. Translocation into the sub-soil: translocation of SOC into the sub-soil can move it away from the zone of disturbance by plowing and other agronomic operations, and minimize the risks removed by erosional processes. Formation of secondary carbonates: land use and management systems in arid and semi-arid regions that enhance formation of secondary carbonates also lead to SIC sequestration (Monger and Gallegos, 2000). Leaching of carbonates into the groundwater is another mechanism of SIC sequestration, especially in irrigated soils (Nordt et al., 2000). The important strategies of enriching C in soil are
65
1. Conservation tillage Plowing the soil is as old as settled farming i.e. way back to 10-13 millenia ago. Yajur veda, the ancient Hindu scripture written between 500 and 1000 B.C states, “let the plowshare turn up the furrow slice in happiness, air and sun nourishing the earth with water and elements”. But the modern day agriculture has changed the meaning of plowing because of ignorance of farmers, not knowing when and how to plow. However, plowing is unlike any natural disturbance because nothing in nature repeatedly and regularly turns over the soil to the specified plow depth of 15-20 cm. Therefore, neither plants nor soil organisms have evolved or adapted to this drastic perturbation. Consequently, plowing renders the soil in a state of an unstable equilibrium, which exacerbates risks of soil erosion by water and wind, disrupts cycles of water and other elements (C, N, P, S), and alters the habitats of soil fauna and flora. Some have called this drastic perturbation the “Rape of the earth” and “Plowman’s folly”. Plowing increases mineralization of SOC by mixing crop residues in soil, bringing it closer to microbes, increasing O2 concentration in soil, disrupting aggregates and exposing physically protected organic matter to microbial and enzyme activity. Conversion of plough-based farmland to conservation tillage reduces risk of soil erosion and can sequester some of the C that would have
66
been otherwise lost. Conventional tillage and erosion deplete SOC pools in agricultural soils. Thus, soils can store C upon conversion from plow till to no till (Fig. 10) or conservation tillage, by reducing soil disturbance, decreasing the fallow period and incorporation of cover crops in the rotation cycle. Eliminating summer fallowing in arid and semi-arid regions and adopting no till with residue mulching improves soil structure, lowers bulk density and increases infiltration capacity (Shaver et al., 2002). However, the benefits of no till on SOC sequestration may be soil/site specific, and the improvement in SOC may be inconsistent in fine textured and poorly drained soils (Wander et al., 1998). Some studies have also shown more N2O emissions in no till (Mackenzie et al., 1998). Similar to the merits of conservation tillage reported in North America, Brazil and Argentina (Lal, 2000; Sa et al., 2001) several studies have reported the high potential of SOC sequestration in European soils (Smith et al., 1998, 2000a,b). Smith et al. (1998) estimated that adoption of conservation tillage has the potential to sequester about 23 Tg C/year in the European Union or about 43 Tg C/year in the wider Europe including the former Soviet Union. In addition to enhancing SOC pool, up to 3.2 Tg C/year may also be saved in agricultural fossil fuel emissions. Smith et al. (1998) concluded that 100% conversion to no till agriculture could mitigate all fossil fuel C emission from agriculture in Europe. In some cases, soil organic
67
carbon can be augmented up to 50 per cent through no-till or reduced tillage practices (IPCC, 1990). Table 13. SOC accumulation due to conservation tillage Treatment
0 - 15cm
15 - 30 cm
No-tillage (4 year)
Tillage (4 year)
Soil organic C (%)
0.24
0.22
0.15
0.15
Hydrolysable-N kg-1)
179.2
168.0
175.0
156.8
(mg
No-tillage (4 year)
Tillage (4 year)
(Singh et al. 1998). Singh et al., (1998) have shown slight improvement in SOC and hydrolysable N after 4 years of practicing no-tillage in arid conditions (Table 13).
Fig. 10. No till farming and other conservation tillage practices eliminate drastic soil disturbance, and enhance soil organic matter in the surface layers. Conversion from plow till to no-till with residue mulch is a viable option for SOC sequestration.
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Table14. SOC gain in different countries by conservation tillage Country Brazil
Location Southern
Morocco Semi-arid
Soil
SOC gain
Reference
Sandy clay loam, Acrisol
44 Mg C/ha over 9 yrs
Bayer et al. (2000)
Calcixeroll
13.6% over
Mrabet et al (2001)
11 yrs USA
Kansas
USA
Eastern corn belt
Argiudoll, Haplustoll, Hapludoll
SOC increased with quantity of residue returned
Havlin et al. (1990)
Miscellaneous
300-600 kg/ha/yr
Dicket et al. (1998)
Different scientists all over the world have shown increasing SOC content by conservation tillage (Table 14). 2. Cover Crops and Crop Rotation The effectiveness of conservation tillage in SOC sequestration is enhanced by use of cover crops and appropriate rotations (Fig 11). Frequent use of sodtype legumes and grasses in rotation with food crops is an important strategy to enhance SOC and improve soil quality. Growing leguminous cover crops enhances biodiversity, the quality of residue input and SOC pool (Singh et al., 1998; Fullen and Auerswald, 1998; Uhlen and Tveitnes, 1995). It is well established that ecosystems with high biodiversity absorb and sequester more C
69
than those with low or reduced biodiversity. Drinkwater et al. (1998) observed that legume-based cropping systems reduce C and N losses from soil. In Georgia, USA, Sainju et al. (2002) observed that practicing no till with hairy vetch can improve SOC. Franzluebbers et al. (2001b) also observed in Georgia, USA that improved forage management can enhance the SOC pool. However, the use of cover crops as a short-term green manure may not necessarily enhance the SOC pool. The beneficial effect of growing cover crops on enhancing SOC pool has been reported from Hungary by Berzseny and Gyrffy (1997), U.K. by Fullen and Auerswald (1998) and Johnston (1973), Sweden by Nilsson (1986), Netherlands by Van Dijk (1982) and Europe by Smith et al. (1997). The practice of summer fallowing observed in semi-arid countries can decrease SOC pool at the rate of 320 to 530 kg C ha-1 (Doran et al. 1998). Gains in SOC by growing cover crops in rotation with food crops in India were also reported. Aggarwal et al., (1997) have shown that three-year cultivation of mungbean and cluster bean increased SOC from 0.22 to 0.26 and 0.28 per cent,
70
respectively.
Fig. 11. Mucuna utilis (velvet bean), a suitable cover crop for the humid tropics of West Africa, and other cover crops enhance SOC pool Table 15. SOC gain in different countries by crop rotation Country
Location
Soil
SOC gain
Reference
Canada
Saskatchewan
Prairie
Minor
Canada
Saskatchewan
Prairie
1.2 – 2.4 Mg C/ha/yr
Curtin et al. (2000)
USA
Colorado
Weld loam, Paleustolls
71
20% in 0-5 cm depth
Curtin et al. (2000) Bowman et al. (1999)
3. Crop Residues, Manure and Fertilizers, Green Manuring Judicious nutrient management is crucial to SOC sequestration. In general, the use of organic manures and compost enhances the SOC pool more than application of the same amount of nutrients as inorganic fertilizers (Gregorich et al., 2001). The fertilizer effects on SOC pool are related to the amount of biomass C produced/returned to the soil and its humification. Adequate supply of N and other essential nutrients in soil can enhance biomass production under elevated CO2 concentration (Van Kessel et al., 2000). Longterm manure applications increase the SOC pool and may improve aggregation (Sommerfeldt et al., 1988; Gilley and Risse, 2000), and the effects may persist for a century or longer (Compton and Boone, 2000). The potential of conservation tillage to sequester SOC is greatly enhanced whereby soils are amended with organic manures (Hao et al., 2002). Smith and Powlson (2000) reported that 820 million metric tons of manure are produced each year in Europe, and only 54% is applied to arable land and the remainder to non-arable agricultural land. They observed that applying manure to cropland can enhance its SOC pool more than it does on pasture land. Smith and Powlson estimated that if all manure were incorporated into arable land in the European Union, there would be a net sequestration of 6.8 Tg C/year, which is equivalent to 0.8% of the
72
1990 CO2-C emissions for the region. Beneficial impacts of manuring for U.S. cropland were reported by Lal et al. (1998). World over 3.4 Pg residues are produced with almost a tenth of it in India (Tandon 1997). On global basis nearly 1.53 Pg C yr-1 is accumulated in residues (Lal, 1995). Returning crop residues to soil increases SOC that shows linear increases with the quantity of residue returned (Duiker and Lal 1999). Returning crop residues to soil has converted many soils from “source” to “sink” of atmospheric CO2 (Rasmussen et al. 1998) by enhancing soil productivity. Sehgal and Abrol (1994) have reported no-addition of crop residues as one of the major reasons of degradation of marginal lands. Higher crop yields and increase in soil organic C has often been reported after residue application from almost all the regions of the world. Aggarwal et al. (1997) have reported that incorporation of cluster bean and mung bean residue and manure along with fertilizer – N increased SOC content and also increased pearl millet production. .Table 16. SOC gain due to mulching Runoff
Soil loss
SOC (%)
N
K2O
Clean cultivation
54.50
5.08
0.28
218.04
136.34
Natural grass
35.08
2.17
0.36
222.21
172.78
Zea mays cultivation
39.65
4.41
0.34
234.72
125.44
Treatment
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Zea mays straw mulch @ 5 t haZea mays straw mulch @ 10 t ha leucocephela mulch @ 5 t ha
-1
leucocephela mulch @ 10 t ha-1
-1
40.25
2.10
0.33
217.33
127.93
32.24
2.20
0.34
234.72
116.87
27.36
1.29
0.30
218.03
157.25
26.17
1.34
0.34
234.72
125.44
(Yadav et al., 2000) SOC (%) reported more than 50 per cent decline in soil loss and increase in SOC by applying crop residues as mulch (Table 16). Table 17. Estimation of C balance (Mg ha-1) in plots with and without fertilizer addition in the rice-wheat-jute cropping system in long-term fertilizer experiment at Barrackpore (based on 29 years data) Source of Carbon Root C (Mg ha-1)
Rhizodeposition C (Mg ha-1)
Gross total C -1
(Mg ha )
Cropping System
Control
100% N
100% NP
100% NPK
NPK + FYM
Rice
0.197
0.341
0.400
0.444
0.551
Wheat
0.052
0.206
0.271
0.238
0.250
Jute
0.099
0.163
0.215
0.231
0.318
Total
0.348
0.688
0.857
0.881
1.035
Rice
0.059
0.102
0.120
0.133
0.153
Wheat
0.016
0.062
0.081
0.071
0.075
Jute
0.030
0.049
0.064
0.069
0.095
Total
0.104
0.207
0.257
0.264
0.311
Rice
0.256
0.444
0.521
0.577
0.665
Wheat
0.068
0.268
0.352
0.309
0.325
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Annual mineralization of C (Mg ha-1)
Net input C (root+Rhizodepo sition) (Mg ha-1)
Net change in Social Organic C
Jute
0.129
0.212
0.279
0.300
0.413
Total
0.452
0.895
1.114
1.145
4.646
Rice
0.159
0.321
0.376
0.417
0.481
Wheat
0.042
0.194
0.255
0.223
0.235
Jute
0.080
0.153
0.202
0.217
0.299
Total
0.282
0.647
0.806
0.828
0.973
Rice
0.096
0.123
0.144
0.160
0.184
Wheat
0.026
0.074
0.098
0.086
0.090
Jute
0.049
0.059
0.077
0.083
0.114
Total
0.171
0.248
0.309
0.317
3.673
11.12
12.32( -3.30)
12.54
15.84
19.58
(-3.08)
(+0.22)
(+3.96)
(Mg ha-1)
(-4.40)
(CRIJAF, 2000) CRIJAF – Central Research Institute for Jute and Allied Fibres Long-term experiments at Ludhiana showed that integrated use of fertilizer and manure for 20 years increased the SOC content in 0 to 10 cm layer of sandy soil by 0.2 per cent. The increase in SOC was 0.6 per cent for a soil in Jabalpur and 0.5 per cent in Bhubaneswar (Nambiar 1994, 1995). Long term experiments at Coimbatore showed that integrated use of fertilizer and manure for 20 years increased the SOC content in Inceptisols.
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Table 18. Organic C content (%) in soil as influenced by treatments and cropping system (Finger millet- Maize- Cowpea) under LTFE over past 20 years Treatment
1972
1977
1987
1992
1997
2000
50% NPK
0.304
0.40
0.44
0.43
0.53
0.55
100% NPK
0.300
0.47
0.45
0.43
0.50
0.51
150%NPK
0.295
0.50
0.48
0.50
0.58
0.57
100%NPK + HW
0.300
0.45
0.45
0.47
0.53
0.56
100% NPK + ZnSO4
0.303
0.48
0.45
0.48
0.52
0.6
100% NP
0.304
0.40
0.46
0.47
0.54
0.54
100% N
0.300
0.50
0.44
0.41
0.57
0.55
100% NPK + FYM
0.300
0.64
0.52
0.53
0.60
0.66
100%NPK – (S)
0.303
0.49
0.46
0.46
0.50
0.6
Control
0.300
0.49
0.43
0.34
0.45
0.46
(Santhy et al., 2001) The rate of increase in SOC pool by fertilizer application depends on the antecedent SOC pool and can be more for initially low-antecedent SOC (Follett, 2001). However, in case of sub-optimal levels of application in tropics fertilizer application may cause reduction in SOC pool.
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Experiments are being conducted at Central Arid Zone Research Institute (CAZRI) to study the possibility of C sequestration in pearl millet based production systems. Calculations are based on C added for crop production as manure of fertilizer, enrichment/depletion of SOC and C stored in crop biomass. Results showed that production systems based on crop rotation, fallowing, annual application of 5 tonnes manure with or without urea were a net contributor of C to atmosphere in arid climate. Grain and biomass yields were however, higher with annual application of 5 tonnes manure. Highest grain yields were recorded after combined application of 5 tonnes manure with 50 kg urea-N but this system was a net contributor of C to atmosphere. Annual application of 2.5 tonnes manures with 40 kg N ha-1, on the other hand, stabilized grain and total biomass statistically at par with highest achieved in experiment and showed a net C sequestration. Results suggested that it is possible to use crop production systems for C sequestration but heavy application of manures must be avoided for this purpose. In irrigated areas of dry sub-humid and per humid climate addition of a green manure crop preferably a legume, in a sequence or addition of farmyard manure or organic manure or green leaf manuring and crop sequencing with legume should be a part of strategy to maintain organic C to permissible higher level.
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Table 19. SOC gain in different countries by manuring or fertilizer use Country
Location
Soil
SOC gain by manuring or fertilizer use
Reference
Denmark Askov
Sandy loam soil
11% over years
Schjonning et al. (1994)
India
Central India
Vertisol (Typic, Haplustert)
85-739 kg C/ha/yr
Kundu et al (2001)
India
Coimbatore
Inceptisol – LTFE 40 % over 30 years
Selvi et al., (2003)
U.K
Rothamsted
Broadbalk site
Jenkinson (1990)
USA
Colorado
Prairie soils
100 % over 144 years
Bowman and Halverson
40% increase in SOC (1998) of 0-5 cm depth
4. Soil water management Similar to the addition of fertilizers and manures in a nutrient – depleted soil, increasing available water in the root zone of a drought-prone soil can enhance biomass production, increase the amount of above-ground and the root biomass returned to the soil and improve SOC concentration. The linear relation between biomass production and transpiration illustrates the important role of
78
soil water. Water management in the root zone may involve soil water conservation in regions with adequate precipitation, supplemental irrigation, and water table and periodic inundation. Supplemental irrigation, applied with an appropriate method at the critical time and at the desired rate, is essential for high yields in arid and semi-arid regions. In Texas, Borodovesky et al. (1999) observed that surface SOC concentration in plots with irrigated grain sorghum and wheat increased with time. Irrigation can also enhance SOC concentration in grassland. In New Zealand, Barkle et al. (2000) reported an increase in SOC pool by land application of dairy farm effluent. Similarly an application of long-term wastewater irrigation (up to 80 years) increased SOC concentration in Vertisols and Leptosols in the valley of Mezquital in Mexico (Friedel et al., 2000). In addition to SOC, irrigation can also impact Soil Inorganic Carbon (SIC) dynamics, while ground water brought to the surface may release CO2, use of good-quality irrigation water may also sequester SIC through leaching. The formation of secondary carbonates, aided by the activity of soil fauna (e.g., termites, earthworms), is an important process of SIC sequestration. Pedogenic carbonates formed from base-rich bedrocks or noncarbonated sediments are a sink, while those formed in calcareous parent material are not.
79
While irrigation is essential to obtaining high yields in drought-prone soils, excessive and inappropriate irrigation can also cause rise of the water table, waterlogging, and salinization. Indeed, secondary salinization is a serious problem in irrigated agriculture. Risks of secondary salinization are greatly enhanced in soils with restricted surface or subsoil drainage and with poor quality of irrigation water (Gupta and Abrol, 1990). Soil salinization reduces biomass productivity and decreases SOC pool. In contrast, reclamation of salt-affected soils can improve soil quality, increase biomass productivity, and enhance SOC pool. 5. Pasture management On a global basis, grassland/grazing lands occupy 3460 Mha. Restoring degraded grazing lands and improving forage species is important to sequestering SOC and SIC. Furthermore, converting marginal croplands to pastures (by CRP and other set-aside provisions) can also sequester C. Similar to cropland, management options for improving pastures include judicious use of fertilizers, controlled grazing, sowing legumes and grasses or other species adapted to the environment, improvement of soil fauna and irrigation (Follett et al., 2001a). Conant et al. (2001) reported rates of SOC sequestration through pasture improvement ranging from 0.11 to 3.04 Mg C/ha/year with a mean of 0.54 Mg C/ha/year.
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Grasses can be grow even on extremely degraded soils and can produce good forage yield. Cadisch et al. (1998) reported that the introduction of improved pastures increased SOC at the rate of 230 to 3300 kg C ha-1 yr-1 in tropics. Long term experiments in Australia showed that soil under long-term pasture (1918 – 1986) contained 11.5 x 109 g ha-1 more organic C than unfertilized cropped plots (Rosenzweig et al. 1990). . Table 20: SOC (%) gain due to pastures (Cenchrus ciliaris)
Soil depth (cm) Treatment 0-15
15-30
Cultivated field
0.25
0.20
4 years with stubble
0.44
0.33
4 years without stubble
0.40
0.28
6 years with stubble
0.53
0.42
6 years without stubble
0.45
0.31
8 years with stubble
0.49
0.39
8 years without stubble
0.39
0.39 (Rao et al., 1997)
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Rao et al. (1997) have reported 50 per cent increase in SOC after eight years of plantation in pastures of Cenchrus ciliaris (Table 20). Calculations suggest that 3.6 x 1011 g C can be sequestered in only 832448 ha of permanent pastures of Rajasthan if proper management practices are followed. 6. Agro-forestry Agro-forestry with proper management of the component parts (e.g. trees, agronomic crops and ungulates) can be a sink of CO2 (Dixon, 1999). Sanchez and Benites (1987) studied the possibility of C sequestration in agro-forestry systems in low altitude nations. Assuming that, (i)
establishment of one ha of agro-forestry in low altitude nations can provide products that would otherwise require 5 ha of deforestation, and
(ii)
establishment of new agro-forestry system in 2 x 106 ha annually they
(iii)
calculated that agro-forestry may help in conserving 10 Pg C which goes to atmosphere due to deforestation will be reduced (Dixon et al. 1999)
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Table 21. SOC gain in various agro-forestry systems
Agro forestry models
Year
SOC
Available N
Available P
Agrisilviculture
1989 – 90
1020.0
85.0
44.2
480.0
1994 – 95
2672.0
183.0
86.3
858.0
1989 – 90
632.0
56.0
42.0
42.0
1994 – 95
1598.0
161.6
89.0
858.0
1989 – 90
262.0
86.0
45.2
576.0
1994 – 95
1251.0
131.0
61.0
585.0
1989 – 90
842.0
81.8
31.4
110.7
1994 – 95
1328.0
131.6
49.8
161.3
1989 – 90
3920.0
158.9
12.8
109.2
1994 – 95
4896.0
206.0
21.2
153.3
Agrihorticultural
Silvi – pastoral
Boundary plantation
Alley cropping
Available K
Solanki et al. (1999) Solanki et al., (1999) compared the extent of enrichment of C, N, P and K under various agro forestry systems and reported highest enrichment of C
83
(377%) under silvi-pastoral system and highest increase in available N, P, and K under agri-horticultural system (Table 21). 7. Forestry Converting degraded soils under agriculture and other land uses into forests and perennial land use can enhance the SOC pool. The magnitude and rate of SOC sequestration with afforestation depends on climate, soil type, species and nutrient management (Lal, 2001c). Despite its significance, a few studies have assessed the C sink capacity of forest soils (Lal, 2001d; Kimble et al., 2002). In east central Minnesota, an experiment by Johnston et al. (1996) showed an average SOC sequestration rate of 0.8–1.0 Mg/ha/ year. Afforestation, however, may not always enhance the SOC pool. In New Zealand, Groenendijk et al. (2002) reported that afforestation of pastures with radiata pine (Pinus radiata) decreased the SOC concentration by 15% to a depth of 12–18 cm. These researchers concluded that afforestation of hill country pasture soils resulted in net mineralization of the SOC pool. In the Cerrado region of Central Brazil, Neufeldt et al. (2002) also observed that reforestation of pasture with pine led to a clear reduction of SOC compared to pasture and eucalyptus plantation. In such cases, agroforestry may be another option of conserving soil and improving the SOC pool. On a Vertisol in Ethiopia, Lulu and Insam (2000) observed positive effects of alley cropping (i.e., agroforestry) with Sesbania on the SOC pool. In
84
Europe, Nabuurs et al. (1997) reported that total SOC pool of soils supporting European forests is 12.0 Pg, but did not provide an estimate of the rate of SOC sequestration in forest soils. Afforestation of marginal agricultural soils or degraded soils has a large potential of SOC sequestration. Bouma et al. (1998) observed that in Europe a major change in land use may occur because of technological, socio-economic and political developments. For example, adoption of RMPs or technical advances in modern agriculture may produce the same yield on 30–50% of the current agricultural land. That being the case, there is a potential for converting spare agricultural land to forestry. With conversion to a permanent land cover, there is a large potential of SOC sequestration through agricultural intensification. Forestry has been proposed as a means to reduce net greenhouse gas emissions, by either reducing sources or enhancing sinks. Globally, forest vegetation and soils contain about 1146 Pg C, with approximately 37 per cent of this C in low-latitude forests, 14 per cent in mid-latitudes, and 49 per cent at high latitudes.
85
Application of optimum forest and agro forestry management practices like selected cutting or application of fertilizers on 500-800 ha in 12-15 key nations could sequester 1-2 Pg C yr-1 using current infrastructure (Dixon et al. 1999). Nilsson et al. (1995) made an analysis of changes that could be achieved in the carbon cycle with a global, large scale afforestation programme that is economically, politically, and technically feasible. They estimated that only about 345 Mha would actually be available for plantation and agro-forestry for the sole purpose of sequestering carbon. The maximum annual rate of carbon fixation (1.48 Pg C yr-1) would only be achieved 60 years after the establishment of the plantations of which 1.14 Pg would be in form of above ground biomass and 0.34 Pa as below-ground biomass. Over the period from 1995 to 2095, a total of 104 Pg of carbon would be sequestered which would be one-third of the C added to atmosphere during this period at current rates of emission. Rising of trees on degraded soils does not result in forest-like situation but still can remarkably enhance SOC.
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Table 22. SOC gain with raising different tree species Species
Original
After 20 years
pH
SOC(%)
pH
SOC (%)
Eucalyptus tereticornis
10.3
0.12
9.18
0.33
Acacia nilotica
10.3
0.12
9.03
0.55
Albizzia lebbek
10.3
0.12
8.67
0.47
Terminalia arjuna
10.3
0.12
8.15
0.47
Prosopis juliflora
10.3
0.12
8.03
0.58
(Singh and singh, 1993) Singh and Singh (1993) reported that raising trees on alkali soils of Karnal increased SOC from 0.12 per cent to a maximum of 0.58 per cent (Table 22). 8. Precision farming/Farming by soil Precision farming, farming by soil or soil-specific management, is another important strategy of decreasing losses and enhancing biomass production through alleviation of soil-related constraints (Larson and Robert, 1991). The application of nutrients and water as required for the specific soil conditions enhances use efficiency of inputs and improves crop yield. A judicious combination of integrated nutrient management and precision farming can enhance SOC concentration and improve soil quality (Leiva et al., 1997).
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9. Land use planning and land management Land use planning is the key for many conservation issues; however, its adoption has become impossible due to population pressure. Utilizing land as per its capacity and treating it as per need are essential facets of planning and management. Exploitation of marginal land for cultivation without serious conservation efforts is bound to accelerate the pace of degradation and unbalancing C cycle. The appropriate management with suitable vegetation and planning would at least partially reduce the problem if not completely remove it. The possibility of C sequestration in biosphere has received mixed response form researchers and is being given only partial or discounted credit (Lal, 2001). Restoring degraded soils Restoring degraded soils and ecosystems has a high potential for sequestrating soil C. Most degraded soils have lost a large fraction of the antecedent SOC pool, which can be restored through adopting judicious land use practices. The CRP has been effective in reducing the sediment load and enhancing the SOC pool. The rate of SOC sequestration under CRP may be 600– 1000 kg C/ha/year (Follett et al., 2001b). In Shropshire, U.K., Fullen (1998)
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observed that mean SOC content increased consistently and significantly on plots set aside under the grass ley system at the rate of 0.78% in 4 years. Three arguments have been advanced: (i)
Because the most important long-term need is to confront CO2 emissions and to reduce emissions from fossil-fuel burning, carbon sequestration in the biosphere is but a temporary diversion that may provide useful service in the short term but could distract attention from the primary, long-term need,
(ii)
As discussed above, there is a large range of uncertainty with regard to assessing the potential for carbon sequestration in vegetation and soils,
(iii)
Carbon sequestered in the soil is inherently more labile and therefore, less securely stored than carbon present in the lithosphere in the form of fossil fuels.
Therefore, consideration of prospects of carbon sequestration is soil and vegetation should be carefully analyzed before implementation. Conclusions All the results in potential sequestration of carbon to mitigate greenhouse effect are satisfactory but the statistics of the greenhouse effect are increasing alarmingly and raises the question of “whether or not we are going to survive in this universe”?
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Growing world population and demand of improving living standards effectively negate the possibility of reducing fossil fuel consumption or deforestation and also the level of political commitment needed for such actions is just not there. Above all these feeding the ever-increasing population in a healthy world will be the main targets as “access to food and a clean living environment are two of the most basic human rights, which must be respected for all citizens of the planet earth”. There are not many troubles in the world more alarming than those caused by fire in the pit of an empty stomach. O. Henry in his book “Heart of the West” appropriately stated “Love and business and family and religion and art and patriotism are nothing but shadows of words when a man’s starving”. Access to food for all can be achieved only by agricultural intensification, which has no place for the above said principles to sequester soil carbon. Jonathan Swift the famous author of Gulliver’s Travels points out, “Whoever could make two ears of corn, or two blades of grass, to grow upon a spot of ground where only one grew before, would deserve better of mankind, and do more essential service to his country, than the whole race of politicians put together”.
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But, agricultural intensification should not mean destruction of natural resources that took long time for its formation and the atmosphere, which protects us all. For both issues, food security and environment improvement, the answer lies in soil. “In old Roman empire all roads led to Rome”. In agriculture “all roads lead back to soil, the media which feeds the world”. The motto of modern civilization should be “In soil we trust” and we should respect the dirt that feeds us. There is indeed a strong need for “a brown revolution”. It can be appropriately stated in London Times, “Will everyone please, at some stage during next few days, take hold of a handful of soil and show it some respect? Grab some, take a long, hard look at it, marvel at its workings, sing in praise of its chemistry, bless its bugs for being, and before placing it back on the ground, say thank you”. Soil C sequestration is a strategy to mitigate global warming through reducing the rate of enrichment of atmospheric concentration of CO2 and achieves food security through improvement in soil quality. While reducing the rate of enrichment of atmospheric concentration of CO2 soil C sequestration improves and sustains biomass/agronomic productivity. It has the potential to offset fossil-fuel emissions by 0.4 to 1.2 Gt C/year, or 5 to 15% of the global emissions. Soil organic carbon is an extremely valuable
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natural resource. Irrespective of the climate debate, the SOC stock must be restored, enhanced, and improved. The soil C sequestration potential of this win-win strategy is finite and realizable over a short period of 20 to 50 years. Yet, the close link between soil C Sequestration and world food security on the one hand and climate change on the other can neither be overemphasized nor ignored. In conclusion, it is clear that atmospheric CO2 enrichment increases plant growth and plant-mediated carbon inputs to soils; and the resulting increased soil carbon contents will likely not be reduced if air temperatures rise (in fact, it is possible they may even be enhanced), allowing soil carbon sequestration to increase with increasing atmospheric CO2 concentration. Finally, with more carbon in soils, soil structure and fertility should be improved, providing a positive feedback that further enhances plant growth and soil carbon sequestration. Adding organic matter to farmland is good for soil quality and crop yields, both short-term and long-term. Continuous no-till is an efficient way of doing this. Cover crops and manure also help raise carbon levels. If you want to sequester carbon to reduce global warming (and possibly receive a small annual payment) think of it as a bonus for being a good farmer. Soil carbon sequestration is a natural, cost-effective, and environmentally-friendly process.
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Once sequestered, carbon remains in the soil as long as restorative land use, continuous no-till, and other Best Management Practices are followed. It is a win-win option. While mitigating climate change by off-setting fossil fuel emissions, it also improves quality of soil and water resources, enhances agronomic productivity, and buys us time to identify and implement viable alternatives to fossil fuel. The way forward i.
Abiotic options like capturing, compressing, transporting and injecting CO2 emitted by point sources into geological strata, saline aquifers, and oceans, which may be very expensive.
ii.
Improving the area under forest, this is the most cost – effective and feasible option for the first half of this 21st century.
iii.
Suitable agro-forestry systems
iv.
Precision farming
v.
Appropriate land use planning and management
vi.
Forestry, Agro- forestry, crop productions and soil conservation must be concentrated.
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The potential of SOC sequestration is finite. Therefore, it is only a shortterm solution. The long-term solution lies in developing alternatives to fossil fuel. Yet, SOC sequestration buys us time during which alternatives to fossil fuel can take effect. It is a bridge to the future. It also leads to improvement in soil quality.
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