Soil organic carbon: Towards better soil health

Climate Change and Environmental Sustainability (April 2015) 3(1): 26-34 DOI: 10.5958/2320-642X.2015.00003.4

REVIEW ARTICLE

Soil organic carbon: Towards better soil health, productivity and climate change mitigation

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Shovik Deb1* • Pratap Bhanu Singh Bhadoria2 • Biswapati Mandal3 • Amitava Rakshit4

Abstract The presence of organic C in soil is a key determinant for soil quality and productivity. Soil, being the largest C sinks, also controls the global warming. In this regard, this review focused onto the soil C dynamics, C sequestration potential of soil and related C pools. Detailed discussion of several researches depicted that, C sequestration depends onto soil C saturation deficit, presence of biochemically protected recalcitrant C fractions, aggregation and aggregate associated physically shielded C. On the other hand, labile organic C is the soil nutrient reservoir and is closely related with diversified soil biology. For the sustainable and holistic soil resource management and to mitigate climate change, this study also highlighted the possible management practices towards longer residence time of C in soil. Keywords Soil organic C, C sequestration, Pools of soil C, Management practices, Soil quality, Environmental sustainability, Climate change. Abbreviations C: carbon; OC: organic C; SOC: soil organic C; LTFE: long-term fertility experiments; SOM: soil organic matter; Ctot: soil total organic C; Cinorg: soil inorganic C; Cmic: microbial biomass C; Cmin: mineralizable C; qCO2: soil metabolic or respiratory quotient; MQ: microbial quotient 1. Introduction The global carbon (C) cycle is one of the most important biogeochemical cycles. Soils are the largest global C sink (Montagnini and Nair, 2004), harboring approximately twothirds of the C in ecosystems (Schimel et al., 1994). Soil stores approximately 615 Gt C in top 0.2 m and 2,344 Gt C up to 3 m depths, which is more than the summation of

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Harikesh Bahadur Singh5

biomass and atmospheric C pool (Fontaine et al., 2007). Dynamics of soil organic C (SOC) is gaining increasing importance for its impact onto climate change and crop production. Soil organic C has the slowest turnover rates in terrestrial ecosystems (Trumbore, 1997) and thus C sequestration in soils has the potential to mitigate CO2 loading in the atmosphere (Paustian et al., 1997). On the other hand, SOC is one of the most important components in soil that positively contributes to soil fertility, tilth and crop production (Bauer and Black, 1994; Lal, 2004). Soil physical environment is controlled by the presence of SOC as it favourably affects soil porosity, aggregation and soil water storage (Benbi et al., 1998; Chenu et al., 2000). Organic C (OC) also exerts a significant influence onto chemical properties of soils (Rowsell et al., 2004; Singh et al., 2010), nutrient availability (Benbi and Biswas, 1997; Nieder and Richter, 2000), cation exchange capacity (van Dijk, 1966) and retention and mobilization of metal ions (Stevenson, 1986). Furthermore, OC serves as the energy and food source for soil microbes (Reeves, 1997). Therefore, soil as a C tank is a matter of prime concern (Lal, 2004; Xiong et al., 2014). This is more important particularly for tropical and sub-tropical regions of the world, where soils are intrinsically low in OC stock. Figure 1 depicted the present-day worldwide distribution of SOC. The worldwide expansion of agriculture in the last decades has been accompanied by a rapid oxidation of OC from soils while burning of fossil fuels from industries and automobiles results elevated levels of atmospheric CO2 (four times of the pre-industrial level) (Vellinga and Wood, 2008). Considering the importance of SOC to maintain soil quality, productivity in a long run and to mitigate global warming and climate change, this review focused onto the C

Assistant Professor, Department of Soil Science and Agricultural Chemistry, Uttar Banga Krishi Viswavidyalaya, Pundibari 736 165, West Bengal, India Professor, Department of Agricultural and Food Engineering, Indian Institute of Technology, Kharagpur 721302, West Bengal, India 3 Professor, Directorate of Research, Bidhan Chandra Krishi Viswavidyalaya, Kalyani 741 235, West Bengal, India 4 Assistant Professor, Department of Soil Science & Agricultural Chemistry, 5Professor, Department of Mycology & Plant Pathology, Institute of Agricultural Science, Banaras Hindu University, Varanasi 221 005, Uttar Pradesh, India; *Corresponding author e-mail id: [email protected] 1 2




Climate Change and Environmental Sustainability (April 2015) 3(1): 26-34

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Figure 1 World map of soil organic C: Topsoil (30 m) Source: Food and Agriculture Organization, United Nations (http://www.fao.org/nr/land/sols/en/)

sequestration potential of soil, soil C pools and proper management practices which lead towards a holistic pathway to maintain an overall sustainable soil environment. 2. Methodology Extensive literature review was done with the help of the academic search engine scholar.google.com. Further, Thomson Reuters Web of Knowledge (http://wokinfo.com) was used to gather necessary information. Hardcopy research articles, books, monographs, and project reports were also used to collect as many as possible required database onto SOC sequestration, dynamics, budgeting, stock, saturation, pools, impact of soil C onto soil health and climate and management of soil C. The following sections of this article discussed about all these aspects in detail. 3. Soil C sequestration Carbon sequestration can be defined as the arrest and storage of C in soil that would else be emitted to or remained in the atmosphere. On the other hand, C saturation implies that, soil cannot store any additional C inputs after it reaches the maximum C stabilization capacity (Paustian et al., 1997; Six et al., 2002). Therefore, determining the relative C status of a soil in comparison to its C saturation capacity is

important to understand the C sequestration potential of that very soil. A soil is rendered as either a source or sink of the atmospheric CO2 by agricultural practices (Piao et al., 2010). However, before soil can be credited as a sink of atmospheric CO2, there are needs to understand the C sequestration process in the soil and develop appropriate methodology for determining how much and for how long C is sequestered in the soil. This means the methods must be able to measure the specific proportion of SOC (or the specific SOC pools) involved in C sequestration before they are useful for global C budgeting purposes. Long-term fertility experiments (LTFE) are the primary source of information to find out the consequences of soil management, cropping systems and other anthropogenic activities onto the quantitative and mechanistic changes in SOC pools (Grace et al., 2006). The LTFEs in both Europe and North America pointed out that intensive cultivation resulted exponential decline of SOC (Robert et al., 2001; Ogle et al., 2003; Shi et al., 2010). In Indian context, Katyal et al. (2001) had proven that treatment and practice that supported soil organic matter (SOM) buildup, also favoured sustainable productivity in 30 years old LTFE in tropical soil. Long-term comparative studies inferred that organic and sustainable systems resulted buildup

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Figure 2 Different pools/ fractions of soil organic C and their relationship with different components of ecology

of SOM and thus improved soils sustainability (Kong et al., 2005; Lal, 2006; Manna et al., 2006; Rudrappa et al., 2006). Observed rates of C sequestration range from 0-150 kg C ha -1 y -1 in humid and cool in dry and warm regions (Armstrong et al, 2003) and 100 to 1000 kg C ha-1 y-1 in humid and cool climate (Sá et al., 2001). In comparison, the sequestration rate of inorganic C (Cinorg) as secondary carbonates is low in soil. However, to understand the soil C sequestration it is indeed important to understand the pools of soil C. 4. Pools of soil C 4.1 Soil organic C Quantitative data on SOC pools and fluxes at scales ranging from pedon or soil-scape to ecosystem and regional, national and global scales, are needed to assess the magnitude of soil C dynamics in relation to the biotic and atmospheric C pool and to evaluate the contribution of the soil C pool to the atmospheric pool (Chan, 1997). As per Majumder et al. (2008a), the SOC stock is comprised of active or labile pool and stable and passive or recalcitrant pool (Figure 2). This active or labile pool of SOC is one of the most important components contributing in soil health, sustainability and crop production (Lal, 2004; Bationo et al., 2007; Ghosh et al., 2012) as well as the food and energy source for soil micro-organisms (Steenwerth et al., 2002). The most labile SOC components are considered sensitive indicators of change in the soil quality (Maia et al., 2007). On the other hand, stable, passive or recalcitrant pool of SOC is biochemically protected (Parton et al., 1987), altered very slowly by microbial activities (Weil et al., 2003). These pools are comprised of a range of resistant and non resistant

aromatic and long chain aliphatic compounds. Labile SOC contains microbial biomass and is closely associated with nutrient mineralization and reflects organic input management over the past years (Barrios et al., 1996). Dissolved organic matter also represents a tiny and very labile C pool (Ellert and Gregorich, 1995). 4.2 C pools related to soil micro-organisms Micro-organisms are essential for all soil biological functions, formation of soil aggregates and they are indispensable for nutrient availability for plants (SalinasGarcia et al., 1997; Stenberg, 1999). The microbial biomass denotes the living component of SOM (Jenkinson and Ladd, 1981). Microbial biomass in soil is composed of bacteria, fungi, algae, actinomycetes, protozoa and some nematodes. Soil microbial biomass serve as: (i) a labile source or an immediate sink of available nutrients and organic substrates on a short term basis, (ii) a driving force of nutrient transformation from stable organic forms to available mineral form over longer periods, (iii) an agent of polysaccharide secretion, a cementing agent of soil aggregates (Kennedy and Papendick, 1995) and (iv) an agent for releasing and containing enzymes which are responsible for nutrient cycling (Srivastava and Singh, 1991). In soil, microbial biomass C (Cmic) and mineralizable C (Cmin) are the pools most commonly used as the representative of biomass and activity (in terms of respiration) of soil microorganisms. Soil Cmic (expressed as mg g-1 soil) comprises 1 to 5% of SOC (w/ w). The soil Cmic is used as a sensitive indicator of changes in soil environment (Paul, 1984). Usually there is a prominent correlation between SOC and soil microbial biomass (Biederbeck et al., 1984). Thus management practices that increase incorporation of organic





residues naturally boost biological activity. On the other hand, soil respiration (CO2–C release) and soil Cmin is an index that reflects the total activity or energy spent by the soil microbial pool (Anderson and Domsch, 1990). This soil CO 2 –C emission depends on several parameters like substrate variability, soil moisture and temperature (Sparling, 1997). Therefore, Cmin is one of the most important parameter to monitor the microbial mediated processes like decomposition of organic matter in soil. Rate of soil respiration depends on the substrate availability, moisture and temperature of the soil (Brookes, 1995). In soils of tropical and subtropical regions, higher SOM turnover rate resulted in higher amount of C min within a short span (Rudrappa et al., 2006; Majumder et al., 2008b; Mandal et al., 2007). Soil metabolic or respiratory quotient (qCO2) (i.e. soil respiration to microbial biomass ratio), widely used to evaluate soil ecosystem, is a very clear-cut soil microbial index (Bastida et al., 2008). Soil microbial quotient (MQ) i.e. percentage of soil total organic C (Ctot) present as Cmic or Cmic: C toc ratio is also used as an indicator of changing soil process. These indices are more useful measure than either Cmic or Ctoc (Anderson and Domsch, 1990; Sparling, 1992) because these are ratios and therefore avoid the problems of working with absolute values and help in evaluating soils with different organic matter content. 4.3 Soil inorganic C In soil, the Cinorg (lithogenic and pedogenic inorganic C) is present mainly in form of calcium carbonate (Mikhailova and Post, 2006). In terms of soil quality, productivity and health, this fraction of soil C is less important than SOC but sequestration of Cinorg in soil plays a considerable role to mitigate global warming. Although the amount and vertical distribution of Cinorg depends on climate, land use and land cover type (Wu et al., 2009), Lettens et al. (2004) found that, the Cinorg values are significantly related to the soil geological characteristics and increase linearly with soil depth. The soil Cinorg has a positive correlation with mean annual temperature and negative correlation with mean annual precipitation. The content of Cinorg generally follows the order: desert, grassland> shrubland, cropland> marsh, forest, meadow (Mi et al., 2008). However, the largest difference between Cinorg stocks has been observed between the continuously cropped field and native grassland. Mikhailova and Post (2006) has found that in the top 2 m soil layer, native grassland contained 107 Mg ha-1 Cinorg; while cut hay field, continuously cropped field and continuous fallow (for 50 years) possessed 91, 242 and 196 Mg ha-1 Cinorg respectively. On the other hand, researches revealed, heavy grazing activities affects Cinorg more than the SOC (Reeder et al., 2004). In analytical viewpoint, the Cinorg

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measurement is necessary to obtain the correct SOC values from combustion method (Sherrod et al., 2002). 4.4 Soil aggregate associated C Aggregation is a soil physical phenomenon of significance which has prominent impact onto soil structure, water balance, aeration, temperature, crop growth, biological activity and soil erosion (Grandy and Robertson, 2006). As terrestrial C sequestration partly is achieved through SOC stabilization by physical occlusion within aggregates, protected from mineralization (Six et al., 2000; Shrestha et al., 2007), judicious soil management considering soil aggregate formation is an important strategy towards C sequestration in soil (Bronick and Lal, 2005; Shrestha et al., 2007). As per hierarchical model, in soil, persistent, transient, and temporary SOM are associated with > 0.25 mm macro-aggregates, 0.05-0.25 mm micro-aggregates, and < 0.05 mm silt and clay, respectively which distinctly indicated that decomposability of SOM decreases with decreasing particle size owing to physical protection (Tisdall and Oades, 1982; Balesdent et al., 2000). According to this model, transient forms of SOM could act as binding agent causing micro-aggregates (0.5-0.25 mm) to form stable macro-aggregates (> 0.25 mm) (Olchin et al., 2008). Increased SOM input can lead to increased aggregate formation (Kong et al., 2005) which in turn enhances C sequestration by physical protection of SOM inside aggregates. The quality and amount of SOC associated with soil aggregates and particles vary with the size of these structural units. It is often observed that C in micro-aggregates is older and less labile than C macro-aggregates (Jastrow, 1996). Microbial derived C is believed to bind micro-aggregates into macro-aggregates (Tisdall and Oades, 1982). Studies indicated that the macro-aggregate structure exerts some physical protection to SOM (Beare et al., 1994), whereas SOM is mostly protected in free micro-aggregates (Six et al., 1998; Lichter et al., 2008) and in micro-aggregates within the macro-aggregates (Denef et al., 2001; Bossuyt et al., 2002; Lichter et al., 2008). 5. Management of soil C for sustainable development Several natural as well as anthropogenic factors influence C stock and pool in soil (Liang et al., 2007) (Figure 3). Among these, impact of climate and soil properties onto soil C stock and dynamics are of beyond control (Senapati et al., 2014). However, practices like suitable cropping system and proper land use is the pathway for soil C management. In this part of the review, influence of natural factors i.e. climate and soil properties on soil C dynamics has been highlighted first followed by a detail discussion about the impact of cropping system and land use onto soil C.

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Figure 3 Factors affecting organic C stock, pools and dynamics in soils

6. Impact of climate and soil properties on soil C dynamics

7. Land use, cropping system and soil C management

Temperature and precipitation (climate) are the most significant factors controlling SOC (Alvarez and Lavado, 1998). Climatic conditions with high precipitation, low temperature and frozen soil in winter favour the high accumulation of organic matter in soil. These conditions slower the breakdown of added SOM through plant residues, roots and hence the C sequestration in soil may be expected to be larger than warmer and drier climates (Desjardins et al., 1991). On the other hand, the potential for C sequestration with crop rotations is greater in the sub-humid than in drier climates and it is established that tropical soil contains the least C in a stable pool (Tiessen et al., 1998). Again, the C sequestration potential of soil is greater in humid temperate areas in comparison to semi-arid and tropical areas (Pretty and Ball, 2001). It is the high annual temperature which consequences the lowest average SOC stock in Indian soils among the Asian countries (Kyuma, 1988). On the other hand, according to Campbell et al. (1996), a linear correlation exists between soil texture and the SOC. Furthermore, the distribution of OC within different soil structural units controls the average residence time of C in soil (Oades, 1998). Organic C content of soil increases with decreasing particle size (Guggenberger et al., 1994). As per Saggar et al. (1996), in general, more than the half of the total SOC is associated with the clay fraction (< 0.002 mm), probably due to the protection effect of clay on SOC mineralization. The degree of protection provided by clay appears to be also dependent onto the type of clays, e.g. higher porous (e.g. allophane) and expanding (eg. montmorillonite) clay minerals offering more protection than kaolinite (Franzluebbers and Arshad, 1996; Zech et al., 1997). This association of organic matter with soil fractions is further related to mean annual temperature. As researches inferred, the average turnover time of silt-associated SOM is highest in temperate soils whereas SOM attached to clay particles is considered as the most recalcitrant fraction in tropical climate (Christensen, 1992).

Changes in land use are extensively identified as a potential driver for the shift into global C dynamics (Houghton et al., 1999). Nature of land cover namely forest, pasture, plantation, fallow or annual agricultural crops has significant influences on the C stock as well as the content of different pools of C in soils (Grace et al., 2006; Kurganova et al., 2010; Banger et al., 2010). Reconstructions of global land use in past few centuries have contributed noteworthy augmentation in CO2 emissions into the atmosphere (Post et al., 1990). Forestry is known as an important land use system for C sequestration in soils. Afforestation results in a considerable amount of new soil C sequestration as well as increases the residence time of already existing C in soil providing physical protection and stabilization in SOM associated with soil aggregates (Galdo et al., 2003). Grasslands have also been revealed as a potential C sink by recent studies (Sampson et al., 2000; Conant et al., 2001) as presence of greater root biomass and higher return ratio of residues resulted higher SOM contents (Lal, 2002). When these natural systems are put into agricultural use, major changes occur in both the C pool size and turnover rates. Mandal et al. (2007) have shown that because of cultivation there is a significant decline in native C content in soils. The magnitude of decline also varies depending upon the nature and intensity of cropping. Crop rotation can modify microbial C dynamics, which determine the SOC sequestration potential (Balota et al., 2004). Field operations like surface mulching, no-tillage and minimum tillage are also important, especially in tropical soils, as they reduces the soil surface temperature resulting less C mineralization (Lal, 1989). Cropping systems also have a prominent mark onto the soil C stock and pools. A number of researchers (Studdert and Echeverria, 2000; Halvorson et al., 2002) reported that cropping system or crop rotation with or without fallow period in LTFE affected the size and distribution of SOC pools. Decline in the fallow practices (Rasmussen et





al., 1980), addition of legumes and cover crops (Kuo et al., 1997) and balanced use of manures and fertilizers (Hartwig and Ammon, 2002) results increase into SOC stock. Legumebased cropping systems have a positive impact to reduce C and N losses. As per Majumder et al. (2008a, b), inclusion of legume in cropping sequence (e.g. rice-barseem) check the loss of applied C significantly vis-à-vis other sequences (e.g. rice-wheat-fallow, rice-wheat-jute) where no such legumes were included. Cultivation of perennial forages in rotation also increases SOM levels (Nilsson, 1986). Most of the results presented above are related to ‘upland crops’ grown under aerobic conditions. However, the SOC cycles in lowland paddy systems vary distinctly from any of the upland crop systems, since it remains under submergence for a certain period of time. Transformation of C in soils under submergence is different from well aerobic soils and therefore, C sequestration and dynamics of its different pools are expected to be different. This was reflected by more SOC in stable fractions under lowland rice-rice ecology due to slow decomposition of organic substances under prolong water-logging (Mandal et al., 2008). 8. Conclusion There is a growing need to look into the detailed dynamics of C stock in soils and the role of soil in harbouring the atmospheric C and its sequestration in a long run. Any attempt to enrich soil C reservoir through increasing its residence time will help to manage global warming and reach towards global food security (Bruce et al., 1999). For a sustainable future, a proper balance should always be maintained between C inputs into soil and exports which create the equilibrium and thus determine the potential for soil to serve as a C sink. References Alvarez R and Lavado RS (1998). Climate, organic matter and clay content relationships in the Pampa and Chaco soils, Argentina. Geoderma, 83: 127–141. Anderson TH and Domsch KH (1990). Application of ecophysiological quotients (qCO2 and qD) on microbial biomasses from soils of different cropping histories. Soil. Biol. Biochem., 22: 251–255. Armstrong RD, Millar G, Halpin NV, Reid DJ and Standley J (2003) Using zero tillage, fertilisers and legume rotations to maintain productivity and soil fertility in opportunity cropping systems on a shallow Vertosol. Aust. J. Exp. Agr. 43: 141-153. Balesdent J, Chenu C and Balabane M (2000). Relationship of soil organic matter dynamics to physical protection and tillage. Soil Till. Res., 53: 215–230. Balota EL, Filho AC, Andrade DS and Dick RP (2004). Long-term tillage and crop rotation effects on microbial biomass and C and N mineralization in a Brazilian Oxisol. Soil Till. Res., 77: 137– 145. Banger K, Toor GS, Biswas A, Sidhu SS and Sudhir K (2010). Soil organic carbon fractions after 16-years of applications of

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