Conservation Research

10

Innovations in Soil and Water Management/Conservation Research through Integrated Approaches of Nuclear and Isotopic Techniques and Precision Agriculture F. Zapata, M. Zaman, M.L. Nguyen, L.K. Heng, K. Sakadevan, G. Dercon, and L. Mabit

CONTENTS 10.1 Introduction...........................................................................................................................248 10.2 Development and Application of NITs in Agriculture..........................................................248 10.2.1 Use of NITs................................................................................................................248 10.2.2 FAO/IAEA Programme of Nuclear Techniques in Food and Agriculture................ 249 10.3 Recent Developments in Soil-Water Management and Conservation Research in Agroecosystems................................................................................................................. 250 10.3.1 Sustainable Intensification of Crop Production......................................................... 250 10.3.1.1 Integrated Nutrient Management................................................................ 250 10.3.1.2 Agricultural Water Management................................................................ 254 10.3.2 Development of Conservation Agriculture Systems.................................................. 257 10.3.2.1 Improving Soil Organic Matter and Enhancing Soil Quality..................... 257 10.3.2.2 Greenhouse Gas Emissions and Mitigation Options Including C Sequestration........................................................................................... 258 10.3.3 Sustainable Land (Soil) and Water Management/Conservation................................ 259 10.3.3.1 Soil Erosion Assessment and Control......................................................... 259 10.3.3.2 Areawide (Watershed) Land and Water Conservation...............................260 10.3.3.3 Areawide Soil-Water Conservation for Pollution Control.......................... 261 10.3.3.4 Areawide Management of Salinization...................................................... 261 10.4 Further Developments on Soil and Water Management and Conservation Research in Agroecosystems................................................................................................................. 262 10.4.1 Basic Considerations.................................................................................................. 262 10.4.2 Development of Integrated Approaches for NIT and PA Applications in Soil and Water Management/Conservation...........................................................264

247

248

Soil-Specific Farming

10.4.2.1 Innovations of the Soil (Nutrient) and Water Management in Agroecosystems...................................................................................... 265 10.4.2.2 Innovations in Areawide (Watersheds) Land (Soil) and Water Conservation Studies.................................................................................. 270 10.5 Future Prospects: Key Challenging Issues............................................................................ 273 10.6 Conclusions............................................................................................................................ 273 References....................................................................................................................................... 274

Downloaded by [Mohammad Zaman] at 06:07 13 October 2015

10.1 INTRODUCTION The world is facing an unprecedented dual challenge of enhancing food security while ensuring environmental sustainability, in particular the conservation of a natural resources base (soil and water) and genetic (plant and animal) resources. The present world population of 7 billion will exceed 9 billion by 2050 (UN-DESA 2013). The majority of this population increase will occur in underdeveloped and developing countries that already face food shortages. Simplistically, a 60% increase in the current agricultural productivity will be required from existing available resources (land and water) to feed the growing human population. Worldwide soil degradation is currently estimated at 1.9 billion hectares and is increasing at a rate of 5 to 7 million hectares each year (Lal 2006). This soil and land degradation causes not only a productivity decline and biodiversity loss but also affects vital soil/water ecosystem services, all of which are intricately linked with long-term social, economic, and environmental impacts (Bruinsma 2003; UNEP 2010). Moreover, several environmental drivers also affect land and water resources. Among these, major impacts are related to climate change and variability (Nguyen et al. 2011). All of these are likely to have negative impacts and induce changes on agroecosystems, thus placing increased pressures on dwindling land and water resources to produce sufficient food for present and future generations (Lal 2004; Verchot and Cooper 2008). Sustainable agricultural development would require the combined use of soil, nutrient, and water management strategies that enhance crop productivity and at the same time promote environmental sustainability. In this context, there is a strong need for high-quality innovative research and soilwater-specific technologies that will address the most strategically important issues of soil/land and water management and conservation in agroecosystems (Nguyen et al. 2011). The objectives of this chapter are to (1) provide an overview of the development and application of nuclear and isotopic techniques (NITs) in soil and water management-conservation in agroecosystems and (2) identify potential areas where the utilization of precision agriculture (PA) technologies can enhance further the effectiveness of NITs in soil and water management, leading to innovative research and soil-water-specific technologies. However, NIT applications in soil and water research can also enhance PA. This analysis is based on the main project activities of the Soil and Water Management and Crop Nutrition (SWMCN) subprogram of the Joint Food and Agriculture Organization (FAO) and International Atomic Energy Agency (IAEA) Division of Nuclear Applications in Food and Agriculture. This chapter is not an exhaustive review of PA and only focuses on NITs that act as building blocks of precise information for PA to address present and emerging issues related to soil (land) and water management/conservation in agricultural research. For detailed information on the publications originating from the research projects of the SWMCN subprogram, readers are referred to their website (http://www-naweb.iaea.org/nafa/swmn).

10.2 DEVELOPMENT AND APPLICATION OF NITs IN AGRICULTURE 10.2.1 Use of NITs NITs, which are also called nuclear-based techniques, comprise the use of stable (natural abundance and enrichment by artificial labeling) and radioactive (radiation-emitting) isotopes as well


Innovations in Soil and Water Management/Conservation Research

249

as radiation sources such as neutron and gamma density probes. For example, the soil moisture neutron probe (SMNP) is portable nuclear equipment used to monitor soil water content changes for constructing field water balance and defining irrigation scheduling. Gamma density probes are used to measure soil bulk density changes resulting from farm management practices such as tillage systems and animal stocking rate. Isotopes are utilized as tracers that provide unique, precise, and quantitative data on nutrient and water pools and fluxes in the soil-plant-water systems and to assess the relative value of selected soil-water management technologies tailored to specific agroecosystems for improving soil fertility, crop productivity, and water use efficiency in crop and livestock production systems. Specific chemical sources and pollutants from these systems can also be traced using NITs. For example, 15N-stable isotopic techniques can be used to measure rates of nitrogen (N) processes such as N mineralization-immobilization, nitrification and denitrification, biological N fixation, N use efficiency, and sources of N pollution in ground- and surface waters. For details on the principles and applications of these NITs in soil, water, and plant nutrient studies in agroecosystems, readers are referred to the IAEA Training Manuals (IAEA 1990, 2001) and a review paper (Nguyen et al. 2011). Nuclear-based techniques are a complement and not a substitute to non-nuclear conventional techniques. They are applied in the context of agricultural research under field and greenhouse conditions when they offer comparative advantages over conventional techniques. However, they demand skilled and trained personnel and adequate laboratory facilities, in particular measurement equipment/techniques or alternatively financial resources for analytical services. In the case of radioactive isotopes, strict compliance with safety regulations and radiation protection procedures are required. Nuclear-based techniques like any other techniques have advantages and limitations (Nguyen et al. 2011). It is, therefore, the task of a research team leader to assess the usefulness and effectiveness of the nuclear-based techniques to meet specific research objectives taking into account the team’s available resources.

10.2.2 FAO/IAEA Programme of Nuclear Techniques in Food and Agriculture In 1964, two United Nations organizations, the Food and Agriculture Organization (FAO) and the International Atomic Energy Agency (IAEA) established the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture at the IAEA headquarters in Vienna, Austria, to create and strengthen capacities for using nuclear-based methods to develop technologies for sustainable food security and to disseminate these through international cooperation in research, training, and outreach activities in Member Countries of the FAO and IAEA (IAEA 2014a; FAO 2014a). To achieve this mission, the Division has five discipline-oriented Sections, namely the SWMCN, Plant Breeding and Genetics, Animal Production and Health, Insect and Pest Control, and Food and Environment Protection. Each Section is linked to a corresponding laboratory located at the Agriculture and Biotechnology Laboratories (ABL) in Seibersdorf, near Vienna, Austria (FAO/IAEA 2014a). This Joint FAO/IAEA Division implements its regular program through medium-term networked research projects called Coordinated Research Projects (CRPs) involving scientists from agricultural institutes in developing countries as well as Consultative Group for International Agricultural Research (CGIAR) institutions and advanced research organizations from industrialized countries (IAEA 2014b). Since its creation, the projects implemented by this Division have addressed priority issues of the agricultural research agenda and developed new technologies using nuclear-based methods for sustainable food security. Furthermore, transfer of the generated technologies and scaling of supportive services are offered by the IAEA and FAO Technical Cooperation Programmes (TCPs) (IAEA 2014c). In its initial period (1964–1990), the Soils subprogram was named Soil Fertility, Irrigation and Crop Production reflecting the discipline-orientation of the projects and its focus on developing management practices for increasing the efficient use of agricultural inputs (soil, water and nutrients for crop production). Early projects dating back to the 1960s and 1970s were related to maximize

250



the nutrient use efficiency of the applied chemical N and phosphorus (P) fertilizers by major grain crops (cereals) using 15N- and 32P-labelled fertilizers. After the 1974 oil (energy) crisis, the scarcity of chemical fertilizers and their high prices led to studies on enhancing the inputs of biological nitrogen fixation (BNF) as a source of N in agricultural systems. In 1995, the subprogram became SWMCN and its objective was to develop and promote the adoption of NITs technologies to diagnose constraints and pilot test suitable interventions/management practices through the integrated management of soil, water, and nutrient resources in agroecosystems (FAO/IAEA 2014b). Since then, the SWMCN subprogram has developed a wide range of NITs technologies and applied them successfully to cropping systems for sustainable soil-water and nutrient management, arresting land degradation, and climate change adaptation and mitigation (Chalk et al. 2002; Nguyen and Zapata 2006; Nguyen et al. 2010).

10.3 RECENT DEVELOPMENTS IN SOIL-WATER MANAGEMENT AND CONSERVATION RESEARCH IN AGROECOSYSTEMS The continuing need to enhance food security and reduce climate change impacts demands an activity program that leads to sustainable soil and water management/conservation. This section provides a brief account of the investigations using NITs grouped into three main project areas: (1) sustainable intensification of crop production, (2) development of conservation agriculture systems, and (3) sustainable land (soil)/water management/conservation. Recent developments in these topics were reported at the international symposium Managing Soils for Enhancing Food Security and Climate Change Mitigation/Adaptation (FAO/IAEA 2014b).

10.3.1 Sustainable Intensification of Crop Production Intensification of agricultural production on prime agricultural land demands more refined management of external inputs of water and nutrients and thus an increased need for both nuclear and nonnuclear methods to develop better water and nutrient management practices in both rainfed and irrigated agricultural systems. Several CRPs were conducted adopting an integrated approach to soil, water, and nutrient management in selected cropping systems of the main agroecological zones of the world. The investigations included judicious management of external inputs (nutrients and water) to improve their use efficiency and enhance soil productivity in cropping systems (Chalk et al. 2002; Nguyen and Zapata 2006; Nguyen et al. 2010). NITs such as 15N, 32P and 35S isotopes were employed as tracers to develop fertilizer management practices (e.g., sources, timing, and placement) tailored to local conditions and specific cropping systems that improve nutrient use efficiency and enhance soil fertility (IAEA 2005, 2006, 2009). Moreover, the use of crop genotypes best adapted to the local soil/climate conditions was found to be a key requirement for ensuring the productivity and sustainability of the cropping systems. These studies using NITs, which initially focused on the search of crop genotypes with superior nutrient use efficiency, have demonstrated great potential to assist in the breeding and selection of suitable germplasm with tolerance to particular abiotic stresses (drought, flooding, low nutrient status, salinity, aluminum [Al] toxicity, etc.) (Nguyen et al. 2011). Recent trends highlight an agroecological management through nutrient recycling from on-farm organic resources (animal manure and crop residues) and inputs of BNF in integrated crop-livestock production systems of smallholder farmers (FAO 2014b). 10.3.1.1 Integrated Nutrient Management Extensive tracts of land worldwide, particularly those in the tropical and subtropical regions of Asia, Africa, and Latin America, contain acid fragile soils with inherent poor soil fertility, where nutrient


251


imbalances, especially low N and P status, are the main factors limiting crop yields in agroecosystems. Moreover, intensive cultivation and inadequate application of nutrient sources (both mineral and organic) can lead to severe land degradation due to rapid decline in soil organic carbon (SOC) and then ultimately to low crop production, food insecurity, and extreme rural poverty. Lal (2001) reported that the depletion of SOC occurs at a rate of 2%–12% per year and the cumulative loss can be as high as 50%–70% of the original carbon (C) pool over a cultivation period of 10 years. Such negative impacts of SOC loss can be reversed through the development and implementation of an integrated nutrient management approach. The development of an integrated nutrient management package involves not only manufactured fertilizers but also natural sources of nutrients such as phosphate rocks, BNF, and animal and green manures, along with the recycling of crop residues to provide a range of nutrients for plant growth and soil microbial activity. These play an important role in enhancing soil fertility–land productivity and improving soil organic matter and biodiversity (Nguyen 2014). 10.3.1.1.1 Nitrogen Among the essential plant nutrients, N is the key component of all agricultural production systems because it is required by plants in large amounts for protein synthesis and virtually all aspects of plant growth. Thus, N plays a major role in crop productivity and profitability. N inputs under most farming systems come predominantly from the application of chemical (manufactured) fertilizers, which are expensive and imported commodities in the majority of developing countries. This is followed by BNF, with excreta of grazing animals and animal manure being the third most common source. In developed countries, modern agriculture is dependent on the use of high-energy-intensive inputs (mechanization, irrigation water, and chemical fertilizers) for achieving and maintaining the high yields of new crop cultivars. It is reported that adequate food production, in particular cereals, for present and future populations will not be achieved without external inputs of N fertilizers (FAO 2012). There has been a steady increase in the use of chemical N fertilizers with a forecast demand of 194 M ton, of N worldwide in 2016. The amount of fertilizer N to be applied (optimum application rate) can be obtained through a response curve by measuring crop yield and total N uptake to increasing application rates of fertilizer N. However to achieve a high fertilizer N use efficiency (FNUE), additional information such as timing, placement, and sources is required. These should be properly evaluated to select the best fertilizer management practices tailored to specific cropping systems and local agroecological conditions. To achieve this, both direct and indirect (reverse) dilution methods using 15N-labeled fertilizers can be deployed in field experiments (Hauck and Bremner 1976; Van Cleemput et al. 2008). The findings from the FAO/IAEA networked research projects worldwide and those of many other researchers using NITs under a wide range of environments suggest that a large percentage of the N fertilizer input is essentially being wasted because the average N recovery by the crop was reported to be less than 50% of the applied N (IAEA 1980, 1984; Zapata and Hera 1996; Mosier et al. 2004). Such low FNUE has been attributed to a number of factors and processes influencing its plant availability and losses from the soil-plant system. Moreover, this low FNUE represents not only an environmental degradation (N pollution) but also a substantial economic loss for the farmer and the country (Keerthisinghe et al. 2003). Quantitative information about the fate of applied fertilizer N in soil-plant-water-atmosphere is therefore critical for enhancing FNUE and identifying appropriate soil and fertilizer management practices that optimize crop production and protect the environment (water bodies and atmosphere). This can be obtained by conducting sequential experiments using NITs to (1) determine the recovery of the applied fertilizer N by the crop as influenced by management practices (FAO 1980), (2) ascertain the fate of the applied fertilizer N in the soil-plant system and obtain an estimate of unaccounted losses, and (3) measure direct losses by gas production processes (e.g., ammonia


252


volatilization and denitrification) and/or leaching to reduce negative impacts on the environment (IAEA 1980, 1984; Follett et al. 1991; Freibauer et al. 2001; Van Cleemput et al. 2008). Most organic residues, particularly animal manures and plant residues, contain valuable plant nutrients that can be recycled back onto farmland to improve soil quality and health and reduce the need for chemical fertilizers (Nguyen 2014). However, the extremely variable physical and chemical characteristics of the organic residues influence to a great extent the rate of release of plantavailable nutrients. Therefore, research attention has focused on the standardization of methods for the characterization of selected quality parameters and the creation of databases for the application of simulation modeling (Cadisch and Giller 1997; Palm 2001; Vanlauwe et al. 2002; ORD 2004). NITs based on 15N are employed for the study of N released of organic residues and its uptake by crops (Hood-Novotny et al. 2008). Nitrogen transformations such as mineralization (i.e., the conversion of organic N to mineral N) and immobilization (N release from organic residues) are complex processes that are influenced by a variety of microbial and enzymatic activities, soil type, organic and inorganic amendments, and environmental/management conditions (Zaman et al. 1999a,b, 2002; IAEA 2003b; van Kessel 2008). Both these processes can occur concurrently and hence nonisotopic techniques cannot precisely measure N release from soil organic N. The use of 15N stable isotope can provide precise quantitative information on the rate of N being released over time and its subsequent consumption processes including plant uptake, soil retention, and losses from the soil-plant systems (Barraclough 1995; Di et al. 2000). In addition, the production of greenhouse gas (nitrous oxide [N2O]) and nongreenhouse gas (N2) and their precise source in the soil can only be determined by using 15N (Mosier and Klemedtsson 1994; Zaman et al. 2008; Müller et al. 2014). Although it is well recognized that the application of chemical fertilizers plays an important role in the intensification of crop production, the lack of affordable and adequate supplies of chemical fertilizers in the developing world remains the major constraint to crop production. Under these circumstances, it is essential to consider the use of alternative N sources such as BNF and organic residues to provide N for plant growth and to enhance soil quality and health. Among the BNF systems, the symbiotic relationship between Rhizobium (bacteria found in the root nodules of the legume) and legume plants can provide significant N inputs to the plants and it is considered the most effective system for enhancing the productivity of agroecosystems. Numerous nonisotopic and isotopic methods have been tested and applied under field conditions to assess the symbiotic N fixation in legumes (Giller 2001; Hardarson and Atkins 2003; Jensen et al. 2008). The 15N isotopic methods both at enrichment and natural abundance levels are described by Jensen et al. (2008). Extensive research using NITs to assess and enhance BNF in agroecosystems has been done under several FAO/IAEA projects (Hardarson and Atkins 2003). Additional studies using NITs have been also conducted to assess the N inputs from BNF to the agroecosystem (Jensen et al. 2008). Some studies using 15N showed that the below-ground N contribution from legumes can be substantial. For instance, Poth et al. (1986) found about 53% to 71% of the N fixed by pigeon pea over a period of 225–252 days (equivalent to 150 to 180 kg N ha−1) was recovered in the soil after removal of the coarse roots. Chalk (1998) reviewed the dynamics of biologically fixed N in legume-cereal rotations and highlighted the need for wider use of 15N-based methodologies to estimate additions of legume N to the soil and its effects on subsequent crops so that more accurate N balances in soil-plant systems can be made. In spite of all these investigations, there is a need for more research targeting specific aspects of BNF oriented to the development and introduction of viable and cost-effective technologies to farming systems considering the particular needs and constraints of small-scale resource-poor farmers in the developing world (Hardarson and Broughton 2003). 10.3.1.1.2 Phosphorous Most acid soils of the tropics and subtropics often have an inherent low plant-available soil P status and a high P sorption capacity, and thus P deficiency becomes the main constraint to agricultural



253

productivity. Moreover, P depletion by continuous cultivation without replenishment is severely affecting agricultural production in sub-Saharan Africa. Approximately 75 kg P ha–1 has been lost over the last 30 years from 200 million ha of cultivated land in 37 African countries (Smaling et al. 1997). Therefore, P application is needed to improve soil P status and to ensure normal plant growth and adequate crop yields. The application of manufactured water-soluble phosphate (WSP) fertilizers such as superphosphate is usually recommended to correct P deficiencies. However, most developing countries import WSP fertilizers, which are often in limited supply and represent a major outlay for resource-poor farmers. Thus, it is imperative to explore alternative P inputs such as reactive phosphate rocks (RPRs). RPRs have been shown to be as effective as WSP fertilizers for arable crops and pasture systems grown under suitable conditions of soil pH and rainfall for RPR dissolution (Bolan et al. 1990; Rajan et al. 1996; Sale et al. 1997; Zapata and Roy 2004; Quin and Zaman 2012). Phosphorus deficiency is also a major constraint for crop-livestock production in temperate grasslands where the low concentration of solution P available for plant uptake limits the continuous production of animal products and crops. Legume-based pastures rely on a regular supply of adequate available P so that the Rhizobium in the legume root nodules is able to fix atmospheric N (Nguyen et al. 1989). In agricultural areas under intensive livestock production, the continuous use of external P inputs can lead to its accumulation in the topsoil and increased nonpoint (diffuse) pollution risks and high potential for eutrophication of water bodies if such accumulated P finds its way to streams and rivers (Sharpley et al. 2001; Chardon and Withers 2003; Hart et al. 2004). Isotopic techniques using radioactive 32P with its short half-life (14.3 days) can provide significant insights on P dynamics under laboratory conditions (Nguyen 2000; Nemery et al. 2005; Stroia et al. 2007) which in turn provide quantitative information on the potential adsorption-desorption of P forms under different simulated field conditions (water depth, P concentration in solution, etc.). Several key issues relating to soil and fertilizer P management for crop production in tropical agroecosystems have been investigated using 32P isotopic techniques under a CRP (IAEA 2002b). The 32P isotope exchange, a kinetics technique, that provides a comprehensive description of soil P status (i.e., intensity [Cp], quantity [E1], and capacity [Q] factors were found to be a very valuable tool to assess P dynamics in the soil with or without the addition of P fertilizers such as WSP fertilizers and phosphate rock [PR]) (Morel and Fardeau 1991; Fardeau et al. 1995; Fardeau 1996). Several soil P testing methods were compared using the 32P isotope technique as a reference. It was concluded that there is no single soil chemical P test that can be universally used to estimate available P in soils amended with PR and WSP fertilizers (IAEA 2002b). Investigations were also conducted to develop standardized protocols to characterize PR sources and evaluate their relative agronomic effectiveness and where necessary to find ways/means of enhancing their effectiveness (IAEA 2002b). 32P and 33P isotopic techniques have been extensively used in both laboratory and glasshouse experiments to measure P uptake and utilization from the applied P fertilizers, in particular RPR (Zapata and Axmann 1995; Zaharah and Zapata 2003). Adequate soil-plant-fertilizer management practices that can be put in place to enhance the efficient use of soil P and added external P inputs, in particular the application of RPR to build up the soil P capital in tropical acid soils, were identified and pilot-tested in field experiments (Zapata 1995, 2002b; IAEA 2002b; Zapata and Roy 2004). All this information was used to develop databases and a decision support system (DSS) to provide better informed recommendations to soil/fertilizer practitioners, land managers, and policy makers (FAO/IAEA 2014c). 10.3.1.1.3 Sulfur Sulfur (S) is an essential nutrient for plant growth and animal production. S deficiencies in crops and pastures can occur when replacement by fertilizers does not meet demands by cropping, such as following intensification of farming systems or change in crop types (e.g., legumes and brassica plants remove more S than cereals) (Nguyen and Goh 1994a).


254


Several studies have used both 35S (radioactive) and 34S (stable) as isotopic tracers to follow the pathways of S in soil-plant or soil-plant-animal systems and to construct S budgets in grazing systems (Till 1981; Chen et al. 1999). A direct method involving the use of 35S-labeled materials has been applied in studies to determine the S uptake and recovery of 35S-labeled fertilizer and animal excreta in flooded rice and pastoral systems (Samosir et al. 1993; Nguyen and Goh 1994b), the availability of subsoil sulfate using 35S-labeled gypsum to crops (Bole and Pittman 1984; Nguyen and Goh 1994a), and the relative performance of different S fertilizers (Goh and Gregg 1982; Nguyen and Goh 1990). When 35S labeling of fertilizers, animal manure, or crop residues is not appropriate, the 35S reverse dilution technique has been employed to determine (1) the ability of plants to acquire S from the atmosphere (Hoeft et al. 1972), (2) the release of S from elemental S sources, sulfate sources, and its uptake by plants (Shedley et al. 1979), (3) the sources of S taken up by ryegrass and measured by chemical extraction (Chinoim et al. 1997), and (4) the time course of S uptake from the S-coated urea by crops (Yasmin 2003). The 34S natural abundance has been used extensively to identify sources and to trace the fate of S in the environment. This approach requires that the S isotopic signatures of the different sources are known so that the contribution of S from the sources can be apportioned (Krouse 1977; Mayer et al. 1995; Alewell et al. 1999; Novak et al. 2001). This technique has been applied to study long-term changes in S deposition in the Broadbalk experiment in the United Kingdom (Zhao et al. 2003). Ion exchange membranes have been successfully used to collect soil water sulfate for investigating its isotopic (sulfur and oxygen isotope fractionation) composition (Kwon et al. 2008). For detailed information, readers are referred to the IAEA publication Guidelines for the Use of Isotopes of Sulfur in Soil-Plant Studies (IAEA 2003a). 10.3.1.1.4 Micronutrients The seven micronutrients, including boron (B), copper (Cu), chlorine (Cl), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn), are equally important to plant growth such as macronutrients in soils; however, they are required in very small quantities of only a few mg/kg in plant tissues. Micronutrients play many key roles in plant nutrition, enzyme systems, oxidationreduction reactions, photosynthesis, and plant production. Increasing micronutrient deficiency is becoming a matter of concern due to their impact on agricultural production, in particular in intensively cultivated systems in Southeast Asia. Among the micronutrients, Zn deficiency is the most acute followed by B. According to Sillanpää (1990), Zn deficiency is the most commonly occurring micronutrient deficiency problem limiting crop growth in many parts of the world. In India, application of Zn resulted in spectacular yield increases in wheat-growing areas (Takkar et al. 1989). Studies using NITs such as 65Zn reported improved Zn nutrition of flooded rice in Southeast Asia (IAEA 1983). In view of the widespread soil degradation and impacts of climate change on nutrient imbalances (deficiencies and toxicities) and their subsequent effects on human malnutrition (also called hidden hunger), especially in infants and children, there is renewed attention on micronutrient studies in soil-plant-man systems. Recent research focuses on the use of isotope and related techniques in micronutrient studies at several levels due to the urgent need to develop cost-effective interventions to control/mitigate these nutrient imbalances in the developing world (IAEA 2014d). 10.3.1.2 Agricultural Water Management Water is the most precious resource that supports life on the planet and connects the various components of the ecosystems. Agriculture is the largest user of freshwater, accounting for about 75% of the global freshwater use. Cropland under irrigated agriculture contributes approximately 40% of world food production while the remaining 60% comes from the cropland under rainfed agriculture. Rising demand for food and livestock feed together with the use of biofuels and



255

impacts from global climate change are placing a tremendous pressure on freshwater resources (Molden 2007). Managing agricultural water to enhance crop water productivity (more crops per drop) and water use efficiency in crop production systems is therefore of paramount importance for both rainfed and irrigated agriculture. This can be accomplished by (1) increasing the marketable yield of crop for each unit of water transpired, (2) reducing all outflows (e.g., soil evaporation, drainage, seepage, and deep percolation), and (3) increasing the effective use of rainfall, stored water, and water of marginal quality (Molden 2007). In agroecosystems, due attention should be paid to the soil-water interactions and their influence not only on food production but also on the provision of essential ecosystem services (UNEP 2011). Therefore, it is envisaged that soil and water management/ conservation for sustainable crop production will need to be smarter and adopt elements of PA. A paradigm shift will be necessary to change from supply-driven to a more demand-driven water management to conserve water and improve its use efficiency on-farm to produce more crops per drop of water in water-limited environments. Under rainfed conditions, the storage of rainwater in the root zone is the critical factor for plant growth, which requires integrated land-water management practices (Rockström et al. 2007). Studies conducted by IAEA using NITs and related techniques have demonstrated that there is considerable scope to improve water use efficiency and crop productivity in rainfed agriculture. This can be achieved through the appropriate integration of soil-water-plant technological options such as conservation agriculture practices (e.g., zero or reduced tillage, mulching, crop residue retention, crop rotation, intercropping), water harvesting techniques, and improvement of the soil fertility status. Rainfed agriculture systems can be further improved by appropriate crop management such as use of crop varieties adapted to drought and saline conditions, use of deep-rooted crops to access soil water storage at deeper soil depths, or using early flowering and shorter season crop varieties, and changing sowing dates or planting density to suit the local conditions (IAEA 2005). Conservation agricultural practices such as no-till, minimum tillage, tied ridge, and mulching can reduce soil or water losses under rainfed agriculture. Developing on-farm water storage facilities or having water conservation zones to provide supplementary irrigation to crops whenever possible can significantly minimize the risks of rainfed agriculture (Rockström et al. 2007). These risks can be further reduced if the preseason rainfall forecasting can be predicted with anticipated outcomes of different management decisions (Cooper et al. 2008). SMNP, which measures slow neutrons after the collision of fast neutrons (emitted by the neutron radioactive source in the SMNP) with hydrogen atoms in the soil water, is an instrument that is well suited to precisely determine field-scale soil water content as well as to evaluate the impacts of different tillage systems in conserving soil moisture for crop production (IAEA 2008b). Isotopic techniques (18O and 2H) that quantify soil evaporation (E) and crop transpiration (T) fluxes are important research tools to determine their relative magnitudes and design better strategies for water management under different rainfed conditions so that such losses can be eliminated completely without affecting the rate of T (Williams et al. 2004; Heng et al. 2014). Under irrigated agriculture, the combined use of PA and NITs may play a key role in improving water conservation and environmental protection (Nguyen 2014). Precision irrigation (PI) is defined here as the efficient, timely, and correct amount of water delivered to fields to maximize crop yield and quality, and to minimize environmental impacts, including the application of variable amounts of water over a field in response to spatial crop and soil heterogeneities (Fereres and Heng 2014). Three important prerequisites are necessary for successful application of irrigation: (1) the use of modern technologies (i.e., changing the method of irrigation to increase the efficiency of application), (2) knowledge of crop water requirements (evapotranspiration) with a certain degree of accuracy, and (3) the ability to effectively monitor the water status of the root zone so that precise irrigation frequency and the depth of application can be determined.


256


Improving irrigation scheduling to precisely determine the date and amount of irrigation is the first step toward optimizing on-farm water management and improving PI. Accounting for soil variability across a field as well as nonuniform crop growing conditions can be met through the option of applying variable amounts of irrigation water within the field to match the requirements of every zone of the field with minimal environmental consequences compared with what a uniform irrigation rate applied over a variable field would have. Changing the method of irrigation from gravity and open channels to pipe network and/or pressurized systems (sprinkler or drip) can significantly reduce irrigation water use. Drip irrigation supplies water directly to the plant rooting zone and can thus cut water use by up to 50% while maintaining or even increasing crop yields (IAEA 2002a). Applying fertilizers and irrigation water together (fertigation) through drip irrigation system has the potential to further maximize irrigation and fertilizer use efficiencies (IAEA 2002a). In recent years, low-cost, small-scale drip-irrigation systems are encouraging farmers to adopt this technology to improve their water use efficiency on farms. Drip irrigation systems have the greatest potential to increase farm incomes, especially those of smallholders in developing countries through reduced water costs and increased crop yields (FAO 2001; Postel et al. 2001). In a recently completed CRP project, integrated soil-water-plant approaches and recent advances in isotope techniques were utilized to better manage irrigation water for enhancing crop productivity under water-limiting conditions. Stable isotopes of water (18O and 2H), SMNP, and related conventional techniques (e.g., microlysimetry, sap flow) were used to quantify soil evaporation and transpiration fluxes at different stages of crop development. Data generated in the project is being used to validate the FAO’s Aquacrop model (Raes et al. 2009; Steduto et al. 2009) for developing improved irrigation water, soil, and crop agronomic practices to achieve water productivity improvements (Heng et al. 2014). Accurate knowledge of water losses via E and T—collectively known as evapotranspiration (ET)—is also necessary for effective water management on-farm. Isotopic techniques (using 18O and 2H) that quantify E and T are important research tools to determine the relative magnitudes of E and T in different situations (Williams et al. 2004; Heng et al. 2014). Quantifying E and T under surface and subsurface drip irrigation systems can lead to developing guidelines that enable farmers to realize water saving by minimizing soil evaporation. Shifting from crops with high irrigation requirements, such as cotton or rice, to those that have much lower water needs, such as high-value vegetable crops, can cut water use, particularly in water-scarce areas where growing of traditional field crops under irrigation is no longer economically viable (Molden 2007). The carbon isotopic discrimination (CID) technique is based on discrimination of the heavy isotope of carbon (13C) in favor of the lighter and more abundant isotope (12C) during the physical diffusion of CO2 through the leaf stomata and subsequent enzymatic decarboxylation. Thus, CID is a time-integrated index of photosynthetic activity and is related to plant water use or transpiration efficiency that can be used as a surrogate marker for crop water use efficiency (Condon et al. 2002, 2004; IAEA 2005). This technique has been employed in a CRP on the selection of wheat genotypes tolerant to drought and irrigated rice genotypes tolerant to salinity for greater agronomic water use efficiency under a wide range of environments (IAEA 2012). Simulation models are utilized to evaluate actual irrigation management and to identify new approaches to improve water use efficiency under both irrigated and rainfed conditions. Simulation models also provide insights to investigate the complexity of different climate change scenarios in terms of altered water and temperature regimes and elevated carbon dioxide concentration in the atmosphere (Steduto et al. 2012). A major challenge is to assess the average soil water status over large areas because the use of point observations of soil water or plant water status is not feasible to reduce variability problems. The Cosmic-Ray Neutron Probe (CRNP) is a new instrument based on the collision of fast neutrons from the upper atmosphere with hydrogen atoms in the soil that offers potential in assessing


257


area-wide soil moisture status (Zreda et al. 2008; Shuttleworth et al. 2010). This CRNP does not contain a radioactive neutron source so it can be transported easily and left in the field unattended, providing measurements of area-wide scale over a circle of about 600m diameter (~30 ha in area) and over depths varying between 15 and 70 cm (Zreda et al. 2008, 2012; Franz et al. 2012). Monitoring soil water status at these scales allows the integration of variations caused by differences in soilcrop properties and distribution of irrigation water. Several other uses could be implemented (e.g., early warning for flood events in mountainous areas). A sequence of observations over time also permits the computation of the components of the field water balance if the appropriate inputs and outputs are recorded in databases. This technique could also be used to calibrate remote sensing data but this aspect is still under investigation. A very recent development of a cosmic-ray rover (Desilets et al. 2010) with a principle similar to that of the stationary CRNP allows the assessment of average soil moisture with high spatial resolution along the path of the probe to be determined (Chrisman and Zreda 2013).

10.3.2 Development of Conservation Agriculture Systems Conservation agriculture (CA) refers to the sustainable intensification of agricultural production systems with an integrated approach to improve crop productivity for food security as well as to restore soil quality and enhance its resilience against degradation and risks associated with climate change impacts. This is achieved following best management practices such as (1) the strategic application of the required amount and ratio of essential plant nutrients at the right timings to meet crop nutrient demand and minimize losses, (2) minimum mechanical soil disturbance by tillage/ cultivation, (3) retention of crop residues (mulching), and (4) the use of crop rotations/plant associations, including cover/green manure crops (FAO 2014c). Such conservation systems are currently practiced in over 100 million ha to enhance the food security of small holders in the developing world (Derpsch and Friedrich 2009). 10.3.2.1 Improving Soil Organic Matter and Enhancing Soil Quality Increasing and preserving soil organic matter (SOM) plays a key role in improving soil fertility and quality and increasing crop production while ensuring long-term sustainability of agricultural ecosystems. Increased SOM also plays a key role in the global C cycle by acting as a sink for atmospheric CO2 (Paustian et al. 1997). This combination of CA practices mentioned above, when applied on a continuous basis over a period of time, provides a means of enhancing soil C sequestration on agricultural lands and thus, sustaining and increasing SOM levels and providing ecosystem services for enhancing crop production and contributing to environmental sustainability (Lal and Kimble 1997; Lal 2007). In an FAO/IAEA CRP entitled “Integrated soil, water and nutrient management in conservation agriculture,” the influence of soil, water, and crop management practices on SOM accumulation and its subsequent impacts on soil water, nutrient, and C dynamics were investigated using NITs in various cropping systems worldwide. The results from this global project demonstrated that CA can bring benefits such as increased soil moisture retention, BNF, N retention, and soil C sequestration. However, these effects were highly variable and site-specific, in some cases the benefits being negated by the influence of crop residues on plant diseases that could reduce crop yields and quality. One of the major lessons, with great implications for adoption strategies, is that CA can only be sustainable and successfully implemented if specific local constraints such as soil compaction, low soil fertility, and lack of SOM are first removed (Dercon et al. 2010). Further research is therefore needed to develop and pilot-test specific packages of integrated technologies and practices tailored to targeted agroecological zones and local agronomic management. Furthermore, this information is required for a comprehensive assessment of socioeconomic and environmental benefits and the development of appropriate policies to facilitate and encourage the adoption of CA by farmers. With this in mind, another CRP entitled “Soil quality and nutrient

258



management for sustainable food production in mulch-based cropping systems in sub-Saharan Africa” was initiated in 2011 to investigate the potential of mulch-based cropping systems to enhance soil resilience against degradation and climate change risks and to increase soil fertility for sustainable food production in sub-Saharan Africa (SSA) (Nguyen et al. 2011). Stable isotope techniques (15N and 13C) at enriched and/or natural abundance levels will enable an in-depth analysis and understanding of basic soil biological-physical processes, including soil C and nutrient cycling in mulchbased cropping systems. The selection and characterization of benchmark sites in the moist and dry savannahs will provide a platform for extrapolation of results to other relevant agroecological zones in SSA (FAO/IAEA 2014b). 10.3.2.2 Greenhouse Gas Emissions and Mitigation Options Including C Sequestration Among the three greenhouse gases (GHGs)—N2O, carbon dioxide (CO2) and methane (CH4)—emission of N2O from agricultural soils has received particular attention. Increased and inefficient use of chemical N fertilizers, dairy manures and irrigation water, high stocking rate (number of grazing livestock per hectare), and intensive cultivation are the major influencing factors for increased N2O emissions into the atmosphere. Soil can be either a source or sink of CO2 emission depending on land use while livestock and flooded rice systems can be a major source of CH4 emissions. Mitigation options to reduce both N2O and CO2 emissions from farmlands need a holistic approach by integrating farm management practices that include enhancing nutrients and water use efficiencies on farm, sequestering more C, managing animal grazing, and improving soil fertility (physical, chemical, and biological). Nitrogen cycling processes occurring in the soil-plant-water interface including N use efficiency and uptake by plants, N retention in soil, and losses to groundwater and atmosphere are very complex and therefore cannot be precisely measured by nonisotopic techniques. NITs such as 15N can help to quantify and identify the relative importance of these processes. For example, strategic use of chemical fertilizers and organic materials (i.e., applying the right amount of N at the right time in the right way) could lead to improved nutrient use efficiency (Zaman et al. 2013). Enhancing biological N fixation through legumes minimizes the use and dependence on chemical fertilizers (Ledgard et al. 1999). N transformations such as nitrification (both autotrophic and heterotrophic), denitrification, dissimilatory nitrate reduction to ammonium (DNRA), and conversion of organic N to mineral N are considered the major microbial processes that produce greenhouse N2O and nongreenhouse gas (N2) and they can occur concurrently in a given soil system. Measuring the contribution of N2O production from each of these microbial processes is a key element for recommending specific mitigation options. Moreover, the International Panel on Climate Change (IPCC) uses a default global emission factor of 1% for fertilizer-induced emission (FIE). However, a number of studies have shown that the amount of N lost as N2O (1% to 20% of applied N) varies with soil type, N inputs, and soil and crop management practices, and is often greater than the default value (1%) of the IPCC. The available conventional techniques such as acetylene (C2H2) inhibition (Zaman and Nguyen 2010) and the closed chamber methods cannot accurately measure both N2O and N2 and their sources in a given soil. The use of 15N offers the best option to quantify both N2O and N2 concurrently and to identify their precise sources in soil (Mosier and Klemedtsson 1994; Zaman et al. 2008; Müller et al. 2014). Soil C sequestration is one of the strategies to offset anthropogenic CO2 emissions by capturing atmospheric C through photosynthesis and storing it in soil. Carbon sequestration is also a key factor in enhancing soil fertility, nutrients, and water retention, and improving soil quality and health. Appropriate farm management practices and land uses can enable agricultural soils to be a net sink for sequestering atmospheric CO2 and other GHG (Lal and Kimble 1997; Paustian et al. 1997; West and Post 2002). For example, CA practices such as mulching, cover crops, zero or minimum tillage, strategic use of manures, and biochar add more C and nutrients into soil, which could lead to improved soil structure and its drainage and hence reduce both N2O and CH4 emissions. With the



259

help of the stable isotope 13C in soil C sequestration studies, one can assess the relative contribution of C from different plant species (C3 and C4) (Nguyen et al. 2011). Enhancing water use efficiency on-farm is another strategy that could potentially reduce GHG emissions from soils. The differences in soil types, fertility, texture, and land use management practices result in significant variability in soil water content after irrigation/rainfall (Zaman et al. 2012). This patchy distribution of applied irrigation water can enhance localized high emission potential for GHG. It is well known that peak emissions of GHG are associated with areas of high fertility (Parkin 1987), so-called hot spots, or temporary areas of high GHG production potential (Groffman et al. 2009; Müller and Clough 2013). Thus it is important to understand the conditions under which these critical source areas of high emission potential develop. Therefore, to optimize agricultural management practices to reduce GHG emissions, a holistic approach is needed. The type of irrigation system is crucial to avoid conditions of high GHG emission potential. Generally, irrigation systems that provide evenly distributed soil water, such as drip irrigation systems, seem to have a lower potential for GHG emissions than systems that are irrigated with a traveling gun (Bruckler et al. 2000; Scheer et al. 2008; Lv et al. 2014). This was confirmed by a recent study where both conventional N treatments compared with a fertigation system (drip irrigation with N fertilizer solution) showed very low emission factors of N2O emissions (