127 6 Carbon Dynamics in Agricultural Soils R. Lal CONTENTS 6.1 Introduction 127 6.2 Soil as Moderator of Earth’s Climate 128 6.2.1 Soils and the Global Carbon Cycle 130 6.2.2 Soil Carbon Dynamics 132 6.3 Soil Carbon Sequestration 134 6.3.1 Aggregation 135 6.3.2 Illuviation 136 6.3.3 Secondary Carbonates 136 6.4 Technological Options of Carbon Sequestration in Agricultural Soils 138 6.5 Rates of Soil Carbon Sequestration 139 6.5.1 Measurement Issues Related to Soil Carbon Storage 139 6.6 Conclusions 141 References 141 6.1 INTRODUCTION Soils are integral component of natural ecosystems. The functioning of ecosystems, including the cycling of elements and transfer of mass and energy, is moderated through soils. Yet, the focus of soil science research during the 19th and 20th centuries has mainly been on two principal soil functions: (1) the medium for plant (crops, pastures, and trees) growth, and (2) the foundation for civil structures (roads, buildings, hydraulic dams). The unprecedented growth in human population during the 20th century led to (1) widespread adoption of the “Green Revolution” technol- ogies based on intensive management of plant nutrients and available water capacity of the soil in the effective root zone for enhancing crop yields, (2) rapid expansion of urban centers, which often necessitated use of fertile topsoil for brick making, along with development of highways, parking lots, airports, and recreational facil- ities, and dams to create water reservoirs and hydroelectric generation facilities. Consequently, the 20th century witnessed rapid advances in soil science with focus on soil as a medium for plant root growth (e.g., edaphology) with specific reference to water and nutrient movement through the soil and uptake by plants, elemental © 2006 by Taylor & Francis Group, LLC 128 Climate Change and Managed Ecosystems balance, aeration for root/microbial respiration, soil reaction, and optimization of these properties and processes through anthropic interventions involving tillage or lack of it, water table management, precision farming, fertigation, etc. Equally impressive advances were made in soil mechanics and soil hydrology to meet the requirements for engineering functions of soil. The human society faces different challenges at the onset of the 21st century. While there is no cause for complacency in the pursuit of agricultural intensification for advancing food production to meet the needs of human population growing at 1.3%/annum and expected to reach 8.5 to 9.0 billion by 2050 and eventually stabilize at the 10 to 11 billion level by 2100. Yet there are other pressing demands on world soil resources that also require attention. In addition to the two usual functions, human needs in the 21st century demand prioritization of research on several other functions of soil. Important among these are (1) repository for industrial, urban, and nuclear waste, (2) storehouse of germ plasm and biodiversity, (3) archive of human and planetary history, (4) biomembrane to denature pollutant and purify water, and (5) moderate the climate. This chapter specifically focuses on the function of soil as moderator of the climate, through its influence on the global C cycle. World soils constitute the third largest global C pool comprising 1550 Pg of soil organic carbon (SOC) and 950 Pg of soil inorganic carbon (SIC). Indeed, the soil C pool of 2500 Pg is 3.3 times the atmospheric pool of 760 Pg and 4.0 times the biotic pool of 620 Pg. Thus, the objective of this chapter is to illustrate the importance of world soils in the global carbon (C) cycle. The focus of the discussions is on the interaction between the soil and the atmospheric C pools under the changing climatic conditions, with specific reference on C dynamics in agricultural soils because of the need for their intense management to meet the needs for food, feed, fiber, and fuel production. 6.2 SOIL AS MODERATOR OF EARTH’S CLIMATE Soil affects and is affected by the climate through numerous interactive processes (Figure 6.1). Soil affects climate by influencing outgoing (albedo and the long wave radiation) and incoming (insolation) radiation through its effects on air quality and the concentration of dust and other particulate materials, and changing amount and distribution of precipitation through its effect on relative humidity and temperature under local conditions. In turn, climate strongly affects soil properties 1–3 through its influences on rate and depth of weathering, intensity and severity of the cycles of erosion and deposition, quantity and quality of soil C pool and its stratification, and soil reaction (pH) and the attendant changes in elemental composition and cycling by flora and fauna (Figure 6.1). Strong interactive processes between soil and climate have long been recognized by the Dokuchaev School in Russia. 4 The interactions between soil and the atmosphere (climate) are closely linked to those between soil and biosphere, soil and lithosphere, and soil and hydrosphere (Figure 6.2). Soil’s effects on climate are through its influence on the global C cycle. Indeed, the interactive processes between soil and the environment (e.g., biosphere, hydrosphere, lithosphere, and the atmosphere) are moderated through changes in soil processes including the global C cycle. Physical, chemical, and biological processes and properties are influenced by the climate (Table 6.1). Important soil processes that © 2006 by Taylor & Francis Group, LLC Carbon Dynamics in Agricultural Soils 129 FIGURE 6.1 Effects of (A) soil on the atmosphere, (B) atmosphere on soil, and of the interacti ve processes on the climate. (A) (B) Soil Reaction Elemental Concentrations and Cycling by Vegetation Interactive Process Soil Erosion and Sedimentation Atmosphere Quantity and Quality of Soil C Pools and Stratification Rate and Depth of Weathering and Horizonation Precipitation • Amount • Distribution Pedosphere Air Quality: Gaseous concentration and particulate matter Outgoing and Incoming Radiation Relative Humidity © 2006 by Taylor & Francis Group, LLC 130 Climate Change and Managed Ecosystems are influenced by climate and have strong impact on the soil C cycles are soil aggregation and erosion, oxidation/mineralization of soil organic matter (SOM), and methanogenesis. In addition, climate also affects the N cycle through its impact on nitrification/denitrification processes and emission of N 2 O into the atmosphere. Thus, natural or anthropogenic change in climate can drastically affect soil properties, 5,6 and the magnitude of change may depend on the antecedent conditions and the degree of climate change. 6.2.1 S OILS AND THE G LOBAL C ARBON C YCLE Soils provide numerous ecosystem services of value to humans and functioning of the biosphere. 7–9 In this regard, the importance of soil in moderating the global C cycle cannot be overemphasized. Historically, soils have been the source of atmo- spheric enrichment of CO 2 ever since the dawn of settled agriculture about 10,000 FIGURE 6.2 Interactive processes in soil with its environment with strong influence on the global C cycle. Atmosphere • Concentration of: (i) Trace gases (ii) Particulate Matter and Soot Pedosphere Lithosphere • Depth distribution of soil C • Organic vs. inorganic C Pool Hydrosphere • Transport of dissolved and suspended C • Precipitation of C Biosphere • NPP • Root: Shoot Ratio • C:N Ratio Precipitous gaseous flux Particulate matter Weathering elemental cycling Leaching Elemental cycling Hypoxia Erosion Sedimentation Biomass Elemental cycling © 2006 by Taylor & Francis Group, LLC Carbon Dynamics in Agricultural Soils 131 years ago. 10 Most agricultural soils have lost 25 to 75% of their antecedent SOC pool due to historic land use. 11 The historic loss of SOC pool is estimated at 66 to 90 Pg C, of which the loss due to accelerated erosion by water and wind is 19 to 32 Pg C. 12 Therefore, the SOC pool in most agricultural soils is drastically below their potential maximum determined by the pedologic and climatic factors. This deficit in SOC pool, which can be filled through conversion to a restorative land use and adoption of recommended land use and management practice, is also called the soil C sink capacity. The C sink capacity of agricultural soils is estimated to be about 35 to 40 Pg over a 50- to 100-year period. 13,14 Sequestration of 1 Pg of atmospheric C in soil is equivalent to reduction of atmospheric CO 2 by 0.47 ppm. Total SOC pool to 1-m depth is 1500 Pg compared to 760 Pg in the atmosphere. 11 TABLE 6.1 Soil Processes and Properties That Are Affected by Climate and That Strongly Affect the Soil Carbon Pool Characteristics Soil Processes Soil Properties I. Physical (a) Structure/aggregation (i) Clay content and mineralogy (ii) Cementing agents (sesquoxides, carbonates, organic polymers) (b) Erosion (i) Soil erodibility (ii) Rate of new soil formation (iii) Transportability and sedimentation (c) Water retention and transmission (i) Plant available water capacity (ii) Least limiting water range (iii) Infiltration rate (iv) Deep percolation (d) Crusting, compaction, and hard setting (i) Bulk density (ii) Porosity and pore size distribution (iii) Soil strength II. Chemical (a) Ion exchange (i) Elemental concentration (ii) Ionic species (b) Leaching (i) Soil reaction (pH) (ii) Charge density (c) Diffusion (i) Concentration gradient (ii) Tortuosity III. Biological (a) Oxidation/mineralization (i) Decomposition constant (ii) C:N ratio and lignin/suberin contents (b) Soil respiration (i) Soil microbial biodiversity (ii) Biomass C (iii) Soil enzymes (c) Methanogenesis (i) Methanogenic bacteria (ii) Substrate composition (d) Nitrification/denitrification (i) Bacterial population (ii) NO 3 concentration © 2006 by Taylor & Francis Group, LLC 132 Climate Change and Managed Ecosystems 6.2.2 SOIL CARBON DYNAMICS The magnitude and rate of depletion of SOC pool depend on land use and soil/plant management practices (Table 6.2). Practices that lead to severe depletion of SOC pool include deforestation, conversion of natural to agricultural ecosystems, biomass burning and residue removal, soil tillage, and extractive or fertility mining practices. These practices set in motion those processes that exacerbate mineralization of SOM and increase the rate and cumulative amount of CO 2 -C emission. Attendant changes in soil properties, with a positive feedback on emission of CO 2 and other greenhouse gases (GHGs), are reduction in the amount and stability of aggregates, increase in bulk density with a decrease in available water-holding capacity, and reduction in hydraulic conductivity and infiltration rate (Table 6.2). The depletion in SOC pool due to conversion of natural to agricultural ecosystems and soil cultivation occurs due to (1) reduction in the amount of biomass returned to the soil, (2) increase in the rate of mineralization usually associated with the change in soil temperature and moisture regimes, and (3) increase in losses of SOC pool due to erosion and leaching. In sloping and plowed soils prone to erosion by water and/or tillage, 15 severe depletion occurs as a consequence of all three processes. The SOC dynamics in agricultural soils can be described by static and dynamic models. The static model has been developed and used for five decades. 16–19 It states that SOC equals gains minus losses of SOC (Equation 6.1). (6.1) where C is the SOC pool, K is the decomposition constant, and A is the amount of C added to the soil through root biomass, crop residue, and other biosolids applied as amendments, and t is time. At steady state, when the addition of SOC by humi- fication equals the loss by decomposition (and other processes), dC/dt = 0 and Equation 6.1 can be rewritten as C = A/K (6.2) The decomposition constant K is influenced by practices, processes, and properties outlined in Table 6.2. Sometimes Equation 6.2 is written in the form C = hA/K (6.3) where h is the humification efficiency, which is to 10 to 12% of the annual biomass addition in temperate climates. 20 In contrast to the static model, the dynamic expo- nential model is an improvement over the static model. 21–29 The two-component dynamic model is shown in Equation 6.4. C t = K 1 A/K 2 (l – e –k 2 t ) + C O e –k 2 t (6.4) dC dt AKC=− © 2006 by Taylor & Francis Group, LLC Carbon Dynamics in Agricultural Soils 133 where C t is the SOC pool at time t, C O is the antecedent SOC pool at time t = O, K 1 is the annual rate at which biomass is humified and added to the soil and is good for SOC sequestration, and K 2 is the annual rate of SOC loss by mineralization and erosion, and K 2 is bad for SOC sequestration. A is the accretion or annual addition of C to the TABLE 6.2 Land Use and Soil/Plant Management Practices That Exacerbate the Emission of CO 2 from Soil to the Atmosphere Practice Processes Affected Properties Altered A. Deforestation 1. Energy balance (i) Soil temperature 2. Water balance (ii) Soil moisture 3. Compaction (iii) Bulk density 4. Erosion (iv) Porosity 5. Shift in vegetation (v) SOC pool 6. Nutrient cycling (vi) Nutrient reserve B. Biomass burning 1. Energy balance (i) Soil temperature 2. Water balance (ii) Soil moisture 3. Nutrient balance (iii) Mineralization rate 4. Runoff/leaching (iv) Soil reaction (pH) 5. Net primary productivity (NPP) (v) Hydrophobicity C. Biomass removal 1. C/elemental cycling (i) SOC pool 2. Crusting/compaction (ii) Nutrient pool 3. Activity of soil fauna (iii) Bulk density 4. Runoff/erosion (iv) Infiltration rate 5. NPP (v) Soil temperature and moisture regimes D. Soil tillage 1. Gaseous flux (i) Bulk density 2. Erosion/runoff (ii) Infiltration rate 3. Aggregation (iii) Stability and amount of aggregation 4. Compaction/crusting (iv) Soil temperature and moisture regimes 5. Diffusion (v) Permeability E. Extractive subsistence farming 1. SOC depletion (i) Soil structure 2. Nutrient depletion (ii) SOC content 3. Elemental cycling (iii) Nutrient reserve F. Drainage 1. Anaerobiosis (i) Soil moisture and temperature regimes 2. Methanogenesis (ii) Rate of mineralization 3. Nitrification (iii) Leaching 4. Denitrification (iv) Soil reaction © 2006 by Taylor & Francis Group, LLC 134 Climate Change and Managed Ecosystems soil as crop residue or other biosolids. Similar to the static model (Equation 6.1), the first term [K 1 A/K 2 (1 – e –k 2 t )] is an estimate of the addition to the SOC pool through crop residue, etc., and the second term (C 0 e –k 2 t ) is an estimate of the decomposition of C 0 . The difference between the two terms is the net amount of C t at any time. Taking the derivative of Equation 6.4 with respect to t leads to Equation 6.5: (6.5) which at equilibrium, when dC/dt = 0, gives Equation 6.6: C = K 1 A/K 2 (6.6) Similar to the K in Equation 6.3, K 1 is strongly influenced by the quality of crop residue (e.g., C:N ratio, lignin and suberin contents). In contrast, K 2 is influenced by soil properties, climatic factors, and management practices. Good and bad farming/land- use practices affecting the magnitude of constants K 1 and K 2 are outlined in Table 6.3. Models described in Equations 6.1 through. 6.6 are based on several assumptions: 30 1. The rate of mineralization depends on the amount of SOC at time t. 2. The rate of mineralization is not limited by lack of other elements (e.g., N). 3. The decomposition constants (K 1 and K 2 ) do not change over time. 4. All components of the SOC pool are equally susceptible to mineralization. The objective of soil and crop management is to maximize C by moderating K 1 , K 2 , and A through tillage methods, residue management, integrated nutrient manage- ment, use of compost and biosolids, and cropping systems based on complex rota- tions and use of cover crops. 6.3 SOIL CARBON SEQUESTRATION Soil C sequestration implies transfer of a fraction of atmospheric CO 2 into soil C pool through conversion of pant residue into humus, and retention of humus-C in soil for a long time. Enhancing the SOC pool of agricultural soils has numerous advantages. In comparison with engineering techniques (e.g., geologic sequestration, mineraliza- tion), SOC enhancement is a natural process, has no adverse ecological impacts, is cost-effective, and improves soil quality. Restoration of soil quality through SOC enhancement improves biomass/agronomic productivity, improves water quality by reducing erosion and sedimentation and non-point-source pollution, improves air qual- ity by reducing wind erosion, and mitigates global warming by reducing the net rate of enrichment of atmospheric CO 2 . In some cases, however, herbicide effectiveness may be decreased in soils containing high SOC concentration. 31,32 Several important mechanisms of protection of SOC sequestered in soil include physical, chemical, and biological processes, 33–38 some of which are described below. dC dt KA KC=− 12 © 2006 by Taylor & Francis Group, LLC Carbon Dynamics in Agricultural Soils 135 6.3.1 AGGREGATION Physical protection of SOC is an important mechanism of increasing the residence time of C in soil, and it involves its encapsulation within a stable aggregate. Humic compounds, comprising long-chain polymers, stabilize micro-aggregates against disruptive forces including chemical, mechanical, and biological processes. Several models have been proposed suggesting the role of SOC in stabilization of soil aggregates. 39 The classical model of Edwards and Bremmer 34 illustrates the mechanism of physical protection of SOC through stabilization of micro-aggregates (Equation 6.7): Micro-aggregate = [(Cl–P–OM) x ] y (6.7) where Cl is clay particle, P is polyvalent cation (e.g., Fe 3+, Al 3+ , Ca 2+ , Mg 2+) , OM is organic molecule, and x and y are the number of these units bonded together by cementing agents to form a secondary particle or a microaggregate. The OM thus TABLE 6.3 Factors Affecting the Value of Constants K 1 and K 2 Parameters Factors Increasing the Value of Constants K 1 Representing Humification Efficiency (good practices) K 2 Representing Loss by Erosion and Mineralization (bad practices) 1. Climate (i) Rainfall High Low (ii) Temperature Low High (iii) Type Temperate, boreal, tundra, taiga Tropics, subtropics 2. Soil (i) Clay High Low (ii) Minerology 2:1, high-activity clays 1:1, low-activity clays (iii) Water retention High Low (iv) Type Heavy texture, poorly drained Light texture, excessively drained 3. Soil Management (i) Tillage No-till, conservation tillage Plow tillage (ii) Residue Surface mulch Incorporation, removal, burning (iii) Fertility Integrated nutrient management Nutrient deficit, fertility mining 4. Crop Management (i) Rotations Complex Simple (ii) Cover crops Winter cover crops Continuous cropping (iii) Agroforestry With tree-based systems Without tree-based systems (iv) Farming systems With animal and ley farming Without animal 5. Landscapes (i) Slope gradient Gentle to none Undulating to steep (ii) Position Foot slopes Summit and shoulder slopes (iii) Shape Concave/depositional Convex (iv) Drainage density Low High (v) Aspect North facing South facing © 2006 by Taylor & Francis Group, LLC 136 Climate Change and Managed Ecosystems sequestered is physically protected against microbial processes and is not mineral- ized. The strong bonding agents (e.g., polyvalent cations, long-chain organic poly- mers) stabilize the aggregate while weak bonds (e.g., Na + ) disperse/slake the aggre- gate (Equation 6.8). (6.8) Dispersion or breakdown of micro-aggregates (such as by raindrop impact or by the shearing effect of flowing water) exposes the OM to microbial processes leading to emission of CO 2 into the atmosphere. Indeed, accelerated soil erosion enhances emission of CO 2 from soil to the atmosphere. 40 Predominant processes are water runoff, soil erosion, gaseous diffusion, crusting and compaction, anaer- obiosis, and depletion of SOC and nutrient pools. Decline in soil quality, caused by a range of degradative processes, exacerbates depletion of the SOC pool and emission of CO 2 . 6.3.2 ILLUVIATION Deep transfer of SOC into the subsoil, away from the surface zone prone to natural and anthropogenic perturbations, is another strategy of increasing the residence time of C in soil. The SOC buried deep in the subsoil is protected against erosion by water and disruption by plowing and animal/vehicle traffic. The rate of mineralization is also lower in the subsoil than in the surface soil. Illuviation of SOC occurs with bioturbation (e.g., earthworms) and movement with percolating water from surface into the subsoil either as dissolved organic carbon (DOC) or suspended colloid along with the clay particles. Reprecipitation of DOC in the subsoil following reaction with silica and other compounds and deposition of clay-humus colloids in the deeper layers is another mechanism of transfer of SOC from surface into the subsoil. 6.3.3 SECONDARY CARBONATES The soil C pool comprises two components: SOC and soil inorganic carbon (SIC) subpools. Agricultural soils in arid and semi-arid regions also have the potential of sequestering SIC. The SIC subpool contains primary carbonates (e.g., calcite, dolo- mite, aragonite, and siderite). These primary carbonates are of lithogenic origin and occur in soil due to weathering of the parent material. In contrast, there are also secondary carbonates that occur in soil due to some pedogenic processes. There are two mechanisms of SIC sequestration: (1) formation of secondary carbonates, and (2) leaching of bicarbonates into the ground water. Visible accumulation of secondary carbonates is a common occurrence in soils of arid and semi-arid climates. 41 Sec- ondary carbonates occur as carbonate films, threads, concretions, and pendants. 42 They may also occur as laminar caps, caliche, and calrete. 43 In gravelly soils, [( ) ]CPOM xy Dispersion Aggregation 1 −−    yC P OM x Dispersion Aggregation ()1 −−   xy CI P OM()−− © 2006 by Taylor & Francis Group, LLC [...]... properties and rainfall Soil Sci 38: 363 –381 4 Neustruev, S.S 1927 Genesis of Soils Russ Pedol Invest USSR Acad Sci., 5 pp 5 Heal, O.W 2001 Potential response of natural terrestrial ecosystems to Arctic climate change Buvsindi 14: 3– 16 © 20 06 by Taylor & Francis Group, LLC 142 Climate Change and Managed Ecosystems 6 Nisbet, T 2002 Implications of climate change: soil and water For Comm Bull 125: 53 67 7... Francis Group, LLC 1 46 Climate Change and Managed Ecosystems 83 Sainju, U.M., B.P Singh, and W.F Whitehead 2002 Long-term effects of tillage, cover crops, and nitrogen fertilization on organic carbon and nitrogen concentrations in sandy loam soils in Georgia, USA Soil Tillage Res 63 : 167 –179 84 Franzluebbers, A.J., F.M Hons, and D.A Zuberer 1994 Long-term changes in soil carbon and nitrogen pools in... 58: 163 9– 164 5 85 Bowman, R.A., R.S Vigil, D.C Nielsen, and R.L Anderson 1999 Soil organic matter changes in intensively cropped dryland systems Soil Sci Am J 63 : 1 86 191 86 Follett, R.F 2001 Soil management concepts and carbon sequestration in cropland soils Soil Tillage Res 61 : 77–92 87 Gregorich, E.G., C.F Drury, and J.A Baldock 2001 Changes in soil carbon under long-term maize in monoculture and. .. 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Kimble and W.D Nettleton, Eds., 3–17 October 1987, USDA-SCS, Lincoln, NE, 79–92 43 Gile, L.H 1993 Carbonate stages in sandy soils of the Leasburg Surface, southern New Mexico Soil Sci 1 56: 101–110 44 Mermut, A.R and A Landi 2004 Secondary/Pedogenic Carbonates In Encyclopedia of Soil Science, R Lal, Ed Marcel Dekker, New York © 20 06 by Taylor & Francis Group, LLC 144 Climate Change and Managed Ecosystems. .. Agroecosystems: Long-Term Experiments in North America CRC Press, Boca Raton, FL 66 Lal, R 2000 World cropland soils as a source of sink for atmospheric CO2 Adv Agron 71: 145–191 67 Izaurralde, R.C., W.B McGill, J.A Robertson, N.G Juma, and J.J Thurston 2001 Carbon balance of the Breton classical plots over half a century Soil Sci Soc Am J 65 : 431– 441 68 Marland, G., T.O West, B Schlamadinger, and . terrestrial ecosystems to Arctic climate change. Buvsindi 14: 3– 16. © 20 06 by Taylor & Francis Group, LLC 142 Climate Change and Managed Ecosystems 6. Nisbet, T. 2002. Implications of climate change: . concentration © 20 06 by Taylor & Francis Group, LLC 132 Climate Change and Managed Ecosystems 6. 2.2 SOIL CARBON DYNAMICS The magnitude and rate of depletion of SOC pool depend on land use and soil/plant management. 69 –79. © 20 06 by Taylor & Francis Group, LLC 1 46 Climate Change and Managed Ecosystems 83. Sainju, U.M., B.P. Singh, and W.F. Whitehead. 2002. Long-term effects of tillage, cover crops, and nitrogen