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soil& Tillage Research Soil & Tillage Research 43 (1997) 81-107 ELSEVIER Residue management, conservation tillage and soil restoration for mitigating greenhouse effect by CO,-enrichment R. La1 * School of Natural Resources, The Ohio State University, Columbus, OH, USA Abstract This manuscript reviews the potential impact of residue management, conservation tillage and soil restoration on carbon sequestration in world soils. The greenhouse effect is among four principal ecological issues of global concern that include: (i) adequacy of land resources to meet needs of present and future generations; (ii) role of world soils and agricultural practices in the ‘greenhouse’ effect; (iii) potential of crop residue management, restoration of degraded soils, and conservation tillage in carbon sequestration in soil; and (iv) minimizing risks of soil degradation by enhancing soil resilience and soil quality. Annual increase in CO, concentration in the atmosphere is 3.2 X 1015 g, and there exists a potential to mitigate this effect through C sequestration in soils. Just as world soils are an important active pool of organic carbon and play a major role in the global carbon cycle, crop residue is a major renewable resource which also has an important impact on the global carbon cycle. I have estimated the annual production of crop residue to be about 3.4 billion Mg in the world. If 15% of C contained in the residue can be converted to passive soil organic carbon (SOC) fraction, it may lead to C sequestration at the rate of 0.2 X lOI5 g/yr. Similarly restoring presently degraded soils, estimated at about 2.0 billion ha, and increasing SOC content by O.Ol%/yr may lead C sequestration at the rate of 3.0 Pg C/yr. Conservation tillage is an important tool for crop residue management, restoration of degraded soil, and for enhancing C sequestration in soil. Conservation tillage, any tillage system that maintains at least 30% of the soil surface covered by residue, was practised in 1995 on about 40 X lo6 ha or 35.5% of planted area in USA. It is projected that by the year 2020, conservation tillage may be adopted on 75% of cropland in USA (140 X IO6 ha), 50% in other developed countries (225 X lo6 ha), and 25% in developing countries (172 X lo6 ha). The projected conversion of conventional to conservation tillage may lead to a global C sequestration by 2020 at a low estimate of 1.5 X 1015 g, and at a high estimate of 4.9 X lOI g of C. These potentials of C ’ Corresponding author. 0167-1987/97/$17.00 PII SO167-1987(97)00036-6 0 1997 Elsevier Science B.V. All rights reserved. 82 R. Lal/Soil & Tillage Research 43 (1997) 81-107 sequestration can be realized through adoption of regional, national and global soil policy that stipulate appropriate use of world soil resources. 0 1997 Elsevier Science B.V. Keywords: ment Greenhouse effect; Aggregation; Soil carbon dynamics; Land restoration; Crop residue manage- 1. Introduction There are four principal issues of global concern with regards to agricultural production. The first is related to the finite extent of land resources, second to the impact of agricultural activities on environmental quality in general, but the ‘greenhouse’ effect in particular, third to the role of residue management and conservation tillage (CT) in carbon sequestration, and fourth to restoration of degraded soils by enhancing soil resilience and quality. An important strategy is to restore degraded lands, and intensify agricultural production while mitigating the greenhouse effect (Fig. 1). More than 97% of the world’s food is produced on land. While the world population is increasing, its land resources are finite and unequally distributed. As recently as 197Os, the increase in food production in most developing countries was achieved by bringing new land under agricultural production. Presently, however, reserves of potentially arable prime agricultural land are rapidly dwindling. Whatever potentially arable land exists is located within fragile or ecologically-sensitive ecoregions, e.g., tropical rainforest, acid savannas, Crop residue management and tillage methods 4 Conventional tillage and residue removed Conservation tillage and residue returned 1 1 - Low risks of soil degradation - Increased soil resilience - Enhanced soil quality * High risks of soil degradation * Decreased soil resilience l Reduced soil quality Degradative effects Fig. I. Interaction residue management * Agricultural sustainability l GHG emissions + Water quality between soil degradation, (GHG = greenhouse). soil resilience I 1 I I Restorative effects and soil quality as influenced by tillage method and R. Lul/ Soil & Tillage Research 43 (1997) 81-l 07 83 steeplands, and the West African Sahel. The number of developing countries with per capita arable land area of less than or equal to 0.10 ha was five in 1990, and will increase to 29 by the year 2025 (Table 1). Therefore, agricultural intensification with improved and science-based technology is inevitable, especially for countries that presently practice predominantly resource-based or subsistence agriculture. Agricultural intensification is also an important factor influencing the greenhouse gas (GHG) emissions (Table 2). A report of the Intergovernment Panel on Climate Change (Intergovernmental Panel on Climate Change, 1995) estimated that 20% of the greenhouse effect is related to agricultural activities. Therefore, producers, scientists and planners are faced with a challenge to increase agricultural production without accentuating risks of GHG emissions. In this regard, the management of soil resources, in general, and that of the soil organic carbon (SOC), in particular, is extremely important. Soil resources of the world may be the key factor in the creation of an effective carbon sink and mitigation of the greenhouse effect. Table 1 Developing Engelman Country Emt China Bangledesh Vietnsm Kenya Sri Lanka Somalia Indonesia Guinea Philippines Tanzania Nepal Haiti El Salvador Yemen Jordan Rwanda Peru Pakistan Malawi Ghana Guatemala Zaire Laos Mozambique Mali Burundi Madagascar Cote d’Ivoire countries with per capita and Le Rov. 1995) arable land area of less than 0.1 ha by the year 2025 (recalculated Per capita arable land area (ha) 1990 2025 0.05 0.08 0.09 0.10 0.10 0.11 0.12 0.12 0.13 0.13 0.13 0.14 0.14 0.14 0.14 0.14 0.17 0.17 0.17 0.18 0.18 0.20 0.21 0.22 0.22 0.23 0.24 0.25 0.31 0.03 0.06 0.05 0.05 0.04 0.08 0.05 0.08 0.05 0.08 0.05 0.07 0.07 0.08 0.05 0.05 0.07 0.10 0.07 0.07 0.07 0.09 0.08 0.09 0.09 0.09 0.10 0.09 0.10 from R. La1 / Soil & Tillage 84 Table 2 Relevant references showing the impact Research of soil processes 43 (1997) on greenhouse 81 -I 07 gas emissions Subject Reference 1, Soils and the greenhouse effect 2. Soil organic carbon pool Bouwman (1989), La1 et al. (19951, Schlesinger Eswaran et al. (1993, 19951, Houghton (19951, Sanchez et al. (19821, Tans et al. (1990) Stevenson (1982). Tate (1987) 3. Properties of SOC and humus (1990, 1995) Post et al. (19901, Soil quality refers to its capacity to produce economic goods and services and regulate environment. Soil resilience is its ability to restore its life support processes and environmental regulatory functions after a major anthropogenic perturbation (Lal, 1993). Soil quality and resilience have a profound impact on productivity and environmental quality, and soil quality is affected by crop residue management and tillage methods (Doran et al., 1994). Agricultural intensification may have adverse impact on environmental quality (Fig. 2) through emissions of greenhouse gases, eutrophication of surface water, and contamination of groundwater. Therefore, the objective of sustainable management systems is to minimize the adverse effects of agricultural intensification. In other words, sustainable use of soil resources involves decreasing the adverse impact of degradative processes, enhancing soil resilience, and improving its quality (Fig. 2). Decreasing Per Capita Arable Land Area Finite + So&-Economic land i-e~oui-ces Agricultural Intensification Emissions of * co, . CH; . NO, . N,O I Fig. 2. Land resilience. shortage and its effect on agricultural sustainability, I environmental quality, soil quality and soil R. Lal/Soil & Tillage Researclt 43 (1997) 85 81-101 The objective of this manuscript is to collate, review and evaluate available information on: (i) per capita land resources of the world and the need for agronomic intensification, (ii) soil resources of the world and the global SOC pool, (iii) renewable crop residue production and its role in carbon sequestration, and (iv) carbon sequestration through adoption of conservation tillage, and restoration of degraded soils. The manuscript is based on literature review, and projections of carbon sequestration on assumptions of the land area under CT, and possible increase in SOC resulting from conversion from conventional to CT. Also included are reviews of subject matter at different scales. While processes of C sequestration are discussed at the scale of soil separate or microaggregates, SOC dynamics in soil under tillage methods are discussed at field plot scale, and projections of carbon sequestration and adoption of CT are made at global scale. 2. Soil resources and the soil organic carbon pool Principal soils of the world are Aridisols, Inceptisols, Alfisols, Entisols, Oxisols and Ultisols, which together account for 81% of the earth’s land area (Table 3, Eswaran et al., 1993; Eswaran et al., 1995). Agriculturally productive and highly fertile soils are Inceptisols, Entisols, Mollisols, Histosols and Andisols that constitute about 33% of the world soils. Predominant soils of the tropics are Oxisols, Ultisols, Aridisols, Alfisols, Inceptisols and Entisols accounting for 88% of tropical land area (Table 4). Agriculturally productive and relatively fertile soils of the tropics cover about 20% of the tropics (Eswaran et al., 1993). The SOC content is an important factor affecting soil quality, and is an important source of plant nutrients, especially in subsistence agriculture. The important effect of SOC on productivity and environmental quality is through its role in supplying nutrients, Table 3 Soil organic Order carbon reserves of world Area lo6 ha Alfisols Andisols Aridisols Entisols Histosols Inceptisols Mollisols Oxisols Spodosols Ultisols Vertisols Miscellaneous Total soils (recalculated 1828.3 255.2 3174.3 1492.1 174.5 2158.0 548.0 1177.2 487.8 1133.0 328.7 744.4 13521.5 from Eswaran Soil organic % 13.5 1.9 23.5 11.0 1.3 16.0 4.1 8.7 3.6 8.4 2.4 5.6 100 O-25 cm et al., 1993, 1995) carbon (10” O-50 73 38 57 37 26 162 41 88 39 74 11 - 100 61 95 52 54 215 52 128 53 96 21 - 652 927 g) cm O-100 136 69 110 98 390 267 12 150 98 101 38 18 1555 cm 86 Table 4 Soil organic Order Alfisols Andisols Aridisols Entisols Histosols Inceptisols Mollisols Oxisols Spodosols Ultisols Vertisols Miscellaneous Total/Average R. Id/Soil carbon reserves & Tillage in soils of the tropics Research (recalculated Land Area 43 (1997) from 81-107 Eswaran Soil organic lo6 ha % of the tropics Total 641.1 168.3 911.7 325.6 28.6 456.5 23.4 1151.2 4.0 901.8 218.9 135.8 4966.9 12.9 3.4 18.3 6.6 0.6 9.2 0.5 23.1 0.1 18.2 4.4 2.7 100 30 47 29 19 100 60 2 119 2 85 11 2 506 (Pg) et al., 1993) carbon in the tropics % of the world soils 24 60 26 13 28 17 3 100 3 81 58 11 32 nutrient recycling, improving soil/plant available water reserves, increasing soil buffer capacity, and stabilizing soil structure (Doran et al., 1994). World soils, an important and active pool of organic carbon, play a major role in the global carbon cycle. The SOC pool, estimated at about 1550 Pg (Pg = petagram = 1015 g, Table 3), is about twice that of the atmospheric pool of 750 Pg and about three times that of the biotic pool of 550 Pg (La1 et al., 1995). The SOC pool of soils of the tropics is about 506 Pg or 32% of the global soil pool (Table 4; La1 and Logan, 1995). The atmospheric pool of carbon has increased steadily, partly at the expense of the SOC pool. Small changes in global SOC pool can have a large effect on atmospheric concentrations of CO,. Since agricultural expansion in the 19th and 20th centuries, decomposition of SOC has contributed to increase in atmospheric CO, (Houghton, 1995). Agricultural activities that enhance CO, emissions include deforestation, biomass burning and tillage (Intergovernmental Panel on Climate Change, 1995). Subsistence agricultural systems based on nutrient-mining and fertility-depleting practices accentuate depletion of SOC and contribute to CO, emissions to the atmosphere. 3. Crop residue and its role in soil organic carbon management Crop residue and other biomass constitute an important resource. Residue management, quantity and quality of biomass applied to the soil, has a significant impact on soil quality and resilience, agronomic productivity, and GHG emissions from soil to the atmosphere. The quantity of crop residue produced depends on the arable land area, crops and cropping systems, and soil and crop management. The global arable land area is about 1.4 billion ha (Table 5) with about 31% in Asia, 20 per cent in North and Central America, 17% in the former USSR, and 12% in Africa (FAO, 1993). Based on the mean residue:grain ratio for different crops, annual production of crop residue is R. IA/Soil Table 5 Arable land use in the world & Tillage in 1992 (FAO, Research 43 (I9971 1993) Reeion Arable Africa North central America South America Asia (excluding USSR) Europe (excluding USSR) Oceania Former USSR (excluding Asia and Europe) World 163.6 264.4 96.8 424.8 122.6 51.6 221.5 1351.3 Table 6 Land area and production production of different Crop Area (lo6 Wheat Rice Barley Maize 221.1 147.5 74.5 127.4 13.3 19.6 35.6 41.8 18.1 9.1 16.0 2.9 1.0 24.0 2.9 8.9 9.9 3.4 57.8 20.5 17.4 8.1 1.4 18.2 20.7 6.2 3.1 1.1 30.8 962.9 Rye Oats Millet Sorghum Potatoes Sweet Potatoes Cassava Yam Taro Beans Broad Beans Peas Chick Peas Lentils Soybeans Ground Nuts Sugar Cane Beet Castor Sunflower Rapeseed Sesame Linseed Safflower Cotton Total Residue estimates are made from crops in the world ha) Production in 1993 (FAO, (lo6 residue:grain Mg) land (lo6 1993), ha) and estimates Estimated residue 847 791 256 471 39 35 40 58 72 31 154 28 6 16 4 16 6 3 111 25 260 70 564.5 527.4 170.4 470.6 26.2 35.4 26.4 57.7 288.2 123.8 153.6 28.1 5.6 16.2 4.0 16.0 6.6 2.7 111.0 25.1 1040.6 281.7 1.2 20.5 26.2 2.5 2.2 0.8 49.1 4084.3 standard 87 81-107 21 26 3 2 1 49 3442 ratios for field crops (Lal, 1995). of crop residue (lo6 Mg) 88 R. La1 / Soil & Tillage Research 43 (1997) 81-107 estimated at 3.4 Pg in the world (Table 6) and 0.4 Pg in the USA. These estimates are similar to those produced earlier for the USA (Larson et al., 1978; USDA, 1978) and the world (Lal, 1995). In terms of soil and water conservation and soil surface management, the most useful crop residue is that of small-grain cereals. The amount of residue produced by cereals is usually high because of a high straw:grain ratio, low decomposition rate, and high C:N ratio. Residue production by all grain cereals is estimated at 2.5 Pg for the world (Table 6) and 0.31 Pg for USA (data not shown). These are undoubtedly large quantities of renewable resources that can be used to enhance soil quality and regulate the environment. There is a potential for C sequestration through management of crop residues. Assuming the mean carbon content of 45%, total carbon assimilated annually in the crop residue is about 1.5 Pg in the world and 0.2 Pg in USA. If 15% of the carbon assimilated in the residue can be converted to humus fraction, it may lead to carbon sequestration at the rate of 0.2 Pg/yr or 5.0 Pg of cumulative C sequestration in the world by the year 2020. If 15% carbon in residue were converted to humus, carbon sequestration in USA may be about 0.75 Pg by the year 2020, an estimate similar to that made by Kern and Johnson (1993). Assuming soil bulk density of 1.5 Mg/m3, an increase of 5.0 Pg of C in world arable land area of 1500 X lo6 ha to l-m depth would increase mean SOC content of O.OOl%/yr. These are realistically attainable goals. 4. Conservation tillage: definitions Conservation tillage (CT) is a practical tool to use crop residues for soil and water conservation and of soil quality enhancement. Understanding the role of CT is important to develop strategies and identify policies for sustainable use of soil and water resources, for mitigating the greenhouse effect, and improving environmental quality. The Conservation Tillage Information Center (CTIC, 1990, 1995) defines CT as “any tillage and planting system that maintains at least 30% of the soil surface covered by residue after planting to reduce water erosion; or where wind erosion is a primary concern, maintain at least 1000 kg/ha of flat, small grain residue equivalent on the surface during the critical wind erosion period.” There has been a change in the definitions and concepts involved in the CT system through its evolution since the 1930s. CTIC has revised its definitions since 1989. In fact, CT is a generic term that refers to “any tillage system that reduces loss of soil or water relative to conventional tillage” (Mannering and Fenster, 1983). Under this generic term, there are several types of CT systems that are based on the principle of crop residue management. The latter includes a year-round system beginning with the selection of crops that produce sufficient quantities of residue including the use of cover crops. Basic concepts and applications of CT systems have been described by Unger and McCalla (1980) Cannel1 (1985) La1 (1989) Blevins and Frye (1993) and others. CTIC (1995) defined a range of CT systems included under CRM as follows: 4.1. No-tillage The soil is left undisturbed from harvest to planting except for plant nutrient application. Any tillage system that causes less than 25% of row width disturbance by R. Lal/Soil & TillageResearch43 (1997)81-107 89 planting equipment (e.g., coulters, disk openers, in-row chisels, roto-tillers) is considered a no-tillage system. Weed control is primarily through herbicides, but cultivation may be used for emergency weed control. 4.2. Ridge-tillage The soil in ridge-tillage is also left undisturbed from harvest to planting, except that planting is completed in a seedbed prepared on ridges with sweeps, disk openers, coulters or row cleaners. Residue is left on the surface between ridges. Weed control may be accomplished with herbicides and/or cultivation. 4.3. Mulch-tillage The soil is disturbed prior to planting by tillage tools such as chisels, field cultivators, disks, sweeps or blades. Weed control is accomplished with herbicides and/or cultivation. 4.4. Reduced-tillage Any seedbed preparation system that leaves 15 to 30% residue cover after planting or 500 to 1000 kg/ha of small grain residue equivalent throughout the critical wind erosion period is considered a reduced-tillage system. 4.5. Conventional-tillage Tillage methods that leave less than 15% residue cover after planting, or less than 500 kg/ha of small grain residue equivalent throughout the critical wind erosion period come under the category of conventional-tillage systems. + &mservation-till > 30% crop residue on soil surface &J&l ~25% of row-width disturbance w Planting is on ridges Fig. 3. Crop residue 15.30% crop residue on soil surface ,&VW,lltiOtl&ilI < 15% crop residue on soil surface @her tillare tm < 30% of crop residue on soil sufwx Mulch-till soil is disturbed by chisel disc etc. management systems and types of tillage methods. R. La1 / Soil & Tilluge 90 Research 43 (1997) N-107 Generic classification of these tillage systems based on crop residue management is outlined in Fig. 3. The term conventional tillage must be differentiated from the traditional tillage. The former refers to motorized tillage operations based on moldboard plow or other soil-inversion tillage tools, and the latter to nonmotorized (manual or animal-drawn) methods of seedbed preparation based on native tools and implements. 5. Conservation tillage Conservation tillage is widely being adopted in North America (Carter, 19941, Europe (Riley et al., 1994; Christian and Ball, 1994; Ehlers and Claupein, 1994; Masse et al., 19941, and the Pacific (Choudhary and Baker, 1994; Steed et al., 1994). However, estimates of area under CT in world and even in USA are difficult to make because of lack of a standard definition, and changes in definition over time. Estimates of area under CT in USA are based on two separate definitions, before and after 1989 (CTIC, 1995). Keeping in view all limitations, there has been a steady increase in the area under Table 1 Arable land area in conservation tillage in USA (the data of 1968-1986 those from 1987-1995 are those compiled by CTIC, 1995) Year Land area (lo6 ha) are those % of planted 1968 2.43 2.0 1969 1970 3.24 4.05 4.45 2.0 3.4 3.6 4.86 4.1 4.1 1971 1972 1973 1974 6.01 6.88 7.29 1975 1976 1911 1978 1979 1980 1981 1982 1983 1984 1985 8.10 9.12 5.9 1.0 12.55 9.2 13.36 15.79 9.5 10.9 17.41 11.8 26.72 28.34 35.22 18.2 22.6 25.3 27.8 38.46 1986 1987 1988 32.9 31.6 32.3 25.7 39.68 34.84 35.64 29.03 29.65 1989 1990 1991 26.1 28.1 31.4 34.9 32.04 35.91 t992 1993 1994 39.33 40.21 40.03 1995 There 5.2 5.6 was a change in definition 35.0 35.5 of conservation tillage from 1989 onward. compiledby S&r& 1988; area R. La1 / Soil & Tillage Research 43 (1997) H-107 91 CT in USA from 2.4 X lo6 ha (2.0% of the planted area in 1968) to 40.0 X lo6 ha (35.5% of the planted area in 1995)(Table 7). There have been several attempts to project the future growth of CT in USA (Schertz, 1988). USDA (1975) estimated that ‘minimum tillage’ would be used on 95% of the cropland in USA by 2010. Crosson (1981) estimated that economic factors could easily induce farmers to adopt CT on 50-60% of the cropland by 2010. The Office of Technology Assessment (1982) estimated that 72% of US cropland would be farmed by a CT system by 2010. Schertz (1988) projected that CT may be adopted on 63-82% of the total planted cropland by the year 2010. Considering all economic and ecologic factors, it is probable that a CT system may be used on about 75% of planted cropland in USA by the year 2020. Spread of CT in Europe, Australia, New Zealand and Canada may be at a slower rate than that of USA, and may be adopted on about 50% of planted cropland in these regions of developed economies by 2020. Gatson et al. (1993) estimated that 86% of agricultural land in the former Soviet Union (181 X lo6 ha) was suitable for no-tillage farming. Spread of CT in developing countries of the tropics and subtropics may be slow because crop residues are used for other purposes. It is expected, therefore, that only about 25% of the planted area in the tropics may be under a CT system by the year 2020. The actual adoption rate may depend on several factors including practices and policies on burning crop residues, availability of herbicides and appropriate machinery, market forces and other socioeconomic factors. 4. Mechanisms of C sequestration in soil The SOC content is a function of soil management, and change in management can alter SOC content. The rate of change (sequestration or release per unit time), however, depends on the net SOC content under the new management system. The net SOC content in soil depends on several interacting mechanisms (Fig. 4), most of which are set in motion by addition of biomass to the soil. The use of a CT system affects C sequestration in soil through its effect on C dynamics, aggregation and soil structure, and interaction with cropping system (Table 8). Carbon sequestration in soil depends on two factors: (i) turnover time, and (ii) physical or chemical protection against microorganisms and soil erosion (Carter, 1995). The turnover time depends on the SOC fraction, its quality and physical location within the soil (Table 9). Labile or rapidly decomposable fractions have low turnover time of less than 5 years and comprise microbial and fresh biomass (Oades and Waters, 1991; Woomer et al., 1993). Fractions with moderate turnover time of 5 to 20 years comprise particulate or light fractions. The SOC fractions with slow turnover time of 20 to 50 years comprise humus and light fractions located between microaggregates. Passive or nonlabile SOC fractions are: (i) protected from microbes, (ii) generally located within the stable microaggregates (< 250 ,um), (iii) physically protected or blocked by the clay domains (adsorbed on the inner clay domains), and (iv) complex polymers have a turnover time of 50 to 100 years. The passive fraction may also be chemically protected by formation of resistant/stable organo-mineral complexes of the form [(clay-P-OM),],, where P is polyvalent cation, OM is organic molecule, and x and y refer to number of molecules joined together 92 R. L.ul/Soil & Tillage Research 43 (1997) 81-107 * More biomass returned to the soil humas fraction Fig. 4. Mechanisms of carbon sequestration in soil (SOC = soil organic carbon). (Tisdall, 1995). Other possible modes of formation of organo-mineral complexes may be clay-P-clay or OM-P-OM (Tisdall, 1995; Elliott and Coleman, 1988; Oades and Waters, 1991; Chaney and Swift, 1986a,b; Collis-George and Lal, 1970; Tisdall and Oades, 1982). In Andosols containing amorphous material, the mechanism of aggregation may be: amorphous Al silicates-Fe (Al) oxides-OM-allophone (Wada and Higashi, 1976). Soil erosion, another degradative process with adverse effect on SOC, can also be curtailed through several soil-protective mechanisms. Formation of stable aggregates is an important mechanism for increasing aggregate strength and reducing soil erodibility. Physical protection by crop residue mulch and CT is the other viable and important Table 8 Conservation tillage, cropping R. Ld/Soil & Tillage systems, and cultivation practices 43 (I9971 93 81-107 for carbon sequestration Reference Subject 1, Soil organic carbon 2. Soil organic carbon, 3. Cropping Research systems content and dynamics aggregation and soil structure and tillage interaction Angers et al. (1993), Arshad et al. (1990) Carter (1995) Doran (1980), Follett et al. (1987), Jenkinson (1991) Chaney et al. (1985), Rasmussen and Rohde (1988), Rasmussen and Collins (1991), Lal et al. (1994) Dalal and Bridge (1995) Feller et al. (1995) Tanchandsponge and Davidson (1970) Albrecht et al. (1986) Goldberg et al. (1988) Lucas et al. (1977), Bouwman et al. (1990) Feller (1988) Chihacek and Ulmer (1995), Glendining and Powlson (1995), Havlin et al. (1990), Huggins et al. (1995) La1 et al. (1990a,b), Unger (1982) Bauer and Black (1981), Mann (1986). Lepsch et al. (1982) Resck and da Silva (1990), Skidmore et al. (1986) option to reduce the impact of erosion (Lal, 1989). Deep placement of SOC beneath the plow zone and useful strategy of physical protection of humus, may be achieved by: (i) growing deep-rooted plants, (ii> translocation of SOC by activity of soil fauna (e.g., earthworms and termites), and (iii) transport of SOC through macropore flow. 7. Soil particle size distribution and C sequestration The humic fraction of SOC is usually associated with fine silt (2-50 urn) and clay fraction (< 2 pm). Clay fraction generally contains more than 50% of the SOC (Bonde et al., 1992). Christensen (1995) observed that between 48 and 69% of the SOC was in clay, 21 to 43% in silt, and 2 to 10% in sand fraction. For soils containing less than 20% clay, the proportion of SOC in clay increases with increasing clay content (Balesdent et al., 1991). Gregorich et al. (19891, Gregorich et al. (1994) observed that after complete Table 9 Turnover time of soil organic carbon depending on quality and physical location within the soil (Carter, 1995; Gregorich et al., 1994; Theng et al., 1989; Stevenson and Elliott, 1989; Oades and Waters, 1991; Woomer et al., 1993; Balesdent et al., 1988) Type of organic Microbial biomass Litter Light fraction Particulate Humus Humus Humus matter Location Turnover Year Category Pores, particle/aggregate surface Soil surface, pores Voids, aggregate surface Voids, biopores Inter-microaggregate Adsorbed on intra-microaggragate Adsorbed on intra-microaggragate 0. I-0.5 l-5 5-15 5-20 20-50 50-1000 1000-3000 Labile Rapid Moderate Moderate Slow Passive Passive time 94 R. L.al/Soil & Tillage Research 43 (1997) 81-107 dispersion in laboratory, SOC content was distributed as 65% with clay (< 2 pm), 30% with silt (Z-50 pm) and 5% with sand (50-2000 pm). Similar trends have been reported by Balesdent et al. (1991) and Morra et al. (1991). Experiments conducted in West Africa by Feller et al. (1991a,b,c,d) showed a strong correlation of SOC with clay, but SOC in cultivated soils being about 60% of that in uncultivated soil. Regression equation relating SOC with clay content for a cultivated soil is shown in Eq. (1): SOC(g/kg) = 0.294(clay%) + 0.31 Y = 0.95 * * (1) Lepsch et al. (1982) observed a similar equation for cultivated soils of Brazil (Eq. (2)): SOC(g/kg) = 0.325(clay%) + 0.77 Y = 0.81* * (2) Tillage reduces SOC content in all size fractions (Tiessen and Stewart, 1983; Cerri et al., 1985). Relative decrease, however, varies with particle size and is generally more in fine clay and sand fractions than in fine silt and coarse clay. In Australia Dalal and Mayer (19&6a,b,c,d) observed that cultivation of a Vertisol caused a rapid decline in SOC content in sand fraction, increase in SOC content of clay, and no change in silt fraction. Similar observations were made on African savanna by Martin et al. (1990) and Balesdent et al. (1988). Possible reasons for tillage-induced decline in SOC are decrease in aggregation, increased rate of decomposition by microorganisms, and accelerated soil erosion. 8. Aggregation and C sequestration A principal mechanism of C sequestration in soil is through the formation of stable microaggregates. The higher the SOC the more and stable are the aggregates. Microaggregates are developed around decomposing particulate organic matter because of the formation of humic polymers and organo-mineral complexes (Elliott and Coleman, 1988; Beare et al., 1994a; Beare et al., 1994b). These micro-aggregates consist of clay particles, clay domains, hydrous oxides of Al and Fe, and organo-mineral complexes. Therefore, a strong correlation exists between aggregation and SOC (Table 10, Hamblin and Davies, 1977; Douglas and Goss, 1982; Chaney and Swift, 1984; Haynes et al., 1991). However, the degree of correlation depends on climate, soil type, texture, clay mineralogy, and cropping history. In pasture soils with high SOC, a substantial portion Table 10 Relationship Regression between aggregation and soil organic Correlation equation % WSA (16% clay) = 2.0.x - 11.5 % WSA (39% clay) = 1.33x - 14.0 %WSA (49% clay) = 1.54.x -58.0 MWD (mm) = 0.24.x +0.31 %WSA > 2 mm =21.5x -20.3 %WSA > 0.25 mm = 158.9x -9.5 %WSA > 20 pm = 74.3 + 6.3 pnX Dispersible clay (%) = 2.39 - 0.42 x Y = organic carbon carbon (%), WSA = water Y= I = Y= R2 R2 R2 R” R2 stable (x) content coefficient 0.86 0.73 0.95 = 0.86 = 0.93 = 0.87 = 0.58 = 0.53 aggregation, Reference Douglas and Goss (1982) Douglas and Goss (1982) Douglas and Goss (1982) Haynes et al. (1991) Tisdall and Oades (1980) Tyagi et al. (1982) DalaJ and Bridge (1995) Dalal and Bridge (1995) MWD = mean weight diameter. R. Lal/Soil & Tillage Researcla 43 (1997) 82-107 95 of §OC is not involved in aggregation, and the correlation is often low. In soils with low SOC, mechanisms of aggregation are different, and the correlation coefficient OF SOC with aggregation is also low. Soils with higher clay content usually require more SOC content for maintaining a given level of aggregation and aggregate stability than those with low clay content (Douglas and Goss, 1982). Differences in clay content also cause differences in soil moisture regime. Aggregate stability often increases with decreasing soil moisture content (Perfect et al., 1990). Similar to the degree of aggregation, aggregate stability is also related to SOC content. Water-stable aggregates usually contain more SOC than those that are unstable (Elliott, 1986). Source of crop residue is also a factor in aggregate stability. Skidmore et al. (1986) observed that application of sorghum (Sorghum bicolor) residue produced more stable aggregates than that of wheat (Triticum aestivum) residue. 9. Conservation tillage and carbon sequestration There are several merits of a CT system (Blevins and Frye, 1993) the principal being a possible increase in SOC content leading to enhancement in soil quality and resilience. Conventional tillage practices involving soil turnover are usually detrimental to SOC. Plowing decreases particulate SOC and the light fraction (Beare et al., 1994a; Beare et al., 1994b; Camberdella and Elliott, 1992; Robertson et al., 1991; Angers et al., 1993). The additional SOC in CT is usually held as inter-macroaggregate material. Conversion to CT may increase macroaggregation and aggregate stability (Haynes and Swift, 1990; Elliott, 1986; Haynes et al., 1991). In general, CT leads to relatively high SOC content near the soil surface compared with the plow-based or conventional tillage (Lal, 1989; Carter, 1992; Dick et al., 1986a,b). However, when comparisons are made on the basis of either the whole soil profile or on mass basis (considering soil bulk density) ‘Yesences in SOC between tillage methods are either less, or in some cases in favor of conventional tillage (Dalal and Mayer, 1986a,b,c,d). In some cases, mixing and soil turnover by plowing may enhance formation of organo-mineral complexes and aggregation, as was observed in soils of the semiarid regions of the West African Sahel (Charreau and Nicou, 1971). For some soils, especially those with coarse texture and in arid climates, conversion to CT when soil has been under cultivation for a long time may, however, have little effect on SOC content (Powlson and Jenkinson, 1981; Haynes and Knight, 1989). Similarly, when soils under native vegetation or pastures are converted to arable land use, the decline in SOC over time is more pronounced with conventional than with a CT system (Blevins et al., 1983a; Blevins et al., 1983b; Dick, 1983; Beare et al., 1992). Higher SOC content in CT may lead to higher and stable aggregation (Home et al., 1992; La1 et al., 1994), because of several mechanisms including the following: (i) fungal dominated microflora (Beare et al., 1993; Beare et al., 199.5) (ii) higher earthworm activity (Edwards et al., 1993) and (iii) formation of platy structure with greater bulk density. Plowing leads to breakdown of aggregates, and conversion to CT can lead to increase in aggregation (Hamblin, 1980; Ike, 1986; Prove et al., 1990; Dalal, 1989). Structural-enhancing effects are, however, generally more pronounced in humid than in arid environments (Lal, 1989). Long-term experiments in Ohio showed higher 96 R. Lal/Soil & Tillage Research 43 (2997) al-107 Table 11 Tillage methods effects on water stable aggregation (WSA) and mean weight diameter (MWD) of aggregates in the row zone of two soils under corn-corn rotation in Ohio (modified from Mahboubi et al., 1993) Tillage method Wooster WSA No-till Chisel plow Moldboard plow LSD (0.05) Tillage (T) Soil (S) TxS silt loam Crosby (%) MWD 44.4 28.4 20.0 1.1 0.7 0.5 6.0 5.5 9.5 0.2 0.2 0.4 WSA (mm) silt loam (%) MWD 61.3 29.8 28.6 (mm) 1.9 0.7 0.5 aggregation and bigger mean weight diameter (MWD) in aggregates from CT than those from conventional tillage (Table 11, Mahboubi et al., 1993). An example of the favorable effects of CT on SOC is shown in Fig. 5 from Ohio. The Soil Organic Carbon (Mg ha-l ) 0 40 2 4 - 6 6 10 Continuous Corn 12 14 16 Corn-Soybean Fig. 5. Tillage method effects on soil organic carbon profile of Wooster Typic Fragiudalf) in Ohio (recalculated from Dick et al., 1986a.b). silt loam (fine-loamy, mixed, mesic R. L&/Soil & Tillage Research 43 (1997) 81-107 97 data in the figure show higher total SOC content with CT than with conventional tillage practices (Dick et al., 1986a,b). Kern and Johnson (1991, 1993) developed the following regression equations relating SOC content in no-till (SOC,,) with that in conventional-till (SOC,,) for O-8 cm depth (Eq. (3)) and 8-15 cm depth (Eq. (4)). SOC,,= (1.283 XSOC,,) SOC,,= (1.16 x SOC,,) -0.180,R2 R2=0.75n= +0.510, =0.89,n 15 (3) = 17 (4) These equations are based on surveys of several long-term experiments conducted throughout the USA (Dick, 1983; Bauer and Black, 1981; Clay et al., 1985; and others). It seems that SOC reserves in no-tillage soil may be about 10 to 20% more in the top l-m depth in soils of the temperate region. The data in Fig. 5 show that SOC content in the top 45 cm was greater in no-tillage than conventional tillage for all rotations, e.g., 59.6 vs. 52.6 Mg/ha for continuous corn, 55.8 vs. 47.8 Mg/ha for corn-soybean, and 61.6 vs. 60.4 Mg/ha for corn-oats-meadow rotation. These levels of increase in SOC content under the no-tillage system may be facilitated by frequent use of cover crops in the rotation cycle (Frye et al., 1988; Utomo, 1986; Utomo et al., 1990). The time required for attainment of the steady state level of SOC when changed over from conventional to no-tillage system may be 10 years in the temperate climate (Kern and Johnson, 199 1, 1993). The data in Fig. 6 from an Alfisol in western Nigeria show that total SOC in the top 15-cm layer was more in no-tillage than conventional tillage treatment. However, total SOC content in the 15-30 cm layer was more in conventional tillage than no-tillage treatment. Higher SOC content beneath the surface layer in conventional tillage treatment may be due to turnover of crop residue in the subsoil and its protection from erosion and microbial decomposition. Is it possible, therefore, that periodic (once every 5 to 7 years) plowing under of no-till plots may sequester SOC accumulated in the Soil Organic 0 Fig. 6. Tillage 1982). method effects 0 2 ~"""',',""""""""""'I on soil organic 4 6 carbon Carbon 6 profile (Mg 10 ha’ 12 for an Alfisol ) 14 in Nigeria 16 (recalculated from Lal, 98 R. L.&/Soil & Tillage Research 43 (1997) 81-107 surface layer into the subsoil? Long-term experiments are needed on different soils and diverse ecoregions to test this hypothesis. 10. Global impact of conservation tillage on C sequestration There are several mechanisms of C sequestration in soil by CT including saving in fuel, application of crop residues, reduced mineralization due to differences in soil temperature and moisture regimes, enhanced aggregation and aggregate stability, and reduction in soil erosion. Projections of C sequestration in world soils is made by estimating the area under CT and assuming a possible increase in SOC content. Estimates reported herein are based on projected land area under CT by the year 2020 shown in Table 12. The increase in land area under CT is estimated about 100 m ha in USA; 180 m ha in Canada, Europe, Australia and New Zealand combined, 80 m ha in Asia, 32 m ha in Africa, and 2.5 m ha in Latin America. Based on the projections made by Gatson et al. (1993) for the former Soviet Union, my estimates may be rather conservative for Europe. Low estimates of C sequestration by conversion from conventional till to CT are made on the basis of a relatively low increase in SOC in the top l-m depth and assuming a bulk density of 1.4-1.45 Mg/m3 (Table 13). These calculations show a potential of C sequestration of about 350 Tg in USA and 1480 Tg in the world. Kern and Johnson (1993) estimated that if CT were adopted on 57% of the arable land by 2020, carbon sequestration in soils of USA would range from 80 to 129 Tg C. In the event of CT being adopted on 76% of the arable land, C sequestration in USA would be 286 to 468 Tg C. A similar approach was used by Lee et al. (1993) who used the EPIC model to evaluate the benefits of erosion control in C sequestration by CT systems. They concluded that adoption of CT would increase SOC content of the top 15 cm of soil by 0.2 kg/m2 during the next 100 years. Adoption of no-tillage or no-tillage plus a cover crop may increase SOC content by 0.4 to 0.8 kg/m2. Gatson et al. (1993) estimated that about 10% increase in SOC by complete conversion to CT in the former Soviet Union would sequester 3.3 Pg of C. High estimate of C sequestration by conversion from conventional tillage to CT are made on the basis of assuming relatively high increase in SOC ranging from 0.05% in the top 1 m depth for tropical and subtropical regions to 0.1% for developed agricultural Table 12 Author’s estimate of projected arable land area under Region/country tillage by 2020 lo6 ha Arable 1. USA 2. Europe, Canada, 3. Asia 4. Africa 5. Latin America Total conservation Australia New Zealand 186 441 425 164 130 1352 land area Area under CT 1995 2020 Change 40 45 20 8 I 120 140 225 100 40 32 531 100 180 80 32 25 417 R. Lal/Soil Table 13 Estimates of potential Region carbon sequestration & Tillage Research by conservation 43 (1997) 99 81-107 tillage Increase in area under conservation tillage (lo6 ha) Change in soil organic oganic carbon in l-m depth (%) A. Low estimates USA Developed countries Asia Africa Latin America Total 100 180 80 32 25 417 2.5 x 3.0x 2.0x 1.5x 2.0x B. High estimates USA Developed countries Asia Africa Latin America Total 100 180 80 32 25 417 +0.1 +0.1 + 0.05 + 0.05 + 0.05 10-2 10-2 10-s lo-’ 10-z Soil bulk density Carbon sequestration (Mg/m3) (Tg) 1.40 1.40 1.45 1.45 1.45 3.50 156 232 IO 13 1481 1.40 1.40 1.45 1.45 1.45 1400 2520 580 232 181 4913 economies in the temperate climates. These calculations show a potential of C sequestration at a global scale ranging from 1.5 Pg in USA to 4.9 Pg in the world (Table 13). 11. Carbon sequestration by restoration of degraded soils Oldeman (1994) estimated that 1965 X IO6 ha of soil are degraded worldwide. This includes 1094 m ha degraded by water erosion, 549 m ha by wind erosion, 239 m ha by chemical degradation, and 83 million ha by physical degradation. Most degraded soils are low in SOC content. Soil restoration, by planting trees or sowing vigorously growing cover crops, would enhance SOC content and lead to improvements in soil quality, If SOC of these soils may be increased by O.Ol%/yr in the top l-m depth, it would lead to carbon sequestration at the rate of 3.0 Pg/yr assuming a mean bulk density of 1.5 Mg/m3 (Eq. (5)). SOC Sequestration = 1.965 X lo9 ha X lo4 m2/ha X 1 m X 1.5 Mg/m3 X 10 A4 g/g = 3 Pg/yr (5) This rate of C sequestration is about equal to the present rate of annual increase in carbon concentration in the atmosphere (La1 et al., 1995). This rate of increase in humus content may be difficult to achieve in arid and semi-arid tropics, and if so only for a limited period of time. 12. Conclusions There are several strategies for carbon sequestration in soil (Fig. 7), and the most effective strategies are based on proper land use and soil management. Inappropriate 100 R. Lal/Soil & Tillage Research I I Strategies for c Sequestration 1 SIrategies Restoration of degraded soils 43 (1997181-107 1 + Improved cultivars and new species 1 4 1 1 Crop residue and biomass management Conservation tillaee 1 1 * High amount of biomass returned to the soil - Deep and extensive root system developmenr * High soil biodiversity and biomass C in soil Effects Fig. 7. Strategies for carbon sequestration in soil through better land use. agricultural activities have been responsible for emissions of GHG into the atmosphere (Houghton, 1995), eutrophication of surface water (La1 and Stewart, 1994), and pollution of the environment. Nonetheless, agricultural intensification is an inevitable consequence of increasing population pressure and decreasing per capita land area. Adoption of improved and science-based agricultural practices can be an important strategy to bring about a quantum jump in per capita productivity and yet enhance environmental quality. Restoration of degraded soils by enhancing soil quality is important to increasing productivity, improving water quality, and mitigating the greenhouse effect. Restoration of vast tracts of these degraded soils can reverse the trends by sequestering carbon into the soil. Crop residue is an important and a renewable resource. Developing techniques for effective utilization of this vast resource is a major challenge. Improper use of crop residues (e.g. removal, burning or plowing under) can accelerate erosion, deplete soil fertility, and pollute environment through burning and eutrophication of surface and contamination of groundwater. Residue management may save energy, recycle nutrients, enhance soil fertility, improve soil structure, sequester carbon, and mitigate the greenhouse effect. The CT system is an ecological approach to soil surface management and seedbed preparation. It minimizes soil erosion risks, conserves soil water, decreases fluctuations in soil temperature of the surface layer, improves SOC content, and enhances soil structure. With the development of appropriate soil-specific package of cultural practices R. La1 /Soil Table 14 Feasibility of carbon sequestration & Tillage by different Research technological 43 (1997) 81-l 07 101 options Technique Global carbon sequestration rate (Pg/yr) Global increase in SOC content to l-m depth (%/yr) Crop residue management Conservation tillage Soil restoration 0.20 0.125 3.0 0.001 0.002 0.01 (e.g., crop rotations, cover crops etc.), CT can lead to C sequestration in soil. The principal environmental and ecological benefit of a CT system lies in carbon sequestration in soil and mitigating the greenhouse effect. Implemented as a science-based technique, conversion from conventional to CT system may increase SOC, improve soil structure, and enhance soil quality and its environmental regulatory capacity. Restoration of degraded soils is an important option for carbon sequestration and mitigating the greenhouse effect. It is a win-win situation. While improving productivity through enhancing soil quality, restoration of degraded soils can also sequester carbon and minimize risks of the greenhouse effect. The data in Table 14 show that goals of increasing SOC content by 0.001 to O.Ol%/yr by residue management, conservation tillage, and restoration of degraded soils can effectively mitigate the current rate of increase of atmospheric CO, concentration estimated at 3.2 Pg/yr. Solution to global ecological issues require: (i) a global agenda, and (ii) implementation of appropriate policies to facilitate adoption of CT technologies. Policy issues that require attention at regional, national and global scale include: (i) techniques of crop residue management, (ii) restoration and rehabilitation of degraded soils, and (iii) adoption of a CT system. 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