Agricultural ecosystems hold large carbon reserves (IPCC, 2001a), mostly in soil organic matter.Historically, these systems have lost more than 50 Pg Carbon, but some of this carbon lost can be recovered through improved management, thereby withdrawing atmospheric CO2 (Paustian et al., 1998; Lal, 1999, 2004a).
Land management options for mitigation fall in the following four categories: a) cropland management; b) grazing land management/pasture improvement; c) management of agricultural lands and d) restoration of degraded lands. This description focuses on the management of croplands.
According to the IPCC the main factors influencing change in the agricultural sector during the last four decades are population pressure, technological change, public policies, economic growth and the cost/price squeeze (IPCC, 2007).
The technical mitigation potential for all agricultural, livestock and land management options is large, as illustrated in Figure 1.
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Because often intensively managed, croplands offer many opportunities to impose practices that reduce net GHG emissions (IPCC, 2007). These opportunities are diverse and are often grouped in terms such as Conservation Agriculture, Sustainable Agriculture, etc. Essentially, these categories aim to minimize soil disturbance while maximizing yield.
Land management practices for mitigation regarding croplands include the following partly-overlapping categories:
a) Agronomy
[8]b) Nutrient management
[9]c) Tillage/residue management
[10]d) Water management
e) Rice management
f) Agro-forestry
[11]g) Land cover change
This description first briefly describes these mitigation practices (mostly taken from the IPCC 2007 report) and then explores the socio-economic and environmental consequences of adoption of conservation agriculture management.
Agronomy
Increased soil carbon storage can be achieved through improved agronomic practices. These practices increase yields while also generating higher inputs of carbon residue. Examples of agronomic practices are: using improved crop varieties; extending crop rotations; avoiding or reducing use of bare fallow.
Adding additional nutrients through fertilizers can also promote soil carbon gains. However, the benefits of increased soil carbon can be (partly) offset by higher N20 emissions from the soil and higher CO2 emissions from the manafacturing of the fertilizer.
Moreover, emissions from the land can also be reduced through the adoption of systems that have a reduced reliance on fertilizers, pesticides and other inputs. Not only does this prevent the greenhouse gas emissions from the manafacturing of these inputs, it also increases soil carbon. An important example is the use of rotations with legume crops. These crops reduce reliance on external nitrogen inputs, which reduces demand for fertilizer.
Another group of agronomic practices are those that provide temporary vegetative cover between successive agricultural crops, or between rows of tree or vine crops. These ‘catch’ or ‘cover’ crops add carbon to soils and may also extract plantavailable N unused by the preceding crop, thereby reducing N2O emissions.
Nutrient management
Improving nutrient use efficiency can reduce N2O emissions and indirectly reduce greenhouse gas emissions from fertilizer manufacturing (IPCC, 2007). This is due to the fact that nitrogen applied in fertilizers, manures and biosolids is not always used efficiently by crops and the remaining nitrogen is susceptible to emission of N2O. Improving nutrient use efficiency can also prevent off-site N2O emissions. This is due to the reduction in nitrogen leaching and volatile losses. Examples of practices that improve nitrogen use efficiency are: precision farming (i.e. adjusting application rates of nutrients based on precise estimation of crop needs); using slow- or controlled- release fertilizer forms or nitrification inhibitors (which slow th microbial processes leading to N2O formation); improved timing of nitrogen application, often just prior to plant uptake; placing the nitrogen more precisely to make it more accesible to crops roots; or avoiding nitrogen applications in excess of immediate plant requirements.
Tillage/residue management
No-tillage systems can reduce greenhosue gas emissions in a variety of ways. The same is true for minimal tillage (also called reduced tillage) systems but to a lesser extent. While previously tillage was an essential feature of farming, advances in weed control methods and farm machinery now allow many crops to be grown with minimal or no tillage. These practices are now increasingly used throughout the world (Cerri et al., 2004).
Soil disturbances tend to stimulate soil carbon loss through enhanced decomposition and erosion. Therefore, reducing soil disturbances through minimal tillage or no-tillage systems reduces soil carbon losses. In addition, no-tillage or minimal tillage systems may affect N2O emissions. However, the net effects on N2O emissions are not yet well-quantified (IPCC, 2007). The effect of reduced tillage on N2O emissions may depend on soil and climatic conditions. In some areas, reduced tillage promotes N2O emissions, while elsewhere it may reduce emissions or have no measurable influence (Marland et al., 2001).No-tillage systems can also reduce greenhouse gas emissions from energy use.
Residue management in the form of the retainment of crop residues also tend to increase soil carbon storage. Increased soil carbon storage occurs as the residue is the precursor for soil organice matter, which is the main carbon store in the soil. Moreover, avoiding the burning of residues also avoids emissions.
Water management
About 18% of the world’s croplands now receive supplementary water through irrigation (Millennium Ecosystem Assessment, 2005). Expanding the use of irrigation or using more effective irrigation measures can enhance carbon storage in soils through enhances yields and residue returns. However, some of these gains may be offset by CO2 from energy used to deliver the water, or from N2O emissions from higher moisture in the soil and increased fertilizer inputs. Quantifying these emissions requires additional research.
Cropland drainage in humid regions can also promote productivity, and hence soil carbon, and perhaps also suppress N2O emissions by improving aeration. However, nitrogen loss through the drainage might be susceptible to loss as N2O.
Rice management
Most rice is grown in flooded paddy fields. When fields are flooded, the decomposition of organic material depletes the oxygen present in the soil and floodwater which results in anaerobic conditions in the soil. Anaerobic decomposition of soil organic matter by methanogenic bacteria results in methane emissions. While part of the methane is oxidized by aerobic methanotropic bacteria in the soil and part is leached away as dissolved methane in the floodwater, the remaining unoxidized methane is emitted from the soil to the atmosphere.
As such, cultivated rice production results in significant emissions of methane by the soil. These emissions can be reduced by various practices: a) draining wetland rice once or several times during the growing season reduces methane emissions. If water is drained and soils are allowed to dry sufficiently, CH4 emissions decrease or stop entirely. However, this benefit may be partly offset by increased N2O emissions and the practice may be constrained by water supply; b) rice cultivars with low exudation rates could also offer an important methane mitigation option. In the off-rice season, methane emissions can be reduced by improved water management. By keeping the soil as dry as possible and avoiding water logging methane emissions are reduced.; c) increasing rice production can enhance soil organic carbon stocks.; d) adjusting the timing of organic residue additions can also reduce methane emissions. For instance, incorporating organic materials in the dry period rather than in the flooded periods reduces emissions.; e) composting the residues before incorporation reduces methane emissions and; f) by producing biogas for use as fuel for energy production.
The United States Environmental Protection Agency (US EPA) (date) concludes that the water management system under which rice is grown is the most important factor affecting methane emissions. Also, the amount of available carbon susceptible to decomposition is also considered critical by the US EPA. In addition to water management, other practices (e.g., tillage, fertilization, manure amendments) will alter the soil environmental conditions (e.g., temperature, moisture, pH) and hence affect the soil carbon- and nitrogen-driving processes such as decomposition, nitrification, and denitrification. The changes in the soil biogeochemical processes will finally affect the availability of soil nitrogen and water to the crops and hence alter the crop yields. Because crop residue is the major source of soil organic carbon, the change in crop yield and litter will redefine the soil organic-matter balance, which is one of the most important factors determining the CH4, soil CO2, and N2O emissions.
Soil temperature is also known to be an important factor regulating the activity of methanogenic bacteria and, therefore, the rate of CH4 production.
Agro-forestry
Agro-forestry is the production of livestock or food crops on land that also grows trees for timber, firewood, or other tree products. It includes shelter belts and riparian zones/buffer strips (see Video 1) with woody species. The standing stock of carbon above ground is usually higher than the equivalent land use without trees, and planting trees may also increase soil carbon sequestration. But the effects on N2O and CH4 emissions are not well known.
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Land cover (use) change
According to the IPCC (2007), one of the most effective methods of reducing emissions is often to allow or encourage the reversion of cropland to another land cover, typically one similar to the native vegetation. Converting land to another land cover can occur over the entire land area (called 'set-asides'), or in localized spots, such as grassed waterways, field margins, or shelterbelts. The introduction image all the way at the top is a small-scale illustration of a set-aside in which a piece of land is no longer cultivated and allowed to grow back to another land cover.
Land cover changes often increase carbon storage. For instance, the conversion of arable cropland to grassland typically results in the increase of soil carbon due to lower soil disturbances and reduced carbon removal in harvested products (IPCC, 2007). Moreover, due to reduced inputs by the farmer, grasslands may also have reduced N2O emissions from lower nitrogen inputs. Similarly, converting drained croplands back to wetlands (as described in this ClimateTechWiki description) results in the rapid accumulation of soil carbon. Planting trees can also reduce emission (as described in this ClimateTechWiki description).
The IPCC (2007) concludes that because land cover (or use) conversion comes at the expense of lost agricultural productivity, it is usually an option only on surplus agricultural land or on croplands of marginal productivity.
Barriers to implementation of sustainable land management practices
A number of studies have sought to identify barriers to implementation of sustainable land management practices:
a) large investment costs may discourage adoption (Wandel and Smithers, 2000).
b) The perceived risk may serve as a barrier (Uri, 1998b; Stonehouse, 1996; McNairn and Mitchell, 1992).
c) Long gestation periods for the benefits to materialize may serve as a barrier to farmers with short-term planning horizons (Tweeten, 1995).
d) Barriers may be particular to culture and recent history (Nyagumbo, 1997).
The aspects that are considered important regarding the implementation of sustainable land management practices are illustrated in Figure 2.
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The technical mitigation potential per region for all management practices (i.e. restoration of degraded lands; cropland management; grazing land management/pasture management; management of agricultural lands) and all affected GHGs is shown in Figure 3.
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While the above Figure aggregates the technical mitigation potential of all management techniques, the Figure below outlines the mitigation potential differentiated between management techniques. It can clearly be seen that the more sustainable management of croplands has a substantial potential for mitigation.
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The consequences for socio-economic development and environmental protection are difficult to establish. As can be seen in Figure 5, the consequences of most mitigation categories are essentially positive. However, it is clear that there are also still several aspects that require further investigation before a positive or negative connotation can be given to that mitigation measure.
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Figure 6 reinforces the complexity with determining the socio-economic and environmental protection benefits from the various mitigation measures outlined in this description. It can be seen that cropland managment practices have an influence on the socio-economic and environmental situation, but it is also clear that a variety of other factors also influence the socio-economic and environmental situation.
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Since it is difficult to quantify the overall effect of the mitigation measures, it is valuable to look at the effects of implementation of these mitigation measures in specific situations. For instance, the Food and Agriculture Organization (FAO) analyzed the effects of the implementation of Conservation Agriculture (which is an grouping of several of the methods outlined in this description).
According to the FAO (2010) data concerning Conservation Agriculture implementation in Lesotho, Africa, Conservation Agriculture results in a variety of benefits:
a) Higher agricultural productivity, due to improved efficiency in the use of inputs and other resources. The increased agricultural productivity improves food security in Lesotho and reduces vulnerability.
b) Greater environmental sustainability, due to improved soil structure and enhanced fertility. This reduces the pressure on the environment and allows for a more beneficial use of the soil.
c) Improved livelihoods and social sustainability, due to the accesibility to the technology by all social categories, including the most vulnerable. Sustainable agriculture is not exclusive to certain social groups, and as such can benefit alls social categories within the country in which it is applied.
According to the FAO (2010) report, long-term yield increases and output stability can be achieved, while at the same time stopping and reversing land degradation. In addition, the larger outputs are achieved by employing fewer inputs and thus reducing costs. Additionally, the FAO (2010) notes that conservation farming techniques which rationalize the use of labour are particularly helpful in those rural areas where migration and health emergencies have reduced the labour supply and contributed to the increasing "feminization" of the agricultural sector.
Table 1 and Table 1 summarize the data found in the 2010 FAO report.
| Agro-ecological benefits | Resulting from... | Due to... |
| Progressive suppression of weed growth | Improved soil structure and stability | Reduced tillage |
| Long-term yield increase | Reduced water and wind erosion Increase in soil fertility, stability and structure Improved retention of water, nutrients and soil moisture |
Reduced tillage and soil cover Reduced tillage, soil cover, mulching, inter- cropping, and crop rotation |
| Reduced run-off |
Decreased erosion, improved soil structure and water retention capacity |
Reduced tillage and soil cover |
|
Improved rooting conditions |
Increase in soil fertility and stability and improved soil structure |
Reduced tillage, soil cover, mulching, intercropping and crop rotation |
|
Improved agro-biodiversity |
Higher biological activity in the soil and in the field |
Soil cover and mulching |
|
Output stability |
Reduced vulnerability to climatic shocks |
Improved rooting conditions |
|
Reduced waste of water and inputs |
Reduced run-off |
Decreased erosion, improved soil structure and water retention capacity |
| Environmental Benefits | Resulting From... | Due to... |
|
Decrease of land degradation |
Reduced erosion, higher soil fertility, improved soil structure |
Reduced tillage, soil cover, mulching, intercropping and crop rotation |
|
Reduced downstream sedimentation and siltation |
Reduced runoff |
Decreased erosion, improved soil structure and water retention capacity |
|
Reduced contamination of soil and surface and ground water |
Reduced runoff |
Decreased erosion, improved soil structure and water retention capacity |
|
Reduction of CO2 emissions to the atmosphere |
Higher carbon sequestration |
Reduced tillage, soil cover, mulching |
|
Conservation and enhancement of terrestrial and soil based biodiversity |
Crop diversification |
Crop rotation and intercropping |
As the IPCC (2007) notes, the role of the alternative strategies for land management change across the range of different prices of carbon, as is illustrated in Figure 7. At low prices, dominant strategies are those consistent with existing production such as changes in tillage, fertilizer application, livestock diet formulation, and manure management. Higher prices elicit land-use changes that displace existing production, such as biofuels, and allow for use of costly animal feed-based mitigation options. A practice effective in reducing emissions at one site may be less effective or even counterproductive elsewhere. Consequently, there is no universally applicable list of mitigation practices; practices need to be evaluated for individual agricultural systems based on climate, edaphic, social setting, and historical patterns of land use and management.
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Figure 8 outlines the results of Conservation Agriculture implementation by the FAO. It is shown that Conservation Agriculture in this case provided economic benefits. Net income from the same area is almost double compared to the income from conventional tillage and the labour hours is also reduced. While reduced labour requirements might be a constraint to the implementation in other situations, the FAO (2010) notes that in Lesotho the rationalization of the use of labour is particularly helpful in the rural areas in Lesotho where migration and health emergencies have reduced the labour supply and contributed to the increasing feminization of the agricultural sector.
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The socio-economic benefits of Conservation Agriculture implementation in Lesotho are illustrated in Table 3. Since this is a case-specific illustration, as it is applicable to Lesotho, it might be that certain benefits do not occur in other situations or that additional benefits can be identified in other cases.
| Socio-economic Benefits | Resulting From... | Due To... |
|
Increased food security |
Long-term yield increase and output stability |
Reduced erosion, higher soil fertility, improved soil structure, |
|
Increased food security |
Crop diversification |
Reduced vulnerability to climatic shocks |
|
Increased net profitability |
Long-term yield increase and output stability |
Reduced erosion, higher soil fertility, improved soil structure, |
|
Increased net profitability |
Reduction of on-farm costs |
Savings in labour, machinery and (in the medium-term) |
|
Technology sustainability |
Suitability to different farming systems and agro-ecological environments |
Appropriate combination of tillage techniques, equipment and inputs |
FAO, 2010. Conservation Agriculture and Sustainable Crop Intensification in Lesotho. Integrated Crop Management Vol . 10-2010. Food and Agriculture Organization of the United Nations, Rome, Italy. Document can be found at: http://www.fao.org/docrep/012/i1650e/i1650e00.pdf [12]
FAO, 2001. The Economics of Conservation Agriculture. Food and Agriculture Organization of the United Nations, Rome, Italy. Document can be found at: ftp://ftp.fao.org/agl/agll/docs/ecconsagr.pdf [13]
Cerri, C.C., M. Bernoux, C.E.P. Cerri, and C. Feller, 2004: Carbon cycling and sequestration opportunities in South America: the case of Brazil. Soil Use and Management , 20, pp. 248-254.
Marland, G., B.A. McCarl, and U.A. Schneider, 2001: Soil carbon: policy and economics. Climatic Change, 51, pp. 101-117.
Millennium Ecosystem Assessment, 2005. Ecosystems and Human Well-Being: Current State and Trends. Findings of the Condition and Trends. Working Group. Millennium Ecosystem Assessment Series, Island press, Washington D.C., 815 pp.
IPCC, 2007. Smith, P., D. Martino, Z. Cai, D. Gwary, H. Janzen, P. Kumar, B. McCarl, S. Ogle, F. O’Mara, C. Rice, B. Scholes, O. Sirotenko, 2007: Agriculture. In Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer (eds)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
Smith,P., D. Martino, Z. Cai, D. Gwary, H.H. Janzen, P. Kumar, B. McCarl, S. Ogle, F. O’Mara, C. Rice, R.J. Scholes, O. Sirotenko, M. Howden, T. McAllister, G. Pan, V. Romanenkov, U. Schneider, S. Towprayoon, M. Wattenbach, and J.U. Smith, 2007: Greenhouse gas mitigation in agriculture. Philosophical Transactions of the Royal Society, B., 363. doi:10.1098/rstb.2007.2184.