8.4.1.1 Cropland management

Because often intensively managed, croplands offer many opportunities to impose practices that reduce net GHG emissions (Table 8.3). Mitigation practices in cropland management include the following partly-overlapping categories:

a. Agronomy: Improved agronomic practices that increase yields and generate higher inputs of carbon residue can lead to increased soil carbon storage (Follett, 2001). Examples of such practices include: using improved crop varieties; extending crop rotations, notably those with perennial crops that allocate more carbon below ground; and avoiding or reducing use of bare (unplanted) fallow (West and Post, 2002; Smith, 2004a, b; Lal, 2003, 2004a; Freibauer et al., 2004). Adding more nutrients, when deficient, can also promote soil carbon gains (Alvarez, 2005), but the benefits from N fertilizer can be offset by higher N₂O emissions from soils and CO₂ from fertilizer manufacture (Schlesinger, 1999; Pérez-Ramírez et al., 2003; Robertson, 2004; Gregorich et al., 2005). Emissions per hectare can also be reduced by adopting cropping systems with reduced reliance on fertilizers, pesticides and other inputs (and therefore, the GHG cost of their production: Paustian et al., 2004). An important example is the use of rotations with legume crops (West and Post, 2002; Izaurralde et al., 2001), which reduce reliance on external N inputs although legume-derived N can also be a source of N₂O (Rochette and Janzen, 2005). 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 (Barthès et al., 2004; Freibauer et al., 2004) and may also extract plant-available N unused by the preceding crop, thereby reducing N₂O emissions.

b. Nutrient management: Nitrogen applied in fertilizers, manures, biosolids, and other N sources is not always used efficiently by crops (Galloway et al., 2003; Cassman et al., 2003). The surplus N is particularly susceptible to emission of N₂O (McSwiney and Robertson, 2005). Consequently, improving N use efficiency can reduce N₂O emissions and indirectly reduce GHG emissions from N fertilizer manufacture (Schlesinger, 1999). By reducing leaching and volatile losses, improved efficiency of N use can also reduce off-site N₂O emissions. Practices that improve N use efficiency include: adjusting application rates based on precise estimation of crop needs (e.g., precision farming); using slow- or controlled-release fertilizer forms or nitrification inhibitors (which slow the microbial processes leading to N₂O formation); applying N when least susceptible to loss, often just prior to plant uptake (improved timing); placing the N more precisely into the soil to make it more accessible to crops roots; or avoiding N applications in excess of immediate plant requirements (Robertson, 2004; Dalal et al., 2003; Paustian et al., 2004; Cole et al., 1997; Monteny et al., 2006).

c. Tillage/residue management: Advances in weed control methods and farm machinery now allow many crops to be grown with minimal tillage (reduced tillage) or without tillage (no-till). These practices are now increasingly used throughout the world (e.g., Cerri et al., 2004). Since soil disturbance tends to stimulate soil carbon losses through enhanced decomposition and erosion (Madari et al., 2005), reduced- or no-till agriculture often results in soil carbon gain, but not always (West and Post, 2002; Ogle et al., 2005; Gregorich et al., 2005; Alvarez 2005). Adopting reduced- or no-till may also affect N₂O, emissions but the net effects are inconsistent and not well-quantified globally (Smith and Conen, 2004; Helgason et al., 2005; Li et al., 2005; Cassman et al., 2003). 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). Further, no-tillage systems can reduce CO₂ emissions from energy use (Marland et al., 2003b; Koga et al., 2006). Systems that retain crop residues also tend to increase soil carbon because these residues are the precursors for soil organic matter, the main carbon store in soil. Avoiding the burning of residues (e.g., mechanising sugarcane harvesting, eliminating the need for pre-harvest burning (Cerri et al., 2004)) also avoids emissions of aerosols and GHGs generated from fire, although CO₂ emissions from fuel use may increase.

d. Water management: About 18% of the world’s croplands now receive supplementary water through irrigation (Millennium Ecosystem Assessment, 2005). Expanding this area (where water reserves allow) or using more effective irrigation measures can enhance carbon storage in soils through enhanced yields and residue returns (Follett, 2001; Lal, 2004a). But some of these gains may be offset by CO₂ from energy used to deliver the water (Schlesinger 1999; Mosier et al., 2005) or from N₂O emissions from higher moisture and fertilizer N inputs (Liebig et al. 2005), The latter effect has not been widely measured. Drainage of croplands lands in humid regions can promote productivity (and hence soil carbon) and perhaps also suppress N₂O emissions by improving aeration (Monteny et al., 2006). Any nitrogen lost through drainage, however, may be susceptible to loss as N₂O.(Reay et al. 2003).

e. Rice management: Cultivated wetland rice soils emit significant quantities of methane (Yan et al., 2003). Emissions during the growing season can be reduced by various practices (Yagi et al., 1997; Wassmann et al., 2000; Aulakh et al., 2001). For example, draining wetland rice once or several times during the growing season reduces CH₄ emissions (Smith and Conen, 2004; Yan et al., 2003; Khalil and Shearer, 2006). This benefit, however, may be partly offset by increased N₂O emissions (Akiyama et al. 2005), and the practice may be constrained by water supply. Rice cultivars with low exudation rates could offer an important methane mitigation option (Aulakh et al., 2001). In the off-rice season, methane emissions can be reduced by improved water management, especially by keeping the soil as dry as possible and avoiding water logging (Cai et al., 2000 2003; Kang et al., 2002; Xu et al., 2003). Increasing rice production can also enhance soil organic carbon stocks (Pan et al., 2006). Methane emissions can be reduced by adjusting the timing of organic residue additions (e.g., incorporating organic materials in the dry period rather than in flooded periods; Xu et al., 2000; Cai and Xu, 2004), by composting the residues before incorporation, or by producing biogas for use as fuel for energy production (Wang and Shangguan, 1996; Wassmann et al., 2000).

f. 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 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 (Oelbermann et al., 2004; Guo and Gifford, 2002; Mutuo et al., 2005; Paul et al., 2003). But the effects on N₂O and CH₄ emissions are not well known (Albrecht and Kandji, 2003).

g. Land cover (use) change: 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. The conversion can occur over the entire land area (‘set-asides’), or in localized spots, such as grassed waterways, field margins, or shelterbelts (Follett, 2001; Freibauer et al., 2004; Lal, 2004b; Falloon et al., 2004; Ogle et al., 2003). Such land cover change often increases carbon storage. For example, converting arable cropland to grassland typically results in the accrual of soil carbon because of lower soil disturbance and reduced carbon removal in harvested products. Compared to cultivated lands, grasslands may also have reduced N₂O emissions from lower N inputs, and higher rates of CH₄ oxidation, but recovery of oxidation may be slow (Paustian et al., 2004). Similarly, converting drained croplands back to wetlands can result in rapid accumulation of soil carbon (removal of atmospheric CO₂). This conversion may stimulate CH₄ emissions because water logging creates anaerobic conditions (Paustian et al., 2004). Planting trees can also reduce emissions. These practices are considered under agro-forestry (Section 8.4.1.1f); afforestation (Chapter 9), and reafforestation (Chapter 9). 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.