S. K. Guru
1.6 Abatement/mitigation
tropical and sub-tropical areas will decrease as irrigation water will become limiting because of additional stress on crops already affected by higher temperatures (Beran and Arnell, 1989). A sub- stantial increase in cost and management of irrigation water is likely to occur in these areas. A northern migration of agriculture would increase irrigation and fertiliser in sandy soils, which may create worse groundwater problems (Wittwer and Robb, 1964).
Such a situation is most likely in Punjab and surrounding areas (Chakravarty and Mallick, 2003). In areas where the amount or intensity of rainfall will increase, management would be oriented in a way to prevent soil erosion. Moreover, increases in fertiliser use may be required in such areas. Thus, the agricultural produc- tivity impacts in most developing countries of Central and South America, Africa, South-East Asia and the Pacific Islands will be minimal through a combination of agricultural zones and adjust- ments in agricultural technology and management (Parry et al., 1990; Wittwer, 1990).
In fact the stability and predictability of the climate and the ability of farmers to adapt their practices to changing climatic conditions will ensure the future of food production and global food security (Chakravarty and Mallick, 2003). The overall level of agricultural GHG emissions will continue to rise for the foreseeable future as agricultural production expands to keep pace with growing food, feed, fibre and bioenergy demand.
Increasing agricultural efficiency is critical to keep overall emissions as low as possible and to reduce the level of emis- sions per unit of agricultural output. Efficient and responsible production, distribution and use of water, fertilisers and other inputs are central to achieving these goals. Agricultural systems can adapt to offset the negative effects of climate change, but not without costs for changes in technology involving research and development and farm-level adoption, including possible physical and human capital investments (Anonymous, 1992;
Rosenberg, 1992; Easterling et al., 1993; Kaiser et al., 1993;
Mendelsohn et al., 1994; Easterling, 1996; Adams et al., 1998).
Mitigation is unlikely to occur without action, and higher emissions are projected in the future if current trends are left unconstrained. Global population will increase by 50% from present, reaching nine billion by 2050 (Lutz et al., 2001; Cohen, 2003). This enormous population pressure will require double production of cereals and other animal-based foods during the coming decades, which will require more use of N fertiliser and livestock increasing N2O and CH4 emissions from enteric fer- mentation unless more efficient fertilisation/management tech- niques and products can be found (Tilman et al., 2001; Mosier, 2002; Roy et al., 2002; Galloway, 2003; Green et al., 2005).
CH4 and N2O emissions vary greatly with land use depending on trends towards globalisation or regionalisation and on the emphasis placed on material wealth relative to sustainability and equity (Strengers et al., 2004). Trends in GHG emissions in the agricultural sector depend mainly on the level and rate of socio-economic development, human population growth and diet, application of adequate technologies, climate and non-cli- mate policies and future climate change. Consequently, mitiga- tion potentials in the agricultural sector are uncertain, making a consensus difficult to achieve and hindering policy making.
Opportunities for mitigating GHGs in agriculture fall into three broad categories (Smith et al., 2007a,b,c; Niggli et al., 2009) based on the underlying mechanisms.
The fluxes of GHGs can be reduced by efficient management of carbon and nitrogen flows in agricultural ecosystems. The Reducing
emissions
practices that deliver added N more efficiently to crops often reduce N2O emissions (Bouwman, 2001), and managing livestock to make most efficient use of feeds often reduces amounts of CH4 produced (Clemens and Ahlgrimm, 2001). The approaches that best reduce emissions depend on local condi- tions and, therefore, vary from region to region.
Agricultural ecosystems stock large carbon reserves mostly in the form of soil organic matter (Anonymous 2001a) which are lost more than 50 Pg C (Paustian et al., 1998; Lal, 1999, 2001a,b;
2002, 2003, 2004a–e, 2005; Lal and Bruce, 1999; Lal et al., 2003). This loss can be recovered through improved manage- ment, thereby withdrawing atmospheric CO2. These practices can be adopted locally to increase the photosynthetic input of carbon and/or slow the return of stored carbon to CO2 through respiration, fire or erosion. This will increase carbon reserves by sequestering carbon or stocking carbon sinks (Lal, 2004a) through agro-forestry or other perennial plantings on agricul- tural lands (Albrecht and Kandji, 2003). Agricultural and forest lands also remove CH4 from the atmosphere by oxidation, but this effect is small compared to other GHG fluxes (Smith and Conen, 2004; Tate et al., 2006).
The combustion of bioenergy feedstock used as a source of fuel either directly or after conversion releases CO2 (Schneider and McCarl, 2003; Cannell, 2003). The net benefit of bioen- ergy sources to the atmosphere is equal to the fossil-derived emissions displaced, which are less than any emissions from producing, transporting and processing. Conserving forest, grassland and other non-agricultural vegetation or discourag- ing further agricultural management practices into new lands can restrict GHG emissions (Foley et al., 2005).
The net benefit of these practices so adopted will depend on the combined effects on all gases (Robertson and Grace, 2004;
Schils et al., 2005; Koga et al., 2006), which may either reduce emissions indefinitely or temporarily (Marland et al., 2001, 2003a; Six et al., 2004). Where a practice affects radiative forc- ing through other mechanisms such as aerosols or albedo, those impacts also need to be considered (Marland et al., 2003b;
Andreae et al., 2005). The broad categories of options men- tioned above can be adopted through any one or combination of the management practices discussed below.
Mitigation practices in cropland management include the fol- lowing practices:
enhanc ing removals
Avoiding/
displacing emissions
Cropland management
Agronomy: Improved agronomic practices such as using improved crop varieties, extending crop rotations especially with perennial crops (produce more below ground carbon), rotation with legumes, growing ‘cover’ or ‘catch’ crops, effi- cient fertiliser/nutrient, pesticides and other input management and avoiding or reducing fallow which not only increases yields but also increases soil carbon storage through higher residue production (Follett, 2001; Izaurralde et al., 2001; West and Post, 2002; Lal, 2003, 2004a; Barthès et al., 2004; Freibauer et al., 2004; Paustian et al., 2004; Smith, 2004a,b; Alvarez, 2005). However, N benefits (also with legume-derived N) can be offset by emissions of higher soil N2O and CO2 from fer- tiliser manufacture (Schlesinger, 1999; Pérez-Ramírez et al., 2003; Robertson, 2004; Gregorich et al., 2005; Rochette and Janzen, 2005). The catch or cover crops can extract available N unused by the preceding crop, thereby reducing N2O emissions (Barthès et al., 2004; Freibauer et al., 2004).
Nutrient management: Crops cannot always use applied nitrogen that emits out of the soil as N2O efficiently (Galloway et al., 2003, 2004; Cassman et al., 2003; McSwiney and Robertson, 2005). Nitrogen-use efficiency can be improved by reducing leaching and volatile losses, applying the precise crop need, using slow/controlled-release forms or nitrification inhibitors (slowing the microbial processes leading to N2O for- mation), applying just prior to plant uptake (least susceptible to loss), placing precisely for accessibility to roots and avoiding excess application during immediate plant requirements, which will directly reduce N2O emissions and indirectly reduce GHG emissions from N fertiliser manufacture (Cole et al., 1997;
Schlesinger, 1999; Dalal et al., 2003; Paustian et al., 2004;
Robertson, 2004; Monteny et al., 2006).
Tillage/residue management: Minimal or zero tillage gen- erally results in soil carbon gain and reduced CO2 and N2O emissions through enhanced decomposition of retained crop residues and erosion due to less disturbance of soil and less energy use (Marland et al., 2001, 2003b; West and Post, 2002;
Cassman et al., 2003; Cerri et al., 2004; Smith and Conen, 2004;
Alvarez 2005; Gregorich et al., 2005; Helgason et al., 2005; Li et al., 2005; Madari et al., 2005; Ogle et al., 2005; Koga et al., 2006). Residue burning should be avoided to prevent emissions of aerosols and GHGs generated from fire (Cerri et al., 2004).
Water management: Supplementary irrigation provides water to 18% of the world’s cropland (Anonymous, 2005a). Improving the efficiency of this irrigation system supplementary through delivery and drainage management along with further extension
of irrigated area will boost yield and residue returns, thereby increasing soil carbon and also suppressing N2O emissions by improving aeration (Follett, 2001; Reay et al., 2003; Lal, 2004a;
Monteny et al., 2006). However, the energy used for water deliv- ery or higher moisture and fertiliser N inputs may offset this gain through CO2 and N2O emissions, respectively (Schlesinger 1999; Liebig et al., 2005; Mosier et al., 2005).
Rice management: CH4 emission from cultivated wetland rice soil can be reduced by growing low exuding cultivars, draining once or several times during the growing season, using efficient water management during off-season by keeping the soil dry or avoiding waterlogging and incorporating properly composted organic materials/residues (may be by producing biogas) during the dry period (Wang and Shangguan, 1996; Yagi et al., 1997;
Wassmann et al., 2000; Aulakh et al., 2001; Cai et al., 2000, 2003; Xu et al., 2000, 2003; Kang et al., 2002; Yan et al., 2003;
Cai and Xu, 2004; Smith and Conen, 2004; Khalil and Shearer, 2006). Frequently, however, draining may be constrained by water supply and may partly offset the reduced CH4 emission benefit by increasing N2O emissions (Akiyama et al., 2005).
These practices will also increase productivity by enhancing soil organic carbon stocks (Pan et al., 2006).
Agro-forestry: Planting trees and other perennial species in an agro-forestry system also increases soil carbon sequestration (Guo and Gifford, 2002; Paul et al., 2003; Oelbermann et al., 2004; Mutuo et al., 2005), but the effects on N2O and CH4 emis- sions are not well known (Albrecht and Kandji, 2003).
Land cover (use) change: Increasing the land cover or chang- ing land use, similar to the native vegetation over the entire land area (‘set-asides’) or in localised spots such as grassed water- ways, field margins and shelterbelts effectively converts drained croplands back to wetlands, reducing emissions and increasing carbon storage (Follett, 2001; Ogle et al., 2003; Falloon et al., 2004; Freibauer et al., 2004; Lal, 2004b; Paustian et al., 2004).
Converting drained croplands back to wetland, however, may stimulate CH4 emissions because waterlogging creates anaero- bic conditions (Paustian et al., 2004).
Globally, the area under grazing lands is more than croplands and is usually managed less intensively (Anonymous, 2006c).
The practices that reduce emissions and enhance removals of GHG are discussed below.
Grazing intensity: Grazing intensity and timing influence the removal, growth, carbon allocation and flora of grasslands, affecting the amount of carbon accrual in soils (Conant et al., Grazing land
management and pasture improvement
2001, 2005; Rice and Owensby, 2001; Conant and Paustian, 2002; Freibauer et al., 2004; Reeder et al., 2004; Liebig et al., 2005). The effects are inconsistent as there are many types of grazing practices involving diversified plant species, soil and climate (Schuman et al., 2001; Derner et al., 2006).
Increasing productivity: Carbon stock of grazing lands can be increased by improving its productivity through alleviating nutrient and moisture deficiencies (Conant et al., 2001; Schnabel et al., 2001). Adding nitrogen and energy use for irrigation stimulates N2O and CO2 emissions, which may, however, offset some of the benefits (Schlesinger, 1999; Conant et al., 2005).
Nutrient management: The practices (discussed for crop- land) that improve the plant nutrient uptake can reduce N2O emissions (Follett et al., 2001; Dalal et al., 2003). Nutrient management on grazing lands is made complicated through deposition of faeces and urine from livestock that, too, are uncontrolled and randomly added (Oenema et al., 2005).
Fire management: Anthropogenic or natural on-site biomass burning either contributes to climate change through GHG emission, production of smoke aerosols (have either warming or cooling effects on the atmosphere), albedo reduction of the land surface for several weeks (causing warming) and disturbed woody versus grass cover proportion, particularly in savan- nahs which occupy about one-eighth of the global land surface (Andreae, 2001; Andreae and Merlet, 2001; Menon et al., 2002;
Anderson et al., 2003; Beringer et al., 2003; Jones et al., 2003;
Van Wilgen et al., 2004; Andreae et al., 2005; Venkataraman et al., 2005). Therefore, reducing the frequency or intensity of fires through more effective fire suppression, reducing fuel load by vegetation management and burning at a time of year when less CH4 and N2O are emitted can restrict these processes along with an increased CO2 sink into soil and biomass (Scholes and van der Merwe, 1996; Korontzi et al., 2003).
Species introduction: Introducing grass species with higher productivity (legumes) or carbon allocation to deeper roots can increase soil carbon (Fisher et al., 1994; Davidson et al., 1995;
Conant et al., 2001; Machado and Freitas, 2004; Soussana et al., 2004), and perhaps also can reduce emissions from fer- tiliser manufacture if biological N2 fixation displaces applied N fertiliser (Sisti et al., 2004; Diekow et al., 2005).
Organic or peaty after draining can be used for agriculture, but the accelerated aeration decomposition in these soils results in high CO2 and N2O fluxes (Kasimir-Klemedtsson et al., 1997).
The drainage of such soils should either be avoided in the first Manage ment of
organic/peaty soils
place or a higher water table be re-established. If not, emissions can be reduced to some extent by avoiding deep ploughing, dis- couraging row crops and tubers and maintaining a shallower water table (Freibauer et al., 2004).
Degraded agricultural lands can be partially restored and car- bon storage can be improved by re-vegetation, nutrient amend- ments, application of organic substrates (manures, biosolids, composts), minimum/zero tillage, retaining crop residues and water conservation (Batjes, 1999; Bruce et al., 1999; Lal, 2001a,b, 2003, 2004b; Olsson and Ardö, 2002; Paustian et al., 2004; Foley et al., 2005). Where these practices involve higher nitrogen amendments, the benefits of carbon sequestration may be partly offset by higher N2O emissions (Smith et al., 2007a).
The practices for reducing CH4 and N2O emissions from live- stock (cattle and sheep) rearing are categorised as improved feeding practices, use of specific agents or dietary additives and long-term management changes and animal breeding (Monteny et al., 2006; Soliva et al., 2006).
Improved feeding practices: Improving pasture quality (in less developed regions to improve animal productivity), replac- ing forages with concentrates, supplementing certain oils or oilseeds to the diet and optimising protein intake (reduce N excretion) can reduce CH4 and N2O emissions, but may increase daily methane emissions per animal (Blaxter and Claperton, 1965; Leng, 1991; Johnson and Johnson, 1995; McCrabb et al., 1998; Machmüller et al., 2000; Phetteplace et al., 2001; Lovett et al., 2003; Beauchemin and McGinn, 2005; Clark et al., 2005;
Alcock and Hegarty, 2006; Jordan et al., 2006a–c).
Specific agents and dietary additives: Dietary additives fed to the animals can suppress methanogenesis to reduce CH4
emissions. These are ionophores (antibiotics—banned in the EU); halogenated compounds (inhibit methanogenic bacte- ria—can have side effects such as reduced intake); novel plant compounds such as condensed tannins, saponins and essential oils (side effect—reduced digestibility); probiotics (yeast cul- ture); propionate precursors (fumarate or malate—expensive);
vaccines (against methanogenic—not yet commercially avail- able) and bovine somatotropin and hormonal growth implants (Wolin et al., 1964; Benz and Johnson, 1982; Rumpler et al., 1986; Johnson et al., 1991; Bauman, 1992; Schmidely, 1993;
Van Nevel and Demeyer, 1995, 1996; McCrabb, 2001; Newbold et al., 2002; Lila et al., 2003; Pinares-Patiño et al., 2003;
McGinn et al., 2004; Wright et al., 2004; Newbold et al., 2005;
Resto ration of degraded lands
Livestock management
Hess et al., 2006; Kamra et al., 2006; Newbold and Rode, 2006;
Patra et al., 2006).
Long-term management changes and animal breeding:
Breeding for high-yielding varieties and better management practices for improved efficiency (producing meat— animals reach slaughter weight at a younger age) and decreasing replacement heifers reduces methane emission per unit of ani- mal product (Lovett and O’Mara, 2002; Boadi et al., 2004;
Kebreab et al., 2006; Lovett et al., 2006).
CH4 or N2O emissions from stored manure can be reduced by cooling, use of solid covers, mechanically separating solids from slurry, composting (solidifying), anaerobical digestion to capture CH4 for renewable energy source or by altering feeding practices (Amon et al., 2001; Clemens and Ahlgrimm, 2001;
Gonzalez-Avalos and Ruiz-Suarez, 2001; Monteny et al., 2001, 2006; Külling et al., 2003; Paustian et al., 2004; Chadwick, 2005; Pattey et al., 2005; Amon et al., 2006; Clemens et al., 2006; Hindrichsen et al., 2006; Kreuzer and Hindrichsen, 2006; Xu et al., 2007). However, globally for most animals there is limited opportunity for manure management, treatment or storage as excretion happens in the field, and handling for fuel or fertility amendments occur when it is dry and methane emissions are negligible.
Facing pollution threats from fossil fuels, forest/agricultural crops and residues are now being increasingly used as green fuel for a viable alternative (Rogner et al., 2000; Cerri et al., 2004; Edmonds, 2004; Hamelinck et al., 2004; Hoogwijk, 2004; Paustian et al., 2004; Richter, 2004; Sheehan et al., 2004;
Dias de Oliveira et al., 2005; Eidman, 2005; Hoogwijk et al., 2005; Anonymous, 2006e; Faaij, 2006). Biofuels also release CO2 but this CO2 is of recent atmospheric origin, which dis- places CO2 released from fossil carbon. The net benefit to atmo- spheric CO2, however, depends on energy used in growing and processing the bioenergy feedstock (Spatari et al., 2005).
Some mitigation measures operate predominantly on one GHG (e.g. dietary management of ruminants to reduce CH4
emissions) while others have impacts on more than one GHG (e.g. rice management). Moreover, practices may benefit more than one gas while others involve a trade-off between gases (e.g. restoration of organic soils). Consequently, a practice that is highly effective in reducing emissions at one site may be less effective or even counterproductive elsewhere. This means that there is no universally applicable list of mitigation practices and Manure
management
Bio energy
the proposed practices will need to be evaluated for individual agricultural systems according to the specific climatic, edaphic, social settings, and historical land use and management (Smith et al., 2007a). The effectiveness of mitigation strategies also changes with time. Some practices such as those which elicit soil carbon gain have diminishing effectiveness after several decades while others can reduce energy use restricting emis- sions indefinitely. For instance, there is a strong time depen- dency of emissions from no-till agriculture, in part because of changing influence of tillage on N2O emissions (Six et al., 2004). Many of the climate change effects have high levels of uncertainty but demonstrate that the practices chosen to reduce GHG emissions may not have the same effectiveness in coming decades. Consequently, programmes to reduce emissions in the agricultural sector will need to be designed with flexibility for adaptation in response to climate change (Smith et al., 2007a).
1.7 Co-benefits and trade-offs of