S. K. Guru
1.9 Agricultural GhG mitigation potential
Farming practices that conserve and improve soil fertility are important for the future of agriculture and food production.
Organic agriculture systems are built on a foundation of con- serving and improving diversity by using diverse crops, rotations and mixed farm strategies. The diversity of landscapes, farm- ing activities, fields and agro-biodiversity is greatly enhanced in organic agriculture (Niggli et al., 2008), which makes these farms more resilient to unpredictable weather patterns that result
from climate change (Bengtsson et al., 2005; Hole et al., 2005).
Enhanced biodiversity reduces pest outbreaks (Wyss et al., 1995; Pfiffner et al., 2003; Pfiffner and Luka, 2003; Zehnder et al., 2007). Similarly, diversified agro-ecosystems reduce the severity of plant and animal diseases while improving utilisa- tion of soil nutrients and water (Altieri et al., 2005). Better soil structure, friability, aeration and drainage, lower bulk density, higher organic matter content, soil respiration (related to soil microbial activity), more earthworms and a deeper topsoil layer are all associated with the lower irrigation need (Proctor and Cole, 2002). Under conditions in which water is limited during the growing period, yields of organic farms are equal or signifi- cantly higher than those of conventional agriculture common in developing countries (Badgley et al., 2007). Water capture in organic plots was twice as high as in conventional plots during torrential rains significantly reducing the risk of floods (Lotter et al., 2003).
In Switzerland and the United States, organic matter, water percolation through top layer and soil structure stability were higher in organically managed soils than in conventional soils (Mäder et al., 2002; Marriott and Wander, 2006), making organic fields less prone to soil erosion (Reganold et al., 1987;
Siegrist et al., 1998) and resulted significantly in higher yields of corn and soya bean in dry years (Lotter et al., 2003; Pimentel et al., 2005). In Tigray Province, one of the most degraded parts of Ethiopia, agricultural productivity was doubled by soil fer- tility techniques such as compost application and introduction of leguminous plants into the crop sequence instead of using purchased mineral fertilisers (Edwards, 2007). These reports recommend the practice of organic farming to improve soil fer- tility through green manuring, leguminous intercropping, com- posting and recycling of livestock manure for reducing GHGs, while also increasing global food productivity.
Eventually, a complete conversion to organic agriculture could decrease global yields by 30–40% in intensively farmed regions under the best geo-climate conditions (Niggli et al., 2009). In the context of subsistence agriculture and in regions with periodic disruptions of water supply brought on by droughts or floods, organic agriculture is competitive to conventional agri- culture and often superior with respect to yields (Halberg et al., 2006; Badgley et al., 2007; Sanders, 2007; Anonymous, 2008c).
Organic agriculture has a huge potential for climate change miti- gation strategies in agricultural production (Pimentel et al., 1995;
Niggli et al., 2008, 2009):
• It reduces wind, water and overgrazing erosion of 10 mil- lion ha annually, essential for ensuring future food security.
• It rehabilitates poor soils, restores organic matter content and brings such soils back into productivity.
• It is inherently based on lower livestock densities and can compensate for lower yields by a more effective vegetable production. Organic agriculture has a land use ratio of 1:7 for vegetable and animal production.
• The potential productivity of organic farms and organi- cally managed landscapes can be improved considerably by scientific agro-ecological research.
• It conserves agricultural biodiversity, reduces environ- mental degradation impacts and integrates farmers into high-value food chains.
Numerous attempts particularly on soil carbon sequestra- tion have been made to assess the technical potential for GHG mitigation in agriculture (Anonymous, 1996; Boehm et al., 2004; Caldeira et al., 2004; Ogle et al., 2004, 2005; Smith et al., 2007b,c). Mitigation potentials for CO2 represent the net change in soil carbon pools reflecting the accumulated difference between carbon inputs to the soil after CO2 uptake by plants and the release of CO2 by decomposition in soil. Mitigation potentials for N2O and CH4 depend solely on emission reductions. As miti- gation practices can affect more than one GHG; it is important to consider the impact of mitigation options on all GHGs (Robertson et al., 2000; Smith et al., 2001; Gregorich et al., 2005).
It was estimated that 400–800 MtC year−1 (equivalent to about 1400–2900 MtCO2-eq. year−1) could be sequestered in global agricultural soils. In addition, 300–1300 MtC (equivalent to about 1100–4800 MtCO2-eq. year−1) from fossil fuels could be offset by using 10–15% of agricultural land to grow energy crops in which crop residues will contribute 100–200 MtC (equiva- lent to about 400–700 MtCO2-eq. year−1) to fossil fuel offsets if recovered and burned. CH4 emissions from agriculture would be reduced by 15–56% through improved nutrition of ruminants and better management of paddy. Improved management would reduce N2O emissions by 9–26%. The global 2030 technical potential for mitigation options in agriculture considering no economic and other barriers for all gases was estimated to be 4500–6000 MtCO2-eq. year−1 or 89% from soil carbon sequestra- tion, 9% from mitigation of methane and 2% from mitigation of soil N2O emissions (Caldeira et al., 2004; Smith et al., 2007b).
The contribution of agriculture to global GHG emissions ranges from 5.1 to 6.1 Gt CO2-eq. The global potential of ara- ble and permanent cropping systems to sequester is 200 kg C ha−1 year−1 and pasture systems is 100 kg ha−1 year−1; the world’s carbon sequestration will amount to 2.4 Gt CO2-eq. year−1 (Lal, 2004a; Niggli et al., 2009). This minimum scenario for a con- version to organic farming would mitigate 40% of the world’s agriculture GHG emissions (Niggli et al., 2009). The sequestra- tion rate on arable land adopting organic farming with reduced tillage techniques will be 500 kg C ha−1 year−1, which will contribute 65% mitigation of the agricultural GHG and, thus, total global organic mitigation would be 4 Gt CO2-eq. year−1. This indicates that application of sustainable management tech- niques to build up soil organic matter have the potential to bal- ance a large part of the agricultural emissions although their effect over time may be reduced as soils are built up (Foereid and Høgh-Jensen, 2004). By a conversion to organic farming, another approximately 20% of the agricultural GHG could be reduced by abandoning industrially produced nitrogen fertilis- ers as is practiced by organic farms. This encouraging figure strongly supports the reality of low GHG agriculture and the possibility of climate neutral farming.
1.10 Agricultural GhG mitigation