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Source: IPCC (2007)

Fig. 15.1 Shape of different sectors in total anthropogenic GHG emission in 2004 in terms of CO2 equivalents (Source: IPCC 2007)

Residual and commercial building 7.

Transport

25.9 % Energy supply

Agriculture 13.5 %

Forestry 17.4 %

Waste and wastewater

31% of total non-CO2 emissions in 2005. Besides, rice production, manure management and biomass burning contribute 11,7 and 7%, respectively, to CH4 annual emissions. Emissions of CO2, mainly from land use change, especially deforestation for agricultural purposes, are estimated to account for 15% of anthropogenic CO2 emissions (FAO 2003). However, a reliable assessment is difficult due to large spatial and temporal variability as well as concomitant emission and consumption of these gases over a large geographical area of the world.

Globally, agricultural emissions have increased by 14% from 1990 to 2005 with an average annual emission of 49MtCO2 eq. yr-1 (US-EPA 2006). N2O from soils and manure management and CH4 from enteric fermentation were the agricultural sources, showing the highest increase in emissions at 21, 18 and 12%, respectively. N2O emissions increased by 31 MtCO2 yr-1, which is almost twice the rate of increase for CH4 emissions. US-EPA forecasts acceleration in the global GHG emissions from agriculture for the period 2005-2020. In the developing countries, the growth is expected to continue at the same rate as in 1990-2005, whereas in the more developed regions, the decreasing trend would be reversed and emission would grow by 8% upto 2020 (US-EPA 2006). Two most significant sources, N2O from soils and CH4 from enteric fermentation, would also increase quite rapidly. N2O emission, which is expected to an average of 49MtCO2 yr-1, would continue to grow faster than CH4 emissions, projected to an average of 35MtCO2 yr-1.

15.3 Mitigation Options

Specific management options can be used to reduce agriculture's environmental impacts. Conservation practices, that help prevent soil erosion, may also sequester soil C and enhance CH4 consumption. Managing N to match crop demands can reduce N2O emission, while manipulating animal diet and manure management can reduce both CH4 and N2O emission from animal husbandry. Thus, all segments of agriculture have the management options which can reduce agriculture's GHG footprints. Opportunities for mitigating GHGs in agriculture can be grouped into three broad categories based on the following principles:

a. Reducing emissions: The fluxes of GHGs can be reduced by managing more efficiently the flows of carbon and nitrogen in agricultural systems. The exact approaches, that best reduce emissions, depend on local conditions and therefore, vary from region to region.

b. Enhancing removals: Agricultural ecosystems hold large reserves of C, mostly in soil organic matter. Any practice, that increases the photosynthetic input of C or slows the return of stored C via respiration, will increase stored C, thereby 'sequestering' C or building C 'sinks'.

c. Avoiding emissions: Using bioenergy feed-stocks would release CO2-C of recent origin and would, thus, avoid release of ancient C through combustion of fossil fuels. Emissions of GHGs, can also be avoided by agricultural management practices that forestall the cultivation of new lands.

15.4 Mitigation Practices for Different Greenhouse Gases

Several practices have been adopted to mitigate GHG emissions based on the principles cited above. Often a practice would affect more than one gas, by more than one mechanism, sometimes in contradicting ways so that the net benefit depends on the combined effects on all gases (Robertson and Grace 2004). Further, the temporal pattern of influence may vary among practices or among gases for a specified practice. The impacts of various mitigation practices considered are summarized in Table 15.2.

15.4.1 Carbon Dioxide

Historically, land-use, land-use change and forestry (LULUCF), residue burning and emission and removal of C from soils are known to positively contribute to the global budget of CO2. Agriculture also consumes fossil fuels during the manufacture of equipments, fertilizers and other chemical inputs as well as during machinery and grain handling operations (e.g. grain drying). Strides have been made to change

Table 15.2 Proposed measures for mitigating GHG emissions from agricultural ecosystems and their apparent effect on reducing emissions of individual gases

Mitigative effects

Mitigation measures Processes CO2 CH4 N2O

Table 15.2 Proposed measures for mitigating GHG emissions from agricultural ecosystems and their apparent effect on reducing emissions of individual gases

Mitigative effects

Mitigation measures Processes CO2 CH4 N2O

Cropland management

Agronomy

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