Mitigation of Greenhouse Gases in Agricultural Ecosystems

Modern cropping systems use substantial amounts of fossil energy in the form of fertilizers, pesticides, and fuel for field operations. An important environmental consequence of this use is the emission of greenhouse gases (GHGs) to the atmosphere, from sources both direct and indirect. Direct sources include fossil fuel used for tillage and other field operations as well as GHGs produced and consumed by microbes in cropped soils. Indirect sources include fossil energy used off-site to produce fertilizers and other agronomic inputs, as well as GHGs produced by microbes in noncropped sites that receive nutrients escaped from cropped fields. Row-crop agriculture can thus be either a net source or sink of GHGs, with the balance (net emission or uptake) influenced greatly by management practices. All three of the major biogenic GHGs are affected by agriculture: carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O). Not including postharvest activities or land-use conversion caused by agricultural expansion, agriculture is responsible for 10–14% of total global anthropogenic GHG emissions (Barker et al. 2007, Smith et al. 2007). This includes ~84% of anthropogenic N 2 O emissions and ~53% of anthropogenic CH 4 emissions (Robertson 2004). The manufacture of agrochemicals adds another 0.6–1.5% to the global total (Vermeulen et al. 2012). Most agricultural CO 2 emissions are from land conversion and fossil fuel use. Methane emissions associated with agriculture are from enteric fermentation by ruminant animals such as cattle, cultivated rice soils, animal wastes, and agricultural biomass burning. In addition, land conversion to agriculture substantially reduces microbial CH 4 oxidation in soil, thereby attenuating an important CH 4 sink and effectively increasing CH 4 in the atmosphere. Nitrous oxide emissions from agriculture are produced mostly from nitrogenous fertilizers, with lesser contributions from animal wastes and biomass burning. Gelfand, I. and G. P. Robertson. 2015. Mitigation of greenhouse gas emissions in agricultural ecosystems. Pages 310-339 in S. K. Hamilton, J. E. Doll, and G. P. Robertson, editors. The Ecology of Agricultural Landscapes: Long-Term Research on the Path to Sustainability. Oxford University Press, New York, New York, USA.