Agriculture Reference
In-Depth Information
12
Mitigation of Greenhouse
Gases in Agricultural
Ecosystems
Ilya Gelfand and G. Philip Robertson
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 atmo-
sphere, 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 bal-
ance (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 posthar-
vest activities or land-use conversion caused by agricultural expansion, agricul-
ture 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 agricul-
tural 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 contribu-
tions from animal wastes and biomass burning.
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