The contribution of maize cropping in the Midwest USA to global warming: A regional estimate
Agricultural soils emit about 50% of the global flux of N2O attributable to human influence, mostly in response to nitrogen fertilizer use. Recent evidence that the relationship between N2O fluxes and N-fertilizer additions to cereal maize are non-linear provides an opportunity to estimate regional N2O fluxes based on estimates of N application rates rather than as a simple percentage of N inputs as used by the Intergovernmental Panel on Climate Change (IPCC). We combined a simple empirical model of N2O production with the SOCRATES soil carbon dynamics model to estimate N2O and other sources of Global Warming Potential (GWP) from cereal maize across 19,000 cropland polygons in the North Central Region (NCR) of the US over the period 1964-2005. Results indicate that the loading of greenhouse gases to the atmosphere from cereal maize production in the NCR was 1.7Â Gt CO2e, with an average 268Â t CO2e produced per tonne of grain. From 1970 until 2005, GHG emissions per unit product declined on average by 2.8Â t CO2eÂ ha-1Â annum-1, coinciding with a stabilisation in N application rate and consistent increases in grain yield from the mid-1970's. Nitrous oxide production from N fertilizer inputs represented 59% of these emissions, soil C decline (0-30Â cm) represented 11% of total emissions, with the remaining 30% (517Â Mt) from the combustion of fuel associated with farm operations. Of the 126Â Mt of N fertilizer applied to cereal maize from 1964 to 2005, we estimate that 2.2Â Mt N was emitted as N2O when using a non-linear response model, equivalent to 1.75% of the applied N.
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- G. Robertson & Peter Grace, 2004. "Greenhouse Gas Fluxes in Tropical and Temperate Agriculture: The need for a Full-Cost accounting of Global Warming Potentials," Environment, Development and Sustainability, Springer, vol. 6(1), pages 51-63, March.
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