Short CommunicationThe contribution of maize cropping in the Midwest USA to global warming: A regional estimate
Research highlights
► Emissions from maize production in the North Central Region (1964–2005) 1.7 Gt CO2e. ► On average 268 t CO2e produced per tonne of grain. ► N2O from N fertilizer inputs represented 59% of these emissions. ► Soil C decline (0–30 cm) is 11% of total emissions, remaining is fuel. ► 2.2 Mt N2O–N emitted, equivalent to 1.75% of the applied N.
Introduction
The North Central Region (NCR) of the USA encompasses 12 Midwestern states (North Dakota, South Dakota, Nebraska, Kansas, Minnesota, Iowa, Missouri, Wisconsin, Illinois, Michigan, Indiana and Ohio), is the major producer of maize and soybean, and produces half of the nation’s wheat. The major Land Resource Region in the NCR is Central Feed Grains and Livestock, more commonly known as the Corn Belt, which extends across all of these states. Donigan et al. (1994) have estimated that the Corn Belt has lost half of its soil organic carbon (SOC) stores since cultivation began in the mid-nineteenth century. Crop residues are essential for sustainable cereal maize productivity for both bioenergy and food production in this region (Wilhelm et al., 2007, Robertson et al., 2008). Our study informs the bioenergy vs. food security debate in that it directly examines the historical impact of cereal maize production in the NCR over the past 40 years on greenhouse gas emissions, including the maintenance of SOC for continued productivity.
Average maize yields in the NCR increased nearly threefold (from 3.6 to 9 t ha−1) from 1964 to 2005, with a 25% increase in the harvested area (Fig. 1) since the introduction of post-Green Revolution maize varieties in the mid-1960’s (United States Department Agriculture (USDA) – National Agricultural Statistics Service (NASS), http://nass.usda.gov). Recommended N application rates in the NCR for maximum yields exceed 250 kg N ha−1 in some states (Vitosh et al., 1995).
Cost-effective and large scale greenhouse gas (GHG) mitigation interventions in agriculture can be identified only if all emissions are evaluated. Whilst the sequestration of atmospheric CO2 into stable SOC pools has been demonstrated through reduced tillage and improved grazing management (Lal, 2004), agricultural systems in the NCR remain net sources of CO2 and N2O and consume a reduced amount of atmospheric CH4 relative to unmanaged ecosystems (Robertson et al., 2000).
The total global flux of N2O attributable to human activity is approximately 1.2 Pg C-equivalents annually (Prinn, 2004, Robertson, 2004). The current loading of CO2 to the atmosphere is 4.1 Pg C y−1 (Canadell et al., 2007), therefore in comparison, N2O represents a major area for both concern and mitigation (IPCC, 2007a). About 85% of the global flux of N2O from human sources is from agriculture (IPCC, 2007b), with about 50% of the global flux from denitrification and nitrification in agricultural soils.
The overall balance of CO2, N2O, and CH4 constitutes the net Global Warming Potential (GWP) impact of the agricultural production system (Robertson and Grace, 2004). Primary influences on SOC dynamics and losses of N2O from soils are climate (temperature and precipitation), soil type, and the quantity and quality of organic material and nitrogen returned to the soil. In concert, climate and soil type have a major impact on N2O loss, with clay soils more conducive to water logging and denitrification (Bouwman et al., 2002).
Regional assessments of SOC change and N2O loss require the synergy of biology, modeling, and geographic data management. A Modeling Applications Integrative Framework (MASIF) (Gage et al., 2001) has been specifically developed to assemble and process the large amounts of spatiotemporal climate, land use, yield, and soil data necessary for regional scale simulation experiments. Here we use MASIF to couple the SOCRATES terrestrial carbon dynamics model (Grace et al., 2006b) with an empirical N2O calculator to estimate the net GWP impact of maize production in the North Central Region (NCR) since the advent of the Green Revolution in the mid-1960’s (Duvich and Cassman, 1999).
Section snippets
Soil organic carbon simulation
We used the USDA/Natural Resources Conservation Service (NCRS) STATSGO soils database (USDA, 1994) to develop a base map of textural properties across the NCR. Each of the STATSGO mapping units was assigned a dominant pre-settlement vegetation type (forest or grassland) based on the potential natural vegetation dataset of Kuchler (1964). The National Land Cover Data (NLCD) (Vogelmann et al., 2001) was then overlaid, producing nearly 19,000 separate cropland polygons across the 1056 counties of
Results and discussion
We estimate the loss of SOC from the top 30 cm of cropping soils in the NCR under cereal maize from 1964 to 2005 to be nearly 52 Mt (Table 1), approximately 1.2 Mt C annum−1 with the largest losses occurring in the states of Iowa, Indiana and Illinois, which represented half of the cereal maize harvested in the Midwest during this time. To put this in perspective, carbon loss associated with the combustion of fuel in farming operations and ancillary inputs associated with cereal maize (154.5 Mt C) is
Conclusions
Total emissions of greenhouse gases from cereal maize production in the North Central Region from 1964 to 2005 were 1.7 Gt CO2e, with N2O production from nitrogen inputs representing 35–59% of these emissions, when using Tiers 1 and 2 approaches respectively. Using a non-linear (Tier 2) model linking N2O loss and the rate of N application, an estimated 2.2 Mt N of the 126 Mt of N fertilizer applied to cereal maize in the NCR from 1964 to 2005 was emitted as N2O–N, equivalent to 1.75% of the applied
Acknowledgements
This study was supported with funding from the Electric Power Research Institute (EPRI), the Michigan Agricultural Experiment Station, the US National Science Foundation LTER Program and the Institute for Sustainable Resources at Queensland University of Technology.
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