The contribution of maize cropping in the Midwest USA to global warming: A regional estimate

Abstract

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 1.7 Gt CO₂e, with an average 268 t CO₂e produced per tonne of grain. From 1970 until 2005, GHG emissions per unit product declined on average by 2.8 t CO₂e ha⁻¹ 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 (0–30 cm) represented 11% of total emissions, with the remaining 30% (517 Mt) from the combustion of fuel associated with farm operations. Of the 126 Mt of N fertilizer applied to cereal maize from 1964 to 2005, we estimate that 2.2 Mt N was emitted as N₂O when using a non-linear response model, 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).