The greenhouse gas cost of agricultural intensification with groundwater irrigation in a Midwest U.S. row cropping system

Bonnie M Mcgill,Stephen K Hamilton,Neville Millar,G Philip Robertson

doi:10.1111/gcb.14472

Bonnie M Mcgill, Stephen K Hamilton + Show 2 more

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https://doi.org/10.1111/gcb.14472

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Abstract

Groundwater irrigation of cropland is expanding worldwide with poorly known implications for climate change. This study compares experimental measurements of the net global warming impact of a rainfed versus a groundwater-irrigated corn (maize)-soybean-wheat, no-till cropping system in the Midwest US, the region that produces the majority of U.S. corn and soybean. Irrigation significantly increased soil organic carbon (C) storage in the upper 25cm, but not by enough to make up for the CO2 -equivalent (CO2 e) costs of fossil fuel power, soil emissions of nitrous oxide (N2 O), and degassing of supersaturated CO2 and N2 O from the groundwater. A rainfed reference system had a net mitigating effect of -13.9 (±31) g CO2 e m-2 year-1 , but with irrigation at an average rate for the region, the irrigated system contributed to global warming with net greenhouse gas (GHG) emissions of 27.1 (±32) g CO2 em-2 year-1 . Compared to the rainfed system, the irrigated system had 45% more GHG emissions and 7% more C sequestration. The irrigation-associated increase in soil N2 O and fossil fuel emissions contributed 18% and 9%, respectively, to the system's total emissions in an average irrigation year. Groundwater degassing of CO2 and N2 O are missing components of previous assessments of the GHG cost of groundwater irrigation; together they were 4% of the irrigated system's total emissions. The irrigated system's net impact normalized by crop yield (GHG intensity) was +0.04 (±0.006) kgCO2 ekg-1 yield, close to that of the rainfed system, which was -0.03 (±0.002) kgCO2 ekg-1 yield. Thus, the increased crop yield resulting from irrigation can ameliorate overall GHG emissions if intensification by irrigation prevents land conversion emissions elsewhere, although the expansion of irrigation risks depletion of local water resources.

Highlights

Global food security depends on irrigation to expand arable land area, intensify crop production, and provide a buffer from increasingly hot and dry growing seasons (Turral, Burke, & Faures, 2011)
Others (Jin et al, 2017; Sainju, 2016; Sainju et al, 2014; Trost et al, 2016) have assessed the global warming impact (GWI) of irrigated cropping systems and found that irrigation accounted for a major portion of emissions, sometimes offset by increases in soil organic C (SOC) accrual, but none have included greenhouse gas (GHG) degassing from groundwater
The GWIs of CO2 and N2O degassing from groundwater at Kellogg Biological Station (KBS) in the average and high irrigation scenarios were 7.0 (±0.2) and 16.8 (±0.6) g CO2‐ equivalent (CO2e) m−2 year−1, respectively (Figure 2)

Summary

Introduction

Global food security depends on irrigation to expand arable land area, intensify crop production, and provide a buffer from increasingly hot and dry growing seasons (Turral, Burke, & Faures, 2011). The recent expansion of irrigated area is expected to continue in coming decades in response to changing climate, growing populations, and increasing food consumption per capita (Konikow, 2011; Wada et al, 2010). Irrigation can increase the global warming impact of agriculture (Mosier, Halvorson, Peterson, Robertson, & Sherrod, 2005; Mosier, Halvorson, Reule, & Liu, 2006; Sainju, 2016; Sainju, Stevens, Caesar‐TonThat, Liebig, & Wang, 2014; Trost et al, 2013, 2016 ), which is a major source of greenhouse gas (GHG) emissions to the atmosphere (IPCC, 2014). The GHG impacts of groundwater irrigation are not included in the GHG inventory methods of the Intergovernmental Panel on Climate Change (IPCC) or the US Environmental Protection Agency (De Klein et al, 2006; USEPA, 2017a)

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