Abstract
Agricultural landscapes are the largest source of anthropogenic nitrous oxide (N(2)O) emissions, but their specific sources and magnitudes remain contested. In the US Corn Belt, a globally important N(2)O source, in-field soil emissions were reportedly too small to account for N(2)O measured in the regional atmosphere, and disproportionately high N(2)O emissions from intermittent streams have been invoked to explain the discrepancy. We collected 3 y of high-frequency (4-h) measurements across a topographic gradient, including a very poorly drained (intermittently flooded) depression and adjacent upland soils. Mean annual N(2)O emissions from this corn-soybean rotation (7.8 kg of N(2)O-N ha(-1)⋅y(-1)) were similar to a previous regional top-down estimate, regardless of landscape position. Synthesizing other Corn Belt studies, we found mean emissions of 5.6 kg of N(2)O-N ha(-1)⋅y(-1) from soils with similar drainage to our transect (moderately well-drained to very poorly drained), which collectively comprise 60% of corn-soybean-cultivated soils. In contrast, strictly well-drained soils averaged only 2.3 kg of N(2)O-N ha(-1)⋅y(-1) Our results imply that in-field N(2)O emissions from soils with moderately to severely impaired drainage are similar to regional mean values and that N(2)O emissions from well-drained soils are not representative of the broader Corn Belt. On the basis of carbon dioxide equivalents, the warming effect of direct N(2)O emissions from our transect was twofold greater than optimistic soil carbon gains achievable from agricultural practice changes. Despite the recent focus on soil carbon sequestration, addressing N(2)O emissions from wet Corn Belt soils may have greater leverage in achieving climate sustainability.