Nutrient cycles across topography in drainage-impaired Corn Belt soils

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2021-12
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Lawrence, Nathaniel
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Hall, Steven J
Crumpton, William
Lu, Chaoqun
McDaniel, Marshall
VanLoocke, Andy
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Ecology, Evolution, and Organismal Biology
Abstract
Greenhouse gas emissions and nutrient leaching from Corn Belt soils pose a threat to both local and global human and environmental health. Topographic depressions in the US Corn Belt have documented impacts on hydrology, crop production, soil characteristics, and soil microbial communities. These topographic features also likely impact nutrient leaching rates and nitrous oxide (N2O) production. Specifically, high soil moisture in depressions may promote denitrification of nitrate (NO3-) to gaseous forms including N2O. As a result, denitrification may also effectively remove NO3- prior to leaching. In-situ measurements of N2O production and nutrient leaching rates are needed to understand both the role of topography in mediating these processes and the management strategies that could reduce them. We improved an automated soil gas chamber system capable of collecting high frequency (4-h) N2O emission measurements and collected 3 y of N2O emissions across a topographic gradient in a conventionally managed corn–soybean (Zea mays–Gycine max) Iowa agricultural field. We also collected 2 y of nutrient leaching dynamics across several topographic gradients. We found no consistent impact of topography on N2O emissions. Rather, we found that mean N2O emissions were high in both the upland and depression. A regional literature synthesis suggested that N2O emissions were similarly high across all drainage classes except for well-drained (including all drainage classes found across our field gradient). The N2O emissions from these drainage-impaired soils were twofold greater than likely soil carbon gains achievable from alternative agricultural management. We also found greater average nitrogen and phosphorus leaching rates in topographic depressions relative to uplands despite evidence for periods of greater denitrification in the depression. Nutrient leaching was especially high after fertilization, when the isotopic signature of nitrate was consistent with a combination of mineralized soil nitrogen and recent fertilizer additions. When considered with the flooding-induced crop mortality in depressions during the study period, these results indicate especially poor depression nutrient use efficiency. Management to reduce nutrient inputs or maintain consistent vegetation may reduce the environmental impact of these features.
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