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Denitrification in Agricultural Depressions by Nitrate Isotope Analysis
YEAR: 2019

Iowa’s agricultural lands are among the most productive in the world, but they are also a large source of nitrate (NO3-)pollution. Ideally, NO3-is incorporated into crop tissues which are removed through harvest. NO3-which is not utilized by crops, or is returned to the soil through mineralization, is primarily lost through leaching and denitrification (Libra et al., 2004). The highest rates of NO3-leaching in Iowa are found in the poorly drained Des Moines Lobe (DML), where extensive tile drainage is thought to promote leaching to streams (Jones et al., 2017). However, wet soil conditions also promote denitrification, where in the absence of oxygen, soil microbes reduce NO3-to gaseous forms, primarily nitrous oxide (N2O) and dinitrogen gas (N2) (Hall et al., in press). Soil water content in the DML is mediated by the presence of landscape depressions (former prairie pothole wetlands), where both infiltration and N2O production may exceed surrounding uplands (N. Lawrence, unpublished). Quantifying how much NO3-is removed through denitrification versus lost through leaching is critical to understanding the role of landscape depressions as a source and sink of NO3-at the landscape level and providing insight into how targeted management of depressions might address NO3-loads. Here, we propose to apply a cutting-edge method—analysis of the natural-abundance isotope composition of NO3-—to address the role of within-field denitrification in attenuating agricultural NO3-loads.