The role of corn and soybean cultivation on nitrate export from Midwestern US agricultural watersheds

Accumulation of nitrogen (N) in reservoirs has negative effects, generating algal blooms, creating hypoxic zones, and degrading drinking water quality. Agricultural land use, specifically corn and soybean cultivation, contribute excess N to aquatic systems. This work examines whether the nitrate as nitrogen (NO₃–N) export (load) from a watershed is correlated to the percentage of land devoted to growing corn or soybeans. To investigate potential relationships, Annual NO₃–N load from within 10 watersheds in the US Midwest were calculated using discharge (Q) and NO₃–N concentration data obtained from USGS gauging stations. The NO₃–N load data were coupled with crop data from the watersheds. Watershed areas ranged from 106 to 154,767 km². Corn was grown on between 14.3 and 56.1% of the land, while soybeans accounted for 7.2–45.4%. Crop percentages were compared to NO₃–N loads per area, the quotient of annual NO₃–N load (kg) to the watershed area (km²), from 2008 to 2017. The analyses of the collective data showed a positive trend between the percentage of land cultivated and the NO₃–N exported. However, as the percentage of corn cultivated increased within the individual watersheds, NO₃–N load per area decreased. At both the collective and individual scale, increases in soybean cultivation resulted in increases in NO₃–N export. Extended cultivation of the same crop lead to higher NO₃–N export. Strategies to decrease NO₃–N export need to focus on enhanced crop rotation.