Cover crops control nitrogen and phosphorus transport from two agricultural watersheds at multiple measurement scales

Environmental impacts on freshwater ecosystems persist due to inputs of excess fertilizer to agricultural landscapes. Conservation efforts, such as cover crops, are being encouraged to reduce nitrogen (N) and phosphorus (P) runoff from fields, but their effects on working lands are rarely documented. We quantified reductions of nitrate-N and soluble reactive phosphorus (SRP) losses from cropland in response to widespread planting of cover crops in two agricultural watersheds (Indiana, USA) over four water years (2016–2019). We collected water samples bimonthly from tile drains and stream sites to measure nitrate-N and SRP losses across scales. Cover crops consistently reduced tile drain nitrate-N loss by 27–72%, while SRP reductions were more variable, ranging from 7%–58%. Subwatershed nitrate-N yields were consistent across each watershed, while headwaters disproportionately contributed SRP to the stream, suggesting targeted cover crop implementation may be required to reduce SRP export. Finally, watershed-scale nitrate-N export was reduced by 2–67% (5/8 site-years) and SRP export by 31–88% (7/8 site-years) in spring. However, given the effect of interannual variability in runoff and spatial heterogeneity in N and P loading, regional-scale planting of cover crops may be needed to confer consistent reductions in annual export, with meaningful impacts on downstream water quality.