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.

由于对农业景观的过量肥料投入，对淡水生态系统的环境影响持续存在。正在鼓励保护工作，例如覆盖作物，以减少田间的氮 (N) 和磷 (P) 径流，但很少记录它们对工作土地的影响。我们量化了两个农业流域（美国印第安纳州）在四个水年（2016-2019）中广泛种植覆盖作物导致农田中硝酸盐氮和可溶性活性磷 (SRP) 损失的减少。我们每两个月从瓷砖排水管和河流站点收集水样，以测量不同尺度的硝酸盐-N 和 SRP 损失。覆盖作物始终将瓷砖排水硝酸盐-N 损失减少 27-72%，而 SRP 减少的变化更大，范围为 7%-58%。亚流域硝酸盐-N 产量在每个流域中是一致的，虽然源头对河流的 SRP 贡献不成比例，这表明可能需要实施有针对性的覆盖作物以减少 SRP 出口。最后，流域规模的硝酸盐-N 出口在春季减少了 2–67%（5/8 个站点年），SRP 出口减少了 31–88%（7/8 个站点年）。然而，考虑到径流年际变化的影响以及氮和磷负荷的空间异质性，可能需要在区域范围内种植覆盖作物以使年出口量持续减少，从而对下游水质产生有意义的影响。