Seasonality interacts with mixed land use and conservation in controlling patterns of nutrient and pathogen export from agricultural watersheds

Fertilizer runoff is a significant source of nutrients to streams draining agricultural watersheds and results in numerous downstream impacts including eutrophication and coastal hypoxia. Additionally, pathogen export linked to manure application poses a threat to human health and results in recreational closures. Agricultural conservation practices, such as the planting of winter cover crops (CC), can reduce nutrient losses to streams, but their impacts on pathogen transport remain understudied. From 2019 to 2022, we sampled every 2 weeks in three subwatersheds of the Paw Paw River Basin (Michigan, USA) to assess the role of CC in reducing dissolved nitrate (NO₃^–-N), ammonium (NH₄⁺-N), soluble reactive phosphorus (SRP), and the fecal bacterium Escherichia coli export. The three subwatersheds contain varying levels of agricultural land use, ranging from 41 to 77 %. Water column NO₃^–-N (mg L^-1) peaked during the winter and spring fallow season, while E. coli (CFU 100 mL⁻¹) peaked during summer, which points to different drivers controlling NO₃^–-N and pathogen export throughout the year. Increased daily yields of dissolved nutrient tended to coincide with more agricultural land cover; however, we measured highest daily yields of E. coli in the watershed with lowest agricultural land cover. Planting CC reduced NO₃^–-N yield by 10–31 %, NH₄⁺-N yield by 19–22 %, SRP yields by 3–11 %, and E. coli yields by 17–48 %, and therefore is effective at mitigating both nutrient and pathogen export from agricultural landscapes, but additional work is required to fully understand the dynamics (timing and drivers) controlling E. coli export in watersheds of mixed land use.