Linking nitrogen management, seep chemistry, and stream water quality in two agricultural headwater watersheds

Riparian seepage zones in headwater agricultural watersheds represent important sources of nitrate-nitrogen (NO₃-N) to surface waters, often connecting N-rich groundwater systems to streams. In this study, we examined how NO₃-N concentrations in seep and stream water were affected by NO₃-N processing along seep surface flow paths and by upslope applications of N from fertilizers and manures. The research was conducted in two headwater agricultural watersheds, FD36 (40 ha) and RS (45 ha), which are fed, in part, by a shallow fractured aquifer system possessing high (3-16 mg L^-1) NO₃-N concentrations. Data from in-seep monitoring showed that NO₃-N concentrations generally decreased downseep (top to bottom), indicating that most seeps retained or removed a fraction of delivered NO₃-N (16% in FD36 and 1% in RS). Annual mean N applications in upslope fields (as determined by yearly farmer surveys) were highly correlated with seep NO₃-N concentrations in both watersheds (slope: 0.06; R² = 0.79; p < 0.001). Strong positive relationships also existed between seep and stream NO₃-N concentrations in FD36 (slope: 1.01; R² = 0.79; p < 0.001) and in RS (slope: 0.64; R² = 0.80; p < 0.001), further indicating that N applications control NO₃-N concentrations at the watershed scale. Our findings clearly point to NO₃-N leaching from upslope agricultural fields as the primary driver of NO₃-N losses from seeps to streams in these watersheds and therefore suggest that appropriate management strategies (cover crops, limiting fall/winter nutrient applications, decision support tools) be targeted in these zones.