Water transport pathways influence the propagation of field‐scale NO₃⁻‐N reductions to the watershed scale

Abstract

Diffuse nutrient runoff from agricultural fields can result in the eutrophication of downstream water bodies, highlighting a need for conservation efforts to reduce dissolved nitrogen (N) and phosphorus (P) loading to adjacent waterways. However, few studies explore how the impacts of field‐scale conservation manifest at the watershed scale. We explored how sources of streamflow and nutrients may influence the magnitude of the impact of conservation practices at a field versus watershed scale at the Shatto ditch watershed (SDW), where the planting of winter cover crops reduced field‐scale nitrate‐N (NO3−‐N) losses from subsurface tile drains by 69%–90%; yet watershed NO3−‐N export only decreased by 13%. To resolve this discrepancy, we used a water budget approach paired with water stable isotope (18O and 2H) analysis to determine the composition of streamflow across seasons, sampling five times from November 2018 to September 2019. While we hypothesized that watershed‐scale patterns in nutrient export were driven by direct groundwater upwelling, we found that this pathway only accounted for 43% of streamflow on average. We also developed a NO3−‐N mass balance for the watershed during our November 2018 sampling; results indicated groundwater upwelling contributed only ~1% of NO3−‐N export at the watershed outlet, while subsurface tile drains contributed the remaining 99%. Specifically, three large county tile drains (with an unknown drainage area and extent of conservation practice implementation) contributed ~69% of the watershed NO3−‐N export, obscuring the impacts of field‐scale conservation in SDW. Our study highlights the importance of cross‐scale analyses to accurately evaluate the effect of conservation given the interactions between sources of streamflow and nutrient loss at the watershed scale.