The Potential of Wetlands to Contain Agricultural Nitrate Under Extreme Streamflow Events

Abstract:

Agriculturally-derived nitrogen in Midwestern US streams and rivers causes water quality impairments at the local, regional, and continental scales. In this region, most of the annual aquatic nitrogen yield occurs during large storm events in the spring when freshly applied fertilizer and legacy terrestrial nitrogen stores are mobilized from soils to stream water as nitrate (NO₃^-). Global climate models predict that extreme precipitation events in the Midwestern USA will increase in frequency and magnitude, which would create wetter conditions that amplify NO₃^- losses and thus environmental consequences. Existing and restored wetlands have the potential to substantially reduce NO₃^- export and are one of the most promising conservation interventions under consideration in the region. However, most research to evaluate wetland performance has focused on NO₃^- reduction across individual wetlands without considering watershed scale effectiveness of a suite of multiple connected wetlands. In this study we evaluated watershed scale effectiveness of wetlands for reducing agricultural nitrogen under a range of climate conditions. We used field observations in the Minnesota River Basin (MRB) and a spatially explicit, reduced complexity network model of the Le Sueur River basin, a sub-watershed of the MRB. Field NO₃^- observations were collected during six synoptic sampling campaigns, over three years and 180 locations, including observations at 83 locations during a 0.1% daily exceedance probability streamflow event. Observational data showed that NO₃^- concentrations decreased exponentially with wetland cover and suggest that during the high streamflow event, wetland margins, floodplains and shallow depressions were actively connected to the network and reducing NO₃^-. The process-based NO₃^- routing model illustrates the dynamic nature of wetland effectiveness under a range of streamflow conditions and was used to explore optimal restored wetland placement for increasingly frequent extreme climatic events. Ultimately, our findings can guide wetland restoration and management by quantifying critical locations and wetland specifications for NO₃^- containment within agriculturally intensive regions under a wetter climate.