Assessing Wetland Effects on Nitrogen Reduction within a Fluvial Network Perspective: A Combined Field and Modeling Approach

Abstract:

Agriculturally-derived excess nitrogen in Midwestern US streams and rivers has contributed to water quality impairments at the local, regional, and continental scales, such as the dead zone in the northern Gulf of Mexico. Rates of biogeochemical processing of nitrogen, through denitrification and assimilation, vary throughout space and time and are dependent on factors such as carbon and nitrogen availability, temperature and water residence time. Through an extensive field sampling campaign in the Minnesota River Basin, a 44,000 km² agricultural basin in the upper Midwest, we investigated the spatial and temporal variability of nitrate concentrations throughout the river network and found that wetlands and lakes, characterized by long residence time and vegetative flow paths, behave as hot spots of nitrogen reduction. We documented that during the critical high loading period of late spring, nitrate concentrations were dependent on the fraction of lakes and wetlands composing the drainage area and that this dependency held even after controlling for the fraction of drainage area in crops. Variability in N uptake within wetlands was characterized through a separate field sampling effort in order to determine first order controls on denitrification and assimilation rates. These observational results were used to develop a spatially explicit, reduced complexity fluvial network model of nitrate transport and uptake which, together with the field measurements, highlights the disproportionately large contribution of wetlands and lakes to nitrate reduction and thus affirms the need to strategically conserve wetlands in intensively managed agricultural landscapes. Ultimately, this effort can guide wetland restoration and management by quantifying critical locations and wetland specifications for achieving desired nitrate reductions within a basin.