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Wetland restoration yields dynamic nitrate responses across the Upper Mississippi river basin
Citation:
Evenson, G., H. Golden, J. Christensen, C. Lane, A. Rajib, E. DAmico, D. Mahoney, E. White, AND Q. Wu. Wetland restoration yields dynamic nitrate responses across the Upper Mississippi river basin. Environmental Research Communications. IOP Publishing, PHILADELPHIA, PA, 3(9):095002, (2021). https://doi.org/10.1088/2515-7620/ac2125
Impact/Purpose:
Uncertainty prevails regarding the capacity of wetlands to mitigate nitrogen losses at large river-basin scales. We simulate wetland restoration impacts on nitrate dynamics across 279 subbasins comprising the ~0.5 million km2 Upper Mississippi River Basin (UMRB). Simulations suggest that restoring ~8,000 km2 of wetlands will reduce mean annual nitrate loads to the UMRB outlet by 12%, a substantial improvement over existing conditions but markedly less than previously published estimates. Our lower wetland efficacy estimates are partly attributed to incorporating model simulations of processes discounted by preceding studies − namely processes reducing wetland capacity to receive nitrate (i.e., subsurface tile drainage) and mediating basin-scale effects (i.e., in-stream nutrient storage and processing). We confirm that wetlands mitigate surplus nitrogen losses but caution that they may not be as universally impactful in tiled landscapes and when considering in-stream processing.
Description:
Wetland restoration is a primary management option for removing surplus nitrogen draining from agricultural landscapes. However, wetland capacity to mitigate nitrogen losses at large river-basin scales remains uncertain. This is largely due to a limited number of studies that address the cumulative and dynamic effects of restored wetlands across the landscape on downstream nutrient conditions. We analyzed wetland restoration impacts on modeled nitrate dynamics across 279 subbasins comprising the ∼0.5 million km2 Upper Mississippi River Basin (UMRB), USA, which covers eight states and houses ∼30 million people. Restoring ∼8,000 km2 of wetlands will reduce mean annual nitrate loads to the UMRB outlet by 12%, a substantial improvement over existing conditions but markedly less than widely cited estimates. Our lower wetland efficacy estimates are partly attributed to improved representation of processes not considered by preceding empirical studies − namely the potential for nitrate to bypass wetlands (i.e., via subsurface tile drainage) and be stored or transformed within the river network itself. Our novel findings reveal that wetlands mitigate surplus nitrogen basin-wide, yet they may not be as universally effective in tiled landscapes and because of river network processing.
URLs/Downloads:
DOI: Wetland restoration yields dynamic nitrate responses across the Upper Mississippi river basin