Strategic placement of wetlands mitigate surplus nutrient losses in tile-drained agricultural landscapes

Extensive resources have been committed to mitigating surplus nutrient losses via a variety of conservation practices, including restored and constructed wetlands. Despite these efforts, some watersheds have failed to show substantial improvement in water quality conditions at the watershed outlet. This phenomenon may be partly attributed to landscape alterations including subsurface tile drainage which, while advantageous for agricultural productivity, promotes transport of surplus nutrients to downgradient waters while often bypassing wetlands and other conservation practices. We apply previously constructed hydrologic models of the Upper Mississippi River Basin (~4.4 x 105 km2) and the Maumee River Basin (~1.7 x 104 km2) two of the most expansively tile-drained landscapes in the United States to ask: To what extent does tile drainage affect the capacity of wetland restoration and construction to mitigate surplus nutrient (nitrogen, phosphorus) export at the watershed scale? Our model simulations demonstrate that tile drains substantially reduce the ability of wetlands to receive, store, and remove surplus nutrients at the watershed scale. However, we additionally show that wetland restoration and construction can be especially advantageous in these landscapes if wetlands are strategically positioned to receive and treat tile-effluent. Our work demonstrates the potential of wetland restoration and construction in tile-drained landscapes to reduce downstream nutrient loads while highlighting the continued challenge of balancing the agricultural production benefits of tile drainage with large-scale water quality mitigation needs.