Agricultural Wetland Restoration: The Role of Sediment Removal, Hydroperiod and Time on Restoration Outcomes

Abstract

Restoring agricultural wetlands to remediate nutrient runoff, decrease flood risk, and improve wildlife habitat are areas of growing interest. One restoration strategy that may improve species diversity, enhance water retention, and decrease nutrient availability is the removal of accumulated eroded sediment from agricultural wetlands prior to restoration. In this work, we the measured physical and chemical characteristics of soils, characterized plant communities, and examined water column nutrient availability and denitrification potential in 54 restored agricultural wetlands in west central Minnesota. In half of the wetlands hydrologic function was restored by removing and plugging drainage tile and ditches, while hydrology was restored in the remaining basins following sediment removal (Excavation treatment), increasing basin depth by an average 30 cm. Excavation primarily influenced the plant community, by delaying the establishment of two invasive emergent macrophytes, hybrid cattail (Typha x glauca) and reed canary grass (Phalaris arundinacea), but the affect only lasted for 6 years. Contrary to expectations, soil properties, water column dissolved nutrients, and denitrification potential were all primarily influenced by hydroperiod – the number of consecutive days with standing water. Wetlands with longer hydroperiods had less bioavailable P in soils, lower dissolved N and P concentrations, and lower denitrification potential. We also found evidence that vegetation likely plays an important role in dissolved nutrient dynamics over time. Our results suggest that excavation may be an important tool in wetland restoration but its influence was lost as wetlands aged in the absence of invasive species management. Moreover, nitrogen and phosphorus dynamics were almost universally controlled by hydroperiod, with tradeoffs between nitrogen removal and phosphorus remineralization.