Assessing Environmental Drivers of Denitrification in Restored Riverine Floodplains

This is an accepted article with a DOI pre-assigned that is not yet published.Restoration of impaired and hydrologically disconnected floodplains is an increasingly prevalent strategy for alleviating water quality concerns and reducing downstream flooding at watershed scales. Hydrologically connected floodplains temporarily store water and slow flow velocity to promote sedimentation during overbank flooding and remove inorganic nitrogen from floodwater and groundwater via denitrification. Evaluating the impacts of different restoration strategies on denitrification can inform more strategic management efforts. Our research links denitrification rates to environmental factors that can be incorporated into restoration design and water resources management. We seasonally measured soil denitrification enzyme activity and various environmental characteristics in four floodplains with different restoration design approaches at the confluence of the Wabash and Tippecanoe Rivers in Indiana, U.S.A. Despite being hydrologically connected, our results showed that denitrification rates in an agricultural floodplain were significantly lower than in restored floodplains with native vegetation. Certain soil conditions were associated with higher denitrification, particularly elevated total nitrogen, moisture, silt, and organic matter contents. Vegetation was also an important predictor of denitrification rates, which may reflect direct effects, such as supplying labile organic carbon, or indirect effects, such as acting as an indicator of hydrologic regime. Denitrification seasonally varied, peaking in winter when nitrate supply from rivers draining agricultural watersheds in the region is also high. Substrate limitations of denitrification were most significant during the summer when overbank flooding, which replenishes soil nitrogen stocks, rarely occurs. Our findings indicate that denitrification capacity will likely be maximized in riverine floodplains with enhanced surface or subsurface hydrologic connectivity and diverse native vegetation, which support elevated moisture, fine sediment deposition, and sufficient organic matter.