Does Denitrification Drive Nitrogen Removal across Restored Floodplain Wetlands?

Abstract

Wetland restoration projects commonly seek to promote denitrification, which permanently removes NO3 from ecosystems by stepwise conversion to N2 gas. Anammox (N2 production from NH4 and NO2) and dissimilatory nitrate reduction to ammonium (NH4 production from NO3) are assumed to be less common than denitrification in freshwater systems. However, the relative importance of these processes may not be consistent across wetlands subject to different vegetation and hydrologic restoration practices. We measured NO3, NH4, NO2, and N2 flux in 18 restored floodplain wetlands across west Tennessee and Kentucky, USA, in summer 2019 and 2020 using flow-through soil core incubations (n > 500). We predict N2 production will be positively correlated with NO3 uptake and sediment oxygen demand if denitrification is the dominant N removal process across our sites. A weak relationship between N2 production and NO3 uptake would indicate that other N cycling processes contribute significantly to NO3 uptake or N2 production. We will use mixed effects models to explore potentially dominant N cycling pathways while accounting for effects of restoration practices. Results will inform future isotopic labeling experiments designed to trace sources and fates of excess N in restored floodplains.