Emerging Wetlands From River Diversions Can Sustain High Denitrification Rates in a Coastal Delta

It is assumed that to treat excess NO 3 − high soil organic matter content (%OM) is required to maintain high denitrification rates in natural or restored wetlands. However, this excess also represents a risk by increasing soil decomposition rates triggering peat collapse and wetland fragmentation. Here, we evaluated the role of %OM and temperature interactions controlling denitrification rates in eroding (Barataria Bay‐BLC) and emerging (Wax Lake Delta‐WLD) deltaic regions in coastal Louisiana using the isotope pairing (IPT) and N 2 :Ar techniques. We also assessed differences between total (direct denitrification + coupled nitrification‐denitrification) and net (total denitrification minus nitrogen fixation) denitrification rates in benthic and wetland habitats with contrasting %OM and bulk density (BD). Sediment (benthic) and soil (wetland) cores were collected during summer, spring, and winter (2015–2016) and incubated at close to in‐situ temperatures (30°C, 20°C, and 10°C, respectively). Denitrification rates were linearly correlated with temperature; maximum mean rates ranged from 40.1–124.1 μmol m −2 h −1 in the summer with lower rates (<26.2 ± 5.3 μmol m −2 h −1 ) in the winter seasons. Direct denitrification was higher than coupled denitrification in all seasons. Denitrification rates were higher in WLD despite lower %OM, lower total N concentration, and higher BD in wetland soils. Therefore, in environments with low carbon availability, high denitrification rates can be sustained as long as NO 3 − concentrations are high (>30 μM) and water temperature is >10°C. In coastal Louisiana, substrates under these regimes are represented by emergent supra‐tidal flats or land created by sediment diversions under oligohaline conditions (<1 ppt).