Denitrification and DNRA in Urban Accidental Wetlands in Phoenix, Arizona

Denitrification and dissimilatory nitrate reduction to ammonium (DNRA) both require low oxygen and high organic carbon conditions common in wetland ecosystems. Denitrification permanently removes nitrogen from the ecosystem as a gas while DNRA recycles nitrogen within the ecosystem via production of ammonium. The relative prevalence of denitrification versus DNRA has implications for the fate of nitrate in ecosystems. Unplanned and unmanaged urban accidental wetlands in the Salt River channel near downtown Phoenix, Arizona, USA receive high nitrate relative to non-urban wetlands and have a high capacity for denitrification, but unknown capacity for DNRA. We conducted in-situ push-pull tests with isotopically labelled nitrate to measure denitrification and DNRA rates in three of the dominant vegetative patch types in these urban accidental wetlands. DNRA accounted for between 2 and 40% of nitrate reduction (DNRA plus denitrification) with the highest rates measured in patches of Ludwigia peploides compared to Typha spp. and non-vegetated patches. The wetland patches were similar with respect to dissolved organic carbon concentration but may have differed in carbon lability or strength of reducing conditions due to a combination of litter decomposition and oxygen supply via diffusion and aerenchyma. The ratio of DNRA to denitrification was negatively correlated with nitrate concentration, indicating that DNRA may become a more important pathway for nitrate attenuation at low nitrate concentration. Although DNRA was generally lower than denitrification, this pathway was an important component of nitrate attenuation within certain patches in these unmanaged urban accidental wetlands.