Denitrification in stormwater constructed wetlands: Temporal variability and trace metal effects

Reduction of nitrous oxide (N₂O) production from nutrient rich stormwater constructed wetland ecosystems will enable a more environmentally sustainable system in urban and unmanaged areas. During the nitrogen metabolic reaction, N₂O can either escape to the atmosphere or be further reduced to N₂ by nitrous oxide reductase, which is the only known enzyme responsible for catalyzing this process. Denitrifiers need copper (Cu) to make this enzyme, but too much will inhibit the process, and too little will have no or few impacts. To investigate the effect of Cu on stormwater-constructed wetlands, two wetlands were selected here in Oklahoma. Laboratory denitrification enzyme activity assays and molecular analysis were performed to see the impact of spatial and temporal variability due to the Cu amendments. The treatment concentrations ranged from 0.01 µM (lowest) to 30 µM (highest). The results demonstrated that the effects of Cu amendments varied due to spatial and temporal variability. The ratio of N₂O to (N₂O+N₂) was also reduced or, in some cases, increased for the Cu treatments with varying levels, but not for the treatment with a fixed level of Cu. Furthermore, the addition of Cu amendments according to the United States Environmental Protection Agency’s requirements for aquatic ecosystem quality may not result in a substantial reduction in N₂O when compared to higher concentrations. For further investigation, Paracoccus denitrificans, a model denitrifier, was selected for Cu amendment experiments. It was found that high concentrations of Cu treatment reduced the N2O production rate in P. denitrificans-incubated systems. The quantitative polymerase chain reaction analysis with nosZ operational taxonomic unit (OTU) 1 and nosZ clade II primers showed that Cu additions have a general inverse relationship between N₂O production rate and nosZ gene abundance.