Water treatment wetlands are increasingly being used to reduce pollutant loads including nitrogen (N) in urban runoff. Processes such as denitrification (DNF) and anaerobic ammonium oxidation (anammox), which remove N, and dissimilatory nitrate reduction to ammonium (DNRA), which recycles N, play an important role in controlling NOx (NO3− + NO2−) removal versus recycling in wetlands. The relative importance of DNF, anammox and DNRA was investigated in four constructed stormwater urban wetlands in Melbourne, Australia. Rates of DNF and DNRA were variable and did not differ significantly among wetlands. However, rates of DNF and DNRA were significantly different (p < 0.05) in different seasons. The relationship between NOx reduction processes and measured concentrations of water column NOx, chlorophyll a (chl-a), sediment organic carbon (OC), porewater ferrous iron (Fe2+) and sulfide (S2−) and water column temperature were examined using multiple regression analysis (MRA). Anammox was an insignificant pathway (< 0.05% of total nitrate reduction). During winter when average water column temperatures were 12 °C, DNRA was consistently higher than DNF averaging 67 ± 23% of total NOx reduction. The MRA revealed that DNF was positively associated with NOx concentration whereas DNRA was negatively associated with temperature, and porewater Fe2+, and positively associated with chl-a. The ratio between DNF and DNF:(DNF + DNRA) showed a positive correlation with both temperature and NOx concentration in the MRA. At higher temperatures and higher NOx concentrations, DNF increased over DNRA. Overall, this study suggests that at low NOx concentrations, N is recycled internally in these urban stormwater wetlands, but the portion of N removed by DNF increases as NOx concentrations increase.
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