Effect of temperature and drying-rewetting of sediments on the partitioning between denitrification and DNRA in constructed urban stormwater wetlands

Abstract

Constructed wetlands are increasingly used within urban areas to reduce pollutant runoff, including nitrogen. These environments are exposed to frequent wetting and drying events, and also increasing temperatures due to the heat island effect and seasonal variation. In this study, we have investigated the role of drying and rewetting of sediments and temperature on the partitioning of two nitrate reduction pathways: denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) in four constructed urban stormwater wetlands in Melbourne, Australia. Our results suggest that both DNF and DNRA decreased due to drying of sediments, but DNRA decreased to a greater extent. Rates of DNRA from the drying-rewetting treatment decreased 5–90% compared to the control. Moreover, a concomitant increase in NH4+ fluxes and a slight change in sediment oxygen demand (SOD) suggest that oxic conditions arising from sediment drying was responsible for the decreased rates of DNF and DNRA. Rates of both DNF and DNRA increased with increasing temperature in slurries. The DNF:DNRA ratio suggests that the relative increase in DNRA was more than DNF with increasing temperature. The mean activation energy of DNF ranged from 41 ± 1 to 64 ± 4 kJ mol−1 and the corresponding temperature coefficient (Q10) values ranged from 1.3 to 2.4. In comparison, the mean activation energy of DNRA was higher and ranged from 50 ± 8 to 107 ± 14 kJ mol−1 with the corresponding Q10 values ranging from 1.2 to 3.4. Overall, our results suggest that drying and re-wetting of sediments decreases nitrate reduction and increases nitrogen retention, whereas increased temperature enhances the recycling of bioavailable nitrogen in wet sediments in constructed urban stormwater wetlands.