Role of organic carbon, nitrate and ferrous iron on the partitioning between denitrification and DNRA in constructed stormwater urban wetlands

Denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) are two competing nitrate reduction pathways that remove or recycle nitrogen, respectively. However, factors controlling the partitioning between these two pathways are manifold and our understanding of these factors is critical for the management of N loads in constructed wetlands. An important factor that controls DNRA in an aquatic ecosystem is the electron donor, commonly organic carbon (OC) or alternatively ferrous iron and sulfide. In this study, we investigated the role of natural organic carbon (NOC) and acetate at different OC/NO3− ratios and ferrous iron on the partitioning between DNF and DNRA using the 15N-tracer method in slurries from four constructed stormwater urban wetlands in Melbourne, Australia. The carbon and nitrate experiments revealed that DNF dominated at all OC/NO3− ratios. The higher DNF and DNRA rates observed after the addition of NOC indicates that nitrate reduction was enhanced more by NOC than acetate. Moreover, addition of NOC in slurries stimulated DNRA more than DNF. Interestingly, slurries amended with Fe2+ showed that Fe2+ had significant control on the balance between DNF and DNRA. From two out of four wetlands, a significant increase in DNRA rates (p