Stable isotopes of nitrate (δN and δO) as functional indicators of nitrogen processing in constructed wetlands.

The effectiveness of nitrogen removal in wetlands relies heavily on the biological processes that control its removal. Here, we used δN and δO of nitrate (NO) to assess the presence and the dominance of transformation processes of nitrogen in two urban water treatment wetlands in Victoria, Australia over two rainfall events. Laboratory incubation experiments were undertaken in both light and dark to measure the isotopic fractionation factor of nitrogen assimilation (by periphyton and algae) and benthic denitrification (using bare sediment). Highest isotopic fractionations were observed for nitrogen assimilation by algae and periphyton in the light, ε = -14.6 to -25 ‰ while the ε = -1.5 ‰ in bare sediment, consistent with that of benthic denitrification. Transect water samplings of the wetlands showed different rainfall patterns (discrete versus continuous) affect the removal capability of the wetlands. During the discrete event sampling, the observed ε of NO (an average of 3.0 to 4.3 ‰) within the wetland falls between the experimental ε of benthic denitrification and assimilation; coinciding with the decrease in NO concentrations, suggesting that both denitrification and assimilation were important removal pathways. Depletion of δN-NO throughout the whole wetland system also suggested the influence of water column nitrification during this time. In contrast, during continuous rain events, no fractionation effect was observed within the wetland and was consistent with limited NO removal. The difference in fractionation factors within the wetland during different sampling conditions suggested that nitrate removal was highly likely limited by changes in overall nutrient inputs, residence time and water temperature which impeded biological uptake or removal. These highlight that consideration of sampling condition is crucial when assessing the efficacy of a wetland in removing nitrogen.