Using stable isotopes to identify nitrogen transformations and estimate denitrification in a semi-constructed wetland

Constructed wetlands are effective at removing nitrate, one of the major pollutants in aquatic ecosystems. In this study, nitrogen and oxygen isotopes (δ¹⁵N and δ¹⁸O) were used to determine the sources and transformations of nitrate (NO₃^-), evaluate isotopic fractionation, and estimate nitrogen (N) removal in the Xixi Wetland, a large semi-constructed wetland in East China. The values of δ¹⁵N and δ¹⁸O in the Xixi Wetland ranged from +3.7‰ to +19.0‰ and from +1.1‰ to +13.7‰, respectively. The main NO₃^- sources included sewage/manure, chemical fertiliser and soil nitrogen. Anthropogenic sources became more significant in the Xixi Westland, especially in autumn, as the increased number of tourists resulted in increased sewage/manure input. The results indicated that nitrification and denitrification were the key processes governing N transformations. The monthly variations in the NO₃^- concentrations and δ¹⁵N values indicated that denitrification was strong from spring to autumn in the Xixi Wetland. Based on measurements of the natural abundance of δ¹⁵N, it was determined that the enrichment factors (ε) of the Xixi Wetland ranged from -0.7‰ to -1.0‰, and were smaller than those in a laboratory denitrification experiment (-1.6‰). In this study, denitrification, nitrification, and assimilation in plants/microbes were responsible for the low enrichment factors. Additionally, the amount of N removal via denitrification varied widely, from 22 kg N·ha^-1 yr^-1 to 798 kg N·ha^-1 yr^-1; thus, significant potential for N removal was found in the Xixi Wetland.