Sources and transformation mechanisms of inorganic nitrogen: Evidence from multi-isotopes in a rural-urban river area

Multi-isotope tracers were applied to quantitatively reveal the sources and transformation mechanisms of inorganic nitrogen both spatially and seasonally in a complex land use area in China. Based on land use and the level of socioeconomic development, the study area was divided into four zones: the rural area, developed urban area, developing urban area and industrial urban area. The redox condition and isotope analysis results indicated that the nitrification process dominated in the Han and Rong River, which were characterized by ammonium nitrogen (NH₄⁺-N) and oxidizing conditions, while neither nitrification nor denitrification occurred in the Lian River. The inorganic nitrogen sources of the four areas were revealed from the results of a stable isotope analysis in R (SIAR) and a two-component mixing model after determining the transformation mechanisms. In the rural area, nitrate nitrogen (NO₃⁻-N) was mainly sourced from the increased fertilization of nitrogen fertilizer (42–56%) to farmland during the wet season, and from soil nitrogen (33–62%) related to increased nitrification during the dry season. In the urban area, the contributions of soil nitrogen, manure and sewage and industrial wastewater to the total inorganic nitrogen exhibited large seasonal and spatial differences, which were distinguished by the environmental management supported by gross domestic production (GDP). In the developed and developing urban areas, soil nitrogen contributed 41% and 47% of the NO₃⁻-N, respectively, during the wet season, and 47% and 54%, respectively, during the dry season. The second highest contribution was from manure and sewage (30–41%) with no seasonal differences. In the industrial urban area, the dominant contribution to the NH₄⁺-N was from manure and sewage (81%) during the wet season, but industrial wastewater (84%) in the dry season. Our findings elucidate the multiplex sources and transformation mechanisms of inorganic nitrogen, and promote the management of nitrogen tracing to control nitrogen pollution in complex land use areas.