[Analysis of Nitrogen Pollution and Its Pollution Sources in the Muli River Basin]

To identify the sources of nitrogen pollutants in the Muli River basin in the Guangdong-Hong Kong-Macao Greater Bay Area, ammonium salt isotope tracer technology, nitrate isotope tracer technology, and a multiple linear mixing model were applied in this study to effectively identify the changes of nitrogen sources in the basin. The results showed that nitrogen pollution in the Muli River basin was serious, and the concentrations of NH₄⁺-N and NO₃^--N in the confluence were higher than in the two tributaries. In addition, although there was no obvious population residence at the upstream or downstream of the Muli River basin, higher nitrogen pollution still occurred at some sites (Dakengshan tributary and aquiculture area). Qualitative analysis of ammonium salt and nitrate isotopes showed that nitrogen pollution in the Muli River basin mainly came from soil, fertilizer, atmospheric particles, and animal and human excreta. The results of the multiple linear mixing model showed that the main source of nitrogen in the upper reaches of the Dakengshan tributary is atmospheric subsidence, with a contribution rate of about 80%. The average contribution rate of soil organic nitrogen in the upper reaches of the Jilongkeng tributary was 33%, higher than those of the Dakengshan tributary (9%) and Muli River (24%). The contribution rate of sewage and manure to nitrogen pollution was up to 70% in the lower reaches of the Dakengshan tributary, the lower reaches of the Jilongkeng tributary, and the middle and upper reaches of the Muli River. In addition, while all forms of livestock and poultry farming and aquiculture activity were stopped, the contribution rate of sewage and manure in Yangzhiqu was found to be still as high as 56%, which is much higher than that at the Danshui River Dam section in the lower part of the Muli River (3%); this may be due to residual livestock waste in the sediment. This study qualitatively and quantitatively analyzed the nitrogen sources of the Muli River basin, which provids a theoretical basis for pollution source management in the Greater Bay Area.