[Spatio-temporal Distributions and Source Analysis of Nitrogen in the Surface Water of Dianbu River Basin]

Dianbu River, flowing into the western part of Chaohu Lake, has been heavily polluted, and nitrogen is one of the key factors. During three periods （wet, normal, and dry）, 30 surface water samples were collected from the Dianbu River Basin as the research objects. The water chemistry, multiple stable isotopes （δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, and δ¹⁵N-NH₄⁺）, and a SIAR mixing model were analyzed not only to understand the spatio-temporal distribution characteristics of nitrogen and its influencing factors but also the sources of nitrogen. The results showed that the concentrations of TN and NH₄⁺-N were higher in dry periods than those in wet and normal periods. Compared with the water quality requirements （GB/T 14848）, 56.67% of samples in the surface water TN were in the situation of Grade Ⅴ-inferior Grade V. The proportion of Ⅳ-inferiorⅤof NH₄⁺-N sections was 10%. Along the main river, the concentrations of TN and NO₃^--N mainly increased and then decreased （in the wet season and normal season） or increased （in the dry season） from upstream to downstream, mainly affected by precipitation, land use types, etc. The ratios of δ¹⁵N-NO₃^- and δ¹⁸O-NO₃ ranged from -4.61‰ to 24.39‰ and from -5.39‰ to 15.18‰, respectively, suggesting the multiple sources of fertilizer, soil organic nitrogen, and manure sewage. Essentially, slight denitrification was observed in surface water. Based on the SIAR model, in different hydrological periods, fertilizer （34.53%-52.39%） and soil organic nitrogen （26.24%-32.29%） were the major sources of nitrate in the upper basin. Manure sewage （30.10%-37.93%） was the primary source of nitrate in the middle and lower reaches of the Dianbu River Basin, followed by soil organic nitrogen （28.78%-31.21%）, fertilizer （25.89%-28.11%）, and precipitation （4.36%-11.92%）. The ratio of δ¹⁵N-NH₄⁺ in surface water during the wet period ranged from -9.16‰ to 2.48‰, mainly indicating that the sources of ammonium salt were ammonium nitrogen fertilizer and precipitation. This study will provide a useful insight for the water environmental management.