基于氮氧同位素示踪的西南低山丘陵农业小流域硝酸盐来源解析OACHSSCD

To accurately identify the sources and transformation characteristics of surface water nitrates in the Quanmin Reservoir watershed,126 samples from four major rivers in the watershed were collected over three consecutive years during both the wet and dry seasons.The nitrogen indicators of the water and the nitrogen and oxygen isotopic composition of nitrates(δ15 N-NO3-and δ18 O-NO3-)were measured.These data were integrated with the Bayesian isotope mixing model(MixSIAR)to quantitatively apportion the contributions of different nitrate sources across the watershed.The results indicated:① The average concentrations of TN,NO3--N,NH4+-N and the mean values of δ15 N and δ18 O in the Quanmin Reservoir watershed showed higher values in the dry season than in the wet season,indicating that the nitrogen pollution problem in the watershed was severe,and the pollution level was even higher during the dry season.NO3--N was the main component of DIN in the watershed.Due to the application of chemical fertilizers and manure and domestic sewage,the concentrations of NO3--N and TN,as well as the δ15 N value in Xiongjiagou River,showed the highest average values.In contrast,the Guxian River,influenced by the effluent from sewage treatment plants,presented a relatively high concentration of NH4+-N.② The transfer and transformation characteristics of nitrate isotopes in the surface water of the watershed were mainly dominated by nitrification,with no significant denitrification occurring.③ Results from the MixSIAR model indicated that overall,in the wet season,the contribution of soil nitrogen to nitrate sources in the watershed's surface water reached 88.6%,and 73.0%in the dry season.For the four rivers in the basin during the wet season,the contribution rates of nitrate sources in surface water from highest to lowest were soil nitrogen＞manure and domestic sewage＞chemical fertilizers＞atmospheric precipitation;in the dry season,the contribution rates of nitrate sources in each river changed significantly,with a decrease in soil nitrogen and an increase in manure and domestic sewage and chemical fertilizers contributions.In the dry season,except for Xiongjiagou River,the other rivers had contribution rates in the order of soil nitrogen＞chemical fertilizers＞manure and domestic sewage＞atmospheric precipitation.The study showed that the contribution rate of soil nitrogen is closely related to the topography and soil characteristics of the Quanmin Reservoir watershed,and agricultural non-point source pollution had a continuous impact on the nitrate levels in surface water.The results can provide a basis for the prevention and control of nitrogen pollution in agricultural watersheds located in hilly areas.