千岛湖水体氮的垂向分布特征及来源解析

选取千岛湖水深0.2，5，10，20，30和40m处水样进行分析，利用氮氧同位素和稳定同位素模型（SIAR）研究千岛湖水体氮（N）的垂向分布特征，分析水体N的来源并计算各N源的贡献率.结果表明，硝酸盐（NO₃^-）和溶解性有机氮（DON）是千岛湖水体总溶解氮（TDN）的主要形式，分别占溶解态N的57.9%和39.7%.千岛湖水体δ¹⁵N-NO₃^-和δ¹⁸O-NO₃^-的平均值分别为4.5‰和4.3‰.上层水体（0~10m）中，硝化作用和浮游植物的同化作用共同控制水体N的形态组成和氮氧同位素值（δ¹⁵N-NO₃^-和δ¹⁸O-NO₃^-）的变化.中层水体（10~30m）中，硝化作用是主要的生物地球化学过程，使得水体NO₃^-含量增加而δ¹⁸O-NO₃^-值减小.底层水体（30~40m）受到硝化作用、底泥N释放和反硝化作用的共同影响.化肥是千岛湖水体NO₃^-的最主要来源，在S1和S2处的贡献率分别为51.9%和30.6%.新安江上游的农业面源污染使得S1处化肥贡献率远高于S2.土壤N是仅次于化肥的第二大水体NO₃^-来源，在S1和S2处的贡献率分别为17.8%和27.8%.此外，底泥对底层水体NO₃^-的贡献不可忽视.

The vertical distribution of nitrogen and the nitrogen sources in Qiandao Lake

Water samples were collected at the depths of 0.2, 5, 10, 20, 30 and 40m in Qiandao Lake. Concentrations of nitrogenous species and dual isotopes of nitrate (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) were analyzed to identify the vertical distribution of nitrogen and the main nitrate sources. The results showed that nitrate (NO₃^-) and dissolved organic nitrogen (DON) were the major nitrogenous species, accounting for 57.9% and 39.7% of total dissolved nitrogen, respectively. The values of δ¹⁵N-NO₃^- ranged from 3.8‰ to 5.8‰ with a mean of 4.5‰, and the values of δ¹⁸O-NO₃^- varied from 2.1‰ to 5.9‰, with a mean of 4.3‰ in Qiandao Lake. In the upper water (0~10m), nitrification and assimilation of phytoplankton were the main biogeochemical processes. In the middle water (10~30m), nitrification was the dominant biogeochemical process with the increasing concentrations of nitrate and the decrease of δ¹⁸O-NO₃^- values. The bottom water (30~40m) was affected by nitrification, nitrogen release from sediment and denitrification. The contributions of external sources (precipitation, sewage/manure, nitrogen fertilizer and soil nitrogen) and endogenous nitrogen source (sediment) were calculated by SIAR. It was showed that nitrogen fertilizer was the most important nitrate source in Qiandao Lake, accounting for 51.9% in S1 and 30.6% in S2. The agricultural non-point source pollution from Xinan River resulted in the higher contribution of nitrogen fertilizer in S1than that in S2. The effect of soil nitrogen was also significant for the NO₃^- in Qiandao Lake, contributing 17.8% in S1and 27.8% in S2. There was more serious soil erosion in S2in central lake which is surrounded by mountains with more cultivated land on hillside. In addition, the results suggested that the endogenous nitrogen source (sediment) can't be neglected in the bottom water of Qiandao Lake.