沁河上游硝酸盐的定量源解析及其季节性差异

沁河流域人为源硝酸盐输入增加了河流富营养化的风险.为识别沁河上游硝酸盐来源，分别于2017年10月、2018年3月和2018年6月在沁河上游采集了28个河流表层水样，联合稳定同位素（δ15N-NO₃-和δ18O-NO₃-、δD-H₂O和δ18O-H₂O）和水化学参数（Cl-、NO₃--N、NH₄+-N）评估了沁河上游河流硝酸盐源与关键过程，并基于贝叶斯混合模型量化了不同硝酸盐来源的贡献.结果表明:①沁河上游溶解无机氮以NO₃--N为主，秋季（2.40±1.17）mg/L]高于春季（2.11±1.03）mg/L]和夏季（1.50±0.61）mg/L]；受积雪融化的影响，春季δ18O-NO₃-（13.0‰±3.13‰）显著高于夏季（2.90‰±3.12‰）和秋季（6.62‰±1.30‰）.②SIAR同位素模型结果表明，沁河上游硝酸盐主要来自土壤氮和化肥.在春、夏、秋3个季节，土壤氮的硝化对沁河上游硝酸盐的贡献比例分别为27.8%、39.5%和39.3%，化肥贡献比例分别为29.1%、40.2%和41.9%.春季（24.7%）大气沉降的贡献比例高于夏季（2.4%）和秋季（10.6%）.③沁河上游硝酸盐主要受硝化过程的影响，没有发生明显的反硝化过程.研究显示，春季、夏季和秋季沁河上游硝酸盐主要来自土壤氮和化肥的硝化，硝酸盐污染防治应考虑化肥的使用效率和抑制硝化过程的发生. 

Quantitative Analysis and Seasonal Differences of Nitrate Sources in Upper Reaches of Qin River

The risk of river eutrophication significantly increased in Qin River Basin with increasing nitrate by anthropogenic activities. In order to identify nitrate sources in the upper reaches of Qin River, twenty-eight surface water samples were collected in October 2017, March 2018 and June 2018. The combined use of stable isotopes (δ15N-NO₃- and δ18O-NO₃-, δD-H₂O and δ18O-H₂O) and water chemistry parameters (Cl-, NO₃--N, NH₄+-N) were used to evaluate nitrate sources and transformation processes. The fractional contributions of potential nitrate sources were calculated using a Bayesian mixture model. The results were as follows: (1) Nitrate is the dominant form of dissolved inorganic nitrogen. The average concentration of NO₃--N in autumn ((2.40±1.17) mg/L) was significantly higher than that in spring ((2.11±1.03) mg/L) and summer ((1.50±0.61) mg/L). δ18O-NO₃- (13.0‰±3.13‰) values in spring were significantly higher than that in summer (2.90‰±3.12‰) and autumn (6.62‰±1.30‰). (2) Nitrate is mainly from soil nitrogen and chemical fertilizer. According to the results of SIAR stable isotopes model, the fractional contributions from soil nitrogen in spring, summer, and autumn were 27.8%, 39.5% and 39.3% respectively, from fertilizer were 29.1%, 40.2% and 41.9%, respectively. The fractional contribution from atmospheric deposition in spring (24.7%) was significantly higher than that in summer (2.4%) and autumn (10.6%). (3) Nitrate in the upper reaches of Qin River is mainly affected by the nitrification process, and is notinfluenced by denitrification process. The research showed that nitrate in the upper reaches of Qin River is mainly from nitrification of soil organic nitrogen and ammonia in chemical fertilizers. Measures for reducing NO₃- inputs to rivers should be considered by improving effectively utilizing rate of chemical fertilizer and inhibit nitrification.