氮和氧同位素示踪伊洛河河水硝酸盐来源及转化过程

Tracking Riverine Nitrate Sources and Transformations in the Yiluo River Basin by Nitrogen and Oxygen Isotopes

河流硝酸盐(NO_(3)^(-))浓度及氮和氧同位素组成(δ^(15)N-NO_(3)^(-)和δ^(18)O-NO_(3)^(-))可以辨识河水NO_(3)^(-)受自然过程和人为输入的影响,但流域不同土地利用方式对河水NO_(3)^(-)来源及转化过程的影响尚不明确,特别是山区人为输入对河水NO_(3)^(-)的影响仍不清楚.选择土地利用空间异质性显著的伊河和洛河作为研究对象,借助水体水化学组成,氢氧同位素(δD-H_(2)O和δ^(18)O-H_(2)O)、δ^(15)N-NO_(3)^(-)和δ^(18)O-NO_(3)^(-),辨识不同土地利用方式影响下河水NO_(3)^(-)来源及转化途径.结果表明:(1)伊河与洛河河水ρ(NO_(3)^(-))均值分别6.57和9.29 mg·L^(-1),δ^(15)N-NO_(3)^(-)均值分别为9.6‰和10.4‰,δ^(18)O-NO_(3)^(-)均值分别为-2.2‰和-2.7‰;结合δ^(15)N-NO_(3)^(-)和δ^(18)O-NO_(3)^(-)分析,伊河和洛河水中NO_(3)^(-)主要受多来源混合控制,同时洛河存在脱氮作用,但伊河生物去除作用较弱.(2)综合考虑干流和支流河水δ^(15)N-NO_(3)^(-)和δ^(18)O-NO_(3)^(-)空间分布,结合贝叶斯同位素混合模型,计算洛河和伊河河水硝酸盐来源,发现洛河上游和伊河上游森林植被广泛,但生活污水和粪肥影响较大;上游土壤有机氮和化学肥料贡献比例大于下游;下游生活污水和粪肥贡献继续增加.研究结果证实点源污染(城镇生活污水和粪肥)对伊洛河流域河水NO_(3)^(-)来源的影响,同时面源污染(化肥)贡献并未随着下游农业活动增加而升高.因此需要重视伊洛河流域内点源污染的有效治理,保障黄河流域生态文明高质量发展.

The impacts of natural processes and anthropogenic input on riverine nitrate(NO_(3)^(-))could be identified by NO_(3)^(-) concentrations and nitrogen and oxygen isotope ratios(δ^(15)N-NO_(3)^(-) and δ^(18)O-NO_(3)^(-));however,the effects of variable land use on riverine NO_(3)^(-) sources and transformations remain unclear.In particular,the human impacts on riverine NO_(3)^(-) in mountain areas are still unknown.The Yihe River and Luohe River were used to elucidate this question due to their spatially heterogeneous land use.Hydrochemical compositions,water isotope ratios(δD-H_2O andδ^(18)O-H_(2)O),and δ^(15)N-NO_(3)^(-) and δ^(18)O-NO_(3)^(-) values were utilized to constrain the NO_(3)^(-) sources and transformations affected by different land use types.The results indicated that(1)the mean nitrate concentrations in the Yihe River and Luohe River waters were 6.57 and 9.29 mg·L^(-1),the mean values of δ^(15)N-NO_(3)^(-) were 9.6‰ and 10.4‰,and the average δ^(18)O-NO_(3)^(-) values were -2.2‰ and -2.7‰,respectively.Based on the analysis of δ^(15)N-NO_(3)^(-) and δ^(18)O-NO_(3)^(-) values,the NO_(3)^(-) in the Yihe and Luohe Rivers were derived from multiple sources,and nitrogen removal existed in the Luohe River,but the biological removal in the Yihe River was weak.(2)The contributions of different nitrate sources were calculated using a Bayesian isotope mixing model(BIMM)based on δ^(15)N-NO_(3)^(-) and δ^(18)O-NO_(3)^(-) values of river water in the mainstream and tributaries with spatial locations.The results revealed that sewage and manure had major impacts on riverine nitrate in the upper reaches of both the Luohe River and Yihe River,where forest vegetation was widely distributed.However,the contributions from soil organic nitrogen and chemical fertilizer were higher in the upper reaches than in downstream ones.The contributions of sewage and manure still increased in the downstream reaches.Our results confirmed the primary impacts of point sources,e.g.,sewage and manure,on riverine nitrate in the studied area,and the contributions of nonpoint sources,e.g.,chemical fertilizer,had not increased as the agricultural activities elevated the downstream.Therefore,more attention should be paid to point source pollution treatment,and the high-quality development of ecological civilization in the Yellow River Basin should be maintained.