中文摘要：

通过分析西安周边4条主要河流（浐河、灞河、涝河、黑河）的溶解无机碳（DIC）浓度和碳同位素组成,初步探讨了西安周边主要河流溶解无机碳（DIC）的浓度变化及碳源.结果表明西安周边主要河流DIC浓度的变化范围为0.34～5.66mmol.L-1,平均为1.23 mmol.L-1,自源头到下游,DIC浓度呈现升高趋势.4条河流δ13CDIC值的变化范围在-13.3‰～-7.2‰之间,平均值约为-10.1‰,4条河流整体表现为δ13CDIC值在源头偏负（平均值约为-12.6‰）,中下游农耕区δ13CDIC值偏正（平均值约为-9.4‰）,靠近入渭河河口的城市区δ13CDIC表现为偏负值（平均值为-10.5‰）.DIC浓度与河流DIC碳同位素组成的变化规律揭示了河流溶解无机碳来源的变化,土壤CO2的输入可能是源头水体DIC的主要来源;中下游农耕区河水δ13CDIC值偏正是由于农业区农作物存在C4植被（如：玉米）,使得农业区土壤CO2和土壤碳酸盐具有偏正的碳同位素组成,进而导致河流水体具有偏正的δ13CDIC值;靠近河口处具有较低δ13C值,污水的大量输入可能导致河水δ13CDIC表现为偏负.结果表明西安周边河流溶解无机碳浓度和同位素组成变化大致指示了河流从源头到下游过程中DIC的可能来源,可为黄土高原小流域河流无机碳来源示踪研究提供参考.

英文摘要：

In this study,the content and isotopic compositions of water dissolved inorganic carbon（DIC） from four typical rivers（Chanhe,Bahe,Laohe and Heihe） around Xi＇an City were studied to trace the possible sources of DIC.The results of this study showed that the content of DIC in the four rivers varied from 0.34 to 5.66 mmol·L-1 with an average value of 1.23 mmol·L-1.In general,the content of DIC increased from the headstream to the river mouth.The δ13 CDIC of four rivers ranged from-13.3‰ to-7.2‰,with an average value of-10.1‰.The δ13 CDIC values of river water were all negative（average value of-12.6‰） at the headstream of four rivers,but the δ13 CDIC values of downstream water were more positive（with an average value of-9.4‰）.In addition,δ13 CDIC of river water showed relatively negative values（the average value of δ13 CDIC was-10.5‰） near the estuary of the rivers.The variation of the DIC content and its carbon isotope suggested that the DIC sources of the rivers varied from the headstream to the river mouth.The negative δ13 CDIC value indicated that the DIC may originate from the soil CO2at the headstream of the rivers.On the other hand,the δ13 CDIC values of river water at the middle and lower reaches of rivers were more positive,and it showed that soil CO2 produced by respiration of the C4 plants（like corn） and soil carbonates with positive δ13 C values may be imported into river water.Meanwhile,the input of pollutants with low δ13 CDIC values may result in a decrease of δ13 CDIC values in the rivers.The study indicated that the DIC content and carbon isotope may be used to trace the sources of DIC in rivers around Xi＇an City.Our study may provide some basic information for tracing the sources of DIC of rivers in the small watershed area in the Loess Plateau of China.