中国西南河流碳、氮运移机制及其对水文条件变化的响应:以西江为例

The Transport Processes of Carbon and Nitrogen under Various Hydrological Conditions in the Carbonate-rich Areas,Southwest China:Example from the Xijiang River

流域碳、氮的生物地球化学循环过程关联着区域/全球尺度的气候变化,同时受到气候条件变化的影响。本次研究以我国西南喀斯特地区的西江为例,通过时间序列的高频次样品采集和分析,揭示水文条件变化对河流碳、氮动态变化的影响及其控制机制。结果表明,在高流量条件下,化学风化的加速、土壤CO2的汇入及流域内有机质的降解导致了水体δ13CDIC偏负,碳酸盐矿物的快速风化致使河水HCO3-浓度表现出强烈的"化学稳定性";对DIC、δ13CDIC、流量、温度的联合分析表明,矿物溶解、土壤CO2的汇入与河流中有机质降解的共同作用导致了西江河水碳动态的季节变化;河水NO3-浓度在高流量条件下并未表现出明显的稀释效应,反应了人为输入和化学转化过程对NO3-浓度的影响;硝酸盐氮、氧同位素的分析结果表明,西江NO3-来源受人为活动影响较大,主要转化过程为硝化作用;同位素和水化学证据表明碳酸、硫酸、硝酸共同参与流域岩石化学风化,雨季外源酸参与风化的减弱是河流中HCO3-稀释的重要原因。

The biogeochemical cycle of carbon/nitrogen in the catchment are related with the climate change on regional/global scale and are also affected by changes in climatic conditions. In the karst area of Southwest China,high-frequency sampling campaigns and analyses were conducted to elucidate carbon/nitrogen dynamics under various hydrological processes in the Xijiang River. The main conclusions are listed below. Higher contributions of δ13 C-depleted soil CO2 fluxing into the river,induced by high discharge,would shift δ13 CDICto more negative values,whereas HCO3- would exhibit stronger chemostatic responses due to fast carbonate dissolution. Mineral weathering,soil CO2 influx,and organic matter degradation in the river are responsible for the carbon temporal dynamics in the Xijiang River through the combined analyses of DIC,δ13 CDIC,discharge and water temperature. Dilution effects were not observed for NO-3 with high-discharge conditions,indicating the attribution of anthropogenic activities and transformation. Carbonic acid,sulfuric acid and nitric acid participate in the chemical weathering of the catchment. The ions produced by the weathering of carbonate rock show obvious chemostatic responses for the increase in discharge,while the decrease in exogenous acid involved in weathering play an important role in the dilution of HCO3- in the river.