The Transport Processes of Carbon and Nitrogen under Various Hydrological Conditions in the Carbonate-rich Areas,Southwest China: Example from the Xijiang River
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 delta~(13)C-depleted soil CO_2 fluxing into the river,induced by high discharge,would shift delta~(13)C_(DIC) to more negative values,whereas HCO_3~- would exhibit stronger chemostatic responses due to fast carbonate dissolution. Mineral weathering,soil CO_2 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,delta~(13)C_(DIC),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 HCO_3~- in the river.