Source and flux of anthropogenically enhanced dissolved inorganic carbon: A comparative study of urban and forest karst catchments in Southwest China

Anthropogenic disturbance has become an accelerator of dissolved inorganic carbon flux (F DIC ), but the magnitude by which F DIC is enhanced by various anthropogenic activities, especially urbanization, remains largely unknown. In this study, rainfall, discharge, pH, and specific conductivity were continuously monitored during a one-year period in two adjacent karst catchments affected by anthropogenic activities in Southwest China. Groundwater, rainwater, and untreated sewage were collected monthly, and their hydrogeochemical characteristics and isotopic compositions (δ 13 C DIC, δ 34 S SO4, δ 18 O SO4, δ 15 N NO3 and δ 18 O NO3 ) were measured. The results showed that the urban catchment exported 70% more F DIC than its forest counterpart, with considerable differences in the concentration and isotopic composition of DIC, SO 4 2− and NO 3 − between the two subterranean streams. Based on the contribution by various SO 4 2− or NO 3 − sources estimated with a Bayesian isotope mixing model, carbonate weathering by H 2 CO 3, H 2 SO 4 and HNO 3 contributed 67.5%, 26.0% and 6.5% of F DIC in the urban catchment and 93.3%, 3.4% and 3.3% in the forest catchment, respectively. Anthropogenic activities enhanced the F DIC by 24.0% in the urban catchment and 5.8% in the forest catchment. Anthropogenically enhanced DIC flux (F DIC-A ) primarily arose from carbonate dissolution due to acid precipitation (especially S deposition), which accounted for approximately 64% of the F DIC-A in the two catchments. Sewage (including manure) and fertilizer applications also enhanced carbonate dissolution through nitrification processes. Furthermore, F DIC-A was much higher (>83%) during the wet and warm season (May to October) in both catchments. The results of this study suggest that anthropogenic impacts on carbonate dissolution should be considered in global carbon cycle assessments, especially in urban areas.