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

• DOI: 10.1016/j.scitotenv.2020.138255
• Publication Date: 2020-03-29T03:06:23Z
• Authors: Yuanzhu Zhang; Yongjun Jiang; Daoxian Yuan; Jian Cui; Yong Li; John Yang; Min Cao
• Keywords: Carbon fibers

Anthropogenic disturbance has become an accelerator of dissolved inorganic carbon flux (FDIC), but the magnitude by which FDIC 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 (δ13CDIC, δ34SSO4, δ18OSO4, δ15NNO3 and δ18ONO3) were measured. The results showed that the urban catchment exported 70% more FDIC than its forest counterpart, with considerable differences in the concentration and isotopic composition of DIC, SO42- and NO3- between the two subterranean streams. Based on the contribution by various SO42- or NO3- sources estimated with a Bayesian isotope mixing model, carbonate weathering by H2CO3, H2SO4 and HNO3 contributed 67.5%, 26.0% and 6.5% of FDIC in the urban catchment and 93.3%, 3.4% and 3.3% in the forest catchment, respectively. Anthropogenic activities enhanced the FDIC by 24.0% in the urban catchment and 5.8% in the forest catchment. Anthropogenically enhanced DIC flux (FDIC-A) primarily arose from carbonate dissolution due to acid precipitation (especially S deposition), which accounted for approximately 64% of the FDIC-A in the two catchments. Sewage (including manure) and fertilizer applications also enhanced carbonate dissolution through nitrification processes. Furthermore, FDIC-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.