Hydrologically-driven variations in the karst-related carbon sink fluxes: Insights from high-resolution monitoring of three karst catchments in Southwest China

Rainfall (P), discharge (Q), water temperature, pH, and specific electrical conductivity (EC) of the three karst catchments, Banzhai, Huanghou and Houzhai, with different land uses and carbonate lithologies but similar subtropical monsoonal climate, in Guizhou Province, Southwest China, were monitored with CTDP 300 high-resolution multi-parameter data loggers during the period of May 2007-October 2013. In addition, HCO3- and calcium concentrations were titrated in the field and the other major ions determined in the laboratory once every one or two months. Simple linear regression models were used to link the continuous chemical data to in situ measured data to estimate the concentration of HCO3-, the CO2 partial pressures and the calcite saturation indices on the high-resolution logger data. Continuous karst-related carbon sink fluxes (CSFs) were also estimated with the continuous Q and HCO3- concentrations in each catchment area. The primary goal of this study is to understand how discharge and HCO3- concentration determine the CSFs at the storm scale, seasonal scale and annual scale, and to estimate the CSFs for the three studied catchments. Results show that the variation in runoff (river discharge) played a more important role in controlling the CSFs than the variation in HCO3- for all the karst catchments, because of the chemostatic behavior of HCO3- in the catchments. Soil coverage, vegetation and bedrock lithology determined the CSFs by controlling the proportion of precipitation that recharges groundwater (and thus Q), and controlling the soil CO2 productivity (and thus HCO3-). The average CSFs in the Banzhai, Huanghou and Houzhai karst catchments were 29 +/- 3, 33 +/- 5 and 39 +/- 8 t-CO2 km(-2) a(-1), respectively, which are 15 times higher than those CSFs by silicate weathering in the silicate-rock catchments with similar hydrology, showing the dominant role of carbonate weathering in the rock-weathering-related carbon sink. The interannual change in CSFs was large (1.7-2.5-fold for this study), depending mainly on the variations of catchment river discharge (e.g. high flow year vs low flow year). This suggests that the conventional view that pre- and post-anthropogenic riverine carbon fluxes are equal is problematic, in that large changes in river hydrology caused by global climate and land use and land cover changes have already been documented. (C) 2015 Elsevier B.V. All rights reserved.