Effects of Land Use on Riverine Dissolved Inorganic Carbon (DIC) and δ13CDIC in a Karst River Basin, Southwestern China

Abstract

As an important part of the riverine carbon cycle, dissolved inorganic carbon (DIC) has attracted continuous attention. The stable carbon isotope (δ13CDIC) of DIC can reflect its sources and transformations. However, the effects of land use on DIC and δ13CDIC are rarely investigated. To study the influencing factors of DIC and δ13CDIC, river water samples were collected and analyzed from the Chishui River basin, a typical karst river basin in southwestern China. The upper reaches of the Chishui River basin were predominantly underlain by carbonate sedimentary rocks, whereas the lower reaches contained mostly siliciclastic sedimentary rocks, and the forest coverage in the lower reaches was relatively high. The elemental compositions of the river water revealed that the hydrochemistry of rivers in the upper reaches of the basin was mainly controlled by carbonate weathering, whereas the hydrochemistry of some tributaries in the lower reaches was mainly affected by silicate weathering. During the wet season, the average values of c (DIC) and the δ13CDIC value were (1940±493) μmol·L-1 and (-9±1) ‰, respectively, whereas the c (DIC) was relatively high, and the δ13CDIC value was more positive in the dry season, with the average values of (2334±626) μmol·L-1 and (-7.3±1) ‰, respectively. The DIC of most samples was mainly controlled by carbonate weathering under an open system and was derived from carbonate minerals and atmospheric and soil CO2. The seasonal variations in the c (DIC) and δ13CDIC values suggested that c (DIC) was regulated by climate, hydrology, and biology. The increasing contribution of biological carbon to DIC in summer was the main cause of the more negative δ13CDIC value in the wet season, whereas the dilution effect of higher discharge was the main cause of the low c (DIC) in the wet season. The samples from three tributaries in the lower reaches with the highest proportion of silicate and forest distribution had the highest dissolved organic carbon concentration[c (DOC)], the lowest c (DIC), and the most negative δ13CDIC value in the wet season. The proportion of carbonate distribution had positive correlations with c (DIC) in the wet and dry seasons, indicating that lithology was the main controlling factor of c (DIC). The rivers draining the carbonate areas had a lower c (DIC) and a more negative δ13CDIC value in the wet season than those in the dry season, whereas for the rivers draining non-carbonate areas with high forest coverage, the c (DIC) was higher and the δ13CDIC value was significantly more negative in the wet season than those in the dry season. This implies that c (DIC) and δ13CDIC are significantly affected by land use when they are less affected by lithology.