Factors controlling the isotope composition of dissolved inorganic carbon in a karst-dominated wetland catchment, Guizhou Province, Southwest China

Abstract

Wetlands contain a large proportion of the world’s carbon, of which dissolved inorganic carbon (DIC) is one of the important fractions in wetland carbon cycle. We collected groundwater, river water, and wetland water from the Caohai catchment to investigate the isotope composition of DIC and inorganic carbon cycling. The main sources of groundwater DIC were carbonate dissolution and soil CO2. Groundwater was the main source of riverine DIC, but the δ13CDIC value in river water showed a more positive excursion than groundwater because of phytoplankton photosynthesis and CO2 evasion. Seasonal variations in DIC concentrations and the δ13CDIC indicate the influence of local rainfall events in the high-flow season and the longer residence time of water in the low-flow season. There was marked seasonal variation in the pCO2 and δ13CDIC in wetland water. During the high-flow season, the pCO2 in the emergent plant zone was higher than that of the atmosphere because of respiration of roots and sediments. Because of photosynthesis, the pCO2 in the submerged plant zone was lower than that of the atmosphere. Furthermore, in this zone, the δ13CDIC ranged from −15.13 to −6.68 ‰ (mean value of −10.79 ‰), and may have been influenced by chemically enhanced fractionation and photosynthesis. In the low-flow season, the pCO2 was mostly higher than atmospheric pCO2 because of organic matter decomposition in sediments. Additionally, during this season, the δ13CDIC ranged from −6.23 to −0.06 ‰ (mean value of −2.52 ‰), and may have been influenced by CO2 evasion, CO2 reduction to methane, and phytoplankton photosynthesis.