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

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 δ¹³C_DIC 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 δ¹³C_DIC 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 δ¹³C_DIC 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 δ¹³C_DIC 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 δ¹³C_DIC 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.