Dynamics of CO₂ in a karst catchment in the southwestern plateau, China

Inland waters, including rivers and lakes, are increasingly recognized as playing significant roles in the transport, mineralization and burial of organic carbon exported from land. However, in many areas, dissolved inorganic carbon (DIC) dominates the carbon export from catchments. Owing to different production processes and turnover times of organic versus inorganic carbon, CO₂ emitted from rivers and lakes may have different impacts on global carbon cycling depending on its origin. Here, pCO₂ and dissolved oxygen concentrations were determined, and the ratios of excess CO₂ to O₂ depletion (ΔCO₂/ΔO₂) were compared in spring water, river water and lake water in a carbonate catchment located in the southwestern plateau region of China. Results show that groundwater CO₂ evasion, at 2.0 g C m⁻²year⁻¹, is insignificant in terms of terrestrial carbon loss compared with soil CO₂ emission. In the rivers, calcite precipitation due to oversaturation is an important mechanism for CO₂ production in some seasons. In the lake, HCO₃ ⁻ contributed approximately 75 % of the total carbon supply to organic matter production and calcite deposition during seasons favoring photosynthesis. The seasons which had high ΔCO₂/ΔO₂ are the main periods of CO₂ emission from the lake, and the extra CO₂ may be produced from HCO₃ ⁻ titration by H⁺. Thus, lake CO₂ evasion was controlled primarily by pH, not respiration. The spring, river, and lake waters mainly process DIC exported from the catchment, of which HCO₃ ⁻ is primarily derived from carbonate weathering by soil CO₂ that, with extraordinarily high ΔCO₂/ΔO₂, may originate from sources including organic matter decomposition, root respiration (autotrophic), and acid dissolution. Therefore, freshwater CO₂ emission is a return pathway of catchment soil CO₂ to the atmosphere more than that of net primary production and net ecosystem production.