CO₂ influx and efflux circadian cycles in bare dry lake sediments

Abstract. Permanent and temporary drying of inland waters expands the surface area of exposed sediments, potentially altering total ecosystem CO₂ fluxes. However, the response of CO₂ fluxes from dry sediments to episodic rewetting, and the relative roles of biotic and abiotic exchange mechanisms in shaping net fluxes, remain poorly understood. Here, we conducted a field rewetting experiment on long-term exposed sediments of Lake Gallocanta (Spain) to assess rewetting-induced variations in CO₂ fluxes. CO₂ exchanges between sediments and the atmosphere were measured before and after rewetting using closed gas chambers, and the isotopic composition of emitted CO₂ (δ¹³C-CO₂) was analyzed. Sediment samples were also collected to characterize key physicochemical properties (e.g., water activity, temperature, pH) and to assess changes in microbial community composition. In addition, a numerical model integrating gas flux and isotope data was developed to disentangle biotic and abiotic contributions to total CO₂ emissions. Contrary to expectations, rewetting had minimal influence on CO₂ flux magnitudes, which instead exhibited a pronounced circadian pattern of efflux and influx. Although rewetting substantially altered microbial community structure, these changes did not indicate that biological carbon fixation explained the observed CO₂ influx. Model simulations incorporating both biotic and abiotic processes indicated that abiotic mechanisms, most likely carbonate weathering, dominated CO₂ influx, whereas aerobic respiration accounted for midday efflux. Overall, our results highlight the critical role of abiotic processes in regulating CO₂ dynamics from long-term desiccated lake sediments and suggest that net CO₂ fluxes may not reliably indicate organic carbon remobilization under dry lake conditions.