Rainfall shapes CO₂ emission and dissolved organic matter dynamics in a karst catchment: Controlling factors revealed by isotopic and molecular signatures.

Rainfall events are key hydrological processes that regulate regional water and carbon cycling, particularly in sensitive terrestrial ecosystems such as karst regions, where groundwater recharge and carbon fluxes are tightly coupled. This study investigates the influence of rainfall on carbon dynamics in the Houzhai River catchment, a representative karst catchment with distinctive hydrological connectivity in Southwest China. By integrating accelerator mass spectrometry (AMS) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analyses, our results demonstrate that rainfall substantially enhances the transfer from terrestrial to aquatic systems. Specifically, such a process alters the age structure of the dissolved organic matter (DOM) pool by increasing the proportion of "younger carbon", primarily derived from surface forest soil. This shift contributes to a DOM composition enrichment of more oxygenated and unsaturated compounds. Additionally, such a typical rainfall event reduced the partial pressure of CO₂ (pCO₂) by 8.19 %, corresponding to a decline in CO₂ emission potential equivalent to approximately 624.0 tons of carbon, with the source-sink dynamics regulated by the inherent physicochemical properties and molecular composition of the catchment area. Notably, low-molecular-weight, oxygen-poor, and easily degradable compounds were identified as robust markers to indicate the CO₂ release potential in karst systems. These findings underscore the influence of episodic hydrological processes and provide detailed insights into how rainfall reshapes the carbon cycling in the karst catchment, which is a traditional carbon sink.