The potential role of carbonate dissolution and cation competition in regulating dissolved organic carbon leaching during wet-dry cycles in Karst soils

Karst soils represent significant carbon sinks, yet their stability is increasingly threatened by intensified wet-dry cycles under climate change. Although Ca 2+ derived from carbonate weathering can stabilize soil organic carbon through cation bridging, the efficacy of this mechanism under conditions of multi-cation competition and varying intensities of Ca 2+ inputs remains poorly understood. To clarify the role of carbonate weathering-derived cations on dissolved organic carbon (DOC) leaching, three sequential leaching experiments (S1–S3) were conducted using a lysimeter device packed with karst soil to model wet-dry cycles. Natural multi-cation karst water was applied via a simulated rainfall apparatus. Leachate was collected at 5-min intervals over 60 min (12 samples per experiment). Results showed that DOC concentrations declined over time by 53.8%–61.9% within 60 min, a pattern potentially linked to the “Birch effect” induced by drying–rewetting. Successive concentration peaks showed a declining trend, potentially reflecting a progressively reduced availability of labile organic matter under repeated leaching. Based on Pearson correlation analysis, calcite saturation index, and ion ratio analysis, three distinct regulatory stages were suggested: during the S1 stage, the relatively high intensity of carbonate dissolution appeared to release substantial Ca 2+ , potentially saturating the soil cation bridging capacity, while adsorbed Na + , K + , and Mg 2+ seemed to displace weakly bound DOC via ion exchange. In S2, as carbonate dissolution appeared to weaken, Ca 2+ release decreased, and the reduced availability of soil Ca 2+ may have led to unsaturated bridging sites, possibly reflecting a transition from Ca 2+ release to retention, while correlations among Na + and K + were decreased. In S3, carbonate dissolution further weakened, Ca 2+ limited, and Na + , K + , and Mg 2+ again drove DOC leaching through ion exchange. Overall, this preliminary study suggests that carbonate dissolution may influence soil Ca 2+ levels and exchange site saturation, possibly affecting the interplay between Ca 2+ cationic bridging and ion exchange competition by Na + , K + , and Mg 2+ in regulating DOC leaching. Site-specific Ca amendment and CO 2 degassing may be worth considering in karst soil carbon management. However, these exploratory findings from a single lysimeter with non-independent sequential leaching and an inferred “Birch effect” require validation through future in situ and long-term studies.