Karst wetland reclamation increases the proportion of inorganic sulfur in total sulfur: associations with sulfate-reducing bacteria abundance and soil enzyme activity

Karst wetlands are fragile and critical ecosystems for soil sulfur biogeochemical cycling. In this study, six typical land-use types in the Huixian karst wetland of Guilin, southern China, were selected to investigate the variations in soil physicochemical properties, four inorganic sulfur fractions, soil enzyme activities, and sulfate-reducing bacteria (SRB) abundance across 0–40 cm soil profiles. The results showed that wetland reclamation significantly reduced soil pH, soil organic carbon (SOC), silt content, total nitrogen, and total sulfur, while increasing soil bulk density, electrical conductivity (EC), and redox potential (Eh). Reclamation reduced the content of total inorganic sulfur, while elevating its proportion in total sulfur; water-soluble sulfur dominated the inorganic sulfur pool. The composition of inorganic sulfur fractions differed markedly across land-use types. Reclamation increased SRB abundance but inhibited soil enzyme activities. Correlation analysis indicated that soil enzyme activities were negatively correlated with inorganic sulfur contents in abandoned land, whereas positive correlations were observed in other land-use types. Close internal correlations existed among different inorganic sulfur fractions. Soil pH and SOC were associated with greater inorganic sulfur accumulation, while Eh showed an inhibitory association. SOC, Eh, EC, pH, and silt content were the key environmental factors associated with SRB abundance and soil enzyme activities. Collectively, wetland reclamation reduced absolute concentrations of both total inorganic sulfur (TIS) and total organic sulfur (TOS), and preferential loss of TOS increased the TIS/TS ratio, altered sulfur transformation processes, and were accompanied by higher SRB abundance and lower soil enzyme activity. This study reveals the responses of soil sulfur biogeochemical processes to land-use conversion in subtropical karst wetlands and provides insights into sulfur cycle regulation under wetland disturbance.