Relationship between soil organic carbon fractions and microbial nutrient limitations among different woodlands in the western karst region of Hubei

In karst regions, soil organic carbon (SOC) stability and microbial activity are vital for ecosystem function, yet their response to nutrient availability remains unclear. This study investigated SOC fractions and microbial nutrient limitations in natural mixed woodland (NW), cypress woodland (CW), and stone dike artificial woodland (SW) in the Xialaoxi watershed, Hubei. Seasonal variations in labile organic carbon (LOC), microbial biomass carbon (MBC), particulate organic carbon (POC), mineral-associated organic carbon (MAOC), and enzyme activities were analyzed. The results showed that SOC levels were mainly controlled by soil microbial activity during spring and summer, whereas plant-derived organic carbon sources were the main contributors during fall and winter. NW and CW exhibited higher active organic carbon and microbial activity than SW, where SOC was predominantly mineral-bound. Deciduous broadleaved woodlands (NW) demonstrated higher carbon and nitrogen enzyme activities and larger vector angles (VA), facilitating soil carbon sequestration. Soil microbial growth was phosphorus-limited across woodlands, with microbial nutrient limitation positively correlated with SOC fractions and amino sugar content, enhancing organic carbon sequestration. Large- and medium-aggregate LAP activity and microaggregate NAG activity were key factors influencing soil aggregate SOC. PLS pathway analysis revealed woodland type affected the total nutrients, thereby altering the contents of SOC fractions, promoting the transformation activities of enzymes and ultimately impacting microbial nutrient limitation. These findings provide insights into carbon sequestration mechanisms and offer guidance for soil nutrient management and ecosystem sustainability in karst woodlands.