Short-term thinning alters labile soil phosphorus fractions and is associated with microbial alpha diversity and phosphatase activity in subtropical conifer plantations.

Introduction: Plantations play important roles in ecosystem functions but long-term high-density management can limit nutrient cycling. Phosphorus is a key limiting nutrient in subtropical forest soils due to strong fixation by Fe/Al oxides. The Hedley fractionation method provides detailed insight into soil P pools, especially labile and moderately labile fractions. Soil microorganisms and phosphatase enzymes are important regulators of P transformation. However, subtropical introduced conifers + paired first-year responses + Hedley fractions + microbial/enzymatic associations. This study investigates thinning effects on P fractions, microbial diversity, and phosphatase activity in two conifer plantations. Methods: Study was conducted in Lushan National Nature Reserve, Jiangxi Province, China. Japanese cedar and Hinoki cypress plantations were used. Treatments included control, light, moderate, and heavy thinning. Soil samples were collected from 0-10 cm and 10-20 cm layers before and one year after thinning. Soil P fractions were measured using Hedley sequential extraction. Microbial diversity was analyzed using 16S rDNA and ITS sequencing. Phosphatase activity was measured using p-nitrophenyl phosphate method. Statistical analyses included ANOVA and redundancy analysis (RDA). Results: Thinning significantly increased labile P and moderately labile P fractions, especially in surface soils. Thinning-related changes were observed in both Japanese cedar and Hinoki cypress plantations, although their magnitudes varied among thinning treatments. Microbial alpha diversity and acid phosphatase activity generally increased under thinning, although responses varied among treatments. RDA showed strong coupling between labile P fractions, microbial diversity, and phosphatase activity, while stable P fractions remained largely unchanged. Discussion: Thinning-related increases in labile P fractions were associated with changes in microbial alpha diversity and phosphatase activity, indicating that microbial and enzymatic indicators were associated with thinning-related changes in soil P fractions. Increases in labile and moderately labile P fractions reflect changes in operationally defined active P pools rather than detectable changes in total P. The observed associations are consistent with the known roles of microbial communities in decomposition and of acid phosphatase in potential organic P mineralization. Species-specific differences indicate that stand structure and litter quality regulate the magnitude of thinning effects. Overall, the coordinated responses of microbial diversity, phosphatase activity, and active P fractions are consistent with a potential microbial-enzyme-P coupling mechanism.