Tree Species Configuration Shapes Soil Phosphorus Fractions and Microbial Communities in Uneven‐Aged Multilayered Chinese Fir–Broadleaved Mixed Forests

ABSTRACT Chinese fir ( Cunninghamia lanceolata ) plantations in southern China are often limited by phosphorus (P) availability, constraining productivity and sustainable management. We hypothesized that multi‐species, multilayered, uneven‐aged configurations would enhance soil P availability by regulating P fractions and microbial P transformation. This study aimed to identify the optimal tree species configuration for alleviating P limitation and improving soil quality. Four configurations were compared: pure Chinese fir (M0), Chinese fir + Phoebe bournei (M1), Chinese fir + P. bournei + Taxus wallichiana var. chinensis (M2), and Chinese fir + P. bournei + T. wallichiana var. chinensis + Schima superba (M3). Soil samples were collected from the 0–20, 20–40 and 40–60 cm layers and analysed for P fractions, acid phosphatase (ACP) activity, soil properties, and bacterial and fungal communities. Multivariate analysis, network analysis, structural equation modelling (SEM), and soil quality index (SQI) analysis were used to identify regulatory pathways. Different tree species configurations significantly altered soil P pool composition and its vertical distribution. Mixed configurations increased organic phosphorus (Po) and available phosphorus (AP), mainly enhanced surface soil organic matter (SOM) and total nitrogen (TN), and reshaped bacterial and fungal communities. M1 was enriched in P‐activating fungi such as Penicillium and Trichoderma , whereas M2 exhibited the highest fungal network complexity and SQI value, as well as higher AP content than the pure plantation. SEM indicated that available Fe (AFe) was an important driver of AP formation, while Al‐bound P (AlP) and occluded P (OP) negatively affected AP. Overall, M2 was the most favorable configuration for improving soil P availability and soil quality.