Microbial mechanisms of mixed planting in regulating soil phosphorus availability across different stand ages in Chinese fir plantations.

The mixed planting of Chinese fir with broadleaf species to increase soil phosphorus (P) availability has been widely adopted in subtropical China. As soil P availability is significantly influenced by tree growth, the microbial mechanisms underlying the effects of mixed planting on soil P availability across different stand ages are not fully understood. In this study, we collected soil samples from mixed-species plantations of Chinese fir and Schima superba (MCP) and pure Chinese fir plantations (PCP) at young (5 years), middle-aged (20 years), and mature (32 years) stages in southeastern China. We determined the soil P fractions, organic P (Po) mineralizing ability, and dynamics of the microbial community associated with Po mineralization in the samples. We hypothesized that the influence of mixed planting on soil P availability is modulated by stand age. Compared with the PCP stands, the young and mature MCP stands exhibited significantly greater contents of labile and moderately labile P, with increases of 13.22% and 8.18%, respectively, in the young stands and 22.20% and 30.52%, respectively, in the mature stands. Conversely, the middle-aged MCP stands exhibited lower contents of labile and moderately labile P, with decreases of 20.93% and 18.16%, respectively. The communities of Po-mineralizing fungi (Pmin-F) and bacteria (Pmin-B) changed not only among the different plantation types but also across the various stand ages. The Pmin-F community contributed mainly to labile P, whereas the Pmin-B community was the primary driver of moderately labile P. Additionally, mixed planting mediated labile P availability through soil pH, accounting for 71% of the variation in this P fraction. Conversely, stand age affected the availability of moderately labile P through soil nitrogen availability and the Pmin-F community, explaining 81% of the variation in this P fraction. Overall, we revealed that the impact of mixed planting on soil P availability is modulated by stand age, with fungi and bacteria fulfilling distinct ecological roles in the process. Our results are highly important for maintaining soil P availability for the sustainable management of Chinese fir plantations.