Available phosphorus (AP) is a critical limiting factor for productivity and nutrient cycling in subtropical Chinese fir (Cunninghamia lanceolata) plantations. However, plantation soils are typically acidic and phosphorus deficient, resulting in persistently low AP levels. Biochar, owing to its distinct physicochemical properties, is considered effective in improving soil conditions and nutrient availability. However, how soil AP responds to biochar over time remains poorly understood, particularly whether early increases can be sustained or instead diminish and reverse over time. In this study, a field experiment was conducted in a subtropical 3-year-old Chinese fir plantation in Fujian Province, China. The experiment was established on an acidic red soil (Ultisol) with a sandy loam texture. Corn straw biochar produced at 400–500 °C under oxygen-limited slow pyrolysis was applied once at four rates (0, 0.5, 1, and 2 kg plant−1) to replicated plots. Soil samples from each plot were collected at 6, 12, and 18 months after biochar application and analyzed for soil AP, phosphorus fractions, pH, Fe/Al oxides, and microbial communities. Biochar application significantly increased soil AP concentrations at 6–12 months, with AP increasing by 18–56 % at 6 months and 10–47 % at 12 months relative to the control and showing a clear dose-dependent pattern. However, by 18 months, soil AP in all biochar treatments had declined markedly and decreased by 72–76 % relative to the control, indicating that the initial AP enhancement was not sustained and reversed over time. Machine learning and structural equation modeling applied to identify drivers of AP showed that early increases in AP were driven by elevated soil pH and weakened Fe/Al fixation, whereas later declines were associated with Ca-P precipitation and sustained Fe/Al-P stabilization. Consistent with this shift, Ca-bound P increased by 22–36 %, while Fe-P and Al-P decreased by 17–60 % at 18 months relative to the control. Biochar also reshaped soil bacterial communities and P-cycling functional groups, with Proteobacteria-dominated phosphate-solubilizing assemblages and certain Actinobacteriota showing treatment- and time-dependent shifts that were linked to changes in soil AP dynamics. Overall, biochar effectively alleviates soil phosphorus limitation in subtropical Chinese fir plantations in the short term, but its effect is strongly time dependent and may diminish and reverse over time. Our results highlight soil pH elevation and Ca-P precipitation as the dominant biochar-associated drivers of AP dynamics. Since a single high-dose biochar application, although highly effective in the short term, tends to accelerate fixation processes in the long term, plantation management should consider low-dose repeated biochar applications or integration with organic amendments to prolong the effect and sustain nutrient supply and ecosystem functioning.
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