ABSTRACTAfforestation plays a crucial role in mitigating climate change, yet the transfer of soil phosphorus (P) to aboveground biomass may lead to soil P limitation. However, the influences of afforestation on soil P fractions and availability remain unclear. Here, we investigated soil P fractions at 144 paired sampling sites by comparing afforested lands with adjacent non‐afforested lands in subtropical China. Afforestation increased the proportions of available P, labile P, and moderately‐labile P, while decreasing inorganic P concentrations, with no significant effect on soil total P. These shifts were strongly coupled with the increase in organic P and the decline in stable P. Notably, afforestation with broad‐leaved species (i.e., Quercus variabilis) increased soil organic P concentration, while afforestation with coniferous species (i.e., Pinus massoniana and Platycladus orientalis) reduced soil inorganic P concentration. Afforestation weakened the effects of soil microclimates on labile P fractions (e.g., decreased with moisture) in non‐afforested soil. In contrast, plant nutrients (e.g., leaf P concentration) exhibited positive effects on inorganic P fractions in afforested soils. Soil iron oxides (Feo) were the primary cause of P variations in both afforested and non‐afforested soils; particularly, lower Feo and soil pH under afforested lands likely promoted the desorption of more non‐stable P fractions compared to the non‐afforested soils. Overall, our findings suggest that afforestation may alleviate soil P limitation by enhancing the concentration of labile and moderately‐labile P fractions, while soil inorganic P fractions are crucial for regulating soil P dynamics, thereby providing insights for optimizing afforestation practices.