Phosphorus (P), a macro-nutrient essential for plant growth, is considered the key factor for maintaining the productivity of subtropical forest ecosystems. Understanding the effects of soil factors on the accumulation of phosphorus species after forest conversion is important in developing management practices that sustain and enhance ecosystem functionality. The objectives of the present study were to evaluate the response of soil phosphorus to forest conversion, and elucidate the mechanisms and environmental factors driving changes in soil phosphorus. The study sites included secondary broadleaved forests of Castanopsis carlesii (SF), human-assisted naturally regenerated forests of Castanopsis carlesii (AR), and plantations of Cunninghamia lanceolata (CF), which were converted from natural forests in the mid-subtropical region of China. Basic physicochemical characteristics, including iron and aluminum oxide content, different phosphorus forms, and acid phosphatase activities, were determined. The results revealed that the soil total P and organic and microbial biomass P levels were significantly higher in AR than in SF or CF. A redundancy analysis indicated that the changes in soil phosphorus were mainly driven by soil moisture, total nitrogen, and amorphous Fe oxides (Feo) in the eluvial horizon, and by acid phosphatase, free Fe (Fed), and total nitrogen in the illuvial horizon. The results suggest that soil biochemical properties and microbial characteristics affect the accumulation of different soil P formations, primarily by changing the soil moisture levels and acid phosphatase activity. In summary, human-assisted naturally regenerated forests converted from natural forests were the most conducive to the storage and supply of nutrients and the sustainable development of ecosystem functionality among the management systems studied. The present study might provide a theoretical basis for assessing forest management practices in mid-subtropical forests.