Mineral phosphate precipitation is often recognized to limit the availability of phosphorus in acid soils of tropical and subtropical forests. We studied extractable phosphorus pools and transformation rates in soils of a tropical evergreen forest at Xishuangbanna and a subtropical montane wet forest at the Ailao Mountains in order to understand biogeochemical processes that regulate phosphorus availability in acid soils. The two forests differ in forest floor-mass with a deep humus layer in the Ailao forest and little presence of humus materials in the Xishuangbanna forest. Resin and sodium-bicarbonate extractable phosphorus pools decreased when soil organic carbon content was low. The lowest levels of extractable P pools occurred in surface (0～10 cm) mineral soils of the Xishuangbanna forest. However, microbial P in the mineral soil of the Xishuangbanna forest was twice that in the Ailao forest. Potential rates of microbial P immobilization were greater than those of organic P mineralization in mineral soils for both forests. Our data suggest that microbial P immobilization plays an essential role in avoiding mineral P precipitation and retain plant available P in tropical acid soils, whereas both floor mass accumulation and microbial P immobilization function to retain plant available P in subtropical montane wet forests.