Changes in soil phosphorus fractions following the conversion of Chinese fir plantations to evergreen broad-leaved forests in subtropical China

Forest conversion is a common management practice in forestry, yet the effects of forest conversion on the transformation of phosphorus (P) fractions remain unclear. This work aims at assessing the changes in soil P fractions, including total P (TP), available P (AP), inorganic P (Pi), organic P (Po) and microbial biomass P (MBP) after natural conversion of Chinese fir plantations (CFP) to evergreen broad-leaved forests (EBLF) in subtropical China. Results showed the contents of soil TP and Po were not affected by the forest conversion, but soil AP and MBP concentrations and their proportion significantly increased with forest conversion from CFP to EBLF. The contents of soil P fractions were significantly accumulated in the topsoil (0–20 cm) in the studied forests. Additionally, soil P contents were higher in the winter–spring seasons than in the summer–fall seasons during the period of study. Soil AP content was positively correlated with TP, Po, Pi, labile organophosphorus (LOP), moderate labile organophosphorus (MLOP), moderate resistant organophosphorus (MROP), aluminum–bound P (Al–P), iron–bound P (Fe–P) and MBP contents in EBLF, while the content of soil AP was positively related to Po, LOP, MLOP, Al–P, Fe–P and MBP contents in CFP. Path analysis showed that the most important factor in affecting soil P availability was Po (LOP and HROP) in EBLF stands and was Al–P in CFP stands. Our study suggested that management practices in conserving soil Pi (especially in Al–P) in the CFP and in increasing Po inputs in the EBLF would improve soil P availability in subtropical forests.