Soil phosphorus fractions and their availability over natural succession from clear-cut of a mixed broadleaved and Korean pine forest in northeast China

To assess phosphorus (P) status of forest soil under naturally restored vegetation, P fractions in the 10-cm soil layer were quantified at different successional stages on the clear-cut site of mixed broadleaved and Korean pine forest. Four communities of shrub, softwood broad-leaved forest, softwood and hardwood broad-leaved forest, and hardwood broad-leaved forest represented different successional stages. A soil sample from a primary broad-leaved and Korean pine stand was the control. A sequential P fractionation scheme extracted empirically defined pools of P and path analysis used to partition the direct and indirect contribution of soil P fractions to available P. The results show that available P increased significantly with long-term succession, while both sodium bicarbonate-extractable P (NaHCO 3 -P) and sodium hydroxide-extractable P (NaOH-P) fractions were reduced in early successional stages and increased in late stages. Compared to the primary forest, concentrations of P fractions in the four stages significantly decreased except for HCl-P, indicating that soil P supplements over the long-term did not return to primary forest levels. The results of related analysis also showed that NaHCO 3 -P i levels were significantly related to available phosphorus. According to the path analysis coefficient, NaHCO 3 -P i exhibited the highest effect on available P among eight P fractions; the indirect effects of other P fractions via NaHCO 3 -P i were larger than those with other P fractions. Overall, this study suggests that soil P bioavailability gradually improved during natural vegetation restoration on clear-cut sites mainly through the increase of NaHCO 3 -P, where phosphorous is immediately available, and subsequently available phosphorus NaOH-P.