Abstract: Phosphorus is a limiting nutrient in forest ecosystems. Little attention has been paid to the effects of gap size on soil phosphorus fractions. In particular, the variation characteristics of phosphorus fractions at different positions in different size gaps are not clear. In this study, we carried out an experiment in a 50-year-old Platycladus orientalisplantation in Zhaotuan Forest Farm, Xuzhou, after opening forest gaps for four years. We examined the changes of soil phosphorus fractions in different positions (D₁ in the gap center, D₂ in the edge of the gap, D₃ in the understory, D₄ in the understory) under control, small, medium and large gaps (radius R was 4, 8 and 12 m, respectively). The results showed that: (1) The effects of different gap sizes on the concentrations of total phosphorus, NaHCO₃-Po, NaOH-Pi, NaOH-Po, HCl-Pi and Residual-P were in the order of medium gap > large and small gaps. The effects of gap size on the NaHCO₃-Pi concentration were in the order of the medium and large gaps > small gap, and the effects on the H₂O-Pi concentration were in the order of small gap > big gap > medium gap. Soil NaHCO₃-Po concentration significantly differed among positions of forest gap (P<0.05), and showed a single peak trend of “low-high-low” at different positions of small and medium gap. (2) Compared with the control, soil pH decreased and water content increased after four years of gap formation. Soil total nitrogen and organic carbon concentration increased in the small gap, but decreased in the medium and large gap. The effects of different gap sizes on microbial biomass phosphorus and alkaline phosphatase activity were in the order of medium gap > large and small gap. (3) Soil water content, organic carbon, microbial biomass phosphorus and alkaline phosphatase activity were important factors affecting soil phosphorus fractions. In conclusion, the medium gap is beneficial to improve soil phosphorus availability. Our results can provide a scientific basis for the determination of reasonable gap size of P. orientalis plantations, and for improving soil phosphorus deficiency, forest productivity, and ecosystem stability.

Key words: gap size, soil phosphorus fraction, alkaline phosphatase, microbial biomass phosphorus, physicochemical properties.