Characteristics and influencing factors of soil inorganic phosphorus across the interfaces of sub-alpine forest–alpine meadow ecosystems at Minjiang headwaters

Soil inorganic phosphorus is a key source of available phosphorus in alpine ecosystems. It is critical to study the dynamics and factors influencing soil inorganic phosphorus fractions across the interface between sub-alpine forest and alpine grassland ecosystem. This is useful to understand their biogeochemical cycle process. We analyzed the seasonal dynamics (the early, peak, and end growth stages) and spatial patterns of inorganic phosphorus fractions and the main factors influencing the soils at different depths (0－15 and 15－30 cm), across the interface between the sub-alpine forest and alpine grassland ecosystem at Kaka and Doujitai Mountains, respectively, of the Min River headwater region. The study was based on the Hedley phosphorus fraction method modified by Tiessen. We found that the content of soil inorganic phosphorus fractions was significantly influenced by growth periods, vegetation, and their interaction (P < 0.05). Soil labile inorganic phosphorus (NaHCO₃-P_i) content was the lowest in the early growing season at Doujitai Mountain (P < 0.05). Labile inorganic phosphorus content was the highest at the end of growing season in the alpine meadows (56.0 mg·kg⁻¹), tree species line (68.4 mg·kg⁻¹), and closed forest (65.7 mg·kg⁻¹) of Kaka Mountain (P < 0.05). Moderately labile inorganic phosphorus (NaOH-P_i) content reached the highest at the end of the growing season in both alpine interfaces (P < 0.05). NaHCO₃-P_i and HCl-P_i were the main soil inorganic phosphorus forms at the closed forest and tree species line sites, respectively. Moreover, NaOH-P_i and residual inorganic phosphorus (Residual-P) were dominant in the alpine meadow area. Soil organic carbon, total nitrogen, total phosphorus, variable water content, pH, nitrogen and phosphorus stoichiometry ratios, and soil carbon were significantly correlated with the content of soil inorganic phosphorus fractions (P < 0.05). The study results indicate that the growth period, vegetation type, and soil physical and chemical properties correlate significantly with a change in inorganic phosphorus fractions in soil.