Distribution of Phosphorus Forms Along the Altitude Gradient in the Soil of the Qinghai–Tibetan Plateau and the Influencing Factors

Phosphorus (P) is a key limiting nutrient in alpine meadows. Analyzing the spatial distribution of soil P and its forms along altitudinal gradients is crucial to understand soil nutrient cycling and sustain productivity under climate change. In this study, changes in the total P, available P, inorganic P (Pi), and organic P (Po) contents in soil along an altitudinal gradient of 4400–5200 m on the Qinghai–Tibetan Plateau were investigated using sequential chemical fractionation and solution 31P nuclear magnetic resonance (NMR). The results showed that the contents of total soil P, available P, Pi, and Po forms showed vertical distribution patterns. At an altitude of 4400–4950 m, the dominance of NaOH-Po was observed, whereas HCl-Pi was predominant at 5200 m. With increasing elevation, total soil P, orthophosphate, NaHCO3-Pi, NaOH-Pi, HCl-Pi, and HCl-Po contents increased gradually. In contrast, the concentrations of available P, H2O-Pi, H2O-Po, NaHCO3-Po, NaOH-Po, pyrophosphate, orthophosphate monoester, and diester initially increased, peaked at approximately 4950 m, and subsequently decreased. Both climatic factors (i.e., mean annual temperature and precipitation) and biological factors (aboveground biomass and enzyme activity) jointly regulated the vertical distribution of soil P forms in the alpine ecosystems.