Adopting an optimal cropping regime is crucial for sustainable soil use. However, how different cropping regimes impact phosphorus (P) availability and the underlying mechanism remain unclear. Here, a 10-year field experiment was performed to examine the influence of different cropping regimes, including maize–soybean rotation (MSR), continuous maize cropping (CMC), and farmland fallow (FALL), under unfertilized and fertilized conditions in Northeast China. The P forms were analyzed using chemical fractionation and solution phosphorus-31 nuclear magnetic resonance. Compared to FALL, total P and different forms of P contents were significantly lower under MSR and CMC systems. Moreover, the contents of total P and different forms of P were higher under MSR than those under CMC. Correlation analysis showed that there were significant and positive correlations between total P and different forms of P contents. Redundancy analysis revealed soil organic carbon (SOC) as the most significant factor influencing total P and different forms of P. Structural equation modeling demonstrated the direct positive impacts of SOC, total nitrogen, total phosphorus, and Olsen phosphorus on phosphatase activity, which exhibited direct positive influence on P availability. In summary, maize–soybean rotation is an effective cropping regime for promoting P accumulation and availability in this region.

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.

The alpine grassland ecosystem of the Tibetan Plateau is a vital base for animal husbandry and a key ecological security barrier in China. Phosphorus (P), an essential nutrient, is among the primary factors limiting grassland productivity. However, the spatial distribution of soil P fractions across alpine grasslands on the Tibetan Plateau and their environmental drivers remain unclear, limiting our understanding of P cycling and grassland productivity. This study examined the composition and distribution of soil P in three representative alpine grasslands (meadow, steppe, and desert) using a combination of chemical fractionation and ³¹P nuclear magnetic resonance (NMR) spectroscopy. The results revealed pronounced spatial heterogeneity, with total soil P content varying by approximately 2.4-fold among the grassland types. Alpine meadows had the highest total P (0.73 g kg⁻¹) and available P (4.02 mg kg⁻¹) concentrations, with the latter being nearly twice that of alpine steppes and deserts. Alpine meadows were characterized by a predominance of labile and moderately labile organic P (e.g., NaOH-Po) and a diverse array of phosphate monoesters and diesters, whereas alpine deserts were dominated by stable, calcium-bound inorganic P (HCl-Pi). Temperature, precipitation, pH, and phosphatase activity were identified as key factors regulating the distribution and transformation of P fractions. The distinct P fractions and availability uncovered in this study are essential for predicting grassland ecosystem responses to environmental change and guiding sustainable pasture management on the Tibetan Plateau.