Long-Term Grazing Altered Soil Phosphorus Fractions via Inorganic Phosphorus Solubilisation in a Semi-Arid Grassland

Phosphorus (P) is a crucial limiting nutrient in grassland ecosystems. Microorganisms play a vital role in soil P cycling and bioavailability. Grassland grazing profoundly affects soil P cycling, while the role of microbial-driven mechanisms in the regulation of soil P fractions by grazing intensity is yet unclear. Here, soil samples, primarily classified as calcic Chernozems, were collected from a long-term grazing experiment with grazing intensity gradients (0.0, 1.5, 3.0, 4.5, 6.0, 7.5, 9.0 sheep ha⁻¹) in a semi-arid grassland of Inner Mongolia. Soil chemical properties, P fractions, phospholipid fatty acid (PLFA) and functional gene abundance of inorganic P (Pi) solubilisation (pqqC) and organic P (Po) mineralisation (phoD) were measured. As grazing intensity increased, soil pH increased while soil dissolved nitrogen (DN) decreased. The marked decline in soil total PLFAs resulted in a linear reduction in soil labile Po. The gradual increase in pqqC gene abundance and constant phoD gene abundance with increasing grazing intensity suggested that the Pi solubilisation processes, not Po mineralisation processes, were pivotal for soil P transformation when the soil microbial community's growth was limited. The enhanced soil Pi solubilisation reduced soil moderately labile Pi, which in turn affected soil moderately resistant Pi and labile Pi. Under grazing intensity gradients, the increased soil pH and decreased soil DN were also involved in the regulation of soil P fractions' transformations via affecting soil total PLFAs and pqqC gene abundance. Our findings underscore the importance of Pi solubilisation processes in regulating soil P cycling under long-term grazing intensity gradients, thereby providing valuable insights for sustainable grassland management and ecosystem conservation in semi-arid regions.