Impact of Dominant Species Shift in Herbaceous Vegetation Beneath Sand-Fixing Plantations on Soil Microbial Communities Involved in Organic P Mineralization and Inorganic P Solubilization

Herbaceous vegetation under sand-fixing plantations is critical for ecosystem stability. Phosphorus (P) represents a key growth-limiting factor for plants in sandy soils, yet systematic understanding of P-cycling microbial communities including their composition, ecological functions, and responses to vegetation succession remains limited. We conducted cross-transplant experiments using dominant species from different successional stages under Caragana microphylla plantations in the Horqin Sandy Land, and assessed soil properties, enzyme activities, organic P mineralization and inorganic P solubilization rates, and phoD/gcd-harboring microbial communities. Encroachment by late-successional species increased soil nutrients, biological activities, phoD/gcd-microbial abundances, and soil P transformation rate, and restructured microbial communities while preserving core taxa. Soil properties (pH, electrical conductivity, organic matter, total nitrogen, total P, available potassium, available P, and NH4+-N) significantly influenced phoD community composition. Notably, P availability was primarily driven by phoD-mediated organic P mineralization, with gcd-mediated solubilization playing a minor role. These changes enhanced alkaline phosphatase activity and P availability, promoting plant growth and new species colonization, thereby establishing a positive plant–soil–microbe feedback that drives succession. The identified dominant phoD/gcd taxa provide candidates for developing microbial inoculants to accelerate vegetation restoration.