Effects of Secondary Salinization on Soil Phosphorus Fractions and Microbial Communities Related to Phosphorus Transformation in a Meadow Grassland, Northeast China

Soil microorganisms play key roles in soil phosphorus (P) mobilization in grassland ecosystems. However, little is known about how bacterial communities involved in P transformation respond to soil secondary salinization. In this study, a meadow grassland with a gradient of secondary salinization in the semi-arid Horqin Sandy Land, Northeast China was selected. Soil properties, P fractions, the P transformation potentials, the community structures, and the abundance of a phosphorus (P)-mineralizing gene (phoD) and a P-solubilizing gene (gcd) were determined. NaHCO₃-Pi and NaOH-Pi declined with salinization, whereas H₂O-P, NaHCO₃-Po, and HCl-Pi increased. However, the available P fractions (H₂O-P and NaHCO₃-Pi) remained largely unaffected. Soil salinization significantly decreased the relative abundance of Xanthomonadales and Caulobacterales and increased that of Pseudonocardiales and Enterobacterales. The P fractions, the abundance of the phoD and gcd genes, and the community structures were all closely associated with soil organic matter, total nitrogen, pH, and soil moisture. Additionally, the structures of the phoD and gcd communities were significantly correlated with NaHCO₃-Pi and NaOH-Pi. Overall, secondary salinization altered bacterial communities related to P transformation by modifying soil properties, leading to decreases in the labile P and moderately labile P fractions.