The microbial cycling of phosphorus on long-term fertilized soil: Insights from phosphate oxygen isotope ratios

Microorganisms mobilize phosphorus (P) from soil and make it available for plants. However, the role of microbial activity in soil P dynamics especially among different P pools is poorly understood largely due to methodological limitations. In this study, we analyzed the oxygen isotope ratios in phosphate (δ18OP) of sequentially extracted inorganic P (Pi) pools (H2O-Pi, NaHCO3-Pi, NaOH-Pi, and HCl-Pi) in a long-term agricultural research field in Henan, China with different fertilization histories and coupled with soil enzyme activity and P-cycling bacteria gene abundance studies. Results showed the dominant enzymes were alkaline phosphatase (APase) and phosphodiesterase (PDE), and the functional genes for P-cycling were bpp, cphy, phoX and pqqC. After long-term P fertilization, the δ18OP values of H2O-Pi, NaHCO3-Pi and NaOH-Pi pools approached to or achieved equilibrium, suggesting that the externally applied P was actively mobilized and cycled by soil microorganisms and speciated into different P pools. Based on the extent of isotope excursion among different Pi pools, the equilibrium fractionation oriented the source signature in the NaHCO3-Pi and NaOH-Pi pools, and HCl-Pi pool could be derived from P fertilizer but through multistep reactions, and mixed with the HCl-Pi from rock weathering product, which constituted HCl-Pi pool in the non-P treatments. Overall, the long-term P fertilization especially the balanced fertilization with nitrogen (N), P and potassium (K) was found to be beneficial for extensive utilization of soil P with abundant biological uptake and cell-internal Pi cycling. Variation partitioning analysis (VPA) indicated that the expression of functional genes may be stimulated to mobilize soil P under specific P pools distribution in this alkaline environment. Overall, our findings provide new insights to understand the roles of microbial activities in soil P biogeochemistry that are useful for agricultural P sustainability.