Phod-harboring bacterial communities mediated slow and fast phosphorus transformation in alkaline soil of a Robinia pseudoacacia afforestation chronosequence

Background and aims: Soil phosphorus (P) availability is a key factor determining primary productivity in forest ecosystems in arid and semiarid regions. Under P deficient conditions, phoD-harboring microorganisms secrete alkaline phosphatase, improving P bioavailability. However, their roles in aging plantations of leguminous trees is still unclear. Methods: Soil samples were collected from 8-, 18-, and 30-year-old stands of a Robinia pseudoacacia afforestation of degraded agricultural land. Soil P fractions, alkaline phosphatase activity, phoD gene abundance, and bacterial community structure were determined. An experiment with P addition in pots with tree seedlings was set up to check for short-term P transformation processes. Results: Soil inorganic P (Pi) and organic P (Po) in R. pseudoacacia afforestation stands mainly existed as stable Pi (NaOH-Pi) and moderately easily available Po (HCl-Po). The contents of all these soil P fractions except for HCl-Pi decreased with stand age, while alkaline phosphatase activity increased. Structural equation modelling (SEM) revealed that soil organic carbon (SOC), available phosphorus (AP) and labile-Po contents mediated the community composition, α diversity or abundance of phoD genes, thereby affecting alkaline phosphatase activity. SOC showed the strongest positive effect on alkaline phosphatase activity. The P addition experiment suggests that alkaline phosphatase activity was mainly regulated via soil C:P stoichiometry. Conclusion: SOC appears to be an important regulator of Po turnover in P deficient soils via phoD gene-harboring microbes.