Long-term green manuring increases phosphorus availability in soil by modifying microbial functional genes and enzyme activities

Substitution of mineral fertilizers with milk vetch ( Astragalus sinicus L.) organic fertilization strongly influences the dynamics and availability of phosphorus (P) in soil. How milk vetch organic fertilization affects the specific roles of P cycling functional genes in soil and, in turn, the mechanisms of altered P turnover remain unclear. Soil samples were taken from a 10-year field experiment that included mineral fertilizer and partial substitution of mineral fertilizer with milk vetch, together with an unfertilized control. The P-cycling functional genes in soil determined by metagenomics were related to phosphatase enzyme activities and P fractions with various availability. Milk vetch substitution decreased soil total P and NaHCO 3 -OP (8 and 65 %) but maintain similar available P and high phosphatase activities compared with mineral fertilizer, indicating a net solubilization/mineralization of soil P occurring in response to milk vetch substitution. The Proteobacteria, followed by Chloroflexi, Acidobacteria, Actinobacteria, Nitrospirae, Verrucomicrobia, and Gemmatimonadetes were the most abundant bacteria phyla involved in soil P-cycling. Shifts in P-cycling functional gene community were more related to soil total N and the ratio of SOC to total P. The relative abundance of P-cycling functional genes, increased the production of PQQ-GDH (gcd , pqqE ), fumaric acid ( fumC ), acetic acid ( aldh ), acid phosphatase ( phoN ), alkaline phosphatase ( phoD ), the catabolism of organophosphonates ( phnJ, phnN , and phnW ) and the dissimilation of glycerol-3-phosphate ( glpB, glpF, and glpR ) by 4 %–80 %. Phosphatase activities increased with milk vetch substitution, because of raised abundance of related genes and gene-harboring communities (especially Chloroflexi). The phoN gene was the key predictor of organic P mineralization and thus available P. Overall, milk vetch substitution increased the P-acquiring enzyme activities and boosted the involvement of the functional genes in P cycling, which in turn stimulated P bioavailability. These results highlight the connections between milk vetch substitution and P cycling functional genes, which also have implications for offsetting the future demand for P mineral fertilizer by milk vetch substitution in agricultural systems. • Milk vetch enhanced the genes related to production of organic acid. • Phosphatase were increased by regulating gene abundance and microbial communities. • Genes related catabolism organophosphonates catabolism and glycerol-3-phosphate catabolism were increased under milk vetch.