Ammonium enhances soil organic phosphorus conversion by regulating phoC- and phoD-harboring bacteria

Ammonium facilitates higher absorption and remobilization of phosphorus (P) in rice compared with nitrate nitrogen; however, the underlying mechanism remains unclear. In the present study, a five-year pot experiment was conducted with ammonium or nitrate applied with or without P fertilizer (NH 4 + +P, NH 4 + -P, NO 3 - +P and NO 3 - -P) to investigate the mechanisms by which ammonium enhances organic P release under P-deficient conditions. The results showed that rice growth and yield, together with total aboveground P content, were significantly higher in the NH 4 + -P treatment than in the NO 3 - -P treatment. Treatment with NH 4 + -P improved the transfer of moderately labile organic P to labile organic P, thereby enhancing the available P content in the rhizosphere soil of rice. In addition, the acid phosphatase activity and the expression of related genes ( phoC ) in the rhizosphere soil of rice was significantly higher in the NH 4 + -P treatment than in the NO 3 - -P treatment. In contrast, alkaline phosphatase activity did not different significantly between the NH 4 + -P and NO 3 - -P treatments, although the expression of its related gene ( phoD ) was significantly higher in the NO 3 - -P treatment than in the NH 4 + -P treatment. The α-diversity indices, such as Shannon, Chao1, abundance-based coverage estimator (ACE) and observed species, of phoC -harboring bacteria were significantly higher under the NH 4 + -P treatment than under the NO 3 - P treatment, whereas no significant differences in α-diversity were observed for phoD -harboring bacteria between these two treatments. Under P-deficient conditions, ammonium had a stronger effect than nitrate on the enrichment of Serratia , Raoultella , and Stenotrophomonas among the phoC -harboring bacteria, as well as Mesorhizobium , Ralstonia , and Rhizobacter among phoD -harboring bacteria. Soil pH was the primary environmental factor significantly influencing the abundance of both phoC- and phoD -harboring bacteria. In conclusion, ammonium was associated with enhanced available P from organic P by regulating the community structure of phoC- and phoD-harboring phosphorus-solubilizing bacteria, with soil pH acting as a key driver of community composition.