Phosphorus Forms Affect the Hyphosphere Microbiome Involved in Soil Organic Phosphorus Turnover

Interactions between bacteria and arbuscular mycorrhizal (AM) fungi play a significant role in mediating organic phosphorus (P) transformations and turnover in soil. The bacterial community in soil is largely responsible for mobilization of the soil organic P pool allowing the extraradical AM hyphae to absorb the released P and mediate its use for plant growth. However, the functional microbiome involved in organic P mineralization in the hyphosphere remains poorly understood. The aim of this study was to understand how AM hyphae-associated bacterial communities related to P turnover in the hyphosphere of leek (Allium porrum) respond to different forms of soil P. Using a compartmented microcosm, leek was grown with the AM fungus Funneliformis mosseae, and the extraradical mycelium of F. mosseae was allowed to grow into a separate hyphal compartment containing either no added P, or P as KH₂PO₄ or phytin. High-throughput sequencing technologies were used to analyze how the phosphorus forms in the hyphosphere affected the bacterial communities harboring alkaline phosphatase (ALP) and β-propeller phytase (BPP) that were associated with the extraradical mycelia of F. mosseae. The ALP-harboring bacterial community was dominated by Sinorhizobium, Bradyrhizobium, Pseudomonas and Ralstonia and was significantly changed in response to different P treatments, with Pseudomonas showing higher relative abundance in organic P treatments than in control and inorganic P treatments. Pseudomonas was also the major genus harboring the BPP gene in the hyphosphere, but the BPP-harboring community structure was not affected by the presence of different P forms. These results demonstrated the profound differences in ALP- and BPP-harboring bacterial communities in the hyphosphere at genus level, providing new insights to link bacteria and biogeochemical P cycling driven in association with mycorrhizal hyphae.