Synergistic Interactions between Nitrogen Fixation and Phosphorus Uptake in Legumes: Insights from the Root Nodule Bacterium <i>En1</i>

The synergistic interaction between symbiotic nitrogen (N) fixation and phosphorus (P) nutrition in legumes remains poorly understood yet critical for mitigating grassland degradation under nutrient limitations. Using 16S rRNA sequencing, we compared bacterial communities in root nodules, rhizosphere, and bulk soils of three wild legumes (<i>Trifolium repens</i> L., <i>Medicago sativa</i> L., and <i>Indigofera amblyantha</i> Craib). Nodules showed reduced bacterial diversity but shared core microbes derived from soil reservoirs, including <i>Ensifer</i>. We isolated an <i>Ensifer</i> strain (<i>En1</i>) and genomic analysis revealed genes specifically involved in nitrogen fixation and phosphate uptake. Functionally, <i>En1</i> solubilized and mineralized phosphate (calcium phytate and calcium phosphate). Inoculation experiments confirmed that <i>En1</i> enhances plant P acquisition. Our findings demonstrate a mechanism whereby nitrogen-fixing rhizobia directly facilitate P uptake, establishing a self-sustaining N-P coprovision system in legume-microbe symbiosis, offering a novel strategy for grassland restoration by concurrently addressing N and P limitations.