Effects of Rock Phosphate Application on the Composition of Bacterial Communities Associated with Arbuscular Mycorrhizal Fungal Mycelia

Background Inter-kingdom interactions between arbuscular mycorrhizal fungi (AMF) and bacteria are increasingly recognized for their potential to enhance fertilizer use efficiency in agroecosystems. Here, we investigated the effects of rock phosphate amendment and AMF inoculation on phosphorus (P) nutrition in leek ( Allium porrum L.), as well as on bacterial communities associated with AMF extraradical mycelium. A bi-compartmental microcosm was used to disentangle root-derived effects from those mediated by AMF mycelium. Results Inoculation with Rhizophagus irregularis significantly increased total plant biomass ( p  < 0.001), while rock phosphate amendment enhanced arbuscule abundance in roots ( p  = 0.03), leading to higher shoot P content ( p  = 0.013) and photosynthetic activity ( p  < 0.0001). Because rock phosphate was the sole P source, these results indicate that P solubilized in the soil was translocated to the host plant via the mycorrhizal pathway. Rock phosphate amendment also significantly altered the composition of bacterial communities associated with AMF mycelium ( p  = 0.01). Across treatments, bacterial assemblages were dominated by Planctomycetota, Pseudomonadota, Chloroflexota, and Bacillota, with enrichment of Planctomyces and Gemmata in AMF mycelium, and Planctomyces , Gemmata , and Bacillus in soil. The core bacteriome associated with R. irregularis was primarily composed of Planctomycetota and Bacillota, taxa known to form biofilms on AMF extraradical hyphae. Conclusion These findings demonstrate the pivotal role of mycorrhizal symbiosis in enhancing P acquisition from rock phosphate and provide new insights into AMF–bacteria interactions that are relevant for developing sustainable fertilization strategies.