Field Evidence of Commercial Mycorrhizal Inoculum Mix Effects on Rhizosphere Microbiome and Lettuce Performance

Arbuscular mycorrhizal fungi (AMF) are widely applied as bioinoculants to enhance crop performance, yet their broader ecological effects on rhizosphere microbial assembly under field conditions remain insufficiently understood. Here, we evaluated the impact of a commercial AMF inoculant and its carrier material on lettuce performance and rhizosphere microbial communities in an open-field experiment. We hypothesized that both viable AMF propagules and formulation components contribute to shifts in rhizosphere processes. Active AMF inoculation significantly increased root colonization and fresh biomass at harvest, confirming successful establishment and enhanced plant performance under field conditions. Colonization levels in the heat-inactivated carrier treatment were comparable to the non-inoculated control, indicating that the carrier did not inhibit indigenous AMF activity or induce nutrient-mediated suppression of symbiosis. Plant physiological responses were stage-dependent, supporting the context-dependent nature of AMF effects in dynamic field environments. High-throughput sequencing revealed no significant treatment effects on bacterial or fungal alpha diversity. However, beta-diversity analyses demonstrated significant compositional restructuring of rhizosphere communities, particularly within the bacterial domain. A stable core microbiome persisted across treatments, yet relative abundances and community evenness were altered by both active inoculation and carrier application. These results suggest that AMF inoculation reorganizes microbial community structure predominantly via shifts in ecological niche occupation. Collectively, our results show that AMF inoculation functions not only as a symbiotic nutrient-acquisition strategy but also as a driver of rhizosphere microbial reorganization under field conditions. Integrating plant performance with microbiome dynamics provides a more comprehensive framework for understanding and optimizing microbial inoculants in sustainable agricultural systems.