Effects of arbuscular mycorrhizal fungus on the functional bacteria assembly in promoting plant growth and nutrient acquisition in karst soil

The poor soil nutrients in the degraded karst region frequently restrict vegetation growth. Soil bacterial communities are essential drivers of biogeochemical cycling and regulate soil nutrient availability. Arbuscular mycorrhizal (AM) fungi play a fundamental role in shaping rhizosphere bacterial communities and promoting plant growth and nutrient acquisition. Nevertheless, how AM fungi mediated bacterial community interaction and assembly involved in nutrient cycling to promote plant growth and nutrient acquisition in karst soil remains unclear. In this study, a greenhouse pot experiment was conducted using γ-sterilized karst soil to cultivate four herbaceous plant species with AM fungal inoculation (AM treatment) or the control of non-inoculation (CK treatment). The results showed that plant biomass, nitrogen and phosphorus accumulation, hyphal length and soil organic carbon were significantly higher in the AM treatment than CK treatment, indicating that AM fungus promoted plant growth. The AM treatment also increased the abundance of specific functional bacterial taxa, including Nocardioides, Sphingomonas, and Massilia. Functional annotation of prokaryotic taxa analysis showed that AM treatment significantly enhanced soil functions related to ureolysis, nitrate reduction, and aromatic compound degradation. Furthermore, the co-occurrence network analysis showed that AM treatment decreased the average clustering coefficient and path length while increasing modularity compared to CK treatment, with Haliangium and Devosia identified as keystone species. In addition, Mantel’s test showed a significant relationship among the abundance of functional and keystone bacteria, soil metabolic functions and nutrients, and plant biomass and nutrient accumulation. In conclusion, AM fungus alters the abundance of specific functional bacteria associated with nutrient cycling, concomitant with the promotion of plant growth and nutrient accumulation. These findings highlight the important role of AM fungi in mediating the assembly of soil bacterial communities and enhancing host growth, and advancing understanding of the application of AM fungi for vegetation restoration in degraded karst ecosystem.