Responses of Robinia pseudoacacia co-inoculated with rhizobia and arbuscular mycorrhizal fungi to cadmium under nitrogen excess condition

Aims Rhizobia and arbuscular mycorrhizal fungi (AMF) are important symbiotic microorganisms promoting plant growth in contaminated soils. This study aims to determine whether the new microbial communities constructed after inoculation contribute to the improvement of cadmium (Cd) resistance of Robinia pseudoacacia L. under nitrogen (N) excess. Methods Effects of rhizobium/AMF inoculation on rhizosphere microbial communities of Cd-exposed R. pseudoacacia under N excess/sufficiency were investigated using high-throughput sequencing of 16 S rRNA genes. Results Inoculating plants with rhizobium and AMF individually can significantly enhance their tolerance to Cd, and when co-inoculated, the improvement is the most pronounced. Co-inoculation increased the biomass, antioxidant enzyme activity, mycorrhizal colonization rate, number of nodules, photosynthetic capacity, acquisition of N, P, Fe and Mg. Co-inoculation enhanced Cd accumulation in roots and the translocation factor (TF) from root to shoot, while significantly reducing MDA and H2O2 contents due to increased antioxidant substances and improved photosynthetic efficiency. Furthermore, co-inoculation significantly changed the structure and diversities of rhizosphere microbes. Specifically, co-inoculation increased Firmicutes abundance under N sufficiency, whereas it significantly decreased Firmicutes abundance under N excess. Results of partial least squares path model (PLS-PM) showed relative abundances of Proteobacteria and Firmicutes after inoculation played a key role in Cd resistance. Conclusion Under N excess, compared with single inoculation or un-inoculation, co-inoculation can significantly promote the growth of R. pseudoacacia in Cd-contaminated soil, enhance Cd absorption capacity, and reduce Cd transport to shoot. This work provides an effective phytoremediation strategy for plant management of heavy metal-contaminated soils under N excess context.