Harnessing a Functional Rhizobial Partnership with <i>Astragalus sinicus</i> L. to Unlock Phytoremediation Potential for Soil Heavy Metals

Phytoremediation of heavy metal-contaminated soils is often limited by phytotoxicity and metal availability. This study evaluated the phytoremediation potential of Astragalus sinicus L. and its symbiotic rhizobia. A nationwide soil survey revealed significantly lower arsenic (As) in planted versus unplanted soils, and key factors governing metal retention were attenuated in planted soils, indicating plant-mediated interference. Pot experiments confirmed that A. sinicus L. cultivation significantly reduced soil cadmium (Cd) (33.33%), lead (Pb, 39.73%), copper (Cu, 12.92%), and As (23.70%). Among rhizobial isolates, Mesorhizobium sp. XS6–2 exhibited the highest heavy metal tolerance. Inoculation with XS6–2 increased plant biomass and specifically enhanced chromium (Cr) and Pb remediation. Microbiome analysis showed that XS6–2 reshaped the rhizosphere community and strengthened microbial interactions. Our findings demonstrate a potent plant–microbe synergy that alleviates phytotoxicity and increases metal availability, offering an effective strategy to advance phytoremediation.