Effects of artificial humic acid on rhizosphere ecology and microbial regulation in a ryegrass-Bacillus cereus symbiotic system for remediating Cr(VI)-contaminated soil

Pollution by heavy metals, particularly hexavalent chromium (Cr(VI)), has become a significant environmental threat. This study aimed to evaluate the synergistic effects of artificial humic acid (A-HA) and Bacillus cereus Q-0 on Cr(VI) remediation in a ryegrass-soil system. A-HA was synthesized, and B. cereus Q-0 was labeled with gfp to enable tracking of its colonization in the soil-plant environment. The results showed that compared with the control group, combined treatment with A-HA and B. cereus Q-0-gfp significantly improved Cr(VI) reduction efficiency, decreasing soil Cr(VI) content from 69.8 mg kg^-1 to 17.07 mg kg^-1. Simultaneously, the total Cr content in plants increased by 37.2 %, indicating enhanced hyperaccumulation capacity in ryegrass. A-HA promoted the enrichment of functional microbial communities associated with heavy metal resistance, such as Proteobacteria and Firmicutes, reflecting an optimized soil microbial structure. In terms of plant growth, the combined treatment increased ryegrass biomass by up to 790.3 %, root length by 310.0 %, soil organic matter content by 650.12 %, and soil enzyme activity. Additionally, A-HA significantly enhanced the colonization ability of B. cereus Q-0 in both the rhizosphere and endophytic compartments of plants. This study highlights the unique potential of combining A-HA and B. cereus for effective and eco-friendly Cr(VI) remediation, offering a novel strategy to enhance phytoremediation efficiency in contaminated soils.