Rhizosphere microbiota modulate cadmium mobility dynamics and phytotoxicity in rice under differential Cd stress

Background and Aims

Cadmium (Cd) stress modulates root-zone biogeochemical processes that influence Cd bioavailability and microbial community structure; however, the integrated effects of these interactions on rice Cd toxicity remain poorly characterized.

Methods

This study examined rhizosphere microbial community dynamics and their interactions with Cd speciation under four soil Cd concentrations: 0 (C0), 1 (C1), 5 (C2), and 20 (C3) mg·kg⁻¹.

Results

Rice biomass decreased by 18.5% and 28.1% under C2 and C3 stress at 35 days, respectively. By 65 days, C2 plants exhibited recovery, whereas C3 plants exhibited exacerbated growth inhibition. Rhizosphere dissolved-Cd concentrations consistently exceeded those in bulk soil across all treatments and declined over time. The rhizosphere-to-bulk soil dissolved-Cd ratio progressively decreased from C0 to C2 but surged at C3, indicating rhizosphere activity alleviated Cd stress at C2 while intensifying it at C3. Accordingly, C3-grown rice accumulated 0.85–8.79 times more Cd than those in C0–C2 soils. Rhizosphere soils exhibited reduced microbial richness and diversity compared to bulk soils. Notably, the C2 rhizosphere displayed maximal microbial richness and diversity at 65 days, with narrowed differences between rhizosphere and bulk soil. Furthermore, temporal divergence in community structure revealed enhanced heterogeneity and intensified diffusion limitation by 65 days. Key biomarkers Fonticella and Tumebacillus demonstrated Cd stress-dependent functional adaptations.

Conclusions

The rhizosphere microbial community's impact on rice health undergoes a concentration-dependent shift from protective to detrimental roles with increasing Cd stress. These results provide novel mechanistic insights into rhizosphere Cd-microbe-plant interactions under differential Cd exposure.