Root exudates recruit microbial allies to override their own mobilization effects for cadmium immobilization in...

Challenging the conventional view that root exudates inevitably exacerbate cadmium (Cd) bioavailability in paddy soils, this study revealed that intercropping high- and low-Cd-accumulating rice cultivars creates a distinct rhizosphere microenvironment wherein specifically enriched metabolites exert a dual function, ultimately driving net Cd immobilization. Through field experiments coupled with metabolomic and microbiome analyses, we showed that intercropping significantly reduces grain Cd concentrations by 18.45% in the high-Cd cultivar and 8.13% in the low-Cd cultivar, accompanied by substantial decreases in acid-extractable and oxidizable Cd fractions in rhizosphere soils. Mechanistic analyses revealed that intercropping reshaped the rhizosphere metabolome, enriching organic acids that exert dual functions: transient mobilization of oxidizable Cd and, more critically, recruitment of functional microorganisms, including sulfate-reducing bacteria (Candidatus Sulfobium mesophilum) and nitrifiers (Nitrospira), that promote Cd immobilization. Structural equation modeling confirmed that microbe-driven immobilization (path coefficient = -0.31) outweighs metabolite-mediated mobilization (path coefficient = 0.25), thereby reducing bioavailable Cd in the rice rhizosphere and suppressing its translocation to grains. A pot validation experiment substantiated this causal pathway: exogenous application of organic acids (citric acid) increased the abundance of the target sulfate-reducing bacterium by over 200% and induced a dose-dependent reduction in grain Cd content ranging from 32.29% to 56.25%. Collectively, our findings uncover a microbial-mediated Cd immobilization strategy triggered by intercropping-induced metabolite shifts, offering a streamlined framework that translates mechanistic insights into the efficient screening of green and cost-effective root metabolites as potential remediation agents for sustainable agriculture.