Effects of straw return on cadmium mobilization in paddy soils and its subsequent accumulation in rice grains: a meta-analysis

Straw return is widely promoted as a sustainable practice to improve soil fertility and reduce residue burning, yet its influence on cadmium (Cd) mobilization and rice grain contamination remains controversial. Here, we conducted a comprehensive meta-analysis of 35 peer-reviewed studies encompassing 414 paired observations to quantitatively evaluate the effects of straw return on soil Cd availability and accumulation in rice tissues. Overall, straw return significantly increased soil available Cd by 12.7% (95% CI: 8.0%–16.7%) and total Cd by 7.7% (95% CI: 3.5%–12.0%) relative to no-straw controls, accompanied by elevated soil organic matter (+12.3%) and pH (+1.8%). Subgroup analyses revealed that initial soil pH was the dominant moderator regulating Cd mobilization, with the strongest enhancement of available Cd occurring under acidic conditions (pH < 6.5) and weakening progressively as pH increased. Straw type also exerted a pronounced influence: rice straw markedly increased both available Cd (+20.0%) and total Cd (+10.1%), whereas wheat and rapeseed straw showed negligible effects, likely attributable to the higher Cd content and slower decomposition rate of rice straw. Furthermore, soil available Cd was positively correlated with Cd concentrations in grain ( p < 0.001), straw ( p < 0.01), and roots ( p < 0.001), whereas total Cd showed no significant association, underscoring the greater relevance of bioavailable fractions for predicting plant uptake. Collectively, these findings demonstrate that straw return may act as a Cd risk amplifier in contaminated and acidic paddy soils, emphasizing the necessity of site-specific straw management strategies to ensure food safety and sustainable rice production.