Combined deep tillage and biochar amendment mitigates cadmium toxicity in paddy soil: Mechanisms of bioavailability reduction, microbial response, and grain safety enhancement

Cadmium (Cd) contamination in paddy soils poses risks to ecosystem functioning and food safety. This study proposes and validates an integrated field management strategy combining physical (deep tillage, DT) and chemical (biochar amendment) interventions to mitigate Cd ecotoxicity. Our results demonstrate that DT alone, through physical dilution, significantly reduced total and bioavailable Cd in the topsoil (0−20 cm) by 24.63% and 80.00%, respectively. The co-application of biochar (Fe-modified, bamboo, or rice straw) increased the relative proportion of Cd associated with Fe−Mn oxide−bound fractions, with the combined treatment reducing bioavailability by up to 91.94%. These changes were accompanied by shifts in microbial taxa potentially involved in Fe cycling. The integrated treatment reduced Cd accumulation in rice grain and straw by up to 58.28% and 51.30%, respectively, bringing the cadmium content in the grains close to the maximum permissible limit stipulated by the national food safety standards. Microbial analyses further indicated treatment-associated changes in fungal alpha diversity and bacterial community structure, together with alterations in microbial co-occurrence patterns. Our findings suggest that deep tillage coupled with biochar amendment may mitigate Cd risks through complementary physical, chemical, and biological effects, highlighting its potential for short-term Cd risk mitigation in contaminated paddy soils.