Biochar incorporation to paddy soils increased Cd accumulation in rice grains through priming effects of rhizosphere...

As a recycling bioresource, biochar has been considered as an effective deactivator for soil heavy metals due to its pore structure and surface chemical properties. However, the interactions between biochar and soil biotics in the rhizosphere remain underexplored in the context of Cd mobility in paddy soils. In this study, biochar was applied to the two paddy fields, which were previously polluted by heavy metals from e-waste dismantling. High-throughput sequencing was used to map the changes of soil microbial communities in response to biochar addition. Results showed that biochar incorporation into the two paddy fields consistently increased the concentration of Cd in the maturation polished rice grains (MP grain Cd), which was contrary to most of previous findings. Overwhelming bacterial taxa were significantly enriched by biochar amendments in the rhizosphere of rice at the maturation stage, with aerobic chemoheterotrophic bacteria, iron respiration bacteria, nitrate reduction bacteria, etc. being significantly increased. Statistical analyses revealed that soil organic matter (SOM) was the primary predictor for the MP grain Cd and was directly linked to the MP grain Cd in biochar-treated samples through network analysis (P < 0.05). Variance partitioning analysis illustrated that soil biotics substantially contributed to the MP grain Cd. These findings in our research indicated that biochar triggered the microbial priming effect on SOM. The microbial priming effect might promote Cd bioavailability through the coupling redox processes of SOM oxidation, iron reduction and nitrate reduction in rhizosphere soils. Our research revealed that the microbial mechanism of biochar potentially induced Cd activation in paddy soil and underscored health risks associated with biochar application.