Role of key microbial modules for soil carbon sequestration effects in biochar-based remediation of...

How changes in soil properties and heavy metal toxicity induced during biochar remediation of cadmium (Cd) contaminated soils induce changes in microbial communities, and further, how this process affects the soil carbon sequestration capacity by microorganisms, has not been explored. We prepared virgin biochar (named BC400 and BC600) using rice straw at 400 °C and 600 °C and modified biochar (named MNT-BC400 and MNT-BC600) using co-pyrolysis of montmorillonite and rice straw at the same two temperatures, in an attempt to create different CO₂ emission backgrounds during the remediation of cadmium contamination in soil, and to explore the mechanism of soil carbon sequestration capacity during remediation. The results showed that MNT-BC600 was effective in reducing the soil available Cd during incubation without increasing soil carbon emission, whereas soil carbon emission was elevated by 83.10%, 50.19%, and 21.53% in BC400, MNT-BC400, and MNT-BC600 treatments compared to the control group (CK). Microbial carbon use efficiency (CUE) was increased by 20.68%, 18.78%, and 12.73% in BC400, BC600, and MNT-BC600 treatments, respectively. We found that the bacterial module (Bmod#2), dominated by the eutrophic bacteria Proteobacteria and Actinobacteriota, controlled the increase of soil carbon emissions; the bacterial module (Bmod#3) composed of both oligotrophic and eutrophic flora were closely related to soil CUE after remediation; the bacterial module (Bmod#4) consisting of oligotrophic and eutrophic bacteria alone was easily replaced by eutrophic bacteria module (Bmod#2) under eutrophic conditions and had a negative linear relationship with soil CO₂ emission, while the switch in Cd form was more likely to affect the community structure of modules containing eutrophic bacteria. These findings provide new insights into the use of biochar for soil remediation and balancing soil carbon sequestration.