nZVI-BC strengthens organic carbon sequestration driven by the modulation of soil microbial community and heavy metal speciation

Nano zero-valent iron modified biochar (nZVI-BC) is widely regarded as an efficient and environmentally friendly material for stabilising heavy metals in soil. However, the intrinsic relationship among soil microbial community structure, heavy metal speciation, and soil organic carbon (SOC) sequestration remains unclear. This study comprehensively evaluated the influence of nZVI-BC on the stabilisation of heavy metals and variation of SOC fractions, and the associated microbial community structure, along with conducting correlation analyses. Results indicated that following nZVI-BC treatment, the available forms of Cd, Pb, and Zn in soil decreased by 42.7 %, 48.0 %, and 39.1 %, respectively, with greater conversion to Fe-Mn oxide-bound, organically bound, and residual forms. In comparison to pristine biochar, nZVI-BC amendment resulted in a greater increase in mineral-associated organic carbon (MAOC), a reduction in particulate organic carbon (POC) content, a lower POC/MAOC ratio, and decreased soil respiration. Furthermore, nZVI-BC treatment significantly increased the relative abundance of the bacterial genera (Brevundimonas and Noviherbaspirillum) associated with SOC stabilisation. The SOC and MAOC content exhibited a positive correlation with these bacterial abundances. The correlation coefficient between SOC and residual Pb was 0.043, while that between MAOC and Fe-Mn-bound Zn was 0.104. The findings suggests that the combined effect of active carbon sequestration by Brevundimonas and Noviherbaspirillum, along with passive carbon sequestration of residual Pb and Fe-Mn-bound Zn from iron oxides introduced by nZVI-BC, has a positive effect on SOC sequestration. This study provides a novel perspective on the comprehensive evaluation of the environmental benefits of nZVI-BC in soil heavy metal remediation.