Cadmium pollution alters the priming effect of biochar application on soil organic carbon mineralization

Biochar has high potential to reduce cadmium (Cd) bioavailability in polluted soils. While effective in Cd remediation, biochar amendment can stimulate native soil organic carbon (SOC) mineralization via the priming effect, resulting in soil C release. However, it remains unclear whether Cd pollution alters the impact of biochar application on SOC mineralization, and how microbes modulate biochar-induced priming effect in Cd-polluted soils. To address these questions, a 120-day laboratory experiment was conducted by applying two types of biochar derived from C4 crop residues to C3 paddy soils under Cd pollution (0, 4 and 8 mg kg^-1). In unpolluted soils, sorghum biochar, with higher C availability and C:nitrogen (N) ratios, triggered stronger positive priming of SOC mineralization than sugarcane biochar. This process was mediated by bacterial groups, where higher N-degrading gene abundances and associated enzyme activities promoted N-mining from soil organic matter (SOM). Low Cd pollution reduced priming responses, resulting in an overall negative priming. This result coincided with increased DOC and mineral N, which mitigated microbial resource demand from SOM. Under high Cd, sorghum biochar induced a positive cumulative priming at the late stage, whereas sugarcane biochar triggered an overall negative priming. This could be because higher labile C in sorghum biochar sufficiently activated microbes to produce enzymes which co-metabolized SOM, a process driven by fungal groups and persistent C-degrading genes. These findings highlight a double-edged sword effect that biochar remediation of highly Cd-polluted soils has the potential to trigger soil C emissions.