From heterotrophic priming to autotrophic CO₂ fixation: biochar-driven shifts in microbial turnover of soil carbon

Pyrogenic carbon (PyC), commonly applied as biochar in agricultural soils, is widely promoted as a stable carbon pool for climate mitigation. However, this passive framing overlooks the central role of microbial metabolism and turnover in governing PyC-associated carbon cycling. Here, our study reveal that biochar should be conceptualized not as an inert carbon reservoir but as a dynamic microbial interface that actively regulates soil carbon turnover through coupled heterotrophic and autotrophic processes. Drawing on incubation experiments, microbial functional profiling, and field-scale analyses, we show that biochar–microbe interactions are driven by integrated physical, chemical, and biological mechanisms. Biochar restructures microbial habitats through pore-mediated colonization, nutrient retention, and pH buffering, while simultaneously enabling extracellular and interspecific electron transfer that mediates redox-sensitive metabolic pathways. These processes directly regulate microbial metabolic activity, community structure, and functional assembly, positioning biochar as an active regulator of soil biogeochemical function. Biochar-induced priming effects on native soil organic carbon (SOC) arise primarily from shifts in microbial metabolic strategies rather than from carbon recalcitrance alone. Biochar promotes SOC persistence by stabilizing soil physicochemical conditions and selectively enriching microbial consortia associated with reduced heterotrophic disturbance and efficient secondary metabolite turnover. These findings identify microbial accessibility and functional redundancy as key determinants of carbon turnover and persistence in biochar-amended soils. Critically, biochar also activates an overlooked autotrophic carbon input pathway. We demonstrate that biochar substantially alters Calvin cycle–mediated CO₂ fixation by regulating the abundance, activity, and community structure of cbbL- and cbbM-containing autotrophic microorganisms. The rhizosphere emerges as a hotspot of biochar-enhanced CO₂ assimilation. This autotrophic CO₂ fixation is tightly coupled with essential elemental cycling in soil, integrating PyC-driven microbial metabolism into broader soil biogeochemical networks. Our study supports a conceptual shift from passive PyC stabilization to microbially regulated carbon turnover, highlighting microbial metabolism and turnover as central controls on long-term soil carbon sequestration and soil function.