Oxidised pyrogenic carbon sustains electron transfer capacity and organo mineral coupling in subsoils

Pyrogenic carbon is widely regarded as a component of soil organic carbon, yet how small amounts of vertically redistributed pyrogenic carbon influence organo mineral interfaces in subsoils remains unresolved. Here we combine a ten year field ageing experiment along a 200 cm calcareous soil profile in China with electrochemical assays, spectroscopy and nanoscale microscopy to link pyrogenic carbon electron transfer capacity to mineral association. Redistributed pyrogenic carbon retains electron accepting and donating capacities in subsoils despite low concentrations and exhibits faster electron transfer kinetics. This retention reflects oxidative surface transformation, enriching quinone and phenolic redox moieties and mineral complexing oxygen groups. Nanoscale observations show oxidised subsoil pyrogenic carbon surfaces with organo mineral coatings associated with iron and calcium bearing phases, consistent with a coupled redox and sorptive interface rather than passive presence alone. These findings suggest that field-aged pyrogenic carbon contributes to redox coupled mineral stabilisation in calcareous subsoils.