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

Pyrogenic carbon (PyC) is widely regarded as a persistent component of soil organic carbon, yet how small amounts of vertically redistributed PyC influence organo mineral interfaces in subsoils remains unresolved. Here we combine a ten year field ageing experiment along a 200 cm soil profile with electrochemical assays, spectroscopy, and nanoscale microscopy to link PyC electron transfer capacity to mineral association with depth. Redistributed PyC retains electron accepting and donating capacities in subsoils despite low concentrations, and exhibits faster electron transfer kinetics. Retention is grounded in oxidative surface transformation, enriching quinone and phenolic oxygen redox moieties and mineral complexing oxygen groups. Nanoscale observations of deep subsoil PyC show oxidised surfaces with organo mineral coatings associated with Fe and Ca bearing phases, consistent with a coupled redox and sorptive interface rather than passive persistence. Surface oxygenation links PyC mediated electron exchange to mineral protection and may extend reactive interfaces into subsoils, where lower oxygen availability and reduced microbial activity could preserve functionality. These findings suggest that field aged PyC contributes to redox coupled mineral stabilisation in subsoils and provide a basis for predicting when effects scale with reactive Fe across soil types.