Carbon dioxide removal efficiency of co-applied and co-pyrolyzed feedstocks for enhanced rock weathering and biochar in a sandy soil

Enhanced rock weathering (ERW) and pyrogenic carbon capture and storage (PyCCS) are promising land based approaches for carbon dioxide removal (CDR), yet their co-deployment remains poorly understood. This study investigates how co-application of minerals with biochar and co-pyrolysing biomass with minerals influence mineral weathering, alkalinity production, and carbon sequestration in a sandy soil with low cation exchange capacity (CEC) and low organic carbon (Corg) content. We compared concrete, steel slag, basanite, serpentinised peridotite, limestone and biochar as pure and co-applications, and rock enhanced biochars (RE-biochars) from dry-mixing and wet-impregnation of biomass and consecutive co-pyrolysis. Downflow columns filled with amended sandy soil were incubated for 75 weeks under elevated pCO2 while monitoring cation fluxes, trace metal release, dissolved organic carbon (DOC) dynamics, and changes in soil CEC. Wet impregnation substantially altered RE-biochar properties by increasing the pyrolysis carbon yield and fraction of highly stable pyrogenic carbon particularly for Mg rich feedstocks. Reduced DOC losses and enhanced alkalinity production, attributed to mineral weathering as the geogenic inorganic carbon sink (IC-Sink), was present for co-applications of biochar with basanite, peridotite, and concrete. In contrast, RE biochars mostly showed reduced geogenic IC-Sinks for industrial materials, but increased geogenic IC-Sinks for peridotite from accelerated Mg2+ release from thermally activated serpentine. Cation pool analyses revealed that most cations were leached rather than being retained in exchangeable pools of the soil. Overall, our evaluation of synergies arising from ERW-PyCCS combinations for terrestrial CDR highlights material-specific advantages and trade-offs, supporting the recommendation of both single application and co-application of biochar with mineral feedstocks.