Enhanced rock weathering (ERW) and biochar are potentially effective and scalable options for large-scale carbon dioxide removal (CDR), required to limit global temperature rise to 1.5 °C. Here we present experimental data on their co-deployment, an urgent and novel research direction that may render even larger CDR on multiple timescales. Two greenhouse pot experiments were conducted growing maize (Zea mays L) on sandy and clayey soils mixed with various doses of crushed dunite rocks (20–220 t ha−1) and a fixed dose of biochar (20 t ha−1) for two months. Furthermore, through a comparison of multiple soil extraction procedures for mass balance construction, our work supports the development of a standardized quantification method for CDR associated with ERW. Based on these elemental mass balances, dunite weathering was found to sequester between 1.06 ± 0.025 and 3.48 ± 0.084 t CO2 ha−1 in sandy soils and between 0.28 ± 0.015 and 1.60 ± 0.051 t CO2 ha−1 in clayey soils, while biochar co-deployment only slightly enhanced dunite weathering in the latter. Soil respiration also significantly increased on both soils, exceeding the achieved inorganic CO2 sequestration in our short-term experiments. However, we observed significant increases in soil pH and amorphous iron (hydr)oxide minerals, the latter known to be important for long-term organic carbon stabilization. We argue that the reduction in soil carbon due to enhanced soil respiration is only short term and is likely compensated for by the promising potential of ERW and biochar combinations for long-term inorganic carbon sequestration and organic carbon stabilization. The observed effects of ERW and biochar co-deployment on soil chemical properties, most notably increases in reactive (hydr)oxide minerals and soil pH, provide a great opportunity to boost CDR, with important differences between soil types.
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