Biochar did not affect inorganic C sequestration through steel slag weathering, but increased soil organic carbon loss in a leaching experiment

Large-scale deployment of carbon dioxide removal (CDR) technologies will be needed to achieve the goals of the Paris Agreement. Enhanced silicate weathering (EW) and biochar are promising CDR strategies, yet little is known about their combined deployment. This study evaluated the effects of steel slag and two contrasting biochar types on weathering dynamics, inorganic C sequestration and soil organic carbon dynamics in a seven-week leaching experiment. Under intensive irrigation steel slag weathered rapidly, increasing soil pH and Ca release, which promoted inorganic C sequestration through both dissolved inorganic carbon export and pedogenic carbonate formation. Co-application with biochar did not significantly affect weathering rates or inorganic C sequestration. However, an interaction effect was observed for soil organic carbon dynamics; although biochar alone increased soil organic carbon stocks from 0.94 ± 0.003 mol to 1.86 ± 0.003 and 1.92 ± 0.003 mol (depending on biochar type), soil organic carbon stocks were 10.2% and 6.3% lower, respectively, when BOF slag and biochar were co-applied (1.67 ± 0.003 and 1.80 ± 0.003 mol) than under biochar application alone, suggesting a priming effect. These findings indicate that under co-deployment of EW and biochar soil organic carbon loss may offset inorganic C sequestration in the short term, highlighting the need for a mechanistic understanding of soil organic carbon cycling under combined application.