The potential of enhanced rock weathering for CO2 removal and soil organic carbon storage via organo-mineral aggregation: the trade-off induced by basaltic rock particle size

Carbon dioxide removal (CDR) via enhanced rock weathering (ERW) strongly depends on rock particle size. While ERW models typically link finer particle size to greater CDR, their tendency to aggregate with soil components such as organic matter (OM) may impede weathering. The inconsistent effects of ERW on soil OM storage in recent studies reinforce the need to clarify underlying mechanisms. We thus tested if finer basaltic rock promotes organo-mineral association while lowering CDR through incubation experiments (rock alone and rock-plant residue-sand mixture) under water regimes with or without weekly leaching. After six months, we analyzed total carbon, extractable metal(loid)s, organo-mineral aggregate formation (by density fractionation), and inorganic carbon contents (by XANES and leachates). Coarse basaltic rock (106–150 μm) showed faster abiotic and biologically induced weathering. Contrarily, fine basaltic rock (20–38 μm) led to greater organo-mineral aggregation and OM accrual, which was attributable to higher particle numbers, geometric surface area, and binding agents (inherent and increased reactive metal(loid)s). The amount of organic carbon stabilized in meso-density aggregates by basaltic rock was one order of magnitude higher than the estimated CDR, regardless of the water regimes. These results exhibit the first direct evidence that rock particle size could induce the trade-off between CO2 removal and OM stabilization, which implies that the current ERW models may severely overestimate CDR potential due to basaltic rock interaction with OM and its weathering products. Further research into rock interactions with soil components is essential for improving model prediction and optimizing ERW applications.