Contribution of primary and secondary mineral phases to organo-mineral aggregation during crushed basalt weathering in the presence of fresh plant residue

The application of crushed basalt to soil receives increasing attention as its chemical weathering can promote carbon dioxide removal. However, its impact on the persistence of organic matter (OM) in soil remains poorly understood due to the complex mineral compositions of basalt particles and their interactions with OM during rock weathering. We hypothesized that the mineral-OM aggregation is promoted by easily weatherable primary minerals together with secondary minerals during basalt weathering. To elucidate if the formation of mineral-OM aggregates was mineral-selective, we characterized the basalt-POM aggregates formed in a 6-month leaching experiment using basalt-plant residue-quartz sand mixtures. The aggregates were isolated with density fractionation, and the mineral compositions and distributions were examined with quantitative X-ray diffractometry (qXRD), X-ray absorption near-edge structure (XANES), micro XANES (μ-XANES), and scanning electron microscopy (SEM). The results showed that the meso-density fraction (MF; 1.8 – 2.4 g cm-3) of the fresh basalt was initially enriched with smectite and amorphous minerals than the bulk basalt as found with qXRD, which might have initiated the mineral-POM aggregation at the early stage of the 6-month incubation. The SEM images showed patchy coatings of POM on the basalt particles, implying the presence of preferential binding sites. We found preferential incorporation of plagioclase and pyroxene into the MF   aggregate relative to other primary minerals present in basalt with increasing amorphous mineral phase. Using μ-XANES, we found Fe(III) secondary minerals, presumably from smectite or amorphous minerals, located on the surface of basalt particles and thus appeared to act as binders between basalt and POM particles. However, smectite could be initially present and incorporated in MF during basalt weathering. 
This study demonstrated the importance of not only the secondary minerals but also the easily weatherable primary minerals for promoting fresh organic matter stabilization under wet-and-dry cycles. The stability of the OM in these meso-density aggregates remains unclear. Further study is needed to evaluate the physical structures of mineral-OM aggregates as well as the biodegradability of the OM therein for determining carbon stability.