Formation of mineral-associated organic matter via rock weathering: an experimental test for the organo-metallic glue hypothesis

Abstract. Mineral-associated organic matter (MAOM), representing the dominant form of relatively stable C in soil, contains high physicochemical heterogeneity. The co-localization of organic matter (OM) with reactive aluminum (Al) and iron (Fe) phases in various MAOM fractions—across a range of natural and cultivated soils from five soil orders—has led to the “organo-metallic glue” hypothesis. The hypothesis proposes that coprecipitates formed between mineral-derived metals and microbially processed OM act as a binding agent, promoting the formation of stable microaggregates and thereby enhancing soil OM persistence. However, the formation mechanism remains unclear as the observed associations reflect multiple soil processes. We thus designed a simple laboratory experiment to test if the supply of metals and metalloids through rock weathering controls MAOM formation and if the OM-to-metal ratio of the material formed is consistent with complexation, sorptive association, or their mixture (i.e., coprecipitates). Two end-member igneous rocks (granite and basalt) crushed to have 38–75 µm size and, additionally, 20–38 µm size for basalt, as well as river sand (100–300 µm) as control were mixed with leaf compost (powdered to 100–250 µm) as single OM source. The mineral-OM mixtures were incubated aerobically at 30 ^oC with the natural soil microbial community and subjected to 8 wet-and-dry cycles using artificial rainwater (pH 4.73) over a 55-day experiment. The mixtures were then fractionated by density to examine the formation of meso-density, organo-mineral aggregates (1.8–2.4 g cm^–3: MF) by distinguishing it from the compost-dominant low-density fraction (< 1.8 g cm^–3: LF) and high-density fraction (>2.4 g cm^–3: HF) consisting of the crushed rock. The MF formation assessed as C content was 1.49 ± 0.06 mg C g^–1 rock (fine basalt), 1.04 ± 0.08 (coarse basalt), and 0.62 ± 0.06 (granite) over the 55 days, while the net MF mass increase was detected only in fine basalt due to the presence of meso-density materials in the crushed rock (< 7 % by mass). Faster chemical weathering of the fine basalt was indicated by a significant increase in extractable Fe and Al phases, largely in MF, and the highest leaching of Fe and base cations (esp. Na and Ca). The organo-mineral aggregates formed in the fine basalt treatment had the C-to-metal (Fe+Al) ratio of 0.36 ± 0.01 (molar basis), consistent with organo-metal coprecipitation. Further analysis focusing on the two basalt treatments revealed a significant decline in C:N ratios by 23–25 units and enrichment of δ¹³C and δ¹⁵N by 0.9–1.2 ‰ and 0.6 ‰, respectively, in MFs compared to LFs, indicating a strong contribution of microbial N-containing compounds to the MAOM formation. While microbial community composition differed among the treatments, no significant difference was found in qPCR-based bacterial number or species richness. Microscopic analyses using SEM and STXM confirmed the presence of shaking-resistant microaggregates and co-localization of C, Fe, and Al in MF from selected MF samples. Together, our results strongly supported the organo-metallic glue hypothesis and provided laboratory evidence of basalt-induced MAOM formation as well as some insights into early pedogenesis and organo-mineral interactions when applying crushed rock to soils.