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

Abstract

Mineral-associated organic matter, the dominant form of relatively stable carbon (C) in soil, often co-occurs with reactive iron (Fe) and aluminum (Al) phases across soils. Yet, how organo-metallic associations at the molecular scale give rise to emergent soil properties such as aggregate formation and the persistence of organic matter (OM) in soil remains unclear. The organo-metallic glue hypothesis proposes that dissolved metal released from weathering and microbially processed OM form cohesive organo-metallic phases that bind other particles into stable assemblages. We tested this concept using an artificial soil system comprising crushed rocks (fine basalt: 20–38 µm, coarse basalt and granite: 38–75 µm, and river sand), mixed with leaf compost and microbial inoculum, subjected to eight wet-dry cycles using artificial rainwater (pH 4.7) over 55 d. Sequential density fractionation after the incubation revealed the formation of meso-density, organo-mineral assemblages (1.8–2.4 g cm−3: MF) in the following order: fine basalt > coarse basalt > granite > sand. The accretion of C and oxalate-extractable Fe, Al, and Si in MF generally followed the same pattern. Fine basalt showed the strongest increase in extractable metals, especially Fe, in MF and the highest leaching of Fe and base cations (esp. Na and Ca). Enrichment of extractable Fe, Al, and Si in MF and their slight depletion in the high-density fraction (> 2.4 g cm−3) suggest that weathering-derived metals first associated with OM, forming organo-metal-rich phases that subsequently bound other particles to form organo-mineral assemblages. MF formed in fine basalt treatment had the C: (Fe+Al) molar ratio of 0.6, consistent with organo-metal coprecipitates. Preferential incorporation of microbially-processed, N-rich OM into MF in the two basalt treatments was indicated by lower C:N ratios by 23–25 units and enrichment of δ13C and δ15N by 0.9 ‰–1.2 ‰ and 0.6 ‰, respectively, relative to low-density fraction (< 1.8 g cm−3). SEM and STXM/NEXAFS analyses of limited MF materials confirmed the presence of shaking-resistant microaggregates and the co-localization of microbially altered C with Fe and Al. Collectively, these results provide experimental evidence supporting the organo-metallic glue hypothesis and demonstrate that basaltic rock weathering can promote organo-mineral assemblage formation. This mechanism links microbial processing, mineral weathering, and reactive metal dynamics, offering insights into early pedogenesis and soil OM formation under rock amendment conditions.