Mineral Weathering Promotes Carbon Storage in Forest Soils Macro to Nanoscale Characterizations of 20-year in situ Weathered Vermiculite Particles

Forest soils are a major contributor to soil organic carbon (C) storage in terrestrial ecosystems and play a key-role in climate change mitigation. Mineral weathering in soils is expected to promote chemical and physical interactions between soil organic matter and mineral phases. These interactions are known to enhance the protection of organic matter from decomposition. The investigation of the mineral-organic associations (MOA) formation mechanisms during weathering is therefore crucial to understand carbon storage processes in soils. Until now studies have been mainly conducted through laboratory experiments in simplified and controlled conditions or over very long-term time scales using pedosequences. But knowledge about MOA formation processes occurring in situ is lacking, notably during the first stage of mineral weathering.

To fill this gap, we performed a mesh bag incubation of large Na-saturated vermiculite particles (100-200 µm in size) in a Typic Dystrochrept soil of a Douglas-fir forest, in the Beaujolais area (France). The incubated particles were deposited at the interface under the forest floor. After 20 years, the weathered vermiculite particles were collected and characterized at the macro-scale (XRD and physico-chemical analysis), at the micro-scale (Scanning Electron Microscopy – SEM, imaging and element mapping) and at the nano-scale (Transmission Electron Microscopy - TEM imaging, element mapping and speciation).

Cation exchange capacity, exchangeable cations and elemental analysis showed significant differences between the mineral structures of the initial (V0) and 20 year incubated (V20) vermiculite particles. The exchangeable Na was completely depleted. Cation exchange capacity strongly decreased in V20 (49.2 cmol_c kg^-1) compared to V0 (173.6 cmol_c kg^-1). The V20 lost its specific interlayer collapsing property (≈1.4 -> ≈1.0 nm) with K saturation. V20 interlayer collapsing was only observed with a 330°C heating treatment, suggesting the interlayer hydroxylation of vermiculite. High sheet dissolution, around 10%, was also observed. All these changes were attributed to chemical weathering, during which total C analysis showed significant enrichment in V20 (5.7 mg g^-1) compared to V0 (0.8 mg g^-1).

Macro, micro and nano-scale images and elemental mapping of V0 particles showed a highly flat, smooth surface morphology with no detected C. In contrast, V20 particles showed irregular outer and inner surfaces marked by multiple cracks of chemical dissolution. We also observed internal nano-sized exfoliation spaces filled with C and enriched in Ca, and micro-sized exfoliation spaces filled with C entrapped in nano-crystalline Mn oxides or K-rich aluminosilicates precipitates. The nature of the organic matter found strongly differed between small and large exfoliation spaces. It was characterized by alcohol, carboxyl functional groups and C=C bonds in small exfoliation spaces, while the obtained EELS spectra were more difficult to interpret in large exfoliations spaces. These results provide new evidence that over 20 years in situ weathering induces a significant dissolution, among other physical and chemical changes in large vermiculite particles. They reveal that the mineral weathering processes are responsible for the organic matter entrapment (i) in the newly formed mineral nano-sized spaces, possibly mediated by Ca, and (ii) in association with secondary minerals deposits in micro-sized spaces.