Nanoscale chemical imaging of soil organo-mineral associations.

Organo-mineral associations drive organic matter (OM) stabilization in soils, but mechanisms controlling their dynamics are still not fully known at micro and nanoscale. Adsorption of OM on minerals’ surfaces is a prevalent viewpoint of OM stabilization processes (Kleber et al., 2007), but Basile-Doelsch et al., (2015) suggested that mineral alteration generating amorphous nanophases and cationic oligomers on minerals’ surfaces is also a driver of OM stabilization through coprecipitation processes. Lab experiments which mimic these processes showed that the nanosized co-precipitates (Nanosized Coprecipitates of inorganic oLIgomers with organiCs: nanoCLICs) are made of inorganic Fe, Al, Si oligomers associated with organic molecules (Tamrat et al., 2019). Andosols are known to have a high OM-stabilization capacity, mostly attributed to associations of OM with nanominerals (imogolite, allophane, proto-imogolite) (Basile-Doelsch et al., 2007; Levard et al., 2012). In the present study, we investigated the presence of nanoCLICs in Andosol fractions from La Martinique (French West Indies). We used Transmission Electron Microscopy (TEM, FEI Tecnai Osiris 200kV) coupled with 4 EDX detectors and EELS to semi-quantify and map major elements. TEM analyzed zones of interest ranged from 5 µm to 10 nm with pixel size from 500 to 1 nm. Few crystallized minerals, particulate OM and amorphous thin fibers that could not be definitively attributed to imogolite nanotubes were observed. However, we mainly observed totally amorphous phases to electron diffraction. Al, Si, C, Fe and O were the main component of the latter amorphous phases. Al, Si and Fe were systematically associated to C even at a size resolution down to 1 nm (semi-quantifications ranged from 11 to 41% of C, 4 to 7% of Fe, 34 to 36% of Al and 22 to 46% of Si). Similar high-resolution images were obtained for the andosol organo-mineral associations and the synthetic nanoCLICs. At the working TEM resolution, the nanoCLICs model proposed by Tamrat et al., (2019) is consistent with the structures observed on the andosol. Based on these results, the majority of C appears to be in nanoCLICs form in these Andosol fractions and confirms the hypothesis puts forward by Basile Doelsch et al., (2015).

Basile Doelsch et al., 2007. Mineral control of carbon pools in a volcanic soil horizon. Geoderma, 137 (3-4), 477-489. ISSN 0016-706.

Basile-Doelsch et al., 2015. Are Interactions between Organic Compounds and Nanoscale Weathering Minerals the Key Drivers of Carbon Storage in Soils? Environ. Sci. Technol. 49, 3997–3998.

Kleber et al., 2007. A Conceptual Model of Organo-Mineral Interactions in Soils: Self-Assembly of Organic Molecular Fragments into Zonal Structures on Mineral Surfaces. Biogeochemistry 85, nᵒ 1 (1 août 2007): 9‑24.

Levard et al., 2012. « Structure and distribution of allophanes, imogolite and proto-imogolite in volcanic soils ». Geoderma 183‑184 (1 août 2012): 100‑108. 

Tamrat et al., 2019. « Soil organo-mineral associations formed by co-precipitation of Fe, Si and Al in presence of organic ligands ». Geochimica et Cosmochimica Acta, 10 juin 2019.