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Soil represents the largest reservoir of terrestrial organic C, and plays a critical role in global C cycling. In light of predicted climate change and a more unified approach to mitigate greenhouse gas emissions, the soil's ability to sequester C, and thus to act as a sink or a source for atmospheric CO2 has received growing interest. Organomineral assemblages are a unique niche in C cycling, with large capacity for storing anthropogenic C. However, the underlying biogeochemical mechanisms for C sequestration through organomineral associations are not yet well understood. One of the major challenges to study C sequestration in organomineral assemblages is lack of non-invasive analytical tools with a potential to obtain molecular-level information about the interactions between C and mineral components in submicron geochemical environments. In the present study, we have effectively employed synchrotron-based STXM–NEXAFS spectroscopy to access the K- and L-edges of biogeochemically relevant elements (C, N, Ca, Fe, Al, Si) to identify and image micro- and nano-C sequestration environments, and conduct submicron-level investigation of the compositional chemistry and other interactive features of C and minerals present in these hotspots using ultrathin section of intact organomineral assemblage. The C K-edge NEXAFS spectromicroscopy micrographs clearly demonstrated the existence of spatially distinct seemingly terminal micro- and nano-C repository zones, where organic C was sequestered in apparent agglomeration in the investigated organomineral assemblage. These submicron-C repository environments were only a few micrometers apart from each other; yet they were considerably different compositionally from each other. The organic C in the first repository environment was pyrogenic in origin, largely composed of quinone, phenols, ketones and aromatic ring structures. However, the second hotspot was dominated by filament-like structure, with striking similarity to the C 1s NEXAFS spectral signatures of organic C isolated from soil fungal and bacteria, and dominated by resonances from aliphatic-C and CN bonds of imidazol structures, carboxyl/carbonyl-C, amide- and O-alkyl-C functionalities. The composition of organic C in the organomineral interface around the strand-like structure was highly complex and composed of polysaccharides, amino sugars, amino acids, nucleic acids, and phospholipid fatty acid structures with polar and non-polar termini. The chemistry of mineral matter in the organomineral interface was also equally complex, ranging from Ca, Fe and Al ions, Fe and Al oxides, hydroxides and oxyhydroxides to phyllosilicates, which could provide a variety of polyvalent cations, hydroxyl surface functional groups and edge sites that can attract and bind microbial biomolecules. Based on the enormous complexity of the organic C functionalities and the coexistence of various inorganic components in the organomineral interface, it is possible to suggest that no single binding mechanism could be accountable for the organic C stored in the investigated submicron-C repository environment. Our results seem to suggest that the apparent C sequestration in the micro- and nano-C repository environment appear to be the cumulative result of physical protection and heterogeneous binding mechanisms ranging from ion exchange, hydrogen bonding, and hydrophobic bonding on silicate clay–organic complexes to adsorption on external and internal surfaces of clay minerals. |
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