Micro- and nano-environments of carbon sequestration: Multi-element STXM–NEXAFS spectromicroscopy assessment of microbial carbon and mineral associations

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 mi