Investigating the complexation of soil organic carbon by calcium in soils developed in the diverse parent materials of Point Reyes, California using synchrotron-based spectro-microscopy.

Organo-mineral association and complexation processes are important for the retention and accumulation of soil organic carbon (SOC). However, most of the research into these processes has focused on the biogeochemical interactions between SOC and Fe or Al, largely overlooking a role of Ca in these complexation processes. Recent studies have shown the Ca can be linked to increased organo-mineral association in soils with a higher pH (> 6), but the mechanisms behind this relationship still require investigation. We used a range of synchrotron-based techniques to investigate the complexation of SOC by Ca in soils developed on different parent materials from the Point Reyes National Seashore, CA. We used synchrotron-based X-ray diffraction to ascertain the mineralogical differences in our soils. Calcium K-edge X-ray absorption spectra were collected on both our bulk soil samples and mineral standards. Additionally, specific elemental associations were investigated using micro-probe analysis coupled X-ray absorption spectro-microscopy. Scanning transmission X-ray microscopy (STXM) was also used to investigate Ca-C complexes (Ca L-edge and C K-edge spectra) in our soil and their associations with other elements such as Fe. Though these soils were acidic in nature, STXM confirmed that Ca had a strong spatial correlation with C, presenting a higher spatial correlation than Ca and Fe. The C spectra associated with Ca differed from those associated with Fe or Fe-Ca-C. This work implies that there is a preferential stabilisation of specific functional groups by Ca, even in moderately acidic soils, where Ca complexation processes could still play an active role in the accumulation of C. This work also highlights that the coupling of these advanced characterisation methods can be used to effectively investigate the bonding environment of metals and carbon in soils.