Advances in Scanning Transmission X-Ray Microscopy for Elucidating Soil Biogeochemical Processes at the Submicron Scale

Organic matter, minerals, and microorganisms are spatially associated in complex organo-mineral assemblages within soils. A mechanistic understanding of processes occurring within organo-mineral assemblages requires noninvasive techniques that minimize any disturbance to the physical and chemical integrity of the sample. Synchrotron-based soft (50-2200 eV) X-ray spectromicroscopic techniques, including scanning transmission X-ray microscopy (STXM), transmission X-ray microscopy (TXM), X-ray photoemission electron microscopy (X-PEEM), and scanning photoelectron microscopy (SPEM), coupled with microspectroscopy (e.g., near-edge X-ray absorption fine structure; NEXAFS) allow for determining the spatial association and speciation of most elements found in soils while maintaining sample integrity. This review highlights application of the four spectromicroscopic techniques mentioned above to soil biogeochemical research, with particular emphasis on STXM-NEXAFS, which has contributed to the greatest set of advancements in the understanding of soil organo-mineral interactions, including mineral control on organic carbon cycling and the mechanisms of biomineral formation.