Nanoscale Biogeocomplexity of the Organomineral Assemblage in Soil

Methodological constraints limit the extent to which existing soil aggregation models explain carbon (C) stabilization in soil. We hypothesize that the physical infrastructure of microaggregates plays a major role in determining the chemistry of the occluded C and intimate associations between particulate C, chemically stabilized C and the soil mineral matrix. We employed synchrotron‐based scanning transmission X‐ray microscopy (STXM) coupled with near‐edge X‐ray absorption fine structure (C 1s‐NEXAFS) spectroscopy to investigate the nanoscale physical assemblage and C chemistry of 150‐μm microaggregates from a Kenyan Oxisol. Ultra‐thin sections were obtained after embedding microaggregates in a sulfur block and sectioning on a cryo‐microtome at −55°C. Principal component and cluster analyses revealed four spatially distinct features: pore surfaces, mineral matter, organic matter, and their mixtures. The occurrence of these features did not vary between exterior and interior locations; however, the degree of oxidation decreased while the complexity and occurrence of aliphatic C forms increased from exterior to interior regions of the microaggregate. At both locations, compositional mapping rendered a nanoscale distribution of oxidized C clogging pores and coating pore cavities on mineral surface. Hydrophobic organic matter of aromatic and aliphatic nature, representing particulate C forms appeared physically occluded in 2‐ to 5‐μm pore spaces. Our findings demonstrate that organic matter in microaggregates may be found as either oxidized C associated with mineral surfaces or aromatic and aliphatic C in particulate form. Using STXM and C 1s‐NEXAFS we are for the first time able to resolve the nanoscale biogeocomplexity of unaltered soil microaggregates.