Spatial complexity of soil organic matter forms at nanometre scales

Organic matter in soil has been suggested to be composed of a complex mixture of identifiable biopolymers(1) rather than a chemically complex humic material(2). Despite the importance of the spatial arrangement of organic matter forms in soil(3), its characterization has been hampered by the lack of a method for analysis at fine scales. X-ray spectromicroscopy has enabled the identification of spatial variability of organic matter forms, but was limited to extracted soil particles(4) and individual micropores within aggregates(5,6). Here, we use synchrotron-based near-edge X-ray spectromicroscopy(7) of thin sections of entire and intact free microaggregates(6) to demonstrate that on spatial scales below 50 nm resolution, highly variable yet identifiable organic matter forms, such as plant or microbial biopolymers, can be found in soils at distinct locations of the mineral assemblage. Organic carbon forms detected at this spatial scale had no similarity to organic carbon forms of total soil. In contrast, we find that organic carbon forms of total soil were remarkably similar between soils from several temperate and tropical forests with very distinct vegetation composition and soil mineralogy. Spatial information on soil organic matter forms at the scale provided here could help to identify processes of organic matter cycling in soil, such as carbon stability or sequestration and responses to a changing climate.