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 rather than a chemically complex humic material. Despite the importance of the spatial arrangement of organic matter forms in soil, 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 and individualmicropores within aggregates. Here, we use synchrotron-based near-edge X-ray spectromicroscopy of thin sections of entire and intact freemicroaggregates 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. Since the first investigations of soil organic matter in the eighteenth century, its composition has mostly been considered to be chemically very complex and to consist of mixtures of large molecules and reassembled microbial and plant debris. This paradigm is about to change on the basis of findings that organic matter in soil extracts can be explained by plant and microbial biopolymers and their degradation products rather than distinct so-called humic macromolecules. Recently, the notion that the spatial organization of soil organic matter within its mineral assemblage holds the key to an understanding of processes controlling its quantity and quality as well as turnover has gained increasing attention. Specific compounds bearing certain chemical functional groups may be responsible for binding to mineral surfaces and form very stable organic carbon pools with long turnover times, whereas others may be present as particulate organic matter within pores of aggregates and can be rapidly mobilized for example by tillage. Different physical location of organic compounds that vary in their molecular structure and therefore chemical recalcitrance will also exhibit differential response to global warming. Such fine-scale differences in organic matter forms may be expressed on a scale of several nanometres, Energy (eV) Ab so rb an ce (a rb . u ni ts ) 289.3 287.3