What are humic substances? : a molecular approach to the study of organic matter in acid soils

Molecular studies on the composition of organic matter in soils are scarce. In this thesis, a molecular approach to the study of organic matter in acid soils is presented, with a focus on andic, i.e. volcanic, soils. Analyses include both chemical extractions as well as pyrolysis-GC/MS and CPMAS 13C NMR. Results are discussed in terms of their implications on the factors that determine the composition of organic matter in the acid soils studied (i.e. input, preservation/degradation and transport). In addition to obtaining molecular insights into SOM, the molecular approach to study SOM is standardized. The work described in this thesis is based on chemical extractions, chemolysis and pyrolysis-GC/MS in combination with CPMAS 13C NMR. Most importantly, it provides new molecular insights into SOM. Analyses of total lipid extracts, in addition to analyses of detailed specific lipid fractions, are shown to provide a more complete insight into the lipid chemistry, and therefore on the total of processes that determine the composition of OM in soils. Research on both the free lipid and the ester-bound fraction, revealed that roots are the main source of OM in acid soils. An input from the surface vegetation is only important within the first few centimeters of the soil. In general, ester-bound SOM is shown to be preserved in acid soils. On a molecular level, however, ester-bound unsaturated and epoxy moieties are shown to be quickly degraded in soils. Ester bound w-hydroxy acids were shown to be a source of free extractable w-hydroxy acids revealing a direct link between the ester-bound end free lipid fractions. Analyses on the bulk of SOM reveal a relatively high contribution of fungal and/or thermally altered plant polysaccharides in addition to polypeptides, aliphatic SOM and a condensed aromatic fraction. The non-hydrolyzable material is shown to consist of a condensed aromatic fraction and an aliphatic fraction. The latter was shown to consist of mono-, di-, tri- and tetra-functionalized aliphatic building blocks. Using the molecular data obtained, the main factors that determine the composition of organic matter in the acid soils studied are discussed, i.e. input, preservation/degradation and transport. It is suggested that humic substances are a group of operationally defined fractions comprised of both recognizable plant material and their degradation products, together with a fraction of SOM that has not been characterized at the molecular level. The preservation of SOM in andic soils is suggested to result from complexation with allophane/imogolite and ferrihydrite, complexation with free aluminum, aluminum toxicity to microorganisms, an acid soil pH and, for some SOM fractions, intrinsic structural resistance. Based on the molecular data presented in this thesis, a model consisting of four different cycles of SOM turnover in an acid andic soils is suggeste