Humic substances (HSs), consisting, on the basis of solubilities in aqueous acid and basic media, of humic acids (HAs), fulvic acids (FAs), and humin (Hu), are the major components of soil organic matter (SOM). Most studies of soil/natural organic matter (SOM/NOM) have been carried out on extracts of soils in dilute sodium hydroxide solutions, the solvent used to extract the Standards of the International Humic Substances Society (IHSS). However, Hu, the major component in the classical definition of HSs, is insoluble in aqueous base and is not isolated by the traditional IHSS method. Recently, a sequential exhaustive extraction (SEE) process has been shown to be capable of isolating and separating the major components of the classically defined HSs from the soils of the temperate and tropical regions. The SEE system was used in the present study to isolate the HA/FA and Hu fractions from a subtropical volcanic Taiwanese soil. Chemical and compositional properties of these extracts were then compared with similarly obtained isolates from soils from the different climatic regions. Increases in the aliphatic relative to aromatic carbon contents were observed for both the HA and FA fractions when the pH values of the extraction media were increased. HAs and FAs isolated using the SEE method have spectroscopic profiles similar to those from the IHSS isolate; however, the cumulative extraction efficiency (%) of the SEE method (65 %) for the volcanic soil was much higher than for the traditional IHSS method (33 %). When the residual volcanic soil, following extractions once, three, and eight times with 0.1 M NaOH were then extracted with dimethyl sulphoxide (DMSO) plus concentrated sulphuric acid (the final solvent in the SEE sequence) it was seen that the content of crystalline polymethylene hydrocarbon (33 ppm (13)C-NMR resonance in the Hu (or DMSO/acid)) extract increased relative to the amorphous methylene (30 ppm). That highlights the difficulty in dissolving the more highly ordered hydrocarbon structures that would be expected to have closer associations with the mineral colloids. Although the SEE procedure isolated all of the HAs and FAs from the Yangmingshan soil, extractability of the Hu from the volcanic soil in the DMSO/acid solvent was low (21 %), and contrasted with the much higher yields from temperate and tropical regions. The decreased Hu extraction may arise from its associations with the extensive iron and aluminium hydroxide mineral colloids in the soil. The Hu from this sub-tropical soil was different from the Hus isolated from other soil types, indicating the need to isolate and characterise these recalcitrant organic material in order to understand the organic carbon components in soils in greater detail. Such results would indicate that more attention should be given to mineral colloids in soils, and to the organo/mineral associations that will have an important role in the stabilities of OM in the soil environment.

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1 Sediment cores (ca. 6 m) from an estuarine environment gave insights into the composition 2 and preservation of recalcitrant organic carbon (OC) in the environment. The coring locations 3 provided organic matter (OM) of terrestrial and of marine origins. Our study specifically 4 focused on the humin (HU); the OM fraction that is most difficult to isolate and to 5 characterise. HU fractions were compared with the total OM recovered after removal of the 6 associated mineral colloids. 7Solid state and multiphase (nuclear magnetic resonance) NMR experiments were 8 carried out on dried and swollen samples to obtain comparative information about the whole 9 and the fractionated samples. The total OM associated with the clay-sized fraction provided a 10 standard that allowed differences between the fractions to be observed. 11The NMR data have provided new insights into the molecular structures that become 12 part of the long term C sink in sediments. The recalcitrant OC in the sediments is composed 13 mainly of aliphatic hydrocarbon material that may be protected from, or otherwise 14 unavailable for degradation. Microbial peptides and carbohydrates were also shown to be 15 important contributors to the C sink and these biomolecules may be from living or preserved 16 necromass. Lignin residues formed only a small part of the OM in the surface sediments but 17 made a larger contribution at depth. Highly ordered components in HU (that resists swelling 18 by dimethylsulphoxide, DMSO) play a major role in C sequestration. 19

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