Characteristics of soil organic matter in temperate soils by curie point pyrolysis mass spectrometry iii. transformations occurring in surface organic horizons
Bracewell, J.M.; Robertson, G.W.
Geoderma 40(3-4): 333-344
In previous parts of this study, transformations of organic matter in mineral A and B horizons from a set of soils of temperate regions were characterised by Curie-point pyrolysis-mass spectrometry (Py/MS) and elucidated in terms of changes in the biopolymer composition. These changes were associated with the processes of eutrophic humification, illuviation and hydromorphism. Within the set (23 profiles from Scotland, chosen vas representative of soil groups of the temperate zone) some 13 podzols and humic gleysols displayed raw humus surface horizons above their mineral A horizons. The L, F and H layers of these organic horizons, where they were morphologically distinct, were successfully differentiated in terms of their biopolymer composition by Py/MS. These differences were of degreee rather than kind. The principal components analysis of 50 ion intensities from the mass spectra demonstrated a single dominant factor of composition. The corresponding reconstructed factor spectrum showed, in the sense L .fwdarw. F .fwdarw. H, the loss of lignin and polysaccharide products derived from raw plant material and the increase of residual or humified structures characterized by homologous alkene and higher benzene pyrolysis products. Peat was found to be similar to the L and F material. The successful observation of these chemical differences by a rapidly applied method is contrasted with alternative indices of organic composition, C%, N%, C/N, and also with pH, none of which showed any significant differences between the L, F and H horizons. The retarded process of humification which occurs in these horizons compares closely in terms of Py/MS with that occurring in anaerobic mineral soils and differs from the more rapid eutrophic humification in aerobic soils. The products resemble those from geochemical sediments.