The characterisation of soluble organic matter from forest soils

The characteristics of water-soluble organic material in soil solutions has been studied extensively 'in situ'. Relatively little attention has focused on isolation of such material, particularly when comparison of a number of samples has been attempted. In this study the water-soluble organic acids leached from the soil horizons beneath eight different tree species were isolated by adsorption onto macroporous resins. Amberlite XAD-8 and XAD-4 resins were used in series therefore two samples, labelled as 'hydrophobic' and 'hydrophilic' acids were obtained. Practical amounts of free organic acids for characterisation purposes were isolated from each resin. Elemental analysis (carbon, nitrogen and sulphur) and acidity (measured by potentiometric titration) reflected the probable state of oxidation of the material. Lower molecular weight material was more abundant with depth. Copper binding ability correlated well with the acidity generally but a group of samples isolated from similar horizons and resin showed high acidity and poor metal binding ability. This was probably due to the inability of the acidic sites to be involved in chelation reactions. Metal binding was measured using gel filtration, dialysis and an ion-exchange method. Chromatography in gel filtration was monitored by direct feed of column effluent into an atomic absorption spectrophotometer, a technique attempted but not used successfully before. The method measured directly metal bound and required only 0.2 mg of sample per determination. For these reasons this method was used for routine analysis whereas dialysis and ion-exchange were only used for comparison purposes. As expected the samples isolated on each resin were very different. Comparison of samples from the horizons showed clearly the increased state of oxidation with depth. It was possible to establish differences in the organic material in the soil horizons on the basis of whether the stands were coniferous or deciduous trees but the similarities between the samples were perhaps equally striking.