The Alkaline Nitrobenzene Oxidation of Soil Organic Matter

Summary The yields of syringaldehyde, vanillin, and p‐hydroxybenzaldehyde obtained by alkaline‐nitrobenzene oxidation of soil organic matter indicate the extent to which lignin‐derived material is present. A number of soils, peats, and peat fractions have been oxidized by this method and the phenolic aldehydes formed have been separated and determined by a‐ micro‐technique. The results indicate the presence in soil organic matter of significant amounts of syringyl, guaiacyl, and p‐hydroxyphenyl residues which are almost certainly derived from lignin. The yields of aldehydes from mineral soils accounted for about 0.5–1.0 per cent. of the total organic carbon; from peats for about 1.0–4.0 per cent. There was evidence of a correspondence between the relative amounts of the three aldehydes obtained from the soil organic matter and from the parent plant material. The results for a peat profile indicated a tendency for the yield of aldehydes to increase with depth of the horizon; the reverse was found with a pine‐forest soil profile. Peat fractions all gave some aldehydes on oxidation, but there were marked differences in the levels; the highest yield, almost 7 per cent., was obtained from a humic‐acid fraction soluble in ethanol‐benzene. Hydrolysis of the humic‐acid fractions with dilute hydrochloric acid markedly depressed the yields of aldehydes obtainable on oxidation.