NaOH and Na₄P₂O₇ extractable organic matter in two allophanic volcanic ash soils of the Azores Islands - A pyrolysis GC/MS study

NaOH and Na₄P₂O₇ extractable organic matter fractions of two volcanic ash profiles (Azores Islands) were studied by pyrolysis gas chromatography/mass spectrometry (Py-GC/MS). The soils did not have melanic horizons and were not affected by burning. The pyrolysates of all samples were dominated by polysaccharide-derived compounds. The Na ₄P₂O₇ extractable fractions were relatively enriched by markers of lignin, proteins, and lipids. Only in the topsoils (A horizons) did lignin appear to be present in significant amounts and, although present with a high degree of side-chain oxidation, the distribution of lignin-derived products could be related to the overlying vegetation. A similar trend was observed for lipids, in particular the high abundance of C ₂₆ alkanol in profile EUR05 clearly reflecting grass. Below the topsoils, lignin and lipids were hardly detectable. With depth, markers of intact polysaccharides decreased relatively to smaller polysaccharide pyrolysis products. The same is true for chitin. Compared with other soils, the relative abundance of lignin and lipids was remarkably low. The occurrence of large amounts of polysaccharides including chitin points to an important in situ production of soil organic matter by fungi and/or arthropods. There is no indication of preservation of plant-derived organic matter by allophane or Al³⁺, but the presence of large amounts of (microbial) polysaccharides and chitin suggests that these secondary organic matter products are indeed stabilized.