EFFECTS OF FIRE ON HUMIC AND LIPID FRACTIONS IN A DYSTRIC XEROCHREPT IN SPAIN

Fire-induced changes in humic and lipid soil fractions were assessed by two approaches. Comparisons were made of the nature and amounts of the two fractions in samples of a Dystric Xerochrept under two pine stands, one that had been burned two years earlier and one that had not. Further comparisons were made of humic and lipid fractions in a soil sample before and after heating in the laboratory. Separation and identification of the compounds in these soil fractions were carried out by preparative liquid chromatography followed by GC-MS in the case of lipids; the humic acids were previous degraded by successive treatments with sodium persulphate and potassium permanganate. The greatest differences were in the distribution patterns of soil lipids (alkanes, fatty acids): accumulation of the homologues of lower molecular weight (< C20) was dominant in the post-fire samples. For soil resin acids, these samples had higher values for dehydroabietic and secodehydroabietic acids and lower ones for pimaric acid. The degradative studies on the humic acids showed that the relative yields of aromatic products were comparatively higher in the samples after burning. Alkanoic diacids were found to be the least resistant to heating, whereas greater amounts of alkanes in the less condensed polymer structures of the humic acids were observed in the samples after burning. The relative amounts and distribution patterns of the fatty acids released showed no great differences in the humic acids.