N‐Alkanes and free fatty acids in humus and A1 horizons of soils under beech, spruce and grass in the Massif‐Central (Mont‐Lozère), France

Abstract

SummarySoil profiles under beech, spruce and a grassland have been analysed to study the evolution of natural n‐alkanes in pollution‐free ecosystems. The soils had all developed on granitic bedrock, at an altitude of 1300–1500 m in the region of Mont‐Lozère (southern Massif‐Central, France). In contrast to the grassland soil, the two forest soils both possessed a well‐developed acidic moder humus‐type horizon. This could be subdivided as follows: fresh litter (OL), fragmentation (OF) and humification (OH) layers; two litters, one fresh (OL1) and one old (OL2) could actually be distinguished in the beech forest soil. The n‐alkane signature of the parent plants was preserved in the top litter. Immediately underneath, in the OF layer(s) the original n‐alkane signatures were progressively but rapidly replaced by a common signature composed of n‐C27 and n‐C25 with larger proportions of the former than of the latter. These two hydrocarbons were most probably produced in situ by fungi. These results appear to illustrate the action of soil microorganisms which metabolize the inherited n‐alkanes and produce new compounds of the same family. Unlike the alkanes and the low molecular weight fatty acids ≤ C20 (which increase greatly in the OL2 layer under beech as a result of intense microbial activity), the heavy fatty acids (> C20) show no significant change in the organic horizon.