Plant versus microbial signature in densimetric fractions of Mediterranean forest soils: a study by thermochemolysis gas chromatography mass spectrometry

Abstract

Soil organic matter (SOM) ageing may be measured by means of molecular signatures, ratios between organic compounds which inform us about the origin and/or the degree of biochemical evolution of (or microbial contribution to) specific groups of compounds: lipids, proteins, carbohydrates, etc. Owing to the biochemical heterogeneity of decomposing substrates, it is unlikely that the degree of biochemical evolution can be approached with a single ratio. Nevertheless, obtaining a wide collection of molecular signatures can be costly. Instead of applying specific methods to obtain a collection of ratios, we apply thermally assisted hydrolysis and methylation (THM), followed by GC-MS, to obtain a panoramic view of SOM composition. From the compounds identified after THM, several ratios were obtained. Three ratios are based on aliphatic compounds: (1) ratio between short-chain (≤ 20 C) and long-chain (> 20 C) alkanoic acids, (2) ratio between branched and long-chain alkanoic acids, (3) ratio between short-chain (C18) and long-chain (C22–C24) alkanols. Four ratios are based on lignin-derived monomers: (4) vanillic acid to vanillin, (5) syringic acid to syringaldehyde, (6) a combination of the two previous, and (7) the syringyl- to total lignin monomers. Finally, three ratios are based on sugar composition: (8) fucose to glucose, (9) xylose to glucose, and (10) fucose to xylose. These ratios were applied to the study of several densimetric fractions from three organic OH horizons from Mediterranean forest soils. The fractionation gave a free-light fraction (LF), of density 2.0. The three lightest fractions (LF, OC1, and OC2) seem the least microbially reworked, whereas the denser fractions, OC3 and DF, seem the most evolved ones. Nevertheless, this is valid only as an average result, for no single fraction is made of fresh compounds only, or of highly evolved compounds only, either. The behaviour of the several ratios was inconsistent because some behave in ways opposite to the expected ones. All fractions show signs of both advanced biochemical evolution and preservation of fresh, plant-derived labile compounds. This, added to the inconsistent behaviour of many signatures, suggests that our views about the biochemical evolution of plant debris during their decomposition and humification are probably too simple.