SOILS, SEC 1 • SOIL ORGANIC MATTER DYNAMICS AND NUTRIENT CYCLING • RESEARCH ARTICLE
Plant versus microbial signature in densimetric fractions
of Mediterranean forest soils: a study by thermochemolysis gas
chromatography mass spectrometry
Received: 25 October 2017 / Accepted: 23 May 2018
Springer-Verlag GmbH Germany, part of Springer Nature 2018
Purpose Soil organic matter (SOM) ageing may be measured by means of molecular signatures, ratios between organic com-
pounds 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.
Materials and methods 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.
Results and discussion These ratios were applied to the study of several densimetric fractions from three organic O
from Mediterranean forest soils. The fractionation gave a free-light fraction (LF), of density < 1.6, three occluded fractions (OC1,
OC2, and OC3, of densities < 1.6, 1.6–1.8, and 1.8–2.0 respectively), and a dense fraction (DF) 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.
Conclusions 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.
Responsible editor: Anja Miltner
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s11368-018-2046-8) contains supplementary
material, which is available to authorized users.
* Pere Rovira
CTFC (Forest Science and Technology Centre of Catalonia),
Carretera St Llorenç de Morunys, km 2, 25280 Solsona,
Fac Chimie, Laboratoire de Synthèse et Réactivité des Substances
Naturelles, CNRS UMR 6514, Université de Poitiers, 40 Ave
Recteur Pineau, 86022 Poitiers, France
Journal of Soils and Sediments