This study was conducted on a mull and a moder from two beech stands, 27 and 87 yr old, respectively, in the Fougeres forest. In each stand, five profiles were subdivided in OL, OF and OH (moder only) layers and A(1) subhorizons. In the mull OL-OF layer, the organic matter was quickly degraded by white-rot fungi together with bacteria and,earthworms. A fungal attack occurred in the moder OL and OF layers, whereas bacteria seemed efficient in the OF layer and especially so in the OH layer, where they appeared responsible for lignin and structural polysaccharide degradation. Lignin degradation was indicated by: (i) the decrease of all phenolic monomers, (ii) methoxyl group loss and (iii) an increase in the vanillic acid/aldhehyde ratio. The production of microbial exo-polysaccharides at the expense of the structural polysaccharides, revealed by an increase of the mannose/xylose ratio, was also supported by transmission electron microscopy observations. The decrease in the lignin phenols and structural polysaccharides continues in the underlying A(11) and A(12) layers. In the mull, earthworm activity results in a complex organo-mineral association, whereas in the moder, enchytraeid worm activity is responsible for mixing of inherited organic aggregates associated with minerals.

L'âge des pâturages d'altitude des Vosges est encore très mal connu. Sur la base de l'analyse des archives historiques, il avait été proposé que les défrichements qui avaient abouti à leur installation fussent l'œuvre des moines qui avaient colonisé les vallées vosgiennes entre les VIIe et VIIIe siècles de notre ère. Notre étude pédoanthracologique remet en cause cette hypothèse, grâce à l'identification de charbons de genévrier (Juniperus communis) datés du IIe ou du Ier siècle av. J.-C. Cette espèce, caractéristique de milieux ouverts en voie d'enfrichements, prouve que des pâturages d'altitude existaient au moins 800 ans avant les dates les plus anciennes avancées, au moins depuis la fin de l'âge du fer. Pour citer cet article : D. Schwartz et al., C. R. Geoscience 337 (2005).

SummaryRock‐Eval pyrolysis was designed for petroleum exploration to determine the type and quality of organic matter in rock samples. Nevertheless, this technique can be used for bulk characterization of the immature organic matter in soil samples and recent sediments. We studied 76 samples from seven soil classes and showed that their pyrograms can be described by a combination of four elementary Gaussian components: F1, F2, F3 and F4. These four components are related to major classes of organic constituents differing in origin and their resistance to pyrolysis: labile biological constituents (F1), resistant biological constituents (F2), immature non‐biotic constituents (F3) and a mature refractory fraction (F4). We discriminated the relative contributions of these components and used them to derive two indices: (i) to quantify the relative contributions of labile and resistant biological constituents and (ii) to quantify the degradation stage of the soil organic matter. The practical applications are illustrated via the influence of vegetal cover on soil organic matter dynamics and peat development in a Holocene sedimentary sequence, but we suggest that the approach is of much wider application.

Contents of preserved lignin structures in Miocene-aged lignite lithotypes were determined. Phenol aldehydes, ketones and acids released from lignin by CuO oxidative hydrolysis were separated by capillary zone electrophoresis (CZE). Products of degradation were separated and identified by GC–MS to confirm CZE data and to complement composition data. The following values for the contents of preserved lignin structures in lignite lithotypes were determined (in wt%): xylain, 8.5; humovitrain, 6.7; liptain, 6.4; and humoclarain, 3.3. Some fatty acids, negligible contents of benzene polycarboxylic acids and their hydroxy/methoxy counterparts were also observed using GC–MS.

Four beech stands of various ages were selected in 1997 to study the evolution of lignins and structural polysaccharides (cellulose, hemicelluloses) in the soil cover. The 10-year-old station has a mull type humus. In the 27-year-old stand there is a mull-moder mosaic whereas in the 87 and 145-year-old stands humus is a moder. Twenty-one soil profiles were sampled by separating the different humus layers, i.e., OL and OF, present in mull and moder, the OH layer, present only in the moder. Organo-mineral A I I and A 12 horizons were also sampled with some A13 horizons in the mull stations. Lignins are abruptly degraded in mull where they represent 52parts per thousand of the total organic carbon (TOC) in OL and only 12parts per thousand, in Al horizons. Similarly, polysaccharides undergo degradation from 236parts per thousand (OL) to 105parts per thousand (A 1) of TOC. The fast decrease in the concentration of these components over a small depth interval is indicative of a strong biological activity. The lignin alteration is evidenced by the decrease of the ratio of syringic compounds over vanillic compounds that reveals metboxyl group losses and the increase of vanillic acid over vanillic aldehyde which indicates oxidative depolymerization of lignins. In old stands (87, 145-y.) where humus was of an uniform moder type, the decrease of the xylose/mannose ratio in the OF and OH layers reveals the production of microbial sugars at the expense of the phyto-inherited polysaccharides, like cellulose and hemicelluloses which decrease, whereas lignins are also strongly degraded. The decrease of the structural polysaccharides continues in the underlying Al horizon, similar to the evolution observed in the transition from mull to moder although at a less degradation rate.

Application of Rock-Eval pyrolysis to soil organic matter (SOM) quantitation and characterization has been explored by the study of about 100 soil samples taken from a variety of soil profiles from different ecosystems at different latitudes. A straightforward illustration of these possibilities can be obtained from a Hydrogen Index (HI in mg hydrocarbons g−1 TOC) vs. Total Organic Carbon (TOC) diagram that effectively allows one to follow simultaneously the main qualitative (SOM hydrogen richness given by HI values) and quantitative (TOC) changes that affect SOM with increasing depth and humification, in the soil profiles. In addition, abnormally high Oxygen Index (OI in mg CO, CO2 or O2 g−1 TOC) values are fully diagnostic of extensive SOM alteration, as frequently observed in podzol B horizons. More detailed information on the heterogeneity of SOM and on its degree of evolution, can be gained from the shape of the pyrolysis S2 peak recorded in the course of programmed pyrolysis in an inert atmosphere (N2) and/or from its maximum temperature “Tpeak”. All these parameters and others, all determined rapidly and automatically, are particularly useful to screen major SOM variations within large sets of samples.

In an area of savanna-forest (S-F) mosaic of Cameroon, at Kandara near Bertoua, an enclosed savanna bordered by young semi-deciduous forests was selected for detailed studies of vegetation and soil carbon isotope compositions with a view to estimating the rate of forest advance into savannas. Forest floristic composition and structure were analysed in small plots along two S-F transects and within two large stands. Tree species counts and basal area (BA) measurements gave convergent results defining (1) an edge forest with low BA values that forms an irregular strip parallel to the S-F border, (2) a large colonization zone zone composed of pioneer species (Albizia species, with individuals of very large diameter) and (3) a mature forest composed of abundant Rinorea individuals and large individuals of Triplochiton scleroxylon and Piptadeniastrum africanum. Carbon stable isotopes were deter mined from organic matter of soil profiles sampled at various depths in savanna, colonization-zone and mature forest. In the deep soil horizons (40-50 cm) of the colonization-zone profiles, δ13C values similar to those of the present savanna reveal the past existence of a large-tree savanna. In subsurface horizons (15-20 cm), δ13C values intermediate between those of savanna and mature forest prove the encroachment of the forest ecosystem on savanna. Using 14C measurements, the mean residence time (MRT) of soil organic matter of these last horizons was determined with precision owing to the atmospheric 14C pulse from nuclear bomb tests prior to 1964. By assuming an exponential age distribution of organic compounds and by taking account of MRT and remaining carbon from the savanna, the coloniza tion zone was found to be 60-80 y old. The age of the colonization zone being the same near the present savanna and near the mature forest, it seems that the forest advance was probably not a linear process but would result from the coalescence of Albizia thickets born in savanna.

In an area of savanna-forest (S-F) mosaic of Cameroon, at Kandara near Bertoua, an enclosed savanna bordered by young semi-deciduous forests was selected for detailed studies of vegetation an soil carbon isotope compositions with a view to estimating the rate of forest advance into savannas. Forest floristic composition and structure were analysed in small plots along two S-F transects and within two large stands. Tree species counts and basal area (BA) measurements gave convergent results defining (1) an edge forest with low BA values that forms an irregular strip parallel to the S-F border, (2) a large colonization zone zone composed of pioneer species (Albizia species, with individuals of very large diameter) and (3) a mature forest composed of abundant Rinorea individuals and large individuals of Triplochiton scleroxylon and Piptadeniastrum africanum. Carbon stable isotopes were determined from organic matter of soil profiles sampled at various depths in savanna, colonization-zone and mature forest. In the deep soil horizons (40-50 cm) of the colonization-zone profiles, delta C-13 values similar to those of the present savanna reveal the past existence of a large-tree savanna. In subsurface horizons (15-20 cm), delta C-13 values intermediate between those of savanna and mature forest prove the encroachment of the forest ecosystem on savanna. Using C-14 measurements, the mean residence time (MRT) of soil organic matter of these last horizons was determined with precision owing to the atmospheric C-14 pulse from nuclear bomb tests prior to 1964. By assuming an exponential age distribution of organic compounds and by taking account of MRT and remaining carbon from the savanna, the colonization zone was found to be 60-80 y old. The age of the colonization zone being the same near the present savanna and near the mature forest, it seems that the forest advance was probably not a linear process but would result from the coalescence of Albizia thickets born in savanna.

In spodosols of Gascony (France). conversion of maritime pine stands into maize cropping leads to an incorporation of maize organic matter, which changed the isotopic (delta C-13) and phenolic signature in A and L horizons of soil. Hydrolysis of phenol lignin in forests and cultivated soils showed the predominance of vanillic units under forest and the early but moderate incorporation of cinnamic acids. Incorporation of syringic units appeared higher, related to a large maize production of stable syringic phenols. Syringic units represented a long-term marker of maize inputs in soils, whereas vanillic units revealed the degradation of forest organic matter. (C) 2001 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.

In spodosols of Gascony (France), conversion of maritime pine stands into maize cropping leads to an incorporation of maize organic matter, which changed the isotopic (δ13C) and phenolic signature in A and L horizons of soil. Hydrolysis of phenol lignin in forests and cultivated soils showed the predominance of vanillic units under forest and the early but moderate incorporation of cinnamic acids. Incorporation of syringic units appeared higher, related to a large maize production of stable syringic phenols. Syringic units represented a long-term marker of maize inputs in soils, whereas vanillic units revealed the degradation of forest organic matter.

Peat samples from a one metre core and living Cyperaceae, collected in Tritrivakely marsh in Madagascar, were studied to determine the organic matter (OM) composition and extent of OM degradation in this core. The study was carried out combining light microscopy observations, bulk analyses, infra-red spectroscopy, hydrolyses of sugars, oxidation of lignin and pyrolyses. In the surface peat, organic matter derived from Cyperaceae undergoes extensive degradation of its basic cell wall components, morphologically revealed by destructuration of plant tissues and their transformation into reddish amorphous organic matter occurring in large amounts all along the core. Two ratios (cinnamic units/lignin and xylose+arabinose/total sugars) were determined as markers of Cyperaceae. It appeared that the vegetation of the marsh remained probably unchanged during the considered accumulation period, i.e. the last 2300 years B.P. Rhamnose, mannose and non-cellulosic glucose probably have a common origin and are mostly derived from bacteria. In contrast, galactose is likely to be a marker of algal source, especially of the diatoms that occur only in the upper part of the core (0–ca. 50 cm). Acid/aldehyde ratios of syringic and vanillic monomers (index of lignin oxidative depolymerisation) and mannose+rhamnose+non-cellulosic glucose/total sugars ratios (reflecting bacterial degradation of hemicelluloses) are positively correlated, and can thus be considered as markers of microbial degradation of the Cyperaceae tissues. The n-alkane/n-alk-1-ene doublets that dominate the pyrolysates of hydrolysed peat samples reflect the contribution of B. braunii algaenan and higher plant suberans, and of condensed lipids mostly derived from higher plants and microalgae. The upper part of the core is characterised by a greater dilution of Cyperaceae-derived compounds by organic matter from microalgae when compared with deeper samples, as recorded by peat bulk features, hydrolysable sugars, lignin oxidation products and pyrolysis products. Two accumulation periods can thus be distinguished in the core: a peaty phase between 2300 years B.P. and ca. 1500 years B.P. (low watertable and strongly limited microalgal growth); a waterlogged marsh, from ca. 1500 years B.P. to the present time, in which a higher water table was longer lasting with a substantial algal production. The environmental variation thus recorded could correspond to a regional climatic change occurring around 1500 years B.P.

Geochemical analysis of sedimentary organic matter in recent lacustrine sediments appears to be a useful tool in providing information concerning past environmental conditions. However, such analysis is often made without knowing the geochemical characteristics of the organic matter derived from the watershed and, more explicitly, its soils. The present work deals with (i) a geochemical investigation (Rock-Eval pyrolysis) of soil organic matter sampled in a lake watershed, and (ii) the study of the sedimentary organic matter trapped in the lake deposits. The research was conducted on Chaillexon Lake which was created by a rock collapse that dammed the palaeovalley of the Doubs River about 12 000 years ago. Since this event, the sediment trap provides a continuous palaeoclimatic record for the Postglacial period.Results obtained lead to two main conclusions. First, the variability of Rock-Eval pyrolysis values observed in soils modifies the common interpretation given to these parameters in the characterization of sedimentary organic matter. Indeed, variations in these parameters point not only to varying proportions of terrestrial and lacustrine organic matter in a lacustrine infilling but also to variations of the terrestrial supply linked with the evolution of vegetal cover in the catchment. The second conclusion is that the story of the Chaillexon lacustrine system is marked by a rather sudden soil and forest development at the Preboreal-Boreal transition (9000BP). (C) 1998 John Wiley & Sons, Ltd.

Capillary zone electrophoresis (CZE) was investigated for the separation of eleven phenolic aldehydes, ketones and acids which are main components of lignin. The influence of the buffer pH, electrolyte composition, acetonitrile amount, temperature and voltage has been investigated to determine the best separation conditions. Using optimized buffer [55 mM borate-phosphate (pH 9.1) / acetonitrile (91.5–8.5 v/v], efficient separations occur in less than 15 min with a good repeatability of the electropherograms, giving a relative standard deviation of each peak of less than 1%. The method was applied to the analysis of phenolic compounds produced by the CuO oxidation of foliar organs of gymnosperm and angiosperm trees. Results are in agreement with data obtained by other chromatographic methods.

The current soil dynamics of a podzol-oxisol climosequence located in Tahiti on basaltic and pyroclastic parent rocks has been investigated using the ''test-mineral'' technique. The test-mineral, a vermiculite with a high exchange capacity (161 meq/100 g) was placed in recoverable bags in the A and B horizons of three members of the sequence, for periods of 1 to 3.5 years.Changes in the chemistry and mineralogy, enabling complexing acid environments to be distinguished from non-complexing, were determined following the in situ reaction with natural soil leachates. A set of leachates from the same horizons were collected and chemically analysed at the end of 3.5 years. The results indicate that the current acid environment is only weakly complexing. This suggests that the podzolisation processes which led to the development of a unique A2 horizon containing gibbsite, anatase and rutile, and a placic horizon, in the upper podzol member of the sequence, is no longer operating.

Chaillexon lake has been created by a rock collapse that has dammed the valley of the Doubs river 12,000 years ago. The available space was and is still filling up by the river and that trap represents a continuous record for the Post-glacial. The study is based on the characterization of geochemical and optical organic markers in the actual watershed (soils and bedrock) and their occuring in the sedimentary organic matter. It appears that the Preboreal/Boreal transition (9000 BP) characterizes a major event in the story of this lacustrin system. A forest and soils development permits a significant protection of the bedrock from erosional processes.

In the Congo, Eucalyptus and Pinus have been planted commercially for 30 yr on savanna. Large differences in the abundance of carbon isotopes of the savanna Gramineae and the trees enabled a study to be made of the soil organic matter (SOM) turnover by means of its carbon isotope composition. The organic carbon content slightly increased with plantation age without any significant differences (90% confidence limit) between plantation types. The δ13C of SOM varies with the plantation age from −14.4%, in the savanna reference plot to values close to −26.5% in the oldest tree plantations. The distribution of δ13C as a function of plantation age was modelled by an exponential function and exhibited no significant variation (90% confidence level) between plantation type. As the relative contribution of organic matter inherited from the savanna and that derived from the tree species was proportional to the δ13C value, the fraction of each organic compartment was evaluated for plantations grouped together without distinguishing between the different tree species. With time, the SOM inherited from the savanna tended to disappear. The half-life was estimated to be 16.5 yr, clearly illustrating the rapid SOM turnover which characterizes the savanna ecosystems in psammitic ferrallitic soils. The fraction of organic C of tree origin progressively introduced into the soils was linearly related to time. The annual increment of organic C was found to be equal to 0.3 mg C g−1 yr−1, but it was clear that the linear evolution of carbon with time could not be extrapolated over more than 30 yr. The various models predicted that after 30 yr there would remain in the soil, 1.8 and 0.8 mg C g−1 of savanna origin and 9.4 mg C g−1 of newly introduced organic C of tree origin respectively.