Bernard Guillet

Cerium anomalies in lateritic profiles

The REE geochemistry and mineralogy have been studied in four lateritic profiles, one derived from a syenite (Akongo, SW Cameroon), the others being developed on a gneissic basement and located along a soil toposequence (Goyoum, E Cameroon). There is a fractionation between LREE and HREE in the lateritic samples during weathering, the weathered residual products being enriched in LREE (from La to Eu) and depleted in HREE (from Gd to Lu); sampled waters are enriched in HREE in relation to the syenite host-rock. A positive Ce-anomaly has been found systematically at the top of the saprolite, beneath a zone of iron oxide accumulation in the laterite. Up to 2000 ppm Ce may be present. In the Akongo profile, cerianite, CeO2, is present as very fine coatings in non-ferruginous clayey domains. Primary REE-bearing accessory minerals are weathered at the bottom of the profile. Specifically, allanite and apatite are transformed into florencite and rhabdophane but these phases have no Ce-anomaly. All the data are interpreted as the result of the following processes: 1. (1) REE leaching in a reducing environment. 2. (2) oxidation of Ce3+ to Ce4+ in an oxidizing environment. 3. (3) deposition of cerium as cerianite whereas the other REE remain in solution.

13C/12C Ratios of soil organic matter as indicators of vegetation changes in the congo

Schwartz, D., Mariotti, A., Lanfranchi, R. and Guillet, B., 1986. 13C/1ZC ratios of soil organic matter as indicators of vegetation changes in the Congo. Geoderma, 39: 9 7-1 03. The 13C/1zC ratios were determined for the organic matter of all horizons of a podzol profile and of the A, horizons of some ferrallitic soils, in some grass shoots and in a fossil root fragment from the Bzh horizon of the podzol. The isotope ratio in the organic matter of the A, horizon of the podzol matches those in grass shoots from the present savanna vegetation. The ratios in the lower horizons match those of organic matter in the A, horizons of soils under forest and that of the fossil root fragment in the Bzh horizon. The ratios thus demonstrate that the humus enrichment of the Blh horizon of the podzol occurred while it was under fores1 vegetation and thl the presenl grass vegetation did not take part in the podzolization process. The differences also indicate that savanna replaced forest vegetation after the profile had been formed. I

Soil organic matter (SOM) characterization by Rock-Eval pyrolysis : scope and limitations

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.

Organic matter sources and early diagenetic degradation in a tropical peaty marsh (Tritrivakely, Madagascar). Implications for environmental reconstruction during the Sub-Atlantic.

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.

Agreement between floristic and soil organic carbon isotope (13C/12C,14C) indicators of forest invasion of savannas during the last century in Cameroon

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, δ 13 C values similar to those of the present savanna reveal the past existence of a large-tree savanna. In subsurface horizons (15-20 cm), δ 13 C values intermediate between those of savanna and mature forest prove the encroachment of the forest ecosystem on savanna. Using 14 C measurements, the mean residence time (MRT) of soil organic matter of these last horizons was determined with precision owing to the atmospheric 14 C 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.

Pollen-rain-vegetation relationships along a forest-savanna transect in southeastern Cameroon.

Modern soil and litter samples from southeastern Cameroon, collected along a continuous forest-savanna transect were analysed for pollen content to define modern pollen-vegetation relationships. The pollen results, completed and compared with botanical inventories, leaf area index and basal area measurements performed in the same area, clearly registered the physiognomy, the main floristic composition and floral richness of the two sampled ecosystems. Distortions were observed between sampled vegetations and their pollen rain, related to important differences in pollen production and dispersal of plant species: this is a general feature in many tropical regions. The pollen data in the area studied reflected well the recent transgression of forest versus savanna. This permitted us to define inside the forest ecosystem more successional vegetation communities than the botanical surveys allowed.

Separation of phenolic aldehydes, ketones and acids from lignin degradation by capillary zone electrophoresis

Abstract 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 m M 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.

Distribution of sulphur forms in soils from beech and spruce forests of Mont Lozère (France)

From a quantitative inventory of sulphur forms and sulphur budget, the relation between the distribution of the various sulphur forms and the sulphate fluxes in three soil profiles has been addressed. These profiles are located in two forested watersheds at Mont Lozère. One has been sampled in a beech forest and the other two in a spruce forest and in a harvested plot of this spruce forest, respectively. The mean annual input-output budgets showed a sulphur immobilization in the soil cover of the three plots. In the preserved spruce forest plot, because of larger dry depositions, the sulphur immobilization is much greater than in other plots and occurs essentially in the B horizons. In the other two profiles, the dominant immobilization occurs in the parent material.The total sulphur content is very high in the forest floor reaching 2065 μg S g-1 in the litter of one of the soils under spruce. In the organo-mineral horizons of soils under spruce, the total sulphur content decreases with depth and ranges from 310 to 520 μg S g-1 in the A horizons to 100–200 μg S g-1 in the parent material. In the profile under beech, the total sulphur content is lower except in the parent material. In all cases, the organic sulphur is the major part of sulphur often representing more than 90% of total sulphur. In organo-mineral soil horizons of the spruce forest, the part of the sulphateesters is more important than in the soil of the beech forest, probably related to the different nature of the microbial activity in the spruce forest. In contrast, the humification processes are more efficient in the soil under beech, which can be due to the greater input of organic sulphur by litterfall. It appears that the dominant organic sulphur form varies as a function of microbial ecology and sulphate flux. The maximum of the inorganic sulphate is located at the base of the B horizons in the soil of the spruce forest and in the parent material of the soil under beech. In these horizons, the high content of inorganic sulphate can be related to the higher amounts of amorphous Fe and Al phases.

Evolution morphologique des litières de hêtre (Fagus sylvatica L.) et transformation des biopolymères, lignine et polysaccharides, dans un mull et un moder, sous climat tempéré (forêt de Fougères, Bretagne, France)

Morphological evolution of beech litter (Fagus sylvatica L.) and biopolymer transformation (lignin, polysaccharides) in a mull and a moder, under temperate climate (Fougeres forest, Britany, France). Can. J. Soil Sci. 85: 405-416. 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 1 1 and A 1 2 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.

Preserved lignin structures in Miocene-aged lignite lithotypes, Bulgaria

Abstract 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.