M. Robert

Weathering of ammonium- or calcium-saturated 2:1 phyllosilicates by ectomycorrhizal fungi in vitro

The ways through which ectomycorrhizal fungi benefit tree growth and nutrition have not been totally elucidated. Our study was therefore aimed at assessing fungal access to soil exchangeable and stable mineral reserves. The growth of different ectomycorrhizal fungi in bi-compartment Petri dishes with NH4+- or Ca-saturated vermiculite led to cation exchange reactions and to crystal lattice weathering. The presence or absence of soluble NH4+ or Ca did not seem to affect fungal mobilization potential. Oxalic acid appears to be involved in vermiculite weathering by Paxillus involutus and Ca ions could limit the acid dissolution of Ca-saturated vermiculite. The quantitative significance of such cation mobilization and mineral weathering still has to be assessed in situ and in association with ectomycorrhizal plants.

Caractérisation des matières colloidales évacuées au cours du drainage agricole : incidence sur l'évolution pédogénétique des sols

Abstract During the drainage season, significant quantities of suspended matter can be exported from a drain collector. Their nature has been determined with a methodology linking chemical analysis, X-ray diffraction, and electronic microscopy associated with image analysis. The majority of these particles are smaller than 200 nm. They are composed of 2:1 phyllosilicates and iron particles usually in amorphous form. 137 Cs, phosphorus and organic matter clearly demonstrate the superficial origin of these particles. Drainage modifies natural leaching of soil by exporting definitively particles from the soil profile.

Biogeochemistry of calcium in a broad-leaved forest ecosystem

This study of the biogeochemical cycle of Ca has been carried out in an oak-hornbeam forest ecosystem on a soil derived from the alteration of Famennian (upper Devonian) shales. The physical nature of the geologic substrate made it possible to use the watershed-ecosystem approach by monitoring the loss of Ca by streamflow. Input of hydrogen ions and Ca from rain were average as well as that of mineral nitrogen of which 50% was ammonium-N. Modelling the biogeochemical cycle of Ca in this ecosystem proved to be difficult owing to the heterogeneity of the geologic substrate.The cationic denudation occured mostly with bicarbonate ion as a carrier of the base cations towards the streamflow. Seasonal effects on streamwater cationic composition were in relation to maximal soil respiration occuring in the autumn. In addition to this effect, the electrolyte concentration of streamwater was determined by rain and by changes in concentration of the soil solution either by evapotranspiration or by freezing. In spite of widely fluctuating values of flow rate, the concentration of the streamflow remained constant within a factor of two or at the most three. The mechanism underlying this buffering action of the soil on the concentration of the solution flowing through it, was shown to be related to ion-exchange processes.

The in Situ Weathering of a Test Mineral (Vermiculite) Introduced in a Podzol (Haplaquod) and in a Hydromorphic Soil (Haplaquept) Compared to the Soil Clay Minerals Evolution

Abstract In the Landes du Medoc (France), the soil mantle is organized in short soil toposequences (10–20 metres) made of podzols (Haplaquods) in the less waterlogged upper part, and of hydromorphic soils (Haplaquepts) in the lower part. In the upper part, podzolization is shown by the strong redistribution of OM-Al complexes in a Spodic horizon. Because of the presence of these complexes in the groundwater, this process is likely to be active at present. In the eluvial horizons of podzols, the study of the clay minerals has revealed a weathering process by acidocomplexolysis. On the other hand, weathering in the hydromorphic soil has given intergrade minerals which reflects a simple acidolysis. Samples of a test-vermiculite were introduced in the main horizons of the soils. They were removed and reexamined after six months (winter period or summer period) or one year of in situ experiment. The weathering of the test-vermiculite showed distinctive features according to the soils and the seasons. After the summer period no change could be identified in the test-vermiculite from any of the two soils. After the winter period a few interlayers were blocked at 14 A (interlayered hydroxy-Al) in the vermiculite from the A 1 horizon of the hydromorphic soil. The phenomenon was more pronounced for the vermiculite from the E and Bh horizons of the podzol where most of the interlayers were blocked. According to experimental models such a behaviour is due to the presence of a non-complexing acidic medium in these horizons: this does not correspond to the results reduced from the study of the soil clay minerals.