Pascal Bonnaud

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.

Effects of the clear-cutting of a Douglas-fir plantation (Pseudotsuga menziesii F.) on the chemical composition of soil solutions and on the leaching of DOC and ions in drainage waters

The effects of the clear-cutting of a 70-year-old Douglas-fir plantation on the chemical composition of soil solutions and on leaching of nutrients in drainage waters were observed by a continuous monitoring, six years before and three years after the cutting. Forest harvesting was made with very limited soil disturbances. Results showed that the concentration of weakly fixed solutions did not change but that the concentration of gravitational solutions of the upper soil layers drastically fell down after the cutting. The limited increase in nutrients leached with drainage waters was only due to the increase in the water flux, which is difficult to quantify because of the presence of ground vegetation. The monitoring of numerous fluxes before and after the clear-cutting could explain the specific behaviour of the soil solutions. The limited losses of nutrients the after clear-cutting in a potentially responsive ecosystem were unexpected. The initial hypothesis was that the decrease in the mineralization and nitrification rates observed after the cutting was related to a stimulating effect of Douglas-fir on the activity of soil nitrifyers.RésuméLes effets de la coupe à blanc d’une plantation de Douglas de 70 ans ont été observés sur la composition chimique des solutions du sol et les pertes d’éléments par drainage, par un suivi mensuel pendant 6 ans avant, et 3 ans après la coupe. L’exploitation du peuplement a été réalisée avec une perturbation minimum du sol. Les résultats montrent que les solutions liées ont peu évolué après la coupe, alors que le changement des solutions libres a été drastique dans les horizons de surface du sol. Malgré des incertitudes sur le rôle de la végétation spontanée, le drainage d’éléments n’a pas fortement augmenté après la coupe. La prise en compte de l’ensemble des flux mesurés dans cette étude semble pouvoir expliquer les observations. Les pertes limitées après la coupe d’une plantation où l’activité nitrifiante était élevée avant la coupe étaient inattendues. L’hypothèse avancée est l’arrêt du contrôle stimulateur des populations nitrifiantes du sol après la coupe du Douglas.

Altération d'un phyllosilicate par des champignons ectomycorhiziens in vitro

Summary Two strains of ectomycorrhizal fungi proved able in vitroto alter ammonium saturated vermiculite, freeing bound ammonium ions. Such release is partly associated with crystal lattice dissolution.

Effects of the clear-cutting of a Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantation on chemical soil fertility

Abstract• Stand harvesting and regeneration were usually considered to be a critical phase for the sustainability of forest soils. The present study concerned the effects on soil chemical fertility of the clear-cutting of a highly productive Douglas-fir stand aged 67 years that was clear-cut with no disturbance.• Results showed that soil changes were rapid in the three-year period following the cutting. The forest floor mass considerably decreased and the mineral soil showed a limited but real acidification. Soil losses represented 4% of the available nutrients over a depth of 60 cm for N, 22% for K, 25% for Ca and 32% for Mg. Only P increased by 11%. Due to the spatial variability of forest soils, and despite regular re-sampling, confidence intervals were large and difficult to reduce.• The reversibility of the effects of the clear-cutting and its consequences on soil functions depend on the element: it should not be a problem for C, N and K, which would recover when the biological cycle is re-established once again in the young stand. Phosphorus is not at issue since it changes form in the soil. The depletion of Ca, and to a lesser extent of Mg, is of some concern as a result of limited soil reserves, the limited flux of cations from the mineral changes in the soil, the relatively large part of Ca and Mg in the forest floor, and the negative input-output budgets for those elements.• The duration of the impact of the clear-cutting on soil requires medium-term observations because it cannot be deduced from the current knowledge of this ecosystem.Résumé• La récolte des peuplements est une phase critique pour la durabilité des sols forestiers. Cette étude concerne les effets de la coupe à blanc d’un peuplement de Douglas de 67 ans, réalisée sans perturbation, sur la fertilité organo-minérale du sol.• Les résultats montrent que l’évolution du sol est rapide dans la période de trois ans après la coupe. La couche organique décroît fortement et le sol minéral se désature et s’acidifie. Les pertes représentent 4 % des éléments disponibles du sol sur 60 cm pour N, 22 % pour K, 25 % pour Ca et 32 % pour Mg. Seul P augmente de 11 %. Cependant, la variabilité spatiale des sols forestiers et la technique de l’observatoire, par échantillonnages successifs, conduisent à des intervalles de confiance larges autour de ces valeurs, intervalles qui s’avèrent difficile à réduire.• La réversibilité des modifications du sol dépend des éléments : elle ne devrait pas poser de problème pour C, N et K. P n’est pas en cause puisqu’il change de forme dans le sol mais aucune perte n’est notée. Les pertes de Ca voire de Mg sont les plus préoccupantes en raison des faibles réserves du sol, du flux limité de cations issu de l’altération des minéraux du sol, de la part relativement élevée des ces éléments dans les couches organiques, et de leur bilan entrées-sorties négatif.• La durée de l’effet de la coupe à blanc nécessite des observations sur le moyen terme car elle ne peut pas être déduite des connaissances actuelles sur cet écosystème.

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.