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Aims: The aim of this study was to assess the seasonal influence on the dynamics of exchangeable nutrients (K, Ca and Mg) in acidic and nutrient-poor forest soils, where nutrients can become limiting for tree growth. Methods: The amounts of exchangeable base cations (K+Ca+Mg) were measured in soil samples collected in three soil compartments (Bulk, Outer Rhizosphere, and Inner Rhizosphere) and in 4 months (November, February, May and August) under two stands of 31-year-old Norway spruce and beech in an acidic temperate forest. Results: In all season, both rhizosphere compartments were enriched in exchangeable nutrients compared to bulk soil. This suggests that tree roots and root-associated microorganisms (bacteria and mycorrhizal fungi) increased nutrient availability through mineral weathering or mineralization processes, and thus could contribute to forest sustainability in nutrient-poor conditions. Interestingly, in contrast to beech, a drastic decrease of exchangeable base cations was observed in bulk soil of spruce between November and February (higher than 80% for K and Mg, and 100% for Ca). The relation between this decrease, Al solubility, and nitrate concentration are evoked in the discussion. Conclusion: This study reveals that processes resulting from interactions between trees, microorganisms and soil influence not only the seasonal dynamics of nutrients in the root vicinity but also the bulk soil function. [ABSTRACT FROM AUTHOR]

The mineralogy of bulk and rhizosphere soils was compared to assess the effect of roots on mineral weathering in a Typic Dystrochrept supporting Norway spruce (Picea abies [L.] Karst) and oak (Quercus sessilifiora Smith). In an experimental forest site (Breuil-Chenue, France), systematic soil sampling was performed in forty pits. The soil adhering to the roots was considered as rhizosphere soil. The remaining material was regarded as bulk soil. The mineralogy of the clay-sized particles of both fractions was determined by x-ray diffraction (XRD). Amorphous solid phases were estimated by extracting Fe and Al with hot Na-citrate and dithionite-citrate-bicarbonate. Total chemical analyzes were performed on the clay-sized particles via BaCl2-saturation and an iso-zirconium weathering balance was calculated. The XRD demonstrated an increase of illite-like minerals in the rhizosphere of both species and the selective extractions revealed a decrease in the amorphous phases. The total chemical analyzes showed that the rhizosphere clay-sized fraction contained significantly more Si and K (for oak only) and less Fe and Al than the bulk soil. By way of the iso-zirconium weathering balance, these losses of Fe and Al in the rhizosphere were estimated at several tens of kilograms by hectare for the surface horizon (0-3 cm). This study demonstrates that, despite the short duration of the contact between the active part of a root and the solid mineral phase, the intensity of the processes occurring in the rhizosphere significantly increases mineral weathering. [ABSTRACT FROM AUTHOR]

The ectomycorrhizal symbiosis alters the physicochemical and biological conditions in the surrounding soil, thus creating a particular environment called ectomycorrhizosphere, which selects microbial communities suspected to play a role in gross production and nutrient cycling. To assess the ectomycorrhizosphere effect on the structure of microbial communities potentially involved in the mobilization of nutrients from the soil minerals in a poor-nutrient environment, we compared the functional diversity of soil and ectomycorrhizosphere bacterial communities in a forest stand. Two hundred and sixty-four bacterial strains and 107 fungal strains were isolated from the bulk soil of an oak ( Quercus petraea) stand and from oak– Scleroderma citrinum ectomycorrhizosphere and ectomycorrhizae, in two soil organo-mineral horizons (0 to 3 cm and 5 to 10 cm). They were characterized using two in vitro tests related to their capacities to mobilize iron and phosphorus. We demonstrated that the oak– S. citrinum ectomycorrhizosphere significantly structures the culturable bacterial communities in the two soil horizons by selecting very efficient strains for phosphorus and iron mobilization. This effect was also observed on the diversity of the phosphate-solubilizing fungal communities in the lower soil horizon. A previous study already demonstrated that Laccaria bicolor–Douglas fir ectomycorrhizosphere structures the functional diversity of Pseudomonas fluorescens population in a forest nursery soil. Comparing to it, our work highlights the consistency of the mycorrhizosphere effect on the functional diversity of bacterial and fungal communities in relation to the mineral weathering process, no matter the fungal symbiont, the age and species of the host tree, or the environment (nursery vs forest). We also demonstrated that the intensity of phosphorus and iron mobilization by the ectomycorrhizosphere bacteria isolated from the lower soil horizon was significantly higher compared to that which was isolated from the upper horizon. This reveals for the first time a stratification of the functional diversity of the culturable soil bacterial communities as related to phosphorus and iron mobilization. [ABSTRACT FROM AUTHOR]

The principal nutrient source for forest trees derives from the weathering of soil minerals which results from water circulation and from plant and microbial activity. The main objectives of this work were to quantify the respective effects of plant- and root-associated bacteria on mineral weathering and their consequences on tree seedling growth and nutrition. That is why we carried out two column experiments with a quartz-biotite substrate. The columns were planted with or without pine seedlings and inoculated or not with three ectomycorrhizosphere bacterial strains to quantify biotite weathering and pine growth and to determine how bacteria improve pine growth. We showed that the pine roots significantly increased biotite weathering by a factor of 1.3 for magnesium and 1.7 for potassium. We also demonstrated that the inoculation of Burkholderia glathei PML1(12) significantly increased biotite weathering by a factor of 1.4 for magnesium and 1.5 for potassium in comparison with the pine alone. In addition, we observed a significant positive effect of B. glathei PMB1(7) and PML1(12) on pine growth and on root morphology (number of lateral roots and root hairs). We demonstrated that PML1(12) improved pine growth when the seedlings were supplied with a nutrient solution which did not contain the nutrients present in the biotite. No improvement of pine growth was observed when the seedlings were supplied with all the nutrients necessary for pine growth. We therefore propose that the growth-promoting effect of B. glathei PML1(12) mainly resulted from the improved plant nutrition via increased mineral weathering. [ABSTRACT FROM AUTHOR]