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To which extent does the apparent negative correlation reported between silicate weathering rates and time result from the spontaneous physicochemical evolution of the fluid-mineral interface? To address this question, labradorite powders inserted in nylon bags and buried into two different topsoil horizons for four years were subjected to nanoscale characterization of their near-surface regions using transmission electron microscopy. These characterizations revealed the occurrence of a 30 to 70 nm-thick discontinuous amorphous silica-rich surface layer (ASSL) with a sharp crystalline-amorphous interfacial boundary between labradorite and the layer. Dissolution experiments conducted in mixed flow reactors at ambient temperature and acidic pH demonstrated that the reactivity of fresh and naturally weathered labradorite powders decreased with time, with the dissolution rate of fresh powders remaining systematically greater than that of naturally weathered powders, all over the duration of the experiments (3 weeks). In addition, the dissolution rate of all labradorite batches was noticeably lower in solutions containing elevated concentrations of SiO2(aq), which we attributed to the passivating effect of the ASSL5. This suggestion was confirmed with a simple passivation model, which enabled to capture (i) the greater reactivity of fresh powders;(ii) the dependence of the dissolution rate on [SiO2(aq)];(iii) the gradual decline of powder dissolution rate with time and (iv) the discontinuous occurrence of ASSL5. The model further supports that surface passivation could be one of the (non-exclusive) mechanisms that could account for the so-called kinetic "field-lab discrepancy". The geochemical implications of the recognition of the passivation mechanism are broad, ranging from the need to revisit the kinetic rate laws implemented in geochemical codes to the questioning of the formalism used for determining weathering rates from the study of U-series nuclides in soils and weathering profiles.

International audience; As soil minerals are the principal input of nutrients in non-fertilized forests, the parameters which influence their dissolution must be determined to predict ecosystem sustainability. Notably, biological activities within the rhizosphere, such as root and micro-organism exudation and respiration, considerably affect mineral dissolution rate. Numerous laboratory studies have even demonstrated that certain biological processes involved in mineral weathering can be stimulated in low-nutrient availability conditions, resulting in an improvement of plant nutrition. The objective of this work was to determine in the field if the mineral dissolution rate linked to root and root-associated micro-organism activity is increased in low-nutrient availability conditions. Here, the impact of the rhizosphere on the dissolution of test minerals containing Ca (fluorapatite and labradorite plagioclase) was assessed in an acid forest soil in two stands of mature beeches (Fagus sylvatica) presenting two levels of Ca availability: a control plot as well as a plot fertilized with Ca. Mineral-test bags were inserted at three different depths (−2.5, −10 and −20 cm) in the control and the Ca-fertilized plots into both a zone with roots as well as a zone where roots had been excluded, thus permitting to assess the effect of the rhizosphere on the mineral dissolution. After four years of incubation in the soil, the minerals were weighed and observed by scanning electron microscope. In the control stand, linear dissolution voids were only observed on the mineral surfaces incubated in the zone with roots, suggesting that local biological activities occurring in the rhizosphere affect mineral weathering. This positive effect of the rhizosphere in the control stand was confirmed by quantification of the mineral dissolution, which revealed an increase of fluorapatite and labradorite weathering, reaching factors 3 to 4 at 20-cm depth. In contrast, the beech rhizosphere did not increase mineral dissolution, hyphae colonisation or linear dissolution marks in the Ca-fertilized stand. These results suggest that the rhizospheric biological activities acting on mineral weathering could be regulated by the nutrient availability in the ecosystem. This plasticity of the rhizospheric biological activities may thus contribute to the maintenance of ecosystem sustainability.

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International audience; With the evolution of ecosystem management and the actual climate change we are facing, there is a need to improve our knowledge of carbon (C) balance and more specifically of C allocation in the plants. In our study, we quantified the seasonal variation of gross primary production (GPP, obtained through eddy covariance measurements) and biomass production (BP, the C fixed into the biomass obtained thanks to inventory campaign) for a 60-year-old even-aged beech stand located in North East of France. We also assessed the seasonal evolution of the BP efficiency (BPE=BP/GPP; Vicca et al., 2012) and its potential determining factors for our site. For 2014, we found a net ecosystem exchange (NEE) of -549 gC m-2, corresponding to a C sequestration. This value breaks down between 1089 gC m-2 for the respiration of the ecosystem and -1639 gC m-2 for the GPP. On the same year, our stand built up 461.6 gC m-2 of tree biomass (leaves, trunk, branches, fine roots), leading to an annual BPE of 0.28, which is within the range of value found on other similar sites. There was a large temporal variation of C allocation to the different parts of the tree biomass during the growth season. Our results show that the growth first happened in the trunk and branches -with a peak value of 74.5 gC m-2 month-1 in May - whereas the fine roots biomass production started later (end of July) and reached a maximum at the end of the growth season (28.49 gC m-2 month-1 for September). The BPE varied also during the year from 0.13 in April to 0.31 in August, where the BP was the same than in July but the cumulated GPP was already decreasing. The seasonal variation may be mainly explained by climatic variations, whereas the shift between woody above-ground biomass and fine roots biomass could be explained by the phenology (linked to physiological mechanisms).

International audience; The physicochemical properties of forest soils affect every aspect of soil fertility and productivity. Soil properties notably determine the availability of nutrients and water, and the ease of their absorption by trees, the ease of root penetration, and the degree to which solution moves both laterally and vertically through the soil. In consequence, soil properties can strongly influence tree nutrition and forest productivity. However, trees have at their disposal several mechanisms to improve nutrient availability in low-nutrient environments, i.e., modification of root architecture and exudation, association with microorganisms... This work aims at determining the impact of soil properties and root distribution on forest current productivity. In this view, the forest of Montiers (North East of France) was chosen to support the study because it presents a great diversity of soils representative of the French temperate forest on which rests a mature beech forest stand (50 year-old). In june 2009, 27 soil profiles were sampled in the rendisol, in the calci-brunisol and in the alocrisol for a total of 81 soil profiles. The physico-chemical properties and the root density were measured in these profiles. In addition, the trunk circumferences at breast level and the height were measured in 2009 and 2013 on about 2500 trees distributed on the three soils. By way of modeling, the biomass in the different tree compartments (0 to 4, 4 to 7 and > 7 cm diameter branches, trunk bark and wood) was determined for both dates. This work will present for the three soils (i) the soil physico-chemical properties, i.e., pH, base saturation, particle size distribution, soil water holding capacity and C, N and nutrient stocks, (ii) the evolution of the tree biomass, and (iii) the root density in the different soil horizons. The relations between soil fertility, forest productivity and soil root colonization are discussed.

International audience; Forests provide many benefits and services to society, including clean water, recreation, wildlife habitat, carbon storage, and a variety of forest products. Because nutrient availability is a major driver of forest stability, intensification of wood exportation and notably small woods (nutrient-rich organs) for bio-energy, threatens forest soil fertility and in consequence ecosystem functioning. Maintaining the management of forests in a sustainable way, while adapting to global change is a big challenge. Here, we use field and modeling studies to assess the relative sensitivity of different types of soils face to increase of biomass exportation in a beech mature forest of the North East of France. The study site with a surface area of 252 km2 is characterized by a great diversity of soils representative of the region, i.e., rendosol, rendisol, calcisol, brunisols, and alocrisols. To achieve this objective, we used a combination of field measurements of soil properties and a mass balance modeling of nutrient dynamics in soils. In this study, we clearly demonstrated that intensification of biomass exportation (wood-energy practice) may drastically reduce soil mineral fertility, even in soils reputed to be nutrient-rich, leading in term to nutrient deficiencies (mainly phosphorus and potassium), and to stand decline for most of the studied soils. In consequence, the choice of forest management (level of biomass exportation, species...) must be reasoned in adequacy with the available soil resources in nutrients, which depends on soil properties. These strategies appear to be more important in a context of global change.

International audience; In our temperate regions, forests ecosystems developed on acidic soils are characterized by a low nutrient-content and an absence of fertilization. In this context, the life (plant, fungi, bacteria) is strongly dependent on the nutrients input coming from soil mineral weathering and atmospheric deposits. However, what do we know about the relative contribution of biotic and abiotic reactions on the mineral weathering process? What is the stability of soil mineral in time? Are there mineral weathering hot spots into the soil? Who are the actors involved in mineral weathering? Our present knowledge highlights that mineral dissolution is increased in the upper soil horizons where life is intense and the organic matter mineralised. Preliminary analyses also show that a large proportion of fine-size minerals evolves during the seasons in relation with the environmental conditions. Notably, the soil under the influence of the tree root systems (ie rhizosphere) appears as an important mineral weathering hot spot due to intense biological activities (root and associated microbes), which take place in.Despite its small size (ca. 1% of soil), it contributes up to half of the weathering flux. Recent results showed an enrichment of efficient mineral weathering bacteria in the rhizosphere, suggesting a functional complementation between the trees and their associated microbes. Future challenges will be to identify the abiotic and biotic parameters impacting weathering reactions, and to develop models integrating life. Multidisciplinary approaches combining isotopic and genetic toolsas well as biogeochemistry and ecology are needed

International audience

International audience

International audience; The role of ectomycorrhizal fungi on mineral nutrient mobilization and uptake is crucial for tree nutrition and growth in temperate forest ecosystems. By using a “mineral weathering budget” approach, this study aims to quantify the effect of the symbiosis with the ectomycorrhizal model strain Laccaria bicolor S238N on mineral weathering and tree nutrition, carrying out a column experiment with a quartz/biotite substrate. Each column was planted with one Scots pine (Pinus sylvestris L.) non-mycorrhizal or mycorrhizal with L. bicolor, with exception of the abiotic control treatment. The columns were continuously supplied with a nutrient-poor solution. A mineral weathering budget was calculated for K and Mg. The pine shoot growth was significantly increased (73%) when plants were mycorrhizal with L. bicolor. Whatever their mycorrhizal status, pines increased mineral weathering by factors 1.5 to 2.1. No difference between non-mycorrhizal and mycorrhizal pine treatments was revealed, however, mycorrhizal pines assimilated significantly more K and Mg. This suggests that in our experimental conditions, L. bicolor S238N improved shoot growth and K and Mg assimilation in Scots pine mainly by increasing the uptake of dissolved nutrients, linked to a better exploration and exploitation of the soil by the mycorrhizal roots.

The impact of the mineral weathering process on tree nutrition in acidic forest soils is well known. However, the quantitative effect of different biological factors on this process remains to be clarified. In this context, the aim of the present work was to quantify the impact of the rhizosphere of forest trees (roots and associated microorganisms) on the weathering of soil minerals. In forest, we observed an increase of the weathering of minerals (dissolution and transformation) in the rhizosphere of trees, suggesting an important impact of the tree rhizosphere on the dynamics of minerals as well as on soil evolution. We characterized in vitro the weathering potential of microorganisms which were isolated in forest from bulk soil and ectomycorrhizas. We showed that the ectomycorrhizal symbiosis significantly structures the functional diversity of culturable bacterial communities and selects strains potentially beneficial to tree nutrition. We developed a column experiment to quantify the respective impact of plant, bacteria and fungi on the weathering process. We demonstrated that tree roots and bacteria significantly increase Mg, K, Fe, Cs, Ni mobilisation via the weathering of a reference mineral. This process is altered in the presence of toxic elements such as uranium. Considering the whole of our results, we can propose the following hypothesis: in forest soils, the nutrient bioavailability determines the mineral weathering due to its impact on the root and microorganism activity. This suggests an important plasticity of forest ecosystems towards nutrition, which could partly explain the sustainability of these ecosystems., L'importance du processus d'altération des minéraux pour la nutrition des arbres en sols forestiers acides est aujourd'hui admise. En revanche, la contribution du facteur biologique dans ce processus reste inconnue. Dans ce contexte, en associant des études et des expérimentations en forêt à des expériences de laboratoire, notre travail a eu pour objectif de quantifier l'effet de la rhizosphère d' arbres forestiers (racines et micro-organismes associés) sur l'altération des minéraux du sol. Nous avons mis en évidence une augmentation de l'altération des minéraux (dissolution et transformation) dans la rhizosphère des arbres en forêt, suggérant un rôle important de ce compartiment dans la dynamique des éléments minéraux et dans l'évolution des sols. Grâce à la caractérisation in vitro du potentiel d'altération de souches microbiennes isolées de sol forestier global et d'ectomycorhizes, nous avons pu montrer une structuration fonctionnelle des communautés bactériennes cultivables, qui suggère une sélection exercée par la symbiose ectomycorhizienne en faveur des souches améliorant la nutrition minérale. Par ailleurs, grâce à un dispositif de culture en colonne que nous avons développé, nous avons démontré lors de différentes interactions plante bactéries - champignons que racines et bactéries augmentent de manière significative la mobilisation de certains cations (Mg, K, Fe, Cs, Ni), via l'altération d'un minéral test. Ce processus biologique est affecté en présence d'éléments toxiques comme l'uranium. Au vu de l'ensemble de nos résultats, notre travail permet de proposer l'hypothèse que, dans les sols forestiers, la disponibilité en nutriments contrôlerait l'altération biologique par son impact sur l'activité des racines et des microorganismes associés. Ceci suggère une forte plasticité des écosystèmes forestiers dans le domaine de la nutrition, qui pourrait expliquer en partie la durabilité de ces écosystèmes.

n° national de thèse : 2006NAN10129 Diplôme : Dr. d'Université; L’importance du processus d’altération des minéraux pour la nutrition des arbres en sols forestiers acides est aujourd’hui admise. En revanche, la contribution du facteur biologique dans ce processus reste inconnue. Dans ce contexte, en associant des études et des expérimentations en forêt à des expériences de laboratoire, notre travail a eu pour objectif de quantifier l’effet de la rhizosphère d’arbres forestiers (racines et micro-organismes associés) sur l’altération des minéraux du sol. Nous avons mis en évidence une augmentation de l’altération des minéraux (dissolution et transformation) dans la rhizosphère des arbres en forêt, suggérant un rôle important de ce compartiment dans la dynamique des éléments minéraux et dans l’évolution des sols. Grâce à la caractérisation in vitro du potentiel d’altération de souches microbiennes isolées de sol forestier global et d’ectomycorhizes, nous avons pu montrer une structuration fonctionnelle des communautés bactériennes cultivables, qui suggère une sélection exercée par la symbiose ectomycorhizienne en faveur des souches améliorant la nutrition minérale. Par ailleurs, grâce à un dispositif de culture en colonne que nous avons développé, nous avons démontré lors de différentes interactions plante – bactéries - champignons que racines et bactéries augmentent de manière significative la mobilisation de certains cations (Mg, K, Fe, Cs, Ni), via l’altération d’un minéral test. Ce processus biologique est affecté en présence d’éléments toxiques comme l’uranium. Au vu de l’ensemble de nos résultats, notre travail permet de proposer l’hypothèse que, dans les sols forestiers, la disponibilité en nutriments contrôlerait l’altération biologique par son impact sur l’activité des racines et des microorganismes associés. Ceci suggère une forte plasticité des écosystèmes forestiers dans le domaine de la nutrition, qui pourrait expliquer en partie la durabilité de ces écosystèmes.

International audience

International audience

International audience