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As forest ecosystems generally develop on poor and unfertilized soils, nutrient cycles and input/output budgets, notably in the humus compartment, are key to determining forest sustainability. Previous studies generally focused on the ecosystem scale or only on C, N, P in the humus layer. Our objectives in this study are to establish these budgets for principal elements, considering dust inputs and, for the first time, particulate outputs. To achieve these objectives, the inputs and outputs of the elements in litterfall, particulates (dust inputs and particulates in the outgoing solution) and solution were quantified in mulls in two contrasting soils in beech ecosystems monitored for 7 years. The concentrations of the elements in the particulate outputs were 3 or 16 times higher than those in dust, because some particles were produced in the humus. The input/output budgets were balanced for K, Na and S in both soils and for P, Si and Mg in one soil. These elements were transported in the humus mainly as a solution or as a soluble form in plant tissues. For Ca and Mn in both soils and for P, Si and Mg in one soil, the elements found in biominerals, the inputs were higher than the outputs, indicating that other outputs were not taken into account in this balance, such as biomineral sedimentation or uptake by fungi. These results show the importance of the humus particles in element cycles. [ABSTRACT FROM AUTHOR]

Calcium (Ca) is an essential macronutrient which plays a fundamental role in ecosystem structure and function. In temperate forest ecosystems, Ca oxalate is ubiquitous, existing in live and decomposing biomass pools. Calcium in minerals occurs principally as carbonate and silicate. Because of structural differences between its different forms, Ca speciation can influence the Ca biogeochemical cycle. In this study, we compared three beech temperate forests of contrasting soil Ca availability, i.e., dystric cambisol (DC), eutric cambisol, and rendzic leptosol (RL), but with similar climates, atmospheric depositions, species compositions and management. The pools and fluxes of total Ca (Cat) and Ca oxalate (Caox) were assessed in plant samples (aboveground and belowground tree tissues, forest floor). Pools and fluxes of Cat were also quantified in the atmospheric inputs (dry and wet deposits), in the soil (total and exchangeable fractions) and in the soil solutions at different depths. The soil solutions were measured monthly for 4 years to study the seasonal dynamics of Ca fluxes. A budget of dissolved Ca was also determined for the forest floor and soil layers. Regarding the global Ca cycle at the stand scale, our study highlighted: (i) the predominant role of biological recycling in the Ca cycle in forest ecosystems for all soil types; the turnover rates of fine roots and leaves were approximately 43 and 39% of the total Ca taken up by trees each year, revealing that only 18% accumulated in the perennial tissues; (ii) the vegetation was Ca-enriched in the high Ca soil (RL) compared to the low-Ca soil (DC); (iii) the existence of an unexpected available Ca source in DC due to upwelling by the diffusion of Ca issued from the dissolution of carbonates in-depth; (iv) a higher production of dissolved Ca (from carbonate dissolution) in RL compared to DC, resulting in an important loss of Ca by drainage (280 kg ha−1 year vs. 14 kg ha−1 year); (v) monthly measurements demonstrated that the seasonal dynamics was mainly linked to biological activity. In addition, our study showed that the distribution of Caox differs greatly between vegetation compartments, ranging from null (stem and branch wood) to up to 40% (stem and branch bark and fine roots) of the Cat in the biomass. Vegetation is a factory of Ca oxalate, and the return of this Caox enriched vegetation to the soil influences the Ca mobility in the forest floor, as demonstrated by a Caox residence time shorter than that of Cat in the DC. [ABSTRACT FROM AUTHOR]

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 SiO 2 (aq), which we attributed to the passivating effect of the ASSLs. 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 [SiO 2 (aq)]; (iii) the gradual decline of powder dissolution rate with time and (iv) the discontinuous occurrence of ASSLs. 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. [ABSTRACT FROM AUTHOR]

In forest ecosystems, the specific influence of soil resources on stand productivity is difficult to assess because many other ecological variables also affect tree growth. In this study, we took advantage of a natural soil gradient, from shallow calcic soil to deep acidic soil, all with similar climate, atmospheric depositions, species composition and management, to determine the relationship between soil nutritive resources and the growth and mineral nutrition of a beech ( Fagus sylvatica ) mature forest of Northeastern France. Soil resources were assessed through the quantification of the stocks of available water and mineral nutrients (Ca, Mg, K and P 2 O 5 ). Beech stand growth and mineral nutrition were determined through the use of several indicators, i.e., standing aboveground biomass and annual biomass production, potential growth index (prediction of the height of dominant trees at 100 years) and foliar nutrient content. We observed a gradient of nutritive resources in soils as well as a gradient of stand growth on the study site: the current aboveground biomass was highest on the calci-brunisol which presented the highest water and nutrient stocks while it was lowest on the rendisol, characterized by a very low water holding capacity and a very low stock of available K. However, the growth of beech trees on the rendisol was equivalent to the highest growth classes of beech trees in France, and K nutrition was optimal. Observations on the study site suggest that, in favorable climate conditions, some biological adaptation processes, such as an efficient root colonization as well as an efficient nutrient cycling may allow to maintain stand growth and nutrition on soils with low water and nutrient reserves. The fertility of forest soils has thus to be assessed in a dynamic way by integrating nutrient fluxes as well as the adaptations of trees to environmental constraints. The biological processes become an increasingly important part in the conservation of soil fertility, notably in the perspective of global changes. [ABSTRACT FROM AUTHOR]

Abstract: As wood harvests are expected to increase to satisfy the need for bio-energy in Europe, quantifying atmospheric nutrient inputs in forest ecosystems is essential for forest management. Current atmospheric measurements only take into account the <0.45μm fraction and dry deposition is generally modeled. The aims of this study were to quantify atmospheric particulate deposition (APD), the >0.45μm fraction of atmospheric deposition, below the canopy, to study the influence of the canopy on APD, and to determine the influence of APD below canopy to nutrient input–output budgets with a focus on base cations calcium, magnesium and potassium, and phosphorus. APD was sampled every four weeks by passive collectors. We divided APD into an organic and a mineral fraction, respectively POM and MDD. MDD was divided into a soluble and a hardly soluble fraction in hydrogen peroxide, referred to as S-MDD and H-MDD, respectively. In order to better understand the influence of the canopy on APD, we studied APD in three pathways below the canopy (litterfall, stemflow and throughfall), and in open field. Our results indicated that APD in throughfall (123±64kgha−1 year−1) was significantly higher and synchronic with that in open field (33±9kgha−1 year−1) in the two study sites. This concerned both POM and MDD, suggesting a large interception of APD by foliar surfaces, which is rapidly washed off by rain within four weeks. Throughfall H-MDD was the main pathway with an average of 16±2kgha−1 year−1. Stemflow and litterfall were neglected. In one study site, canopy intercepted about 8kgha−1 year−1 of S-MDD. Although base cations and phosphorus inputs by APD are lower than those of <0.45μm deposition, they contributed from 5 to 32% to atmospheric deposition and improved the nutrient budget in one of the study sites. [Copyright &y& Elsevier]

Abstract: The release of nutritive elements through apatite dissolution represents the main source of phosphorus, calcium, and several micronutrients (e.g., Zn, Cu) for organisms in non-fertilized forest ecosystems. The aim of this study was to quantify, for the first time, the dissolution rate of apatite grains by tree roots that were or were not associated with a mineral weathering bacterial strain, and by various acids known to be produced by tree roots and soil bacterial strains in open-system flow microcosms. In addition, we explored whether the mobilization of trace elements (including rare earth elements) upon apatite dissolution was affected by the presence of trees and associated microorganisms. The dissolution rate of apatite by Scots pine plants that were or were not inoculated with the strain Burkholderia glathei PML1(12)Rp, and by inorganic (nitric) and organic (citric, oxalic and gluconic) acids at pH 5.5, 4.8, 3.8, 3.5, 3.0, and 2.0 was monitored in two controlled experiments: “plant–bacteria interaction” and “inorganic and organic acids”. Analyses of the outlet solutions in the “plant–bacteria interaction” experiment showed that Scots pine roots and B. glathei PML1(12)Rp produced protons and organic acids such as gluconate, oxalate, acetate, and lactate. The weathering budget calculation revealed that Scots pines (with or without PML1(12)Rp) significantly increased (factor>10) the release of Ca, P, As, Sr, Zn, U, Y, and rare earth elements such as Ce, La, Nd from apatite, compared to control abiotic treatment. Scanning electron microscopy observation confirmed traces of apatite dissolution in contact of roots. Most dissolved elements were taken up by Scots pine roots, i.e., approximately 50% of Ca, 70% of P, 30% of As, 70% of Sr, 90% of Zn, and 100% of U, Y, and rare earth elements. Interestingly, no significant additional effect due to the bacterial strain PML1(12)Rp on apatite dissolution and Scots pine nutrition and growth was observed. The “inorganic and organic acids” experiment demonstrated that the apatite dissolution efficacy of organic acids was higher than for the inorganic acid and varied in function of the acids: oxalic acid>citric acid>gluconic acid>nitric acid for pH ⩽3.5. In addition, apatite dissolution increased with increasing acidity for each acid. Only oxalic acid generated non-stoichiometric release of calcium and phosphorus from apatite in the solution at pH ⩽3.5, due to the precipitation of Ca-oxalate crystals at apatite surfaces. Comparison of the experiments revealed that the apatite dissolution rate by Scots pines supplied with nutritive solution at pH 5.5 reached 2.0×10−13 molcm−2 s−1 and was equivalent to rates with nitric acid at pH 3.2, gluconic acid at pH 3.5, citric acid at pH 3.7, and oxalic acid at pH 3.8. Altogether our results highlight that, through the production of weathering agents, notably protons and organic acids, tree roots and root-associated microorganisms are able to significantly increase the release of macro- and micro-nutrients from apatite, thus maintaining high-nutrient conditions to support their growth. [Copyright &y& Elsevier]

Ni phytoextraction processes need further understanding of the interactions between Ni availability in soils and its absorption by plant roots. The large metal uptake and root exudation by hyperaccumulator species could accelerate the weathering process of Ni-bearing phases in the rhizosphere. The aim of this work was to quantify the weathering of a Ni-bearing mineral phase in the rhizosphere of the Ni-hyperaccumulator Leptoplax emarginata. The studied mineral was chrysotile which was characterized by a low Ni solubility. Column experiments were performed to assess the effect of the Ni-hyperaccumulator L. emarginata and the contribution of rhizobacteria on the dissolution rate of chrysotile. Mineral weathering was monitored by measuring Ni and Mg transferred to leachates or plants throughout the experiment. Results showed that L. emarginata increased chrysotile dissolution by more than 2-fold . The hyperaccumulator L. emarginata accumulated 88% on average of total mobilized Ni. Inoculation with Ni-resistant bacteria in the rhizosphere of L. emarginata had no significant effect on chrysotile dissolution or plant accumulation of Ni in this context. Finally, after 15 weeks of culture, 1.65% of total Ni in the system was mobilized in the planted treatments compared with 0.03% in the unplanted treatments. [ABSTRACT FROM AUTHOR]

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]

Trees play a crucial role in nutrient cycling and ecosystem fertility, notably through rhizosphere processes. The aim of this study was to compare soil physicochemical properties between bulk soil and rhizosphere of several tree species, and to compare rhizosphere properties between fertilized and non-fertilized conditions. The soil sampling was performed in Breuil-Chenue forest (North-East of France) in seven stands: native forest (old beech ( Fagus sylvatica L.) and oak ( Quercus sessiliflora Smith) coppice with standards; CwS), beech, oak ( Quercus petraea [Matt.] Liebl.), Douglas-fir and fertilised Douglas-fir, Norway spruce ( Picea abies Karst.) and fertilised Norway spruce. Systematic soil sampling was performed at 0-3, 3-10, and 10-23 cm in 20 calibrated pits. The rhizosphere of the different species was generally enriched in C, N, Ca, Mg, and K. Interestingly, the same positive effect was observed in the fertilised plots. The rhizosphere effect varied between tree species for C, 'base' cations, pH and cation exchange capacity. This study reveals that interactions between roots, microorganisms and soil can enrich the pool of nutrients in the rhizosphere compared to bulk soil whatever the soil fertility conditions, and that the magnitude of the rhizosphere effect depends on tree species. [ABSTRACT FROM AUTHOR]

In acidic forest soils, availability of inorganic nutrients is a tree-growth-limiting factor. A hypothesis to explain sustainable forest development proposes that tree roots select soil microbes involved in central biogeochemical processes, such as mineral weathering, that may contribute to nutrient mobilization and tree nutrition. Here we showed, by combining soil analyses with cultivation-dependent analyses of the culturable bacterial communities associated with the widespread mycorrhizal fungus Scieroderma citrinum, a significant enrichment of bacterial isolates with efficient mineral weathering potentials around the oak and beech mycorrhizal roots compared to bulk soil. Such a difference did not exist in the rhizosphere of Norway spruce. The mineral weathering ability of the bacterial isolates was assessed using a microplaque assay that measures the pH and the amount of iron released from biotite. Using this microplate assay, we demonstrated that the bacterial isolates harboring the most efficient mineral weathering potential belonged to the Burkholderia genus. Notably, previous work revealed that oak and beech harbored very similar pHs in the 5- to 10-cm horizon in both rhizosphere and bulk soil environments. In the spruce rhizosphere, in contrast, the pH was significantly lower than that in bulk soil. Because the production of protons is one of the main mechanisms responsible for mineral weathering, our results suggest that certain tree species have developed indirect strategies for mineral weathering in nutrient-poor soils, which lie in the selection of bacterial communities with efficient mineral weathering potentials. [ABSTRACT FROM AUTHOR]

Soil microbes play an essential role in the environment by contributing to the release of key nutrients from primary minerals that are required not only for their own nutrition but also for that of plants. Although the role of fungi in mineral weathering is beginning to be elucidated, the relative impact of bacteria in this process and the molecular mechanisms involved remain poorly understood. Here, we discuss the ecological relevance of bacterial weathering, mainly in the soil and especially in acidic forest ecosystems, which strongly depend on mineral weathering for their sustainability. We also present highlights from recent studies showing molecular mechanisms and genetic determinants involved in the dissolution of complex minerals under aerobic conditions. Finally, we consider the potential applications of genomic resources to the study of bacterial weathering. [Copyright &y& Elsevier]

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]

Forest canopy is a complex interface between the atmosphere, the biosphere and the lithosphere, exerting a strong influence on forest durability through element recycling. Leaves are filters of the low atmosphere and can capture dry deposition (DD). Trees have developed strategies, such as resorption (R) during senescence, for nutrient conservation. These strategies seem to depend on the soil type. The chemical composition evolution of leaves is conceptualized as a function of four fluxes: foliar absorption (FA) foliar leaching, (FL), accretion (A), and resorption (R). The objective of this study is to evaluate these five fluxes, i.e., DD, FA, FL, A, and R for different elements (N, P, K, S, Ca, Mg, Mn, and Si) at the experimental forest site in Montiers which includes three plots with contrasting soils and similar climate and stand characteristics. The percentage of resorption was 63% for N, 37% for S, 35% for K and 23% for P, regardless of the soil type. Resorption represents the most efficient recycling mechanism for ecosystems. Accretion during senescence occurs in two elements that are also found as biominerals in leaves: Ca (13–26 kg.ha−1.y−1) and Si (3–6 kg.ha−1.y−1). The effect of soil is limited to leaf concentrations and exchanges, and only for three elements (Mn, Ca and Si) that are not growth-limiting elements in our study site. This study shows that, except for N, taking into account the exchanges between living foliar tissues and leaching solutions is fundamental to accurately estimate the resorption rate. [ABSTRACT FROM AUTHOR]

A correction to the article "Laccaria bicolor S238N improves Scots pine mineral nutrition by increasing root nutrient uptake from soil minerals but does not increase mineral weathering," by Christophe Calvaruso, that was published in the 2011 issue is presented.