Ectomycorrhizal weathering of the soil minerals muscovite and hornblende

Ectomycorrhizal fungi are hypothesized to enhance mineral weathering in forest soils. Several studies have shown an increased uptake of mineral-derived nutrients by trees when in symbiosis with ectomycorrhizal fungi. However, it is difficult to determine from these studies if the improved nutrient uptake is the result of increased weathering or better exploitation of the substrate by the ectomycorrhizal fungi. In a pot experiment, Pinus sylvestris (Scots pine) seedlings were grown with or without ectomycorrhizal fungi, and with or without the mineral muscovite as the only potassium (K) source or the mineral hornblende as the only magnesium (Mg) source. After 27 wk, all pools of non-mineral-bound K or Mg were determined. The ectomycorrhizal fungus Paxillus involutus increased weathering of muscovite but not hornblende. The other ectomycorrhizal fungi tested, Piloderma croceum and Suillus bovinus, did not increase weathering of either muscovite or hornblende compared with the nonmycorrhizal trees. The P. involutus-mediated mobilization of K from muscovite resulted in increased K content of root plus adhering hyphae, but not of shoots. In conclusion, ectomycorrhizal fungi may increase weathering of minerals in response to nutrient deficiencies, but this response is species specific.     

Root-induced increases in soil pH and nutrient availability to field-grown cereals and legumes on acid sandy soils of Sudano-Sahelian West Africa

A field experiment with millet (Pennisetum glaucum L.), sorghum [Sorghum bicolor (L.) Moench], cowpea (Vigna unguiculata L.) and groundnut (Arachnis hypogeae L.) was conducted on severely P-deficient acid sandy soils of Niger, Mali and Burkina Faso to measure changes in pH and nutrient availability as affected by distance from the root surface and by mineral fertiliser application. Treatments included three rates of phosphorus (P) and four levels of nitrogen (N) application. Bulk, rhizosphere and rhizoplane soils were sampled at 35, 45 and 75 DAS in 1997 and at 55 and 65 DAS in 1998. Regardless of the cropping system and level of mineral fertiliser applied, soil pH consistently increased between 0.7 and two units from the bulk soil to the rhizoplane of millet. Similar pH gradients were observed in cowpea, but pH changes were much smaller in sorghum with a difference of only 0.3 units. Shifts in pH led to large increases in nutrient availability close to the roots. Compared with the bulk soil, available P in the rhizoplane was between 190 and 270% higher for P-Bray and between 360 and 600% higher for P-water. Exchangeable calcium (Ca) and magnesium (Mg) levels were also higher in the millet rhizoplane than in the bulk soil, whereas exchangeable aluminium (Al) levels decreased with increasing pH close to the root surface. The results suggest an important role of root-induced pH increases for crops to cope with acidity-induced nutrient deficiency and Al stress of soils in the Sudano-Sahelian zone of West Africa. This revised version was published online in June 2006 with corrections to the Cover Date.     

The role of fungi in biogenic weathering in boreal forest soils

In this article we discuss the possible significance of biological processes, and of fungi in particular, in weathering of minerals. We consider biological activity to be a significant driver of mineral weathering in forest ecosystems. In these environments fungi play key roles in organic matter decomposition, uptake, transfer and cycling of organic and inorganic nutrients, biogenic mineral formation, as well as transformation and accumulation of metals. The ability of lichens, mutualistic symbioses between fungi and photobionts such as algae or cyanobacteria, to weather minerals is well documented. The role of mycorrhizal fungi forming symbioses with forest trees is less well understood, but the mineral horizons of boreal forests are intensively colonised by mycorrhizal mycelia which transfer protons and organic metabolites derived from plant photosynthates to mineral surfaces, resulting in mineral dissolution and mobilisation and redistribution of anionic nutrients and metal cations. The mycorrhizal mycelia, in turn provide efficient systems for the uptake and direct transport of mobilised essential nutrients to their host plants which are large sinks. Since almost all (99.99 %) non-suberised lateral plant roots involved in nutrient uptake are covered by ectomycorrhizal fungi, most of this exchange of metabolites must take place through the plant–fungus interface. This idea is still consistent with a linear relationship between soil mineral surface area and weathering rate since the mycelia that emanate from the tree roots will have a larger area of contact with minerals if the mineral surface area is higher. Although empirical models based on bulk soil solution chemistry may fit field data, we argue that biological processes make an important contribution to mineral weathering and that a more detailed mechanistic understanding of these must be developed in order to predict responses to environmental changes and anthropogenic impact.     

Scale matters? Exploring the effect of scale on fungal–mineral interactions

Oxalate is considered to be an important fungal weathering agent, but measured bulk soil concentrations in boreal forest soils are too low to expect a significant effect on mineral dissolution. On the other hand, it is expected that oxalate is heterogeneously distributed in the soil, with localized higher concentrations. Here, a model is presented to explore the effects of an oxalate distribution on feldspar dissolution dynamics. First, fungal distribution was studied in a boreal podzol, using fluorescent techniques. Fungi selectively colonized specific mineral species in the soil, with fungal cover on plagioclase > K-feldspar > quartz. Based on the modelling results, oxalate will not have any major effect on feldspar weathering, unless oxalate stays within a few microns of the surface. The results can be extrapolated to other fungal weathering agents, including protons because of the similarities in the dissolution kinetics. The general conclusion is that local concentration of a weathering agent does not have any substantial positive effect on mineral dissolution unless it is preferentially exuded near specific mineral species or very close, within 10 μm, to the mineral surface.     

Microbial Complexes Occurring on the Apogeotropic Roots and in the Rhizosphere of Cycad Plants

The microbial complexes of soil, the rhizosphere, and the rhizoplane of the apogeotropic (coralloid) roots of cycad plants were comparatively studied. The aseptically prepared homogenates of the surface-sterilized coralloid roots did not contain bacterial microsymbiont, indicating that in the root tissues the symbiosis is a two-component one (plant–cyanobacteria). At the same time, associated bacteria belonging to different taxonomic groups were detected in increasing amounts in the cycad rhizoplane, rhizosphere, and the surrounding soil. The bacterial communities found in the cycad rhizoplane and the surrounding soil were dominated by bacteria from the genus Bacillus. The saprotrophic bacteria and fungi colonizing the cycad rhizosphere and rhizoplane were dominated by microorganisms capable of degrading the plant cell walls. The local degradation of the cell wall was actually observed on the micrographs of the thin sections of cycad roots in the form of channels through which symbiotic cyanobacterial filaments can penetrate into the cortical parenchyma.     

Chemical weathering of soil minerals and the role of biological processes

Existing weathering models based on direct, abiotic factors, and biological factors through systemic feedbacks seem to describe the weathering rates observed in the field with good accuracy. The Swedish weathering rate model, PROFILE is the only existing model with this capacity. The PROFILE model is widely used and has been used successfully in sustainability assessments for forests and agriculture. The author has reviewed available experiments, as well as existing knowledge on chemical weathering kinetics and concludes that the physical and chemical conditions existing in forest soils do not allow any significant direct surface actions on minerals by microorganisms or tree roots. The reported tracks and holes in minerals have been mistakenly identified as being produced by roots, but this misunderstanding is caused by a lack of understanding of crystallography and chemical dissolution mechanisms. The holes and tracks have been produced by known chemical mechanisms.     

Microbial complexes from apogeotropic roots and from rhizosphere of cycad plants

The microbial complexes of soil, the rhizosphere, and the rhizoplane of the apogeotropic (coralloid) roots of cycad plants were comparatively studied. The aseptically prepared homogenates of the surface-sterilized coralloid roots did not contain bacterial microsymbiont, indicating that it was absent in the root tissues. At the same time, associated bacteria belonging to different taxonomic groups were detected in increasing amounts in the cycad rhizoplane, rhizosphere, and the surrounding soil. The bacterial communities found in the cycad rhizoplane and the surrounding soil were dominated by bacteria from the genus Bacillus. The saprotrophic bacteria and fungi colonizing the cycad rhizosphere and rhizoplane were dominated by microorganisms capable of degrading the plant cell walls. The local degradation of the cell wall was actually observed on the micrographs of the thin sections of cycad roots in the form of channels, through which symbiotic cyanobacterial filaments can penetrate into the cortical parenchyma.     

Survival of genetically modified Pseudomonas fluorescens introduced into subtropical soil microcosms

Abstract A genetically modified strain of Pseudomonas fluorescens and its parent showed grossly similar decline rates following introduction into subtropical clay and sandy soils. In unplanted clay soit at pH 6.9 and 25°C, population densities declined progressively from about 10⁸ to 10³ colony forming units (cfu) g⁻¹ dry soil over 75 days, but in unplanted sandy soil the introduced populations could not be detected after 25 days. In clay soil at pH 8.7 or 4.7, or at environmental temperature, decay rates were enhanced as compared to those at pH 6.9 and 25°C. Counts of introduced strains in clay bulk soil and in rhizosphere and rhizoplane of maize suggested that the introduced bacteria competed well with the native bacteria, and colonized the roots at about 10⁶ cfu g⁻¹ dry root at 25°C, over 20 days. However, rhizoplane colonization was lower at environmental temperature. The decay rate of both strains was slower in planted than in unplanted sandy soil. The population densities in the rhizosphere and rhizoplane in the sandy soil were significantly lower than those in the clay soil. Both introduced strains colonized the maize roots in both soils, using seeds coated with bacteria in 1% carboxymethyl cellulose. Introduced cells were localized at different sites along the roots of plants developing in clay soil, with higher densities in the original (near the seeds) and root hair zones as compared to the intermediate zones. No significant difference was observed between the extent of root colonization of the genetically modified strain and its parent.     

LuxAB标记的N2106-L在小麦根圈的定植动态

目的研究小麦PGPR（植物根际促生菌）菌株的个体生态学及其在小麦根圈的定植动态。方法采用三亲本杂交法将发光酶基因luxAB转人具有固氮能力的小麦根际促生菌Azotobacter N2106菌株中,获得标记菌株N2106-L,将标记菌株接种到灭菌和非灭菌的黄褐土、红壤和黄潮土中研究其存活状况,采用根盒试验追踪标记菌株在小麦根圈的定植动态。结果标记菌株N2106-L具有发光活性和对km、str、tet三种抗生素的抗性,且具有较好的遗传稳定性。N2106-L在灭菌土壤中的数量稍高于非灭菌土壤;在3种土壤中的数量依次为：黄褐土〉黄潮土〉红壤。N2106-L在小麦根表定植密度大于根际定植密度;在小麦根际,小麦播种10d时标记菌株在0-2cm深度根际土壤定植达到最大值（2．17±0．25）×10^6CFU／g土,20d时在2-4cm深度的根际土壤中达到最高定植水平（3．92±0．47）×10^5CFU／g土;在小麦根表,标记菌株在小麦播种10d时在所有深度的根段均达到最高定植水平,0-2cm根段定植密度为（3．60±0．60）×10^6CFU／g鲜根,12cm以下根段达到（2．78±0．56）×10^4CFU／g鲜根。结论标记菌株随着根的伸长不断向根尖方向扩散,且较为稳定地在小麦根圈定植,研究结果为小麦PGPR菌株的应用提供了可靠实验数据。     

Changes in the extractability of cations (Ca,Mg and K) in the rhizosphere soil of Norway spruce (Picea abies) roots

Nutrient concentrations in the rhizosphere soil can be higher, lower or remain unchanged compared to the bulk soil, but relatively little is known about such changes for basic cations in the rhizosphere of tree roots. A modified root container technique of studying rhizosphere processes was employed. Plexiglas cylinders were horizontally split by a membrane with 30 M mesh size into an upper compartment for root growth and a root-free lower compartment, each with an inner diameter of 5 cm and a height of 10 cm. One 2-year-old Norway spruce (Picea abies) seedling was transplanted from a nursery into each cylinder. Plants were not specifically inoculated, but roots were colonised by a mix of ectomycorrhizal fungi originating from the nursery. The nutrient poor mineral soil used in the experiment was taken from a forest site in Bayerischer Wald, southern Germany. The soil was either supplied with a mix of Ca, Mg and K, or not supplied with these cations. Plants were harvested 30 weeks after transplanting. The nylon membrane between the root compartments restricted root growth to the upper compartment, so that by the end of the experiment a root mat was formed at the top side of the membrane. In the lower compartment, soil nearest to the root mat was regarded as rhizosphere soil while soil in a distance from the root mat was regarded as bulk soil. In the upper compartment, rhizosphere soil was obtained at the end of the experiment by gently shaking the roots. The soils were analysed for Ca, Mg and K contents following two different soil extraction methods. In the fertilised treatment, H₂O-extractable Ca and Mg were accumulated in the rhizosphere. In contrast, K (NH₄Cl-extraction) was depleted in the rhizosphere. In the bottom tube, the depletion of K (NH₄Cl-extraction) was restricted to 1 cm distance from the root mat. In unfertilised soil, Ca, Mg and K concentrations did not differ clearly between rhizosphere and bulk soils. The results indicated that the occurrence of cation gradients in the rhizosphere depended on the level of soil nutrient supply. Distinct rhizosphere effects were measured by conventional soil extraction methods only when the soil was freshly fertilised with mineral elements prior to the experiment. In this case, K depletion in the rhizosphere reflected higher K uptake by the fertilised Norway spruce plants. For low-nutrient soils, novel techniques are required to follow subtle changes in the rhizosphere.     

刺槐林魔芋健康高产的土壤微生态机制

通过对刺槐林和农田魔芋健株根区、根表及根外土壤微生物区系及养分含量比较,探索刺槐林魔芋健康高产的土壤微生态机制。结果表明:(1)刺槐林魔芋根外和根表土壤细菌数量分别较农田增加11.8%和588.9%,根区土壤真菌数量较农田显著减少74.4%。(2)刺槐林魔芋根区、根表及根外土壤中的有益优势微生物数量及其比例远高于农田魔芋,有害微生物数量远低于农田魔芋;在刺槐林魔芋根区、根表及根外土壤中,3种优势细菌为放射型根瘤菌(Rhizobium radiobacter)、苏云金芽孢杆菌(Bacillus thuringiensis)及摩氏假单胞菌(Pseudomonas mosselii),其中,根表土壤中放射型根瘤菌及苏云金芽孢杆菌数量分别为农田的25.7倍及13.0倍;2种优势真菌为黒附球菌(Epicoccum nigrum)和疣孢青霉(Penicillium verruculosum),1种优势放线菌为绿淀粉酶链霉菌(Streptomyces viridodiastaticus),其中刺槐林魔芋根表和根外土壤中黒附球菌数量分别较农田高159.2%和120.3%;大量存在于刺槐林下魔芋根外土壤中的疣孢青霉、以及根区、根表、根外土壤中的绿淀粉酶链霉菌在农田魔芋相应部位均未检出。(3)刺槐林下魔芋根外、根区土壤有机质含量分别较农田显著增加167.6%、39.6%,但速效P、K含量较农田分别显著降低85.6%~91.3%、12.4%~13.0%。研究认为,刺槐林魔芋健康高产与其根区、根表及根外土壤中特有的有益优势微生物数量多、有害微生物数量少以及土壤有机质含量高密切相关。     

Relationships between plant roots,proteolytic organisms and activity of protease

Proteolytic bacteria represented 18–58% of the bacterial population isolated from the rhizoplane of different crops. The activity of protease was considerably higher on roots of wheat growing in the soil than in the rhizosphere or free soil. However, only a slightly positive rhizosphere effect in the relative occurrence of casein-hydrolyzing bacteria could be observed. An indirect relationship between numbers of bacteria hydrolyzing casein and the activity of the enzyme could be found. The activity of protease related to a unit of culturable proteolytic bacteria was considerably higher on the root than in the rhizosphere and in the soil. A relationship between characteristics of the production of the enzyme by proteolytic bacteria and the protease activity on the surface of roots was demonstrated. The resulting enzyme activity on the surface of roots depended apparently on growth conditions of the plant and nature of root exudates and was influenced both by inactivation and protection due to adsorption of the enzyme by roots.     

Identifying calcium sources at an acid deposition-impacted spruce forest: a strontium isotope,alkaline earth element multi-tracer approach

Depletion of calcium from forest soils has important implications for forest productivity and health. Ca is available to fine feeder roots from a number of soil organic and mineral sources, but identifying the primary source or changes of sources in response to environmental change is problematic. We used strontium isotope and alkaline earth element concentration ratios of trees and soils to discern the record of Ca sources for red spruce at a base-poor, acid deposition-impacted watershed. We measured ⁸⁷Sr/⁸⁶Sr and chemical compositions of cross-sectional stemwood cores of red spruce, other spruce tissues and sequential extracts of co-located soil samples. ⁸⁷Sr/⁸⁶Sr and Sr/Ba ratios together provide a tracer of alkaline earth element sources that distinguishes the plant-available fraction of the shallow organic soils from those of deeper organic and mineral soils. Ca/Sr ratios proved less diagnostic, due to within-tree processes that fractionate these elements from each other. Over the growth period from 1870 to 1960, ⁸⁷Sr/⁸⁶Sr and Sr/Ba ratios of stemwood samples became progressively more variable and on average trended toward values that considered together are characteristic of the uppermost forest floor. In detail the stemwood chemistry revealed an episode of simultaneous enhanced uptake of all alkaline earth elements during the growth period from 1930 to 1960, coincident with reported local and regional increases in atmospheric inputs of inorganic acidity. We attribute the temporal trends in stemwood chemistry to progressive shallowing of the effective depth of alkaline earth element uptake by fine roots over this growth period, due to preferential concentration of fine roots in the upper forest floor coupled with reduced nutrient uptake by roots in the lower organic and upper mineral soils in response to acid-induced aluminum toxicity. Although both increased atmospheric deposition and selective weathering of Ca-rich minerals such as apatite provide possible alternative explanations of aspects of the observed trends, the chemical buffering capacity of the forest floor-biomass pool limits their effectiveness as causal mechanisms.     

Effects of Pinus sylvestris root growth and mycorrhizosphere development on bacterial carbon source utilization and hydrocarbon oxidation in forest and petroleum-contaminated soils

The hypothesis that Pinus sylvestris L. root and mycorrhizosphere development positively influences bacterial community-linked carbon source utilization, and drives a concomitant reduction in mineral oil levels in a petroleum hydrocarbon- (PHC-) contaminated soil was confirmed in a forest ecosystem-based phytoremediation simulation. Seedlings were grown for 9 months in large petri dish microcosms containing either forest humus or humus amended with cores of PHC-contaminated soil. Except for increased root biomass in the humus/PHC treatment, there were no other significant treatment-related differences in plant growth and needle C and N status. Total cell and culturable bacterial (CFU) densities significantly increased in both rhizospheres and mycorrhizospheres that actively developed in the humus and PHC-contaminated soil. Mycorrhizospheres (mycorrhizas and extramatrical mycelium) supported the highest numbers of bacteria. Multivariate analyses of bacterial community carbon source utilization profiles (Biolog GN microplate) from different rhizosphere, mycorrhizosphere, and bulk soil compartments, involving principal component and correspondence analysis, highlighted three main niche-related groupings. The respective clusters identified contained bacterial communities from (i) unplanted bulk soils, (ii) planted bulk PHC and rhizospheres in PHC-contaminated soils, and (iii) planted bulk humus and rhizosphere/mycorrhizosphere-influenced humus, and mycorrhizosphere-influenced PHC contaminated soil. Correspondence analysis allowed further identification of amino acid preferences and increased carboxylic/organic acid preferences in rhizosphere and mycorrhizosphere compartments. Decreased levels of mineral oil (non-polar hydrocarbons) were detected in the PHC-contaminated soil colonized by pine roots and mycorrhizal fungi. These data further support our view that mycorrhizosphere development and function plays a central role in controlling associated bacterial communities and their degradative activities in lignin-rich forest humus and PHC-contaminated soils.     

Mobilization of aluminium in the rhizosphere soil solution of growing tree roots in an acidic soil

Chemical conditions in the rhizosphere in many respects are different from the bulk soil. Especially in acid forest soils aluminium chemistry at the soil root interface is of particular interest because of its importance for evaluating the risk of rhizotoxicity. In the present study we have used micro suction cups to collect soil solution from the rhizosphere of oak seedlings (Quercus robur L.) in high spatial resolution and capillary electrophoresis for the determination of major ions and Al³⁺. While the concentrations of nutrient cations, especially Ca²⁺ and Mg²⁺, decreased in the vicinity of growing roots the concentrations of Al³⁺ significantly increased. Al³⁺-ions were probably released when root-exuded protons were buffered by the soil. Their occurrence indicates, that the oak roots in our experiments had only limited capabilities to detoxify Al in their rhizosphere. The restriction of this effect to the very small soil compartment close to the roots suggests, that common soil analysis which neglect rhizosphere processes might greatly underestimate the in situ concentration of Al³⁺ near tree roots. Our experiments furthermore indicate, that also suberized roots have a significant influence on rhizosphere soil solution chemistry. This revised version was published online in June 2006 with corrections to the Cover Date.     

Induced Reporter Gene Activity, Enhanced Stress Resistance, and Competitive Ability of a Genetically Modified Pseudomonas fluorescens Strain Released into a Field Plot Planted with Wheat

The fates of Pseudomonas fluorescens R2fR and its mutant derivative RIWE8, which contains a lacZ reporter gene responsive to wheat root exudate, were compared in a field microplot. Inoculant survival, root colonization, translocation, resistance to stress factors, and reporter gene activity were assessed in bulk and wheat rhizosphere soils. Populations of both strains declined gradually in bulk and wheat rhizosphere soils and on the wheat rhizoplane as determined by specific CFU and immunofluorescence (IF). In samples from both bulk soil and wheat rhizosphere, IF cell counts were up to 3 orders of magnitude greater than the corresponding numbers of CFU after 120 days, indicating the presence of nonculturable inoculant cells. Estimates of RIWE8-specific target DNA molecule numbers in bulk soil samples 3 and 120 days after inoculation by most-probable-number PCR coincided with the corresponding CFU values. Transport of both strains to deeper soil layers was observed by 3 days after introduction into the microplot. Both strains colonized wheat roots similarly, and cells were seen scattered on the surface of 1-month-old wheat seedling roots by immunogold labelling-scanning electron microscopy. On average, reporter gene activity was significantly higher in wheat rhizosphere soil containing RIWE8 cells than in bulk soil or in soils containing R2fR cells. For both strains, resistance to the four stress factors ethanol, high temperature, high osmotic tension, and oxidative stress increased progressively with residence in soil. Cells from the rhizosphere of 11-day-old seedlings showed similar levels of resistance to osmotic and oxidative stresses and enhanced resistance to ethanol and heat as compared to cells from bulk soil. By 37 days, populations of R2fR and RIWE8 in the rhizosphere were significantly more sensitive to osmotic stress than were populations in bulk soil, whereas differences in response to the other stress factors were less evident. Hence, except for the induction of reporter gene expression in strain RIWE8 in the wheat rhizosphere, the data indicated that there were no great differences in the ecological properties in soil between the lacZ-modified and parental strains.     

Bacterial origin and community composition in the barley phytosphere as a function of habitat and presowing conditions

An understanding of the factors influencing colonization of the rhizosphere is essential for improved establishment of biocontrol agents. The aim of this study was to determine the origin and composition of bacterial communities in the developing barley (Hordeum vulgare) phytosphere, using denaturing gradient gel electrophoresis (DGGE) analysis of 16S rRNA genes amplified from extracted DNA. Discrete community compositions were identified in the endorhizosphere, rhizoplane, and rhizosphere soil of plants grown in an agricultural soil for up to 36 days. Cluster analysis revealed that DGGE profiles of the rhizoplane more closely resembled those in the soil than the profiles found in the root tissue or on the seed, suggesting that rhizoplane bacteria primarily originated from the surrounding soil. No change in bacterial community composition was observed in relation to plant age. Pregermination of the seeds for up to 6 days improved the survival of seed-associated bacteria on roots grown in soil, but only in the upper, nongrowing part of the rhizoplane. The potential occurrence of skewed PCR amplification was examined, and only minor cases of PCR bias for mixtures of two different DNA samples were observed, even when one of the samples contained plant DNA. The results demonstrate the application of culture-independent, molecular techniques in assessment of rhizosphere bacterial populations and the importance of the indigenous soil population in colonization of the rhizosphere.     

Influence of Forest Trees on the Distribution of Mineral Weathering-Associated Bacterial Communities of the Scleroderma citrinum Mycorrhizosphere

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 Scleroderma 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.The mobilization of nutrients via the biotic and abiotic weathering of soil minerals is crucial to satisfying plant nutritional needs (2, 17), especially in acidic forest soils, which are mainly nonfertilized and nutrient poor. Besides the physicochemical weathering reactions, evidence is presently accumulating which indicates that certain soil bacterial strains increase mineral weathering and improve tree nutrition (5, 9, 32, 39-41).By way of their root exudates, plants alter the structure and activity of microbial communities (6, 25, 51) and selectively favor certain ones that are potentially beneficial to them (15, 16, 21, 45, 46). A hypothesis for sustainable forest development proposes that tree roots select from the soil efficient mineral weathering bacterial communities that may contribute to nutrient mobilization and tree growth (20). In this manner, recent studies (10, 46) have revealed that the oak-Scleroderma citrinum ectomycorrhizal symbiosis selects bacterial communities that are more efficient in mineral weathering than those of the surrounding soil, suggesting that the mycorrhizal symbiosis has an indirect effect on plant nutrition through its selective pressure on the functional diversity of the mycorrhizosphere bacterial communities.Distinct impacts of the tree species on the soil bacterial community structure have been previously reported (23, 38), suggesting that the composition and activity of soil bacterial communities depend on tree physiology and notably on its impact on the soil physicochemical properties and nutrient cycling (24, 26, 37). However, no study has ever addressed the question of the impact of tree species on the structure of forest soil bacterial communities involved in mineral weathering. This question regarding the impact of tree species on the functional diversity of the bacterial communities remains a major issue in forestry, especially in the context of today''s climate change, which will give rise to a shift in the spatial distribution of forest tree species.To appreciate the effect of tree species on mycorrhizosphere bacterial communities, we focused on a single but ubiquitous mycorrhizal fungus, S. citrinum, which forms mycorrhizae with different tree species. Since no functional genes have been identified to date, a cultivation-dependent analysis was developed in this study. A total of 155 bacterial isolates were randomly chosen among a collection of 400 bacterial isolates from the soil-Scleroderma citrinum mycorrhiza interface (ectomycorrhizosphere), the extramatrical mycelium (hyphosphere), and the surrounding soil (bulk soil) in 28-year-old stands of oak (Quercus sessiliflora Smith), beech (Fagus sylvatica L.), and Norway spruce (Picea abies Karst.). The mineral weathering potential of each bacterial isolate was evaluated by way of an in vitro microplate assay, putting in interaction a calibrated bacterial suspension and the biotite, a mineral widespread in soils (46). The bacterial isolates were genotypically characterized by amplifying and sequencing a portion of the 16S rRNA gene. Their mineral weathering efficiencies and the functional structure of the bacterial communities were compared with the physicochemical characteristics of the surrounding soil.     

A Model Sheet Mineral System to Study Fungal Bioweathering of Mica

The general objective of this research was to examine fungal interactions with silicate minerals within the context of their roles in bioweathering. To achieve this, we used muscovite, a phyllosilicate mineral (KAl₂[(OH)₂|AlSi₃O₁₀]), in the form of a mineral sheet model system for ease of experimental manipulation and microscopic examination. It was found that test fungal species successfully colonized and degraded the surface of muscovite sheets in both laboratory and field experiments. After colonization by the common soil fungus Aspergillus niger, a network of hyphae covered the surface of the muscovite, and mineral dissolution or degradation was clearly evidenced by a network of fungal “footprints” that reflected coverage by the mycelium. For natural soil incubations, microorganisms associated with muscovite sheet material included biofilms of fungi and bacteria on the surface, while mineral encrustation or adhesion to microbial structures was also observed. Our results show that muscovite sheet is a good model mineral system for examination of microbial colonization and degradation, and this was demonstrated using laboratory and field systems, providing more evidence for the bioweathering significance of fungal activities in the context of silicate degradation and soil formation and development. The approach is also clearly applicable to other rock and mineral-based substrates and a variety of free-living and symbiotic microbial systems.     

Influence of Two Plant Species (Flax and Tomato) on the Distribution of Nitrogen Dissimilative Abilities within Fluorescent Pseudomonas spp

The distribution of nitrogen-dissimilative abilities among 317 isolates of fluorescent pseudomonads was studied. These strains were isolated from an uncultivated soil and from the rhizosphere, rhizoplane, and root tissue of two plant species (flax and tomato) cultivated on this same soil. The isolates were distributed into two species, Pseudomonas fluorescens (45.1%) and Pseudomonas putida (40.4%), plus an intermediate type (14.5%). P. fluorescens was the species with the greatest proportion of isolates in the root compartments and the greatest proportion of dissimilatory and denitrifying strains. According to their ability to dissimilate nitrogen, the isolates have been distributed into nondissimilatory and dissimilatory strains, nitrate reducers and true denitrifiers with or without N(inf2)O reductase. The proportion of dissimilatory isolates was significantly enhanced in the compartments affected by flax and tomato roots (55% in uncultivated soil and 90 and 82% in the root tissue of flax and tomato, respectively). Among these strains, the proportion of denitrifiers gradually and significantly increased in the root vicinity of tomato (44, 68, 75, and 94% in uncultivated soil, rhizosphere, rhizoplane, and root tissue, respectively) and was higher in the flax rhizoplane (66%) than in the uncultivated soil. A higher proportion of N(inf2)O reducers was also found in the root compartments. This result was particularly clear for tomato. It is hypothesized that denitrification could be a selective advantage for the denitrifiers in the root environment and that this process could contribute to modify the specific composition of the bacterial communities in the rhizosphere.