Long term impact of mineral amendment on the distribution of the mineral weathering associated bacterial communities from the beech Scleroderma citrinum ectomycorrhizosphere

Liming is a known forest management procedure used to amend nutrient-poor soils such as soils of acidic forests to rectify cation deficiencies and to restore soil pH. However, although this procedure is well known for its beneficial effect on the forest trees, its relative impact on the functional and taxonomic diversity of the soil bacterial communities has been poorly investigated. In this study, we characterized the ability of the soil bacteria to weather soil minerals and to hydrolyze chitin. A collection of 80 bacterial strains was isolated from the Scleroderma citrinum ectomycorrhizosphere and the adjacent bulk soil in two stands of mature beeches (Fagus sylvatica) developed on very acidic soil and presenting two levels of calcium (Ca) availability: a control plot as well as a plot amended with Ca in 1973. All the bacterial isolates were identified by partial 16S rRNA gene sequence analysis as members of the genera Burkholderia, Bacillus, Dyella, Kitasatospora, Micrococcus, Paenibacillus, Pseudomonas, and Rhodanobacter. Using a microplate assay for quantifying the production of protons and the quantity of iron released from biotite, we demonstrated that the bacterial strains from the amended plot harbored a significant higher mineral weathering potential that the ones isolated from the control plot. Notably, the weathering efficacy of the ectomycorrhizosphere bacterial isolates was significantly greater than that of the bulk soil isolates in the control treatment but not in the amended plot. These data reveal that forest management, here mineral amendment, can strongly affect the structure of bacterial communities even over the long term.     

Efficient mineral weathering is a distinctive functional trait of the bacterial genus Collimonas

The mineral weathering ability of 45 bacterial strains belonging to the genus Collimonas and coming from various terrestrial environments was compared to that of 5 representatives from the closely related genera Herbaspirillum and Janthinobacterium. Using glucose as the sole carbon source in a microplate assay for quantifying the release of iron and protons from biotite, all Collimonas strains proved to be very efficient weathering agents, in contrast to the Herbaspirillum and Janthinobacterium strains. The weathering phenotype was also evident during growth of collimonads on mannitol and trehalose, but not on gluconic acid. All Collimonas strains were able to solubilize inorganic phosphorus and produce gluconic acid from glucose, suggesting that acidification is one of the main mechanisms used by these bacteria for mineral weathering. The production of siderophores may also be involved, but this trait, measured as the ability of collimonads to mobilize iron, was shared with Herbaspirillum and Janthinobacterium strains. These findings are discussed in an ecological context that recognizes collimonads as mycophagous (fungal-eating) and efficient mineral weathering bacteria and suggests that this ability has evolved as an adaptation to nutrient-poor conditions, possibly as part of a mutualistic relationship with mycorrhizal fungi.     

Decomposition of beech leaves (Fagus sylvatica) and spruce needles (Picea abies) in pure and mixed stands of beech and spruce

The decomposition of spruce needles and beech leaves was investigated in a 30- and 120-yr-old beech, spruce and mixed (beech/spruce) forest using 1 mm mesh litterbags. The mass loss, content of C, N and water and microbial biomass, basal respiration and specific respiration of the litter materials were analyzed after exposure for 1.5, 3, 6, 9, 12, 18 and 24 months in the field. Decomposition of both types of litter was faster in beech than in spruce stands and after 24 months loss of C from litter materials was at a maximum in beech stands (>60%) and considerably less in the spruce and mixed stands (ca. 40%). Generally, spruce needles decomposed more rapidly than beech leaves, but the faster decay was not associated with higher N concentrations. Rather, N was accumulated more rapidly in beech leaves. Concomitantly, in beech stands microbial biomass of beech leaves exceeded that of spruce needles indicating that beech leaves consist of more favorable resources for microorganisms than spruce needles. Differences in decomposition between beech leaves and spruce needles were most pronounced in beech stands, intermediate in mixed stands and least pronounced in spruce stands. Decomposition, N content and microbial biomass in litter materials exposed in the 120-yr-old stand consistently exceeded that in the 30-yr-old stand indicating adverse conditions for litter decay in regrowing stands. Generally, mixed stands ranked intermediate between spruce and beech monocultures for most of the variables measured indicating that the adverse conditions for litter decay and microorganisms in spruce forest are effectively counteracted by admixture of beech to spruce monocultures. It is concluded that the accumulation of litter materials in spruce forests is not due to the recalcitrance of spruce needles to decay. Rather, adverse environmental conditions such as high polyphenol contents in the litter layer of spruce stands retard decomposition processes; spruce needles appear to be more sensitive to this retardation than beech leaves.     

Spatial variability of photosynthetically active radiation in European beech and Norway spruce

The vertical and horizontal variability of solar radiation within a mature European beech (Fagus sylvatica L.)-Norway spruce (Picea abies [L.] Karst) mixed stand in Southern Germany is investigated. A large dataset with more than one million spectral measurements of photon fluence rates at six vertical levels within the stand is analyzed with respect to tree species, meteorological sky conditions, and the influence of solar elevation angle on canopy penetration. Irradiance probability density functions of the photosynthetically active waveband are used to describe the three-dimensional radiation field. For a quantification of umbra, penumbra, and sunfleck frequencies, in-canopy fractions of photon fluence rates within the photosynthetically active waveband are investigated. Different phenological stages of beech and their effects on the in-canopy light climate are compared. The results show that during overcast conditions (OVC) fractions of photosynthetically active radiation (PAR) are higher at all canopy levels than during clear sky (CS) conditions due to their exclusively diffuse character. The lowest median PAR level of less than 1% of above-canopy PAR can be observed in the shade crown of beech and at ground level. More PAR can penetrate the canopy at a higher solar elevation under CS conditions. This effect is more pronounced for spruce than for beech due to the conical crown shape of the conifers that allows photons from higher angles to enter the gaps inbetween trees in contrast to the more homogeneously closed beech canopy. Solar elevation is not an important factor at uniformly overcast conditions. Differences in the vertical distribution of umbra and penumbra can be detected when comparing species or different sky conditions. The frequency of sunflecks differs more by species and by the vertical position within the canopy than by sky condition.     

The Collembola community of pure and mixed stands of beech (Fagus sylvatica) and spruce (Picea abies) of different age

The idea of establishing mixed forests that are better adapted to site conditions than spruce monocultures has attracted increasing attention of forest owners and governmental institutions over the last decades. Currently, beech is being replanted and an increasing proportion of German forests are mixed stands. Focusing on the reaction of the soil fauna to forest conversion, this study investigates the response of the Collembola community to replacement of beech by spruce or by mixed stands of beech and spruce. Stands of different age were investigated in a factorial design with the factors stand type (beech, spruce and mixed stands) and stand age (30 and 120 years). Collembola communities did not differ strongly between stand types and stand age and were dominated by Folsomia quadrioculata and Mesaphorura species (e.g. Mesaphorura macrochaeta). Moreover, neither total abundance of Collembola nor densities of the fungal feeding euedaphic Onychiurinae and Tullbergiinae significantly responded to stand type and stand age. The density of the epedaphic and partly herbivorous groups Symphypleona/Neelipleona and Entomobryidae in the 120-year-old stands significantly exceeded that in the 30-year-old stands; presumably, this was due to the well developed herb layer in the 120-year-old stands with more open canopies. Canonical correspondence analysis (CCA) of the Collembola community of the L/F horizon also indicated that most of the epigeic species were associated with the 120-year-old stands. Moreover, the diversity of Collembola significantly increased with forest age which likely reflects increased amount and diversity of food resources in the 120-year-old stands. The density of the hygrophilous species Fo. quadrioculata was significantly higher in the spruce than in the beech stands; probably this was due to the higher water content in litter of the spruce stands. Moreover, the results of the CCAs indicated that soil pH is an important structuring force for the Collembola communities. Overall, the results suggest that stand type and forest age impact Collembola communities, presumably via changes in the amount and quality of food resources, such as living plant and herb litter materials. The pronounced changes which occurred with forest age likely were related to the development of more dense and diverse herb layer in mature forests which provides additional food resources in particular for epedaphic species. On the other hand, dominant species/functional groups of Collembola, such as hemiedaphic species, appear to depend predominantly on abiotic factors, most importantly soil pH and soil water content.     

Changes in humus properties and collembolan communities following the replanting of beech forests with spruce

Differences of collembolan communities within the organo-mineral A layer were studied in relation to physico-chemical changes in humus at nine sites of beech forests (Fagus sylvatica L.) and first generation spruce stands (Picea abies (L.) Karst.), planted on former beech forest 30 years ago (Central Pyrenees, France). Changes in humus form were caused by the spruce plantation and occurred mainly within the fermentation horizon where acidifying litter accumulation increased the horizon depth. The recent replacement of beech by spruce induced a shift from mull towards moder humus forms, which is explained by the decreasing organic matter turnover rate. A significant decrease in the three exchangeable cations and pH under spruce was also observed. Collembolan species diversity within the A horizon was significantly lower under spruce at this early stage of the silvogenetic cycle. Differences between species composition of Collembola between the two forest stands is related to changes in environmental conditions (e.g. nutrient availability, soil porosity, soil moisture). This study shows how forest management practices are susceptible to modify biological activity within the A horizon under exotic conifer plantations.     

Analysis of bacterial communities in soil by PCR-DGGE targeting protease genes

We studied the effects of the application of organic (OM) and inorganic fertilizer (CF) on soil protease activity and proteolytic bacterial communities in rhizosphere and bulk soil on an experimental lettuce field in Hokkaido, Japan. The protease activity always was higher in soils of the OM than with the CF treatment, and also higher in the rhizosphere than in the bulk soil. We analyzed proteolytic bacterial communities by denaturing gradient gel electrophoresis (DGGE) of the alkaline metalloprotease (apr) and neutral metalloprotease (npr) genes. Most apr forms detected were closely related to apr of Pseudomonas fluorescens, and all npr variants closely resembled the gene of Bacillus megaterium. These results were consistent with findings from tests using cultured bacterial communities, indicating a high specificity of our PCR-DGGE for amplifying apr and npr genes. The community compositions of proteolytic bacteria were assessed by principal component analysis of the DGGE profiles. There were significant differences in the effects of CF and OM on the community compositions of apr- and npr-expressing bacteria, and the communities of the two types of bacteria played different roles in rhizosphere and bulk soil. We found significant correlations between the protease activity and the communities of the two types of bacteria. The results indicate that different proteolytic bacteria release different amounts or activities of protease, and that the composition of proteolytic bacterial communities may play a major role in determining overall soil protease activity.     

Fluazifop-P-butyl (herbicide) affects richness and structure of soil bacterial communities

Fusilade (fluazifop-P-butyl) is an extensively used herbicide in legumes cropping systems; nevertheless, the extent of its ecological risk remains unknown. The aim of this work is to estimate the impact of this herbicide on some key enzyme activities, namely acid phosphatase, urease and FDA, and the effect on soil bacterial communities in the rhizosphere of pea (Pisum sativum). Acid phosphatase, urease and FDA showed a high decrease following the application of fluazifop-P-butyl mainly at ten-fold field rate. Grain yield was also severely affected at this concentration (−50%). T-RFLP analysis of 16S rDNA revealed a significant effect of fluazifop-P-butyl application on richness and structure of soil bacterial communities. Putative taxonomic assignation of induced TRFs revealed a marked pattern of acidogenic bacteria like as Staphylococcus, Escherichia, Desulfobacter, Clostridium, Paludibacter and other sulfate-reducing bacteria like as Desulfocella halophila and Desulfocurvus vexinensis. These bacteria may participate in the acidogenesis and the production of propionic acid from the fluazifop-P-butyl after hydrolytic cleavage. They are also known to play important roles in the global biogeochemical sulfur cycles. Unfortunately, some potential human pathogens were also stimulated. Moreover, many putative plant-growth-promoting bacteria belonging to the genera Streptomyces and Bacillus were found to be inhibited in the bulk soil and rhizosphere. The fluazifop-P-butyl effect on bacterial diversity was more pronounced in the rhizosphere compared to bulk soil or uncultivated soil which may suggest that a major component of this effect is mediated by the root system.     

Determination of the impact of continuous defoliation of Lolium perenne and Trifolium repens on bacterial and fungal community structure in rhizosphere soil

To determine the effects of defoliation on microbial community structure, rhizosphere soil samples were taken pre-, and post-defoliation from the root tip and mature root regions of Trifolium repens L. and Lolium perenne L. Microbial DNA isolated from samples was used to generate polymerase chain reaction–denaturing gradient gel electrophoresis molecular profiles of bacterial and fungal communities. Bacterial plate counts were also obtained. Neither plant species nor defoliation affected the bacterial and fungal community structures in both the root tip and mature root regions, but there were significant differences in the bacterial and fungal community profiles between the two root regions for each plant. Prior to defoliation, there was no difference between plants for bacterial plate counts of soils from the root tip regions; however, counts were greater in the mature root region of L. perenne than T. repens. Bacterial plate counts for T. repens were higher in the root tip than the mature root region. After defoliation, there was no effect of plant type, position along the root or defoliation status on bacterial plate counts, although there were significant increases in bacterial plate counts with time. The results indicate that a general effect existed during maturation in the root regions of each plant, which had a greater impact on microbial community structure than either plant type or the effect of defoliation. In addition there were no generic consequences with regard to microbial populations in the rhizosphere as a response to plant defoliation.     

Long-term impact of fertilization on activity and composition of bacterial communities and metabolic guilds in agricultural soil

To explore long-term impact of organic and inorganic fertilizers on microbial communities, we targeted both the total bacterial community and the autotrophic ammonia oxidizing bacteria (AOB) in soil from six treatments at an experimental field site established in 1956: cattle manure, sewage sludge, Ca(NO₃)₂, (NH₄)₂SO₄, unfertilized and unfertilized without crops. All plots, except the bare fallows, were cropped with maize. Effects on activity were assessed by measuring the basal respiration and substrate induced respiration (SIR) rates, and the potential activity of the AOB. To determine the bacterial community composition, 16S rRNA genes were used to fingerprint total soil communities by terminal restriction fragment length polymorphism analysis and AOB communities by denaturing gradient gel electrophoresis. The fertilization regimes had clear effects on both activity and composition of the soil communities. Basal respiration and r, which was kinetically derived as the exponentially growing fraction of the SIR-response, correlated well with the soil organic C content (r=0.93 and 0.66, respectively). Soil pH ranged from 3.97 to 6.26 in the treatments and was found to be an important factor influencing all microbial activities. pH correlated negatively with the ratio between basal respiration and SIR (r=0.90), indicating a decreased efficiency of heterotrophic microorganisms to convert organic carbon into microbial biomass in the most acid soils with pH 3.97 and 4.68 ((NH₄)₂SO₄ and sewage sludge fertilized plots, respectively). The lowest SIR and ammonia oxidation rates were also found in these treatments. In addition, these treatments exhibited individually different community fingerprints, showing that pH affected the composition of AOB and total bacterial communities. The manure fertilized plots harbored the most diverse AOB community and the pattern was linked to a high potential ammonia oxidation activity. Thus, the AOB community composition appeared to be more strongly linked to the activity than the total bacterial communities were, likely explained by physiological differences in the populations present.     

盐地碱蓬根际土壤细菌群落结构及其功能

盐地碱蓬作为生物改良盐碱地的理想材料,其根际土壤微生物对土壤改良发挥着重要作用。为了深入探索环渤海滨海盐碱地碱蓬根际土壤细菌群落结构组成及其功能,采用Illumina Misep高通量测序平台对环渤海地区滨海盐碱地盐地碱蓬根际土壤和裸地土壤进行测序。从16个样本中获得有效序列734 792条, 4 285个OTUs,归属于41门、100纲、282目、400科、892属、1 577种。盐地碱蓬根际土壤细菌群落由变形菌门(Proteobacteria)、放线菌门(Actinobacteria)、绿弯曲门(Chloroflexi)、拟杆菌门(Bacteroidetes)、芽单胞菌门(Gemmatimonadetes)、酸杆菌门(Acidobacteria)、厚壁菌门(Firmicutes)、蓝藻细菌门(Cyanobacteria)、髌骨细菌门(Patescibacteria、浮霉菌门(Planctomycetes)组成。Alpha多样性计算结果表明,盐地碱蓬根际土壤细菌群落结构多样性高并与裸地土壤间差异显著;LEfSe(LDAEffectSize)分析发现,盐地碱蓬与裸地差异指示种明显不同。PCoA与相关性Heatmap表明,盐地碱蓬、速效氮、速效钾、速效磷、电导率是影响土壤细菌目类水平群落组成的主要因子。PICRUSt(Phylogenetic InvestigationofCommunitiesbyReconstructionofUnobserved States)分析表明微生物群落在新陈代谢等40个功能方面盐地碱蓬根际土壤比裸地土壤高。本研究表明盐地碱蓬覆盖能够降低土壤盐分,增加土壤养分,对土壤细菌群落多样性及其功能有积极作用。     

中国北方森林坡向对土壤细菌和从枝菌根真菌群落的影响

The effects of slope aspects on soil biogeochemical properties and plant communities in forested environments have been studied extensively; however, slope aspect influence on soil microbial communities remains largely unexamined, despite the central role of soil biota in ecosystem functioning. In this study, the communities of both soil bacteria and arbuscular mycorrhizal fungi (AMF) were investigated using tagged pyrosequencing for three types of slope aspects (south-facing aspect, north-facing aspect and flat area) in a boreal forest of the Greater Khingan Mountains, China. The bacterial and AMF community composition differed with slope aspects. Bacterial diversity was the lowest on the north-facing aspect, and AMF diversity was the lowest on the flat area. Aspects also had a significant impact on soil pH and available phosphorus (P) and shrubby biomass. Soil pH and understory shrub biomass were significantly correlated with bacterial communities, and soil available P and shrub biomass showed significant correlations with AMF communities. Our results suggested that slope aspects affected bacterial and AMF communities, mediated by aspect-induced changes in plant community and soil chemical properties (e.g., pH and available P), which improved the knowledge on the effects of forest slope aspects on aboveground and belowground communities.     

Rapid changes in the rhizosphere bacterial community structure during re-colonization of sterilized soil

Diversity has been shown to be pivotal in ecosystem stability and resilience. It is therefore important to increase our knowledge about the development of diversity. The aim of this study was to investigate the temporal dynamics of the bacterial community structure in the rhizosphere of wheat plants growing in a soil in which the initial conditions for bacterial re-colonization were modified by mixing different amounts of sterilized with native soil at ratios of 19:1, 9:1, 4:1 and 1:1. Additional treatments comprised sterilized soil or native soil. Plant dry weight at day 20 decreased with increasing percentage of native soil in the mix. The bacterial community structure in the rhizosphere was assessed by polymerase chain reaction-denaturing gradient gel electrophoresis (DGGE) at days 3, 14 and 20 after planting. The bacterial community in the sterilized soil had a lower diversity and evenness than the native soil. Both diversity and evenness increased with time in the sterilized soil. Community structure in the different mixes changed over time and the changes were mix-specific. Principal component analyses of the DGGE banding patterns showed clear differences between the treatments particularly at day 3 and day 14 and revealed changes in community structure within a few days in a given treatment. The results of the present study show that bacterial communities rapidly re-colonize sterilized soil. During re-colonization, the community structure changes rapidly with a general trend towards higher diversity and evenness. The changes in community structure over time are also affected by the amount of sterile substrate to be re-colonized.     

Structure and function of bacterial communities in ageing soils: Insights from the Mendocino ecological staircase

The ecological staircase of Mendocino (California, USA) is characterized by a succession of uplifted marine terraces that are derived from the same mineralogical parent material but have different ages, levels of fertility, and types of vegetation, from grassland in the youngest and most fertile terrace to a pygmy forest in the older terraces. Such conditions present a unique opportunity to determine how the structure, abundance, and function of bacterial communities vary with soil fertility along this natural chronosequence. Pyrosequencing analysis of 16S rRNA gene amplicons revealed that Acidobacteria, Proteobacteria, Actinobacteria, and Bacteroidetes were the most abundantly represented phyla. Bacteroidetes, Firmicutes, Verrucomicrobia were significantly enriched in the grasslands, while the less fertile forested terraces showed higher abundance of Acidobacteria Gp2 and Alphaproteobacteria. The pygmy forest soil harboured significantly more Actinobacteria and OP10 than the non-pygmy forest. Between samples from different terraces, the structure of the bacterial community clearly correlated with soil characteristics. Notably, the number of operational taxonomic units was greater in the fertile terrace, as was the density of culturable bacterial populations. Functional characterization of the soil culturable bacteria from the pygmy and non-pygmy forest terraces revealed that the soil bacteria from the non-pygmy terrace were significantly more effective in solubilizing minerals and more abundant than in the pygmy terrace. Our results provide new information on bacterial community structure as a function of soil age, land cover and fertility, which improve our understanding of soil evolution.     

Effect of calco-magnesian amendment on the mineral weathering abilities of bacterial communities in acidic and silicate-rich soils

Liming of forest ecosystem is recognized to increase nutrients availability in soil and water and to enhance the biomass and bacterial activities in soil. However, little studies have investigated on change induced on bacterial ability to alter silicate mineral after soil liming. Thus, this study is carried out in experimental conditions using podzol collected in the winter (2007), spring and summer (2008) in small forest catchments in Vosges Mountains amended in 2003 by dolostone and limestone. Bacterial communities, extracted from various horizons of these soils amended or not, were put in contact with a phyllosilicate (phlogopite as sole source of Mg and Fe) in miniaturized bioassays in aerobic conditions. A weathering phenotype was determined through the quantification of (i) protons and organic acids released in assay solution by bacteria (ii) iron leached from phlogopite lattice into solution by bacteria and (iii) the carbon source consumption (i.e. glucose). These results were then compared to empirical model based on chemical leaching experiments realized in the same conditions in order to simulate the processes involved. In parallel, the carbon source utilization patterns of bacteria were investigated in order to discriminate the bacterial communities from amended and non-amended soil horizons. The results indicate that (1) the total bacterial biomass was unaffected by the Ca–Mg amendment, whereas the cultivable bacterial biomass increased after the amendment, and (2) the weathering and the carbon source utilization patterns of bacterial communities differs from one soil horizon to another and among soil types. The metabolic profiles analyzed indicated significant differences in organic C substrate usage depending on season and Ca–Mg amendment. Bacterial communities, extracted from the amended soil, are enable greater iron leaching compared to those found in the control soil, suggesting a greater release of organic acids and/or a more highly chelating organic acid release. The process developed by bacteria to alter the phillosilicate is complexolysis. We conclude that the Ca–Mg amendment had a positive effect on the functional richness of bacterial communities extracted from soil and on their potential to weather minerals that was present after several years.     

菌肥对青稞根际土壤理化性质以及微生物群落的影响

应用化学分析、聚合酶链反应-变性梯度凝胶电泳(PCR-DGGE)技术和DNA测序技术,研究了西藏棕色砂壤土中微生物肥料不同施用量和施用期对青稞根际土壤理化性质和细菌群落多样性的影响。结果表明,施用谷特菌肥能显著提高土壤全氮、全磷、全钾、有机质、碱解氮、有效磷和速效钾的水平,如播前施用菌肥浓度750 ml hm-2的处理较不施用菌肥的处理上述指标分别提高13.32%、28.42%、16.20%、9.81%、21.36%、39.35%和30.48%,拔节期施用菌肥浓度2 250 ml hm-2的处理较不施用菌肥的处理分别提高7.25%、29.35%、18.04%、12.86%、15.90%、43.27%和53.99%。DGGE分析表明,相同施用方式中不同施用量土样中微生物的DGGE图谱相似。非加权组平均法(UPGMA)聚类分析将DGGE图谱分为2大类群。Shannon-Wiener指数表明,施用菌肥的土壤细菌多样性先增加后逐渐降低,播前以喷施谷特菌浓度750 ml hm-2时的细菌多样性最高;拔节期则以喷施谷特菌浓度2 250 ml hm-2处理的细菌多样性最高,且两种施用方式土壤养分的释放与Shannon指数的变化规律均为播前﹥拔节期。测序结果表明,不同施肥浓度土样微生物种群分布较为广泛,其中Actinobacteria纲细菌种类略多,少数菌种为未经培养菌种(Uncultured bacterium)。典型对应分析(CCA)表明,DGGE图谱条带分布与土壤理化性质密切相关,碱解氮、全磷和全氮是影响微生物群落的主要环境因子。研究结果表明,施用谷特菌肥可明显改善青稞根际土壤理化性状,提高土壤细菌多样性。     

Impact of artificial root exudates on the bacterial community structure in bulk soil and maize rhizosphere

Bacterial densities, metabolic signatures and genetic structures were evaluated to measure the impact of soil enrichment of soluble organic carbon on the bacterial community structures. The exudates chosen were detected in natural maize exudates (glucose, fructose, saccharose, citric acid, lactic acid, succinic acid, alanine, serine and glutamic acid) and were used at a rate of 100 μg C g⁻¹ day⁻¹ for 14 days. Moreover two synthetic solutions with distinct carbon/nitrogen ratios (20.5 and 40.1), obtained by varying carboxylic and amino acids concentrations, were compared in order to evaluate the potential role of organic N availability. The in vitro experiment consisted of applying exudate solutions to bulk soil. In the case of the control, only distilled water was added. Both solutions significantly increased bacterial densities and modified the oxidation pattern of Biolog® GN2 plates with no effect of the C/N ratio on these two parameters. Genetic structure, measured by means of ribosomal intergenic spacer analysis (RISA), was also consistently modified by the organic amendments. N availability levels led to distinct genetic structures. In a second experiment, one of the previous exudate solutions (C/N 20.5) was applied to 15-day-old maize plants to determine the structural influence of exudates on the rhizosphere microbial community (in situ experiment). Bacterial densities were significantly increased, but to a lesser extent than had been found in the in vitro experiment. Metabolic potentials and RISA profiles were also significantly modified by the organic enrichment.     

Coarse particulate organic matter is the primary source of mineral N in the topsoil of three beech forests

Forest soils contain a variable amount of organic N roughly repartitioned among particles of different size, microbial biomass and associated with mineral compounds. All pools are alimented by annual litter fall as main input of organic N to the forest floor. Litter N is further subject to mineralization/stabilization recognized as the crucial process for the turnover of litter N. Although it is well documented that different soil types have different soil N stocks, it is presently unknown how different soil types affect the turnover of recent litter N. Here, we compared the potential mineralization of the total soil organic N with that of recent litter-released N in three beech forests varying in their soil properties. Highly ¹⁵N-labelled beech litter was applied to stands located at Aubure, Ebrach, Collelongo, which differ in humus type, soil type and soil chemistry. After 4-5 years of litter decomposition, the upper 3 cm of the organo-mineral A horizon was sampled and the net N mineralization was measured over 112 days under controlled conditions. The origin of mineralized N (litter N versus soil organic N) was calculated using ¹⁵N labeling. In addition, soils were fractionated according to their particle size (>2000 μm, 200-2000 μm, 50-200 μm, <50 μm) and particulate organic matter (POM) was separated from the mineral fraction in size classes, except the <50 μm fraction. Between 41 and 69% of soil organic N was recovered as POM. Litter-released ¹⁵N was mainly to be found in the coarse POM fractions >200 μm. On a soil mass basis, N mineralization was two-fold higher at Aubure and Collelongo than at Ebrach, but, on a soil N basis, N mineralization was the lowest at Collelongo and the highest at Ebrach. On a soil N (or ¹⁵N) basis, mineralization of litter ¹⁵N was two to four-fold higher than mineralization of the average soil N. Furthermore, the δ¹⁵N of the mineral N produced was closer to that of POM than to that of the mineral-bound fraction (<50 μm). Highest rates of ¹⁵N mineralization happened in the soil with the lowest N content, and we found a negative relationship between accumulations of N in the upper A horizon and the mineralization of ¹⁵N from the litter. Our results show that mineral N is preferentially mineralized from POM in the upper organo-mineral soil irrespective of the soil chemistry and that the turnover rate of litter N is faster in soils with a low N content.     

Density, structure, and diversity of the cultivable arylsulfatase-producing bacterial community in the rhizosphere of field-grown rape and barley

In this study we investigated the arylsulfatase-producing bacterial community (ARS-BC) in the rhizosphere soils of field-grown rape in comparison with that of barley. For this, the rhizosphere soils from both plant species were sampled four times during plant growth. Soil arylsulfatase (ARS) activity and the density and the structure of the cultivable ARS-BC on M9-Xsulf medium were then determined. ARS activity in rape rhizosphere was greater than in barley rhizosphere and evolved along the phenology in the two rhizosphere soils. In parallel, the average density of ARS-BC in the rape rhizosphere was higher than that in the barley rhizosphere. Moreover, ARS activity is correlated with ARS-BC density both in rape and barley rhizosphere soils. The structure of the ARS-BC in the rape rhizosphere was different from that in the barley rhizosphere. In the rape rhizosphere, the ARS-BC was substantially more structured than in the barley rhizosphere. Among the ARS-BC, Actinobacteria and Pseudomonads were significantly present in the both rhizosphere soils. Actinobacteria predominated in the barley rhizosphere while Pseudomonads were mostly represented under rape. It is possible that the differences in ARS activity observed between rape and barley can be attributed to a different ARS-BC size and/or a different ARS-BC structure under these two plant covers. This impact of rape may be connected to a selective effect of rhizodeposits released by rape roots to the functional bacterial community. Our findings suggest that plant species, via their rhizodeposits, may affect the functional bacterial community and thus influence the dynamic of S in soil.     

喀斯特生态系统中乔木和灌木林根际土壤微生物生物量及其多样性的比较

我国喀斯特区域面积分布较广,而喀斯特生态系统的退化已成为当前西南地区面临的严重的生态问题。本研究选取贵州中部两种不同植被类型的生态系统—乔木林和灌木林,以乔木林中的白栎、园果化香和灌木林中的火棘、竹叶椒等主要优势树种为对象,研究不同的植物树种对根际土壤微生物生物量及其细菌群落结构的影响。结果显示:乔木林系统中根际土壤微生物生物量碳、氮显著性高于灌木林,植物的根际效应在乔木林中表现更为显著;同时乔木林中的优势树种通过根系分泌物的作用显著提高根际土壤细菌多样性指数,而灌木林中优势树种的根际土壤微生物量及多样性均未表现出明显的根际效应。因此,植被的演替通过改变土壤微生物的特性影响植物-微生物-土壤之间的物质和能量循环,进一步影响喀斯特生态系统的稳定和健康功能。