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Soil Microbes (soil + microbe)
Selected AbstractsThe unseen majority: soil microbes as drivers of plant diversity and productivity in terrestrial ecosystemsECOLOGY LETTERS, Issue 3 2008Marcel G. A. Van Der Heijden Abstract Microbes are the unseen majority in soil and comprise a large portion of life's genetic diversity. Despite their abundance, the impact of soil microbes on ecosystem processes is still poorly understood. Here we explore the various roles that soil microbes play in terrestrial ecosystems with special emphasis on their contribution to plant productivity and diversity. Soil microbes are important regulators of plant productivity, especially in nutrient poor ecosystems where plant symbionts are responsible for the acquisition of limiting nutrients. Mycorrhizal fungi and nitrogen-fixing bacteria are responsible for c. 5,20% (grassland and savannah) to 80% (temperate and boreal forests) of all nitrogen, and up to 75% of phosphorus, that is acquired by plants annually. Free-living microbes also strongly regulate plant productivity, through the mineralization of, and competition for, nutrients that sustain plant productivity. Soil microbes, including microbial pathogens, are also important regulators of plant community dynamics and plant diversity, determining plant abundance and, in some cases, facilitating invasion by exotic plants. Conservative estimates suggest that c. 20 000 plant species are completely dependent on microbial symbionts for growth and survival pointing to the importance of soil microbes as regulators of plant species richness on Earth. Overall, this review shows that soil microbes must be considered as important drivers of plant diversity and productivity in terrestrial ecosystems. [source] The turnover of carbohydrate carbon in a cultivated soil estimated by 13C natural abundancesEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 4 2006D. Derrien Summary Understanding the chemical composition of soil organic matter (SOM) requires the determination of the dynamics of each class of compounds. We measured the dynamics of carbon in neutral carbohydrates by use of natural 13C labelling in an experimental wheat and maize sequence extending over 23 years. The isotopic composition of individual neutral monosaccharides was determined in hydrolysed particle-size fractions by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) of trimethylsilyl (TMS) derivatives. The sensitivity in terms of 13C/12C ratios ranged between 1 and 2, depending on the monosaccharide. The age distribution of neutral sugar carbon was very similar to that of total soil carbon. Particulate organic matter (POM) was characterized by the predominance of glucose and xylose of vegetal origin. In POM >,200 µm, the mean age of sugar-C (5 years) was slightly less than that of total carbon (7 years). Xylose was younger than glucose. The fine fraction 0,50 µm contained mainly glucose, arabinose, galactose, xylose, fucose and mannose, which had predominantly microbial origins. The mean age of carbohydrate carbon in the fraction 0,50 µm was between 60 and 100 years and was similar to that of total organic carbon (OC). No difference in the age of carbon between the individual monosaccharides was found. The POM fraction 50,200 µm had an intermediate signature and turnover. Considering the typical lability of carbohydrates, the relatively great age of carbohydrate carbon may be explained by physical or chemical protection from degradation, as well as by recycling of soil organic matter carbon by soil microbes. [source] Regionalisation of chemical variability in European mountain lakesFRESHWATER BIOLOGY, Issue 12 2009LLUÍS CAMARERO Summary 1. We carried out a coordinated survey of mountain lakes covering the main ranges across Europe (including Greenland), sampling 379 lakes above the local tree line in 2000. The objectives were to identify the main sources of chemical variability in mountain lakes, define a chemical classification of lakes, and develop tools to extrapolate our results to regional lake populations through an empirical regionalisation or upscaling of chemical properties. 2. We investigated the main causes of chemical variability using factor analysis (FA) and empirical relationships between chemistry and several environmental variables. Weathering, sea salt inputs, atmospheric deposition of N and S, and biological activity in soils of the catchment were identified as the major drivers of lake chemistry. 3. We tested discriminant analysis (DA) to predict the lake chemistry. It was possible to use the lithology of the catchments to predict the range of Ca2+ and SO42, into which a lake of unknown chemistry will decrease. Lakes with lower SO42, concentrations have little geologically derived S, and better reflect the variations in atmospheric S loading. The influence of marine aerosols on lakewater chemistry could also be predicted from the minimum distance to the sea and altitude of the lakes. 4. The most remarkable result of FA was to reveal a factor correlated to DOC (positively) and NO3, (negatively). This inverse relationship might be the result either of independent processes active in the catchment soils and acting in an opposite sense, or a direct interaction, e.g. limitation of denitrification by DOC availability. Such a relationship has been reported in the recent literature in many sites and at all scales, appearing to be a global pattern that could reflect the link between the C and N cycles. 5. The concentration of NO3, is determined by both atmospheric N deposition and the processing capacity of the catchments (i.e. N uptake by plants and soil microbes). The fraction of the variability in NO3, because of atmospheric deposition is captured by an independent factor in the FA. This is the only factor showing a clear pattern when mapped over Europe, indicating lower N deposition in the northernmost areas. 6. A classification has been derived which takes into account all the major chemical features of the mountain lakes in Europe. FA provided the criteria to establish the most important factors influencing lake water chemistry, define classes within them, and classify the surveyed lakes into each class. DA can be used as a tool to scale up the classification to unsurveyed lakes, regarding sensitivity to acidification, marine influence and sources of S. [source] Seasonal variation in enzyme activities and temperature sensitivities in Arctic tundra soilsGLOBAL CHANGE BIOLOGY, Issue 7 2009MATTHEW D. WALLENSTEIN Abstract Arctic soils contain large amounts of organic matter due to very slow rates of detritus decomposition. The first step in decomposition results from the activity of extracellular enzymes produced by soil microbes. We hypothesized that potential enzyme activities are low relative to the large stocks of organic matter in Arctic tundra soils, and that enzyme activity is low at in situ temperatures. We measured the potential activity of six hydrolytic enzymes at 4 and 20 °C on four sampling dates in tussock, intertussock, shrub organic, and shrub mineral soils at Toolik Lake, Alaska. Potential activities of N -acetyl glucosaminidase, ,-glucosidase, and peptidase tended to be greatest at the end of winter, suggesting that microbes produced enzymes while soils were frozen. In general, enzyme activities did not increase during the Arctic summer, suggesting that enzyme production is N-limited during the period when temperatures would otherwise drive higher enzyme activity in situ. We also detected seasonal variations in the temperature sensitivity (Q10) of soil enzymes. In general, soil enzyme pools were more sensitive to temperature at the end of the winter than during the summer. We modeled potential in situ,-glucosidase activities for tussock and shrub organic soils based on measured enzyme activities, temperature sensitivities, and daily soil temperature data. Modeled in situ enzyme activity in tussock soils increased briefly during the spring, then declined through the summer. In shrub soils, modeled enzyme activities increased through the spring thaw into early August, and then declined through the late summer and into winter. Overall, temperature is the strongest factor driving low in situ enzyme activities in the Arctic. However, enzyme activity was low during the summer, possibly due to N-limitation of enzyme production, which would constrain enzyme activity during the brief period when temperatures would otherwise drive higher rates of decomposition. [source] Effect of seed coating on plant growth and soil conditions: A preliminary study for restoration of degraded rangeland in the Qinghai,Tibetan Plateau, ChinaGRASSLAND SCIENCE, Issue 3 2010Yingchun Liu Abstract Rangeland degradation is a significant problem in the Qinghai,Tibetan Plateau, China. Restoration of the degraded rangelands through reseeding is being undermined by poor seedling growth under adverse soil moisture and nutrition conditions. Accordingly, seeds of Italian ryegrass (Lolium multiflorum) and Chinese milk vetch (Astragalus sinicus) were coated with a mixture of hygroscopic and plant-derived polysaccharide gums and alga powder (Phaeocystis sp. necolon-1), and inoculated with spores of microorganisms (Aspergillus sp. and Streptomyces sp.), serving as coating mixture decomposers, in order to improve rhizosphere moisture and nutrition. In a growth chamber simulating spring climate on the Plateau, seedling emergence of the coated seeds was 60,80 and 48,82%, respectively, for the two plant species in comparison to 38 and 24% for the uncoated seeds, which was due to moisture being supplied by the coat. In the outdoor pots with soil texture and moisture conditions similar to those of the degraded rangelands on the Plateau, dry weight of the plants from the coated seeds was 109,184 and 118,156 mg plant,1 for the respective plant species, while that of the plants from uncoated seeds was 18,20 and 10,11 mg plant,1. The number of Rhizobium sp. nodules on Chinese milk vetch plants from the coated seeds was 21,25 plant,1 while 0,2 in plants from uncoated seeds. Enhancement of plant growth was caused by increased activity of soil microbes. Plant growth on the soil after sampling plants from the coated seeds was also enhanced, indicating sustainability of improvement of soil conditions. Thus seed coating with hygroscopic nutrient-source materials and inoculation with microorganism spores as slow decomposers of the coating materials is a promising method for restoring degraded rangelands. [source] Conspecific plant,soil feedbacks reduce survivorship and growth of tropical tree seedlingsJOURNAL OF ECOLOGY, Issue 2 2010Sarah McCarthy-Neumann Summary 1.,The Janzen,Connell (J,C) Model proposes that host-specific enemies maintain high tree species diversity by reducing seedling performance near conspecific adults and promoting replacement by heterospecific seedlings. Support for this model often comes from decreased performance for a species at near versus far distances from conspecific adults. However, the relative success of conspecific versus heterospecific seedlings recruiting under a given tree species is a critical, but untested, component of the J,C Model. 2.,In a shade-house experiment, we tested plant,soil feedbacks as a J,C mechanism in six tropical tree species. We assessed effects of conspecific versus heterospecific cultured soil extracts on seedling performance for each species, and we compared performance of conspecific versus heterospecific seedlings grown with soil extract cultured by a particular tree species. Additionally, we tested whether soil microbes were creating these plant,soil feedbacks and whether low light increased species vulnerability to pathogens. 3.,Among 30 potential comparisons of survival and mass for seedlings grown in conspecific versus heterospecific soil extracts, survival decreased in seven and increased in two, whereas mass decreased in 13 and increased in 1. To integrate survival and growth, we also examined seedling performance [(mean total mass × mean survival time)/(days of experiment)], which was lower in 16 and higher in 2 of 30 comparisons between seedlings grown with soil extract cultured by conspecific versus heterospecific individuals. Based on performance within a soil extract, conspecific seedlings were disadvantaged in 15 and favoured in 7 of 30 cases relative to heterospecific seedlings. 4.,Species pairwise interactions of soil modification and seedling performance occurred regardless of sterilization, suggesting chemical mediation. Microbes lacked host-specificity and reduced performance regardless of extract source and irradiance. 5.,Synthesis. These results, along with parallel research in temperate forests, suggest that plant,soil feedbacks are an important component of seedling dynamics in both ecosystems. However, negative conspecific feedbacks were more prevalent in tropical than temperate species. Thus, negative plant,soil feedbacks appear to facilitate species coexistence via negative distance-dependent processes in tropical but not temperate forests, but the feedbacks were mediated through chemical effects rather than through natural enemies as expected under the J,C Model. [source] Carbon flux from plants to soil: roots are a below-ground source of phenolic secondary compounds in an alpine ecosystemJOURNAL OF ECOLOGY, Issue 3 2008Courtney L. Meier Summary 1Phenolics are an important, biologically reactive component of the carbon (C) pool that moves from plants to soil. Once in soil, phenolics can regulate plant,soil feedbacks because of their influence on soil nitrogen biogeochemistry. 2Roots are a largely overlooked potential source of below-ground phenolic C. We examined phenolic fluxes from plants to soil in an alpine ecosystem, where phenolics are associated with slow rates of nutrient cycling. Using a phenolic-rich forb (Acomastylis rossii) and a grass with low tissue phenolics (Deschampsia caespitosa), we asked whether leaves, leaf litter or roots are the dominant source of soil phenolics during the growing season. We also determined whether the composition of root-derived phenolics differed from that of leaf litter. 3Both labile low molecular weight phenolics and tannins disappeared from A. rossii leaf litter over the winter. Evidence from this study and others indicates litter phenolics are not a significant source of labile C for soil microbes throughout the growing season. 4In the field, levels of phenolics were higher under A. rossii canopies than under D. caespitosa canopies throughout the growing season. We also estimated significantly higher phenolic fluxes into soils for A. rossii than for D. caespitosa in the glasshouse. Field and glasshouse results suggest roots are an important source of these compounds. Furthermore, the phenolic chemistry of roots was different from that of leaf litter, indicating that the effects of root phenolics on soil processes and neighbouring plant growth may differ from those associated with leaves. 5Synthesis. Based on our results, labile phenolic inputs from roots are likely to have a more important influence on soil nutrient dynamics during the alpine growing season than phenolic inputs from leaf litter. We suggest that roots may be the dominant input of labile phenolics to soil during the growing season in other ecosystems with seasonal patterns of plant growth and senescence. These observations are critical to our understanding of how phenolic-rich species may interact with soil microbes to influence soil nutrient cycling and shape the soil resource environment. [source] Field measurements of the water content in the top soil using a new capacitance sensor with a flat sensitive volumeJOURNAL OF PLANT NUTRITION AND SOIL SCIENCE, Issue 2 2005Bernhard Ruth Abstract Water content directly near the soil surface plays an essential role for degradation of natural organic material and agrochemicals by soil microbes. Furthermore, the water losses by evaporation depend sensitively on the top-soil water content. Rain, irrigation, evaporation, and the water flow between the soil horizons together with the natural inhomogeneity of soils cause a high spatial gradient and a pronounced time dependence of the water content in the top soil. To understand processes in top soil such as redox gradients, the knowledge on ecological conditions in the top soil, which is subject to rapid changes, is essential. In order to meet the requirements for such field measurements, a capacitance sensor with a depth resolution of 1,cm and an active area of 7.5,cm × 14,cm was constructed and operated by a special electronic circuit. Field measurements using these sensors at 1,cm depth showed the high dynamics when measurements were carried out every 10,min. As simultaneous measurements of the soil temperature at 1,cm depth exhibit large temperature variations during the day, its influence on the measurements must be compensated for. As the data, measured during drying periods, allow the assessment of the temperature coefficient, the water content at a reference temperature can be calculated. The course of the water content reflects precipitation events and quantifies the drying of the soil, providing these parameters for process evaluation. Furthermore, the diurnal variation exhibits the drying during the day and the possible rewetting from deeper horizons during the night. Freilandmessungen des Wassergehalts im Oberboden mit einem neuen Kapazitätssensor mit flachem sensitiven Volumen Der Wassergehalt direkt an der Bodenoberfläche spielt für den mikrobiellen Abbau natürlicher organischer Substanz und von Agrochemikalien eine bedeutende Rolle. Darüber hinaus hängen die Wasserverluste durch Evaporation empfindlich vom Wassergehalt an der Bodenoberfläche ab. Regen, Bewässerung, Evaporation und die Wasserbewegung zwischen den Bodenhorizonten, sowie die natürliche Inhomogenität des Bodens verursachen einen großen Gradienten und eine ausgeprägte Zeitabhängigkeit des Wassergehalts und entsprechender Stofftransformationsprozesse im Oberboden. Für das Verständnis der Prozesse im Oberboden, wie z.,B. der Redox-Gradienten, ist die Kenntnis der ökologischen Bedingungen in dem sich schnell verändernden Oberboden unerlässlich. Um die Anforderungen für solche Feldmessungen zu erfüllen, wurde ein Kapazitätssensor mit einer Tiefenauflösung von 1,cm und einer aktiven Fläche von 7.5,cm × 14,cm konstruiert und mit einem speziellen elektronischen Schaltkreis betrieben. Feldmessungen in der Tiefe von 1,cm zeigen eine große Dynamik, wenn alle 10 min ein neuer Messwert erfasst wird. Da simultane Messungen der Bodentemperatur in 1,cm Tiefe hohe Variationen zeigen, muss deren Einfluss auf die Messung kompensiert werden. Da die Messungen während der Trockenperioden die Abschätzung des Temperaturkoeffizienten erlauben, kann der Wassergehalt bei einer Referenztemperatur errechnet werden. Die Messergebnisse korrespondieren mit Regenereignissen und erfassen die Austrocknung des Bodens, so dass damit Parameter für die Prozessberechnung zur Verfügung gestellt werden. Der Tagesgang zeigt Austrocknung während des Tages und die mögliche Wiederbefeuchtung aus tieferen Horizonten während der Nacht. [source] Comparative effect of biofertilizers on fodder production and quality in guinea grass (Panicum maximum Jacq.)JOURNAL OF THE SCIENCE OF FOOD AND AGRICULTURE, Issue 9 2008Seema Mishra Abstract BACKGROUND:Panicum maximum Jacq. is one of the most important fodder grasses of the tropics. For good production and growth it requires high amounts of N fertilizer. Chemical fertilizers have a deleterious effect on soil fertility and are not economical for resource-poor farmers. Utilization of plant growth-promoting bacteria and fungi proved to be beneficial for improving crop production as well as the soil fertility. In the present paper the effect of inoculation of N fixer (Azospirillum brasilense) and P solubilizers (arbuscular mycorrhizal (AM) fungi consortia and inoculum of Glomus intraradiaces) was studied in single as well as mixed inoculation on forage yield and quality in guinea grass (Panicum maximum Jacq.) at different cuttings under the cut-and-carry system. RESULTS: Overall five cuttings were obtained during the year. Dual inoculation, i.e., Azospirillum with indigenous AM consortia, significantly improved fodder growth, production and quality in terms of crude protein (CP) content, while neutral detergent fiber (NDF) and acid detergent fiber (ADF) content decreased after this treatment. The density of soil microbes (number of Azospirillum colony-forming units, number of AM spores) was enhanced in mixed inoculation. Chemical fertilizer improved fodder production and CP content significantly over control but also enhanced NDF and ADF content and suppressed the Azospirillum colony-forming units, AM spores and AM root infection. CONCLUSION: It could be concluded from the present study that inoculated N fixer and P solubilizer have a synergistic effect which enhanced overall fodder production, quality and also beneficial microflora in the rhizosphere soil, which also demonstrated the sustainability of biofertilizers. Copyright © 2008 Society of Chemical Industry [source] Understorey plant and soil responses to disturbance and increased nitrogen in boreal forestsJOURNAL OF VEGETATION SCIENCE, Issue 2 2009O.H. Manninen Abstract Question: How do N fertilization and disturbance affect the understorey vegetation, microbial properties and soil nutrient concentration in boreal forests? Location: Kuusamo (66°22,N; 29°18,E) and Oulu (65°02,N; 25°47,E) in northern Finland. Methods: We conducted a fully factorial experiment with three factors: site (two levels), N fertilization (four levels) and disturbance (two levels). We measured treatment effects on understorey biomass, vegetation structure, and plant, soil and microbial N and C concentrations. Results: The understorey biomass was not affected by fertilization either in the control or in the disturbance treatment. Fertilization reduced the biomass of deciduous Vaccinium myrtillus. Disturbance had a negative effect on the biomass of V. myrtillus and evergreen Vaccinium vitis-idaea and decreased the relative proportion of evergreen species. Fertilization and disturbance increased the biomass of grass Deschampsia flexuosa and the relative proportion of graminoids. The amount of NH4+ increased in soil after fertilization, and microbial C decreased after disturbance. Conclusions: Our results suggest that the growth of slow-growing Vaccinium species and soil microbes in boreal forests are not limited by N availability. However, significant changes in the proportion of dwarf shrubs to graminoids and a decrease in the biomass of V. myrtillus demonstrate the susceptibility of understorey vegetation to N enrichment. N enrichment and disturbance seem to have similar effects on understorey vegetation. Consequently, increasing N does not affect the rate or the direction of recovery after disturbance. Moreover, our study demonstrates the importance of understorey vegetation as a C source for soil microbes in boreal forests. [source] Belowground carbon allocation by trees drives seasonal patterns of extracellular enzyme activities by altering microbial community composition in a beech forest soilNEW PHYTOLOGIST, Issue 3 2010Christina Kaiser Summary ,Plant seasonal cycles alter carbon (C) and nitrogen (N) availability for soil microbes, which may affect microbial community composition and thus feed back on microbial decomposition of soil organic material and plant N availability. The temporal dynamics of these plant,soil interactions are, however, unclear. ,Here, we experimentally manipulated the C and N availability in a beech forest through N fertilization or tree girdling and conducted a detailed analysis of the seasonal pattern of microbial community composition and decomposition processes over 2 yr. ,We found a strong relationship between microbial community composition and enzyme activities over the seasonal course. Phenoloxidase and peroxidase activities were highest during late summer, whereas cellulase and protease peaked in late autumn. Girdling, and thus loss of mycorrhiza, resulted in an increase in soil organic matter-degrading enzymes and a decrease in cellulase and protease activity. ,Temporal changes in enzyme activities suggest a switch of the main substrate for decomposition between summer (soil organic matter) and autumn (plant litter). Our results indicate that ectomycorrhizal fungi are possibly involved in autumn cellulase and protease activity. Our study shows that, through belowground C allocation, trees significantly alter soil microbial communities, which may affect seasonal patterns of decomposition processes. [source] Evidence for top predator control of a grazing ecosystemOIKOS, Issue 11 2008Douglas A. Frank The importance of top predators in controlling ecological processes in large, intact ecosystems is unclear. In grasslands that support abundant ungulates, top,down control by predators may be particularly important, because of the tight biogeochemical linkages of ungulate prey with plants and soil microbes. Here, I examined the effects of the recent reintroduction of the gray wolf Canis lupus on ecosystem processes in Yellowstone National Park, where herds of grazing ungulates previously have been shown to stimulate several processes, including soil net nitrogen (N) mineralization. Rates of ungulate grazing intensity and soil net N mineralization were compared before and after wolf reintroduction in grasslands ranging five-fold in aboveground production. Grazing intensity and grassland net N mineralization declined after wolf reintroduction, a likely partial function of fewer ungulates; wolf predation has been one of several factors implicated in causing the decline in Yellowstone ungulates. In addition, the spatial pattern of grazing and net N mineralization changed after reintroduction. A shift in the spatial patterns of grazer-associated processes is consistent with a growing body of work indicating that wolves have changed habitat use patterns of ungulates in Yellowstone National Park. These findings suggest widespread wolf effects on ungulate prey, plants, and microbial activity that have spatially reorganized grassland energy and nutrient dynamics in Yellowstone Park. [source] The effect of incorporating straw or manure into the soil on the natural microflora of winter wheatPLANT PATHOLOGY, Issue 5 2001B. S. Rodgers-Gray Over 3 years, five crops of winter wheat with different fertilization or pesticide treatments were grown in replicated plots in which straw, cattle manure or nothing was incorporated into the soil. It was previously shown that severities of several foliar diseases were reduced by the treatment with straw. Treatments with straw slightly increased leaf microbial numbers at growth stage (GS) 10, and significantly but slightly decreased numbers at GS 60 and 70, when averaged over all crops. There was no evidence of differences in potential for biological control between plots, because no differences were demonstrated between treatments in the proportion of phylloplane residents able to produce chitinase, siderophores or antibiotics. There was no link between soil and leaf nitrogen and microbial populations or soil amendment. The two youngest leaves of straw-treated plants remained green longer than leaves from untreated or manure-treated plants. Microbial numbers differed between crops, but within crops were more stable on leaves and ears than in soil. Soil populations were larger before the crops were sown. Bacteria predominated over fungi in all habitats throughout the growing season. The ratio of Gram-negative to Gram-positive bacteria recovered was similar on leaves and in soil, but not on ears. Soil and leaf microbes utilized similar carbon sources. More leaf epiphytes than soil microbes produced siderophores. The majority of identified organisms belonged to only a few species. Chromogenic organisms were more abundant on aerial surfaces. A fungicide, chlorothalonil, was applied in one crop; it reduced the microbial population but did not alter its composition (without affecting the reduction in disease due to straw). It is postulated that straw acts in the field by altering plant physiology and thus altering both microbial colonization and disease. [source] ,15N of soil N and plants in a N-saturated, subtropical forest of southern ChinaRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 17 2010K. Koba We investigated the ,15N profile of N (extractable NH, NO, and organic N (EON)) in the soil of a N-saturated subtropical forest. The order of ,15N in the soil was EON,>,NH,>,NO. Although the ,15N of EON had been expected to be similar to that of bulk soil N, it was higher than that of bulk soil N by 5,. The difference in ,15N between bulk soil N and EON (,15Nbulk-EON) was correlated significantly with the soil C/N ratio. This correlation implies that carbon availability, which determines the balance between N assimilation and dissimilation of soil microbes, is responsible for the high ,15N of EON, as in the case of soil microbial biomass ,15N. A thorough ,15N survey of available N (NH, NO, and EON) in the soil profiles from the organic layer to 100,cm depth revealed that the ,15N of the available N forms did not fully overlap with the ,15N of plants. This mismatch in ,15N between that of available N and that of plants reflects apparent isotopic fractionation during N uptake by plants, emphasizing the high N availability in this N-saturated forest. Copyright © 2010 John Wiley & Sons, Ltd. [source] Soil fertility, heterogeneity, and microbes: towards an integrated understanding of grassland structure and dynamicsAPPLIED VEGETATION SCIENCE, Issue 1 2009Heather L. Reynolds Abstract Objective: To highlight the need and the potential for an integrated understanding of three key soil-based drivers of plant community structure and dynamics , soil fertility, soil heterogeneity, and microbes. Location: European and North American grasslands. Methods: Review and discussion of conceptual models and empirical literature, including examples of observational and manipulative studies from both natural and restored grassland communities. Results and Conclusions: In general, the results of empirical studies on soil fertility, soil heterogeneity, and soil microbes in grassland communities do not support expectations of common conceptual models. Ecological theory assumes a unimodal relationship between soil fertility and plant community diversity, yet empirical relationships from grassland communities are variable, the mechanisms underlying these variable patterns are not yet well understood, and there is mixed success at manipulating soil fertility to facilitate restorations. While theory predicts that increased soil heterogeneity will lead to increased plant community diversity, results of experimental manipulations of soil heterogeneity often show the opposite. Of two major conceptual models proposed for how microbes structure plant communities, there is little support for the hypothesis of microbially mediated niche partitioning. Plant-microbe feedbacks do have significant empirical support to date and there is increasing application of positive feedback dynamics in restoration, yet field tests of feedback dynamics remain limited. We suggest that an understanding of interactions between these soil drivers may help to resolve discrepancies between conceptual models and empirical results, improving our understanding of grasslands and our ability to restore them. [source] Diversity and salt tolerance of native Acacia rhizobia isolated from saline and non-saline soilsAUSTRAL ECOLOGY, Issue 8 2009PETER H. THRALL Abstract Re-establishing native vegetation in stressed soils is of considerable importance in many parts of the world, leading to significant interest in using plant,soil symbiont interactions to increase the cost-effectiveness of large-scale restoration. However, effective use of soil microbes in revegetation requires knowledge of how microbe communities vary along environmental stress gradients, as well as how such variation relates to symbiont effectiveness. In Australia, shrubby legumes dominate many ecosystems where dryland salinity is a major issue, and improving plant establishment in saline soils is a priority of regional management agencies. In this study, strains of rhizobial bacteria were isolated from a range of Acacia spp. growing in saline and non-saline soils. Replicates of each strain were grown under several salinity levels in liquid culture and characterized for growth and salt tolerance. Genetic characterization of rhizobia showed considerable variation among strains, with salt tolerance and growth generally higher in rhizobial populations derived from more saline soils. These strains showed markedly different genetic profiles and generic affiliations to those from more temperate soils, suggesting community differentiation in relation to salt stress. The identification of novel genomic species from saline soils suggests that the diversity of rhizobia associated with Australian Acacia spp. is significantly greater than previously described. Overall, the ability of some symbiotically effective strains to tolerate high salinity is promising with regard to improving host plant re-establishment in these soils. [source] Variations of Microbial Communities and the Contents and Isotopic Compositions of Total Organic Carbon and Total Nitrogen in Soil Samples during Their PreservationACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 1 2009Qianye TAO Abstract: Semi-sealed preservation of soil samples at different moisture of 4% and 23%, respectively, was simulated to observe the variations of soil microbial communities and determine the contents and isotopic compositions of the total organic carbon and total nitrogen on the 7th and 30th day, respectively. The results show that during preservation, the quantity of microbial communities tended to increase first and then decrease, with a wider variation range at higher moisture (23%). At the moisture content of 23%, the microbial communities became more active on the 7th day, but less after 30 days, and their activity was stable with little fluctuation at the moisture content of 4%. However, there were no significant changes in the contents and isotopic compositions of the total organic carbon and total nitrogen. During preservation, the responses of soil microbes to the environment are more sensitive to changes in the total nitrogen and organic carbon contents. It is thus suggested that the variations of microbial communities have not exerted remarkable impacts on the isotope compositions of the total nitrogen and total organic carbon. [source] | ||||||||||||||||||||||||||||