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1.
Plants often impact the rate of native soil organic matter turnover through root interactions with soil organisms; however the role of root-microbial interactions in mediation of the “priming effect” is not well understood. We examined the effects of living plant roots and N fertilization on belowground C dynamics in a California annual grassland soil (Haploxeralf) during a two-year greenhouse study. The fate of 13C-labeled belowground C (roots and organic matter) was followed under planted (Avena barbata) and unplanted conditions, and with and without supplemental N (20 kg N ha−1 season−1) over two periods of plant growth, each followed by a dry, fallow period of 120 d. Turnover of belowground 13C SOM was followed using 13C-phospholipid fatty acid (PLFA) biomarkers. Living roots increased the turnover and loss of belowground 13C compared with unplanted soils. Planted soils had 20% less belowground 13C present than in unplanted soils after 2 cycles of planting and fallow. After 2 treatment cycles, unlabeled soil C was 4.8% higher in planted soils than unplanted. The addition of N to soils decreased the turnover of enriched belowground 13C during the first treatment season in both planted and unplanted soils, however no effect of N was observed thereafter. Our findings suggest that A. barbata may increase soil C levels over time because root and exudate C inputs are significant, but that increase will be moderated by an overall faster C mineralization rate of belowground C. N addition may slow soil C losses; however, the effect was minor and transient in this system. The labeled root-derived 13C was initially recovered in gram negative (highest enrichment), gram positive, and fungal biomarkers. With successive growing seasons, the labeled C in the gram negative and fungal markers declined, while gram positive markers continued to accumulate labeled belowground C. The rhizosphere of A. barbata shifted the microbial community composition, resulting in greater abundances of gram negative markers and lower abundances of gram positive, actinobacteria and cyclopropyl PLFA markers compared to unplanted soil. However, the longer-term utilization of labeled belowground C by gram positive bacteria was enhanced in the rhizosphere microbial community compared with unplanted soils. We suggest that the activities of gram positive bacteria may be major controllers of multi-year rhizosphere-related priming of SOM decomposition.  相似文献   

2.
To assess the impact of a transgenic crop on soil environment, we compared soil bacterial communities from the rhizospheres of cucumber green mottle mosaic virus (CGMMV)-resistant transgenic watermelon (Citrullus vulgaris [Twinser] cv. Gongdae) and non-transgenic parental line watermelon at an experimental farm in Miryang, Korea. Soil microbial community structure was studied using terminal restriction fragment length polymorphism (T-RFLP) using HaeIII and HhaI enzymes on products from polymerase chain amplification reactions (PCR) of total DNA from rhizosphere. We used principal component analyses (PCA) to reduce dimensionality of T-RFLP profiles before comparison. On these PCA scores, we conducted discrimination analyses to compare soil microbial communities from the rhizosphere of transgenic and non-transgenic. Discriminant analyses indicate that microbial communities from rhizosphere of transgenic and non-transgenic watermelon did not differ with significance at 95% level. Our study could be used as a model case to assess the environmental risk assessment of transgenic crops on soil microbial organisms.  相似文献   

3.
In order to assess the diversity of culturable Burkholderia populations in rhizosphere and bulk soil and to evaluate how different agricultural management regimes and land use history affect this diversity, four treatments were evaluated: permanent grassland; grassland converted into maize monoculture; arable land and arable land converted into grassland. Burkholderia isolates obtained on PCAT medium were grouped in 47 clusters using 16S ribosomal RNA gene based PCR-DGGE combined with BOX genomic fingerprinting (DGGE-BOX). The distribution of the isolates in the DGGE-BOX clusters was used to calculate the Shannon diversity index per treatment. Interestingly, we observed that the Burkholderia diversity was affected by changes in the agricultural management, since the highest diversity was observed in permanent grassland and in continuous arable land. In addition, the diversity tended to be higher in the rhizosphere than in the corresponding bulk soil. The use of species abundance models indicated that rhizosphere communities had more even distributions than communities collected from the bulk soil. Identification of isolates revealed that only 2% of these belonged to the B. cepacia complex and that the majority was assigned to either (1) new Burkholderia species or (2) Burkholderia species that had originally been isolated from soil. Isolates classified as B. hospita, B. caledonica and Burkholderia sp. ‘LMG 22934’ and ‘LMG 22936’ were found mainly in the arable land, while isolates belonging to Burkholderia sp. ‘LMG 22929’ and B. phytofirmans were associated with the grassland area. Another potentially new Burkholderia species, ‘LMG 22932’, was found in both areas, in close association with the maize rhizosphere.  相似文献   

4.
Invasive plant species have been shown to alter the microbial community composition of the soils they invade and it is suggested that this below-ground perturbation of potential pathogens, decomposers or symbionts may feedback positively to allow invasive success. Whether these perturbations are mediated through specific components of root exudation are not understood. We focussed on 8-hydroxyquinoline, a putative allelochemical of Centaurea diffusa (diffuse knapweed) and used an artificial root system to differentiate the effects of 8-hydroxyquinoline against a background of total rhizodeposition as mimicked through supply of a synthetic exudate solution. In soil proximal (0-10 cm) to the artificial root, synthetic exudates had a highly significant (P < 0.001) influence on dehydrogenase, fluorescein diacetate hydrolysis and urease activity. In addition, 8-hydroxyquinoline was significant (p = 0.003) as a main effect on dehydrogenase activity and interacted with synthetic exudates to affect urease activity (p = 0.09). Hierarchical cluster analysis of 16S rDNA-based DGGE band patterns also identified a primary affect of synthetic exudates and a secondary affect of 8-hydroxyquinoline on bacterial community structure. Thus, we show that the artificial rhizosphere produced by the synthetic exudates was the predominant effect, but, that the influence of the 8-hydroxyquinoline signal on the activity and structure of soil microbial communities could also be detected.  相似文献   

5.
Previous work in our laboratory showed that several bacterial strains, either singly or in association with other bacteria (community migration), were capable of migrating together with the saprotrophic fungus Lyophyllum sp. strain Karsten through soil microcosms. A possible involvement of the type III secretion system (TTSS) in migration was indicated. In this study, we addressed the basis of the community migration, which might lie in a migration helper effect exerted by particular single-strain migrators on other members of the community. Different culturing (plating) as well as culture-independent (PCR-DGGE) methods were applied to assess the effects of putative bacterial helpers in the migration. We used, as a model, the migration-proficient Burkholderia terrae BS001 as the canonical helper strain. PCR-DGGE analysis of the soil system with or without added strain BS001, revealed that the latter consistently stimulated the migration of different bacterial species through the soil. This was observed both following introduction of the organism to a bacterial community from soil and on the basis of a similar organism that was naturally present. One strain, Dyella japonica BS003, was identified as an avid comigrator with B. terrae BS001, although it appeared to lag behind the latter strain in its migration speed. Further examination of the B. terrae BS001/D. japonica BS003 interaction at Lyophyllum sp. strain Karsten hyphae showed that the presence of the D. japonica strain did not negatively affect the growth and migration of B. terrae BS001 with the fungus. A biofilm of B. terrae BS001 was formed on the fungal hyphal front, and we postulate a role for this biofilm in the migration helper effect.  相似文献   

6.
In acid soil, low pH, reduced availability of nutrients, and toxicity of Al and Mn limit plant growth and the survival and effectiveness of rhizobia. The symbiosis between legumes and rhizobia is particularly sensitive to acid soil stress. A pot experiment evaluated whether Bradyrhizobium japonicum strain growth on acidic agar media would predict ability to colonize the rhizosphere and form effective nodules in acidic soils. Three Indonesian strains of B. japonicum with similar effectiveness at neutral pH in sand culture but with different tolerance of acid soil stress factors in agar media, and an acid-tolerant commercial strain (CB1809) of comparable effectiveness, were tested in three acid soils using the Al tolerant soybean (Glycine max cv PI 416937). At 7 days after inoculation all strains had achieved large rhizosphere populations, but by day 14 the rhizosphere population of the acid-sensitive strain had decreased, while the more acid-tolerant strains increased. The acid-tolerant strains had significantly greater nodulation and symbiotic effectiveness than plants inoculated with the acid-sensitive strain. Laboratory prescreening of B. japonicum for acid, Al and Mn tolerance in acid media successfully identified strains which were symbiotically competent in low pH soils.  相似文献   

7.
We examined whether grass species and soil nitrogen (N) availability could enhance Carbon (C) and N turnover during root litter decay in grassland. Three species with increasing competitiveness (Festuca ovina, Dactylis glomerata and Lolium perenne) were grown at two N fertiliser levels in an undisturbed grassland soil, in which soil organic fractions derived for the last 9 years from Lolium root litter which was 13C-depleted. During the subsequent experimental year, the C turnover was calculated using the respective δ13C values of the old and new C in the root phytomass, in two Particulate Organic Matter (POM) fractions above 200 μm and in the lightest part of the aggregated soil fraction between 50 and 200 μm. Soil N availability was monitored during the regrowth periods with ion exchange resins (IER). The C decay rates of each particle size fraction were calculated with a simple mechanistic model of C dynamics. The N mineralisation immobilisation turnover (MIT) was characterised by dilution of 15N-labelled fertiliser in the N harvestThe C:N ratio and the residence time of C in the fractions decreased with particle size. The presence of a grass rhizosphere increased the decay rate of old C. Accumulation of new C in particle size fractions increased with species competitiveness and with N supply. Species competitiveness increased C turnover in the aggregated fraction, as a result of greater accumulation of new C and faster decay of old C. Fertiliser N increased N turnover and C mineralisation in the SOM. Species competitiveness decreased soil -N exchanged with the IER and increased dissolved organic C (DOC) content. The nature of the current rhizosphere is thus an important factor driving C and N transformations of the old root litter, in relation with grass species strategy. Plant competitiveness may stimulate the C and N turnover in the more evolved SOM fractions in a similar way to the mineral N supply.  相似文献   

8.
岩溶区不同恢复阶段檵木根际土壤生态化学计量学特征   总被引：3，自引：0，他引：3  
[目的]探讨不同植被恢复阶段中檵木(Loropetalum chinense)根际土壤生态化学计量学特征,揭示岩溶石漠化区不同恢复阶段下檵木养分利用规律及限制因子,为岩溶区植被恢复与重建提供科学依据。[方法]利用生态化学计量学方法,分析桂西北岩溶区不同植被恢复阶段(灌木林、原生林)檵木根际土壤生态化学计量学特征。[结果]檵木根际土壤C、N、P含量表现为灌木林阶段高于原生林阶段,而C∶N,C∶P,N∶P比值则刚好相反;同一植被恢复阶段,不同坡位间檵木根际土壤C∶P与N∶P比值存在显著差异;相关性分析表明,土壤C含量与土壤N∶P含量存在显著正相关关系,土壤N,P均与C∶N,C∶P,N∶P存在显著正相关关系,土壤C∶P与C∶N,N∶P存在显著正相关关系。[结论]檵木根际养分供应状况与其生境密切相关,在植被恢复前期(灌木林)檵木生长容易受N素限制,到植被恢复后期檵木生长易受P素限制。  相似文献   

9.
Colonization and survival of the inoculated bacteria in rhizosphere of maize were investigated in field and pot experiments conducted for 3 consecutive years under rainfed conditions of Himalayan region. The effect of bacterial inoculations on growth and yield related parameters of maize were also evaluated. While three bacterial species, viz. Bacillus megaterium, Bacillus subtilis and Pseudomonas corrugata were tested in 1st year experiments, P. corrugata (based on the 1st year results) was chosen for inoculation in the subsequent experiments. All the three bacterial inoculants showed good rhizosphere competence giving high inoculum numbers (log10 11.13-11.34 cfu g−1). The bacterial inoculations by B. megaterium, B. subtilis and P. corrugata resulted in an increment in grain yield of maize up to 122.4%, 135.2% and 194.3%, respectively, as compared to respective control. In 1st year, the antibiotic marked (Nalr Rifr) inoculant P. corrugata resulted in the highest increase in grain yield, statistically significant (P<0.05) as compared to control, B. megaterium and B. subtilis. In 2nd and 3rd year experiments, P. corrugata increased the grain yield up to 147.28% and 149.93%, respectively, as compared to control. The best performance and consistent trend of P. corrugata to increase plant yields was credited to its initial isolation from rhizosphere of maize growing under temperate conditions. The overall beneficial effects of bacterial inoculations on maize were contributed to (1) the colonization and survival of the introduced bacteria, and (2) stimulation of the indigenous microflora in the rhizosphere. Based on the comprehensive results obtained in this study, P. corrugata may be recommended as suitable bioinoculant for maize fields of temperate climate grown under rainfed conditions.  相似文献   

10.
The diversity of environmental mycobacteria was studied in water-logged acidic forest soil. Mycobacteria were assessed in upper and lower soil horizons and summer and winter seasons using T-RFLP and sequencing of 16S rRNA gene produced with Mycobacterium-specific primers. Mycobacteria diversity differed between both the two seasons and soil horizons. Cloning revealed the presence of mycobacteria belonging to three major clusters recognized within the genus, i.e. fast-growing, intermediate, and slow-growing species, with unprecedented abundance and diversity of the latter. Two novel clusters of sequences unrelated to the known mycobacteria were identified. This study raises the possibility that forest wetlands could serve as environmental reservoirs for an unexplored diversity of mycobacteria including those related to pathogenic species.  相似文献   

11.
The relative importance of specific plant properties versus soil characteristics in shaping the bacterial community structure of the rhizosphere is a topic of considerable debate. Here, we report the results of a study on the bacterial composition of the rhizosphere of the wild plant Carex arenaria (sand sedge) growing at 10 natural sites in The Netherlands. The soil properties of the sandy soils at these sites were highly disparate, most notably in pH, chloride and organic matter content. Rhizosphere and bulk soil bacterial communities were examined by culture-independent means, namely, 16S rDNA-directed PCR-DGGE profiling. Large differences were observed between the bacterial communities of the different sites for both bulk and rhizosphere soil. Cluster analysis of bacterial profiles revealed that the rhizosphere community of each site was generally more closely related to the bulk soil community of that site rather than to rhizosphere communities of other sites. Hence, bacterial community structure within the rhizosphere of C. arenaria appeared to be determined to a large extent by the bulk soil community composition. This conclusion was supported by a reciprocal planting experiment, where C. arenaria shoots of different sites yielded highly similar rhizosphere communities when planted in the same soil.  相似文献   

12.
This study quantifies the influence of Poa alpina on the soil microbial community in primary succession of alpine ecosystems, and whether these effects are controlled by the successional stage. Four successional sites representative of four stages of grassland development (initial, 4 years (non-vegetated); pioneer, 20 years; transition, 75 years; mature, 9500 years old) on the Rotmoos glacier foreland, Austria, were sampled. The size, composition and activity of the microbial community in the rhizosphere and bulk soil were characterized using the chloroform-fumigation extraction procedure, phospholipid fatty acid (PLFA) analysis and measurements of the enzymes β-glucosidase, β-xylosidase, N-acetyl-β-glucosaminidase, leucine aminopeptidase, acid phosphatase and sulfatase. The interplay between the host plant and the successional stage was quantified using principal component (PCA) and multidimensional scaling analyses. Correlation analyses were applied to evaluate the relationship between soil factors (Corg, Nt, C/N ratio, pH, ammonium, phosphorus, potassium) and microbial properties in the bulk soil. In the pioneer stage microbial colonization of the rhizosphere of P. alpina was dependent on the reservoir of microbial species in the bulk soil. As a consequence, the rhizosphere and bulk soil were similar in microbial biomass (ninhydrin-reactive nitrogen (NHR-N)), community composition (PLFA), and enzyme activity. In the transition and mature grassland stage, more benign soil conditions stimulated microbial growth (NHR-N, total amount of PLFA, bacterial PLFA, Gram-positive bacteria, Gram-negative bacteria), and microbial diversity (Shannon index H) in the rhizosphere either directly or indirectly through enhanced carbon allocation. In the same period, the rhizosphere microflora shifted from a G− to a more G+, and from a fungal to a more bacteria-dominated community. Rhizosphere β-xylosidase, N-acetyl-β-glucosaminidase, and sulfatase activity peaked in the mature grassland soil, whereas rhizosphere leucine aminopeptidase, β-glucosidase, and phosphatase activity were highest in the transition stage, probably because of enhanced carbon and nutrient allocation into the rhizosphere due to better growth conditions. Soil organic matter appeared to be the most important driver of microbial colonization in the bulk soil. The decrease in soil pH and soil C/N ratio mediated the shifts in the soil microbial community composition (bacPLFA, bacPLFA/fungPLFA, G−, G+/G−). The activities of β-glucosidase, β-xylosidase and phosphatase were related to soil ammonium and phosphorus, indicating that higher decomposition rates enhanced the nutrient availability in the bulk soil. We conclude that the major determinants of the microflora vary along the successional gradient: in the pioneer stage the rhizosphere microflora was primarily determined by the harsh soil environment; under more favourable environmental conditions, however, the host plant selected for a specific microbial community that was related to the dynamic interplay between soil properties and carbon supply.  相似文献   

13.
The objective of this study was to evaluate the effect of sewage sludge on soil suppressiveness to the pathogens Fusarium oxysporum f. sp. lycopersici on tomato, Sclerotium rolfsii on bean, Sclerotinia sclerotiorum on tomato, Rhizoctonia solani on radish, Pythium spp. on cucumber, and Ralstonia solanacearum on tomato. Soil samples were collected from an experimental corn field in which sewage sludge had been incorporated once a year, since 1999. Sludge from two sewage treatment stations in Brazil (Franca and Barueri, SP) were applied at the rates of one (1N), two (2N), four (4N) and eight (8N) times the N recommended doses for the corn crop. Soil suppressiveness was evaluated by methods using indicator host plants, baits and mycelial growth. There was no effect of sewage sludge on soil suppressiveness to Fusarium oxysporum f. sp. lycopersici in tomato plants. For S. rolfsii, reduction of the disease in bean was inversely proportional to the dose of Franca sludge. The incidence of dead plants, caused by S. sclerotiorum, was directly proportional to sludge doses applied. For R. solani and R. solanacearum, there was a linear trend with reduction in plant death in soils treated with increasing amounts of sludge from Franca. There was an increase in the pathogen community of Pythium spp., proportional to the amounts of sewage applied. The effects of sewage sludge varied depending on the pathogen, methodology applied and on the time interval between the sewage sludge incorporation and soil sampling.  相似文献   

14.
Elevated aluminum (Al) availability limits plant growth on acidic soils. Although this element is found naturally in soils, acidic conditions create an environment where Al solubility increases and toxic forms of Al impact plant function. Plant resistance to Al is often attributed to organic acid exudation from plant roots and the chelation of cationic Al in the rhizosphere. The association of arbuscular mycorrhizal (AM) fungi with the roots of plants may alleviate Al toxicity by altering soil Al availability or plant exposure through the binding of Al to fungal structures or through the influence of fungi on exudation from roots. Diverse communities of AM fungi are found in soil ecosystems and research suggests that AM fungi exhibit functional diversity that may influence plant performance under varying edaphic environments. In the present study, we evaluated acidic isolates of six AM species in their responses to Al. Andropogon virginicus (broomsedge), a warm-season grass that commonly grows in a range of stressful environments including acidic soils, was used as a plant host for Acaulospora morrowiae, Glomus claroideum, Glomus clarum, Glomus etunicatum, Paraglomus brasilianum, and Scutellospora heterogama. Fungal spores were germinated and exposed to 0 or 100 μM Al on filter paper in sand culture or were grown and exposed to Al in sand culture in association with A. virginicus. Short- and long-term responses to Al were evaluated using direct measurements of fungal spore germination, hyphal elongation, and measurements of A. virginicus colonization and plant growth as a phytometer of AM function in symbio. Spore germination and hyphal elongation varied among AM species in response to Al, but patterns were not consistent with the influences of these AM species on A. virginicus under Al exposure. Exposure to Al did not influence colonization of roots, although large differences existed in colonization among fungal species. Plants colonized by G. clarum and S. heterogama exhibited the least reduction in growth when exposed to Al, produced the highest concentrations of Al-chelating organic acids, and had the lowest concentrations of free Al in their root zones. This pattern provides evidence that variation among AM fungi in Al resistance conferred to their plant hosts is associated with the exudation of Al-binding organic acids from roots and highlights the role that AM fungal diversity may play in plant performance in acidic soil environments.  相似文献   

15.
Fungi are key to the functioning of soil ecosystems, and exhibit a range of interactions with plants. Given their close associations with plants, and importance in ecosystem functioning, soil-borne fungi have been proposed as potential biological indicators of disturbance and useful agents in monitoring strategies, including those following the introduction of genetically modified (GM) crops. Here we report on the impact of potato crop varieties, including a cultivar that was genetically modified for its starch quality, on the community composition of the main phyla of fungi in soils, i.e. Ascomycota, Basidiomycota and Glomeromycota in rhizosphere and bulk soil. Samples were collected at two field sites before sowing, at three growth stages during crop development and after the harvest of the plants, and the effects of field site, plant growth stage and plant cultivar (genotype) on fungal community composition assessed using three phylum-specific T-RFLP profiling strategies and multivariate statistical analysis (NMDS ordinations with ANOSIM test). In addition, fungal biomass, arbuscular mycorrhizal colonization of roots and activities of extracellular fungal enzymes (laccases, Mn-peroxidases and cellulases) involved in degradation of lignocelluloses-rich organic matter were determined. Fungal community compositions, densities and activities were observed to differ significantly between the rhizosphere and bulk soil. The most important factors determining fungal community composition and functioning were plant growth stage for the rhizosphere communities and location and soil properties for the bulk soil communities. The basidiomycetes were the most numerous fungal group in the bulk soils and in the rhizosphere of young plants, with a shift toward greater ascomycete numbers in the rhizosphere at later growth stages. There were no detectable differences between the GM cultivar and its parental cultivar in terms of influence on fungal community structure of function. Fungal community structure and functioning of both GM- and parental cultivars fell within the range of other cultivars at most sampling moments.  相似文献   

16.
Phytoextraction of soil Cd and Zn may require reduction in soil pH in order to achieve high metal uptake. Reducing the pH of high metal soil, however, could negatively affect soil ecosystem function and health. The objectives of this study were to characterize the quantitative causal relationship between pH and soil biological activities in two Zn and Cd contaminated soils and to investigate the relationship between metals and soil biological activities under low pH. Soils were adjusted to five or six different pH levels by sulfur addition, followed by salt leaching. Thlaspi caerulescens was grown for 6 months, and both the rhizosphere and non-rhizosphere soil biological activities were tested after harvest. Reducing pH significantly lowered soil alkaline phosphatase activity, arylsulphatase activity, nitrification potential, and respiration. However, acid phosphatase activity was increased with decreasing pH. The relationship between soil biological activities and pH was well characterized by linear or quadratic regression models with R2 values ranging from 0.57 to 0.99. In general, the three enzyme activities, nitrification potential, and the ratio of alkaline phosphatase to acid phosphatase activity were very sensitive indicators of soil pH status while soil respiration was not sensitive to pH change. The rhizosphere soil had higher biological activities than non-rhizosphere soil. The negative effects observed in the non-rhizosphere soil were alleviated by the rhizosphere influence. However, rhizosphere soil after 6 months phytoextraction showed lower nitrification potential than non-rhizosphere soil, probably due to substrate limitation in our study.  相似文献   

17.
Bacterial associations with plant roots are thought to contribute to the success of phytoremediation. We tested the effect of addition of a polycyclic aromatic hydrocarbon contaminated soil on the structure of the rhizosphere microbial communities of wheat (Triticum aestivum), lettuce (Lactuca sativa var. Tango), zucchini (Cucurbita pepo spp. pepo var. Black Beauty), and pumpkin (C. pepo spp. pepo var. Howden) 16S rDNA terminal restriction fragment length polymorphism (T-RFLP) profiles of rhizosphere microbial communities from different soil/plant combinations were compared with a pairwise Pearson correlation coefficient. Rhizosphere microbial communities of zucchini and pumpkin grown in the media amended with highest degree of contaminated soil clustered separately, whereas communities of these plants grown in unamended or amended with lower concentrations of contaminated soil, grouped in a second cluster. Lettuce communities grouped similarly to cucurbits communities, whereas wheat communities did not display an obvious clustering. The variability of 16S rDNA T-RFLP profiles among the different plant/soil treatments were mostly due to the difference in relative abundance rather than presence/absence of T-RFLP fragments. Our results suggest that in highly contaminated soils, the rhizosphere microbial community structure is governed more by the degree of contamination rather than the plant host type.  相似文献   

18.
Apple replant disease (ARD) is a complex syndrome that affects young trees in replanted orchard sites causing necrotic lesions on feeder roots, stunted tree growth and reduced cumulative yields. Use of ARD-tolerant rootstocks is an emerging control strategy. We studied the bacterial, fungal, and oomycetes populations in the rhizosphere of five rootstock cultivars (M.7, M.26, G.16, G.30 and CG.6210) planted into the old tree row or grass lanes of a previous orchard in Ithaca, NY, to better understand the role of rhizosphere microbial communities in the prevalence and control of ARD. The possible involvement of antagonistic Pseudomonas species, Pythium spp., Phytophthora spp. and rhizosphere cyanide concentrations in ARD were also examined. The rootstocks M.7, M.26 and G.16 were susceptible to ARD, while G.30 and CG.6210 were more tolerant. Tree growth on the rootstocks M.7, M.26 and G.16 was reduced by 10% when planted in the old tree rows, but this did not significantly reduce yields in the first fruiting year. The susceptible rootstocks, M.7 and M.26, supported higher densities of culturable rhizosphere fungi and bacteria than G.16, G.30 and CG.6210. Over 2 years, microbial densities were highest in July, lower in May and lowest in September. The composition of bacterial and fungal communities in the rhizosphere was highly variable and changed over seasons and years, as assessed by terminal restriction fragment length polymorphism (T-RFLP) analyses. Initial differences in fungal rhizosphere communities between the two planting positions converged 2 years after the trees were replanted. In contrast, the bacterial rhizosphere community composition still differed significantly between the two planting positions 3 years after the orchard was replanted. The bacterial and fungal rhizosphere community compositions of susceptible rootstocks, M.7 and M.26, differed from those of the tolerant rootstocks, G.30 and CG.6210; G.16, differed from all the other rootstocks. The observed effects of rootstocks, planting positions and time on microbial community composition were small relative to the high variability observed overall. Pythium spp. and Phytophthora spp. infestations were high and similar for all rootstocks and planting positions. Neither potentially antagonistic Pseudomonas nor rhizosphere cyanide concentrations appeared to be involved in the ARD-complex at the studied site. Avoiding replanting into the old tree rows coupled with use of tolerant rootstocks appear to be the best strategies for reducing ARD in replanted orchards. Changes in rhizosphere microbial communities are among the many factors that contribute to improved tree growth when these management strategies are used.  相似文献   

19.
The impact of exotic plant invasions on soil communities and nutrient cycling processes has received an increasing attention in recent years. To test whether the exotic plant invasions affect nematode communities through altering litter quality, we compared mass loss and nematode colonization during the stem litter decomposition of invasive Spartina alterniflora and native Phragmites australis in salt marshes of the Yangtze River estuary, China. Plastic drinking straws were synchronously used as controls. The addition of plant residues was found stimulating the growth of nematodes, particularly bacterial feeders on day 16 after burial. A top-down control of bacterivous nematodes by carnivores existed in nematode succession during the litter decomposition. With higher nitrogen content and lower C:N ratio, stem litter of the invasive S. alterniflora decayed faster and supported more abundant nematodes than the native P. australis. The greater nematode abundance in S. alterniflora was mainly due to two dominant genera of bacterial nematodes, namely Diplolaimelloides and Diplolaimella. Lower values of maturity index and structure index in S. alterniflora than in P. australis litter indicate that a more degraded food web condition resulted from the faster litter decay. A considerable difference in nematode community structures between two litter types only occurred in a certain period of the decomposition (from 8 to 32 days after burial), suggesting that the changes in faunal community structure are time dependent. In summary, this study confirmed the hypothesis that the invasion of S. alterniflora stimulates the growth of bacterial nematodes by producing higher quality of litter than the native P. australis. The results obtained here suggest that the invasion of exotic plant is likely to alter ecosystem functions indirectly through exerting its effect on soil decomposer communities such as nematodes.  相似文献   

20.
Arbuscular mycorrhizal (AM) fungi have been shown to induce the biocontrol of soilborne diseases, to change the composition of root exudates and to modify the bacterial community structure of the rhizosphere, leading to the formation of the mycorrhizosphere. Tomato plants were grown in a compartmentalized soil system and were either submitted to direct mycorrhizal colonization or to enrichment of the soil with exudates collected from mycorrhizal tomato plants, with the corresponding negative controls. Three weeks after planting, the plants were inoculated or not with the soilborne pathogen Phytophthora nicotianae growing through a membrane from an adjacent infected compartment. At harvest, a PCR-Denaturing gradient gel electrophoresis analysis of 16S rRNA gene fragments amplified from the total DNA extracted from each plant rhizosphere was performed. Root colonization with the AM fungi Glomus intraradices or Glomus mosseae induced significant changes in the bacterial community structure of tomato rhizosphere, compared to non-mycorrhizal plants, while enrichment with root exudates collected from mycorrhizal or non-mycorrhizal plants had no effect. Our results support that the effect of AM fungi on rhizosphere bacteria would not be mediated by compounds present in root exudates of mycorrhizal plants but rather by physical or chemical factors associated with the mycelium, volatiles and/or root surface bound substrates. Moreover, infection of mycorrhizal or non-mycorrhizal plants with P. nicotianae did not significantly affect the bacterial community structure suggesting that rhizosphere bacteria would be less sensitive to the pathogen invasion than to mycorrhizal colonization. Of 96 unique sequences detected in the tomato rhizosphere, eight were specific to mycorrhizal fungi, including two Pseudomonas, a Bacillus simplex, an Herbaspirilium and an Acidobacterium. One Verrucomicrobium was common to rhizospheres of mycorrhizal plants and of plants watered with mycorrhizal root exudates.  相似文献   

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