Impact of Fumigants on Soil Microbial Communities

Agricultural soils are typically fumigated to provide effective control of nematodes, soilborne pathogens, and weeds in preparation for planting of high-value cash crops. The ability of soil microbial communities to recover after treatment with fumigants was examined using culture-dependent (Biolog) and culture-independent (phospholipid fatty acid [PLFA] analysis and denaturing gradient gel electrophoresis [DGGE] of 16S ribosomal DNA [rDNA] fragments amplified directly from soil DNA) approaches. Changes in soil microbial community structure were examined in a microcosm experiment following the application of methyl bromide (MeBr), methyl isothiocyanate, 1,3-dichloropropene (1,3-D), and chloropicrin. Variations among Biolog fingerprints showed that the effect of MeBr on heterotrophic microbial activities was most severe in the first week and that thereafter the effects of MeBr and the other fumigants were expressed at much lower levels. The results of PLFA analysis demonstrated a community shift in all treatments to a community dominated by gram-positive bacterial biomass. Different 16S rDNA profiles from fumigated soils were quantified by analyzing the DGGE band patterns. The Shannon-Weaver index of diversity, H, was calculated for each fumigated soil sample. High diversity indices were maintained between the control soil and the fumigant-treated soils, except for MeBr (H decreased from 1.14 to 0.13). After 12 weeks of incubation, H increased to 0.73 in the MeBr-treated samples. Sequence analysis of clones generated from unique bands showed the presence of taxonomically unique clones that had emerged from the MeBr-treated samples and were dominated by clones closely related to Bacillus spp. and Heliothrix oregonensis. Variations in the data were much higher in the Biolog assay than in the PLFA and DGGE assays, suggesting a high sensitivity of PLFA analysis and DGGE in monitoring the effects of fumigants on soil community composition and structure. Our results indicate that MeBr has the greatest impact on soil microbial communities and that 1,3-D has the least impact.     

Qualitative microbiological changes during decomposition of plant material in a sandy sierozem soil

The successive qualitative microbial changes during the decomposition of bajra stalk in a sandy sierozem soil were studied.Alternaria spp.,Aspergillus spp.,Cladosporium spp.,Fusarium spp.Gliocladium spp.,Mucor spp. andRhizopus spp. were most common fungi. The bacteria observed wereAchromobacter, Arthrobacter, Bacillus, Micrococcus, Pseudomonas andXanthomonas. Cellvibrio andCellulomonas were also observed.     

Effect of biofumigation with manure amendments and repeated biosolarization on <Emphasis Type="Italic">Fusarium</Emphasis> densities in pepper crops

In the region of Murcia (southeast Spain), sweet pepper has been grown as a monoculture in greenhouses for many years. Until 2005, when it was banned, soils were disinfested with methyl bromide (MB) to control pathogens and to prevent soil fatigue effects. The genus Fusarium plays an important role in the microbiological component associated with yield decline in pepper monocultures. In the present study, soils were treated with manure amendments, alone (biofumigation, B) or in combination with solarization (biosolarization, BS), with or without the addition of pepper plant residues. The B and BS treatments were compared with a treatment using MB. The extent of disinfestation was measured from the density of Fusarium spp. isolated from the soil before and after the respective treatments. Three different species were systematically isolated: Fusarium oxysporum, Fusarium solani and Fusarium equiseti. The repeated use of manure amendments with pepper crop residues, without solarization, was unable to decrease the Fusarium spp. density (which increased from 2,047.17 CFU g⁻¹ to 3,157.24 CFU g⁻¹ before and after soil disinfestation, respectively), unlike MB-treated soil (in which the fungi decreased from 481.39 CFU g⁻¹ to 23.98 CFU g⁻¹). However, the effectiveness of the repeated application of BS in diminishing doses (with or without adding plant residues) on Fusarium populations (reductions greater than 72%) was similar to or even greater than the effect of MB.     

Cross-Site Soil Microbial Communities under Tillage Regimes: Fungistasis and Microbial Biomarkers

The exploitation of soil ecosystem services by agricultural management strategies requires knowledge of microbial communities in different management regimes. Crop cover by no-till management protects the soil surface, reducing the risk of erosion and nutrient leaching, but might increase straw residue-borne and soilborne plant-pathogenic fungi. A cross-site study of soil microbial communities and Fusarium fungistasis was conducted on six long-term agricultural fields with no-till and moldboard-plowed treatments. Microbial communities were studied at the topsoil surface (0 to 5 cm) and bottom (10 to 20 cm) by general bacterial and actinobacterial terminal restriction fragment length polymorphism (T-RFLP) and phospholipid fatty acid (PLFA) analyses. Fusarium culmorum soil fungistasis describing soil receptivity to plant-pathogenic fungi was explored by using the surface layer method. Soil depth had a significant impact on general bacterial as well as actinobacterial communities and PLFA profiles in no-till treatment, with a clear spatial distinction of communities (P < 0.05), whereas the depth-related separation of microbial communities was not observed in plowed fields. The fungal biomass was higher in no-till surface soil than in plowed soil (P < 0.07). Soil total microbial biomass and fungal biomass correlated with fungistasis (P < 0.02 for the sum of PLFAs; P < 0.001 for PLFA 18:2ω6). Our cross-site study demonstrated that agricultural management strategies can have a major impact on soil microbial community structures, indicating that it is possible to influence the soil processes with management decisions. The interactions between plant-pathogenic fungi and soil microbial communities are multifaceted, and a high level of fungistasis could be linked to the high microbial biomass in soil but not to the specific management strategy.     

Bacterial inoculation speeds zinc release from ground tire rubber used as Zn fertilizer for corn and sunflower in a calcareous soil

Background and Aims

We tested the utility of some biological treatments to hasten degradation of waste tire rubber in soil and thus the release of zinc and sulfur for plant uptake.

Methods

Three rates of ground tire rubber (0, 150, and 300?mg?kg^?1) were incorporated into a Zn-deficient calcareous soil. Before addition to the soil, ground rubber was given four microbial treatments including no inoculation, inoculation with Rhodococcus erythropolis, inoculation with R. erythropolis+Escherichia coli, and inoculation with R. erythropolis+E. coli+Acinobacter calcoaceticus. In the pot experiment, corn (Zea mays L. Hybrid Single Cross 500) and sunflower (Helianthus annuus L. cv. Record) plants were exposed to three rates of ground rubber (0, 150, and 300?mg?kg^?1) or 3?mg zinc kg^?1 as ZnSO₄. Before addition to the soil, ground rubber and ZnSO₄ were inoculated or non-inoculated with R. erythropolis+E. coli+A. calcoaceticus.

Results

Ground rubber and microbial inoculation treatments reduced soil pH and the magnitude of this reduction increased over time. Ground rubber in combination with microbial inoculation increased DTPA-extractable soil Zn and Fe. The amount of DTPA-extractable Zn and Fe of rubber-amended soils increased over time so that the highest concentration of available Zn and Fe was found at week 10. Application of microbial inoculated ground tire rubber significantly increased shoot Zn concentration of each plant species.

Conclusions

Bacterial inoculation of ground rubber was effective in hastening increase in DTPA-extractable Zn in the studied calcareous soil and in enhancing Zn uptake by plants.     

Effects of vanillin on the community structures and abundances of Fusarium and Trichoderma spp. in cucumber seedling rhizosphere

Soil microbial communities are crucial to the functioning of agricultural systems but little information is available on the effects of allelochemicals on soil microorganisms in vivo. Cucumber seedlings grown in soil were treated with different concentrations of vanillin (0.02–0.2?μmol/g soil), a phenolic compound with autotoxic activity. The community structures and abundances of Fusarium and Trichoderma spp. in cucumber rhizosphere were analyzed by polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis and quantitative PCR, respectively. Results showed that vanillin changed the community structures of Fusarium and Trichoderma spp. Vanillin decreased the number of bands of Fusarium spp., but increased the number of bands, Shannon–Wiener and evenness indices of Trichoderma spp. (p?Fusarium and Trichoderma spp. (p?Fusarium and Trichoderma spp., and that these two microbial groups showed different responses to vanillin.     

Effect of Metal-Rich Sludge Amendments on the Soil Microbial Community

The effects of heavy-metal-containing sewage sludge on the soil microbial community were studied in two agricultural soils of different textures, which had been contaminated separately with three predominantly single metals (Cu, Zn, and Ni) at two different levels more than 20 years ago. We compared three community-based microbiological measurements, namely, phospholipid fatty acid (PLFA) analysis to reveal changes in species composition, the Biolog system to indicate metabolic fingerprints of microbial communities, and the thymidine incorporation technique to measure bacterial community tolerance. In the Luddington soil, bacterial community tolerance increased in all metal treatments compared to an unpolluted-sludge-treated control soil. Community tolerance to specific metals increased the most when the same metal was added to the soil; for example, tolerance to Cu increased most in Cu-polluted treatments. A dose-response effect was also evident. There were also indications of cotolerance to metals whose concentration had not been elevated by the sludge treatment. The PLFA pattern changed in all metal treatments, but the interpretation was complicated by the soil moisture content, which also affected the results. The Biolog measurements indicated similar effects of metals and moisture to the PLFA measurements, but due to high variation between replicates, no significant differences compared to the uncontaminated control were found. In the Lee Valley soil, significant increases in community tolerance were found for the high levels of Cu and Zn, while the PLFA pattern was significantly altered for the soils with high levels of Cu, Ni, and Zn. No effects on the Biolog measurements were found in this soil.     

Seasonal Dynamics of Soil Labile Organic Carbon and Enzyme Activities in Relation to Vegetation Types in Hangzhou Bay Tidal Flat Wetland

Soil labile organic carbon and soil enzymes play important roles in the carbon cycle of coastal wetlands that have high organic carbon accumulation rates. Soils under three vegetations (Phragmites australis, Spartina alterniflora, and Scirpusm mariqueter) as well as bare mudflat in Hangzhou Bay wetland of China were collected seasonally. Seasonal dynamics and correlations of soil labile organic carbon fractions and soil enzyme activities were analyzed. The results showed that there were significant differences among vegetation types in the contents of soil organic carbon (SOC) and dissolved organic carbon (DOC), excepting for that of microbial biomass carbon (MBC). The P. australis soil was with the highest content of both SOC (7.86 g kg^-1) and DOC (306 mg kg^-1), while the S. mariqueter soil was with the lowest content of SOC (6.83 g kg^-1), and the bare mudflat was with the lowest content of DOC (270 mg kg^-1). Soil enzyme activities were significantly different among vegetation types except for urease. The P. australis had the highest annual average activity of alkaline phosphomonoesterase (21.4 mg kg^-1 h^-1), and the S. alterniflora had the highest annual average activities of β-glycosidase (4.10 mg kg^-1 h^-1) and invertase (9.81mg g^-1 24h^-1); however, the bare mudflat had the lowest activities of alkaline phosphomonoesterase (16.2 mg kg^-1 h^-1), β-glycosidase (2.87 mg kg^-1 h^-1), and invertase (8.02 mg g^-1 24h^-1). Analysis also showed that the soil labile organic carbon fractions and soil enzyme activities had distinct seasonal dynamics. In addition, the soil MBC content was significantly correlated with the activities of urease and β-glucosidase. The DOC content was significantly correlated with the activities of urease, alkaline phosphomonoesterase, and invertase. The results indicated that vegetation type is an important factor influencing the spatial-temporal variation of soil enzyme activities and labile organic carbon in coastal wetlands.     

Assessment of Parasitic Activity of Fusarium Strains Obtained from a Heterodera schachtii-Suppressive Soil

This study assessed the potential impact of various Fusarium strains on the population development of sugarbeet cyst nematodes. Fungi were isolated from cysts or eggs of Heterodera schachtii Schmidt that were obtained from a field suppressive to that nematode. Twenty-six strains of Fusarium spp. were subjected to a phylogenic analysis of their rRNA-ITS nucleotide sequences. Seven genetically distinct Fusarium strains were evaluated for their ability to influence population development of H. schachtii and crop performance in greenhouse trials. Swiss chard (Beta vulgaris) seedlings were transplanted into fumigated field soil amended with a single fungal strain at 1,000 propagules/g soil. One week later, the soil was infested with 250 H. schachtii J2/100 cm³ soil. Parasitized eggs were present in all seven Fusarium treatments at 1,180 degree-days after fungal infestation. The percentage of parasitism ranged from 17 to 34%. Although the most efficacious F. oxysporum strain 471 produced as many parasitized eggs as occurred in the original suppressive soil, none of the Fusarium strains reduced the population density of H. schachtii compared to the conducive check. This supports prior results that Fusarium spp. were not the primary cause of the population suppression of sugarbeet cyst nematodes at this location.     

氮磷添加对内蒙古温带典型草原土壤微生物群落结构的影响

选取内蒙古温带典型草原,进行连续6a氮磷添加试验,采用土壤特征微生物PLFA生物标记技术,研究6个氮添加水平N0(0 kg N hm-2a-1)、N1(56 kg N hm-2a-1)、N2(112 kg N hm-2a-1)、N3(224 kg N hm-2a-1)、N4(392 kg N hm-2a-1)、N5(560 kg N hm-2a-1)和6个磷添加水平P0(0 kg P hm-2a-1)、P1(15.5 kg P hm-2a-1)、P2(31 kg P hm-2a-1)、P3(62 kg P hm-2a-1)、P4(93 kg P hm-2a-1)、P5(124 kg P hm-2a-1)对土壤特征微生物PLFA生物标记数量和土壤微生物群落结构的影响。结果表明:(1)随氮添加量增加,土壤微生物总磷脂脂肪酸(PLFA)含量和土壤细菌PLFA生物标记数量、放线菌PLFA生物标记数量呈上升趋势,土壤G+/G-呈增加趋势;各氮添加水平对土壤真菌PLFA生物标记数量无显著差异,随氮添加量增加,土壤真菌/细菌比降低。(2)随磷添加量增加,土壤总磷脂脂肪酸(PLFA)含量、土壤细菌PLFA生物标记数量、放线菌PLFA生物标记数量、真菌PLFA生物标记数量及真菌/细菌比值呈先上升后下降趋势,均以P3水平(62 kg P hm-2a-1)处理最高,说明适宜的磷添加对内蒙古温带典型草原土壤微生物繁殖和菌落结构有显著影响。     

Microbial Community Dynamics during the Bioremediation Process of Chlorimuron-Ethyl-Contaminated Soil by Hansschlegelia sp. Strain CHL1

Long-term and excessive application of chlorimuron-ethyl has led to a series of environmental problems. Strain Hansschlegelia sp. CHL1, a highly efficient chlorimuron-ethyl degrading bacterium isolated in our previous study, was employed in the current soil bioremediation study. The residues of chlorimuron-ethyl in soils were detected, and the changes of soil microbial communities were investigated by phospholipid fatty acid (PLFA) analysis. The results showed that strain CHL1 exhibited significant chlorimuron-ethyl degradation ability at wide range of concentrations between 10μg kg^-1 and 1000μg kg^-1. High concentrations of chlorimuron-ethyl significantly decreased the total concentration of PLFAs and the Shannon-Wiener indices and increased the stress level of microbes in soils. The inoculation with strain CHL1, however, reduced the inhibition on soil microbes caused by chlorimuron-ethyl. The results demonstrated that strain CHL1 is effective in the remediation of chlorimuron-ethyl-contaminated soil, and has the potential to remediate chlorimuron-ethyl contaminated soils in situ.     

Characterizing cultivable soil microbial communities from copper fungicide-amended olive orchard and vineyard soils

Use of copper-based fungicides has led to an increase in the total Cu content in agriculture soils. The focus of this study was to determine fractionation of Cu and to investigate the structure and the diversity of cultivable bacterial communities in two vineyards (one 25 years old and one 2 years old), one olive orchard and two forest soils. All soils developed on an Oligocene sandstone. The concentration of total Cu in the old vineyard (176.6 mg kg⁻¹) and olive orchard (145.5–296.7 mg kg⁻¹) was from 5 to 10 times higher than in forest soils. The major amount of Cu was found bound to the humic substances in cultivated soils, whereas in forest soils Cu was found in the residual mineral fraction. A relationship was found between the number of cultivable Cu-tolerant bacteria and total Cu content in soil. In the cultivated soils, Cu had a toxicological effect on bacterial community, and thereby Cu-levels > to 145 mg kg⁻¹ could be a risk to soil biota. Microbial communities were analysed by community level physiological profiling (CLPP), using the Biolog system, and by the amplified ribosomal DNA restriction analysis (ARDRA) approach. Only when cell suspensions containing 10⁴ colony-forming units (c.f.u.) were inoculated in each well of Biolog EcoPlates it was possible to discriminate microbial communities from different soil samples. As expected, 16S ARDRA showed that cultivated soils had a lower microbial diversity in respect to forest soils.     

Successional changes in the soil microbial community along a vegetation development sequence in a subalpine volcanic desert on Mount Fuji, Japan

Aims

To study the relationship between vegetation development and changes in the soil microbial community during primary succession in a volcanic desert, we examined successional changes in microbial respiration, biomass, and community structure in a volcanic desert on Mount Fuji, Japan.

Methods

Soil samples were collected from six successional stages, including isolated island-like plant communities. We measured microbial respiration and performed phospholipid fatty acid (PLFA) analysis, denaturing gradient gel electrophoresis (DGGE) analysis, and community-level physiological profile (CLPP) analysis using Biolog microplates.

Results

Microbial biomass (total PLFA content) increased during plant succession and was positively correlated with soil properties including soil water and soil organic matter (SOM) contents. The microbial respiration rate per unit biomass decreased during succession. Nonmetric multidimensional scaling based on the PLFA, DGGE, and CLPP analyses showed a substantial shift in microbial community structure as a result of initial colonization by the pioneer herb Polygonum cuspidatum and subsequent colonization by Larix kaempferi into central areas of island-like communities. These shifts in microbial community structure probably reflect differences in SOM quality.

Conclusions

Microbial succession in the volcanic desert of Mt. Fuji was initially strongly affected by colonization of the pioneer herbaceous plant (P. cuspidatum) associated with substantial changes in the soil environment. Subsequent changes in vegetation, including the invasion of shrubs such as L. kaempferi, also affected the microbial community structure.     

Growth Inhibition of Phytopathogenic Fungi and Oomycetes by Basidiomycete Irpex lacteus and Identification of its Antimicrobial Extracellular Metabolites

In dual culture confrontation assays, basidiomycete Irpex lacteus efficiently antagonized Fusarium spp., Colletotrichum spp., and Phytophthora spp. phytopathogenic strains, with growth inhibition percentages between 16.7–46.3%. Antibiosis assays evaluating the inhibitory effect of soluble extracellular metabolites indicated I. lacteus strain inhibited phytopathogens growth between 32.0–86.7%. Metabolites in the extracellular broth filtrate, identified by UPLC-QTOF mass spectrometer, included nine terpenes, two aldehydes, and derivatives of a polyketide, a quinazoline, and a xanthone, several of which had antifungal activity. I. lacteus strain and its extracellular metabolites might be valuable tools for phytopathogenic fungi and oomycete biocontrol of agricultural relevance.     

Evaluation of Mercury Contamination in Fungi Boletus Species from Latosols,Lateritic Red Earths,and Red and Yellow Earths in the Circum-Pacific Mercuriferous Belt of Southwestern China

For the first time, highly elevated levels of mercury (Hg) have been documented for several species of the edible Fungi genus Boletus growing in latosols, lateritic red earths, and red and yellow earths from the Yunnan province of China. Analysis of Hg concentrations in the genus suggests that geogenic Hg is the dominant source of Hg in the fungi, whereas anthropogenic sources accumulate largely in the organic layer of the forest soil horizon. Among the 21 species studied from 32 locations across Yunnan and 2 places in Sichuan Province, the Hg was found at elevated level in all samples from Yunnan but not in the samples from Sichuan, which is located outside the mercuriferous belt. Particularly abundant in Hg were the caps of fruiting bodies of Boletus aereus (up to 13 mg kg^-1 dry matter), Boletus bicolor (up to 5.5 mg kg^-1 dry matter), Boletus edulis (up to 22 mg kg^-1 dry matter), Boletus luridus (up to 11 mg kg^-1 dry matter), Boletus magnificus (up to 13 mg kg^-1 dry matter), Boletus obscureumbrinus (up to 9.4 mg kg^-1 dry matter), Boletus purpureus (up to 16 mg kg^-1 dry matter), Boletus sinicus (up to 6.8 mg kg^-1 dry matter), Boletus speciosus (up to 4.9mg kg^-1 dry matter), Boletus tomentipes (up to 13 mg kg^-1 dry matter), and Boletus umbriniporus (up to 4.9 mg kg^-1 dry matter). Soil samples of the 0–10 cm topsoil layer from the widely distributed locations had mercury levels ranging between 0.034 to 3.4 mg kg^-1 dry matter. In Yunnan, both the soil parent rock and fruiting bodies of Boletus spp. were enriched in Hg, whereas the same species from Sichuan, located outside the mercuriferous belt, had low Hg concentrations, suggesting that the Hg in the Yunnan samples is mainly from geogenic sources rather than anthropogenic sources. However, the contribution of anthropogenically-derived Hg sequestered within soils of Yunnan has not been quantified, so more future research is required. Our results suggest that high rates of consumption of Boletus spp. from Yunnan can deliver relatively high doses of Hg to consumers, but that rates can differ widely because of large variability in mercury concentrations between species and locations.     

Species of Root-knot Nematodes and Fungal Egg Parasites Recovered from Vegetables in Almería and Barcelona,Spain

Intensive vegetable production areas were surveyed in the provinces of Almería (35 sites) and Barcelona (22 sites), Spain, to determine the incidence and identity of Meloidogyne spp. and of fungal parasites of nematode eggs. Two species of Meloidogyne were found in Almería—M. javanica (63% of the samples) and M. incognita (31%). Three species were found in Barcelona, including M. incognita (50%), M. javanica (36%), and M. arenaria (14%). Solanaceous crops supported larger (P < 0.05) nematode numbers than cucurbit crops in Almería but not in Barcelona. Fungal parasites were found in 37% and 45% of the sites in Almería and Barcelona, respectively, but percent parasitism was never greater than 5%. Nine fungal species were isolated from single eggs of the nematode. The fungi included Verticillium chlamydosporium, V. catenulatum, Fusarium oxysporum, F. solani, Fusarium spp., Acremonium strictum, Gliocladium roseum, Cylindrocarpon spp., Engiodontium album, and Dactylella oviparasitica. Two sterile fungi and five unidentified fungi also were isolated from Meloidogyne spp. eggs.     

Detection of Fusarium wilt pathogens of Psidium guajava L. in soil using culture independent PCR (ciPCR)

Traditional culturing methods take a long time for identification of pathogenic isolates. A protocol has been developed for the detection of Fusarium from soil samples in the early stage of infection. Seventeen soil samples from different locations were collected before the onset of rains to find out the presence of Fusarium spp. population present in the soil of guava orchards and to correlate its presence with incidence of wilt. A PCR based method was developed for the molecular characterization of Fusarium using Fusarium spp. specific primer. DNA extracted by this method was free from protein and other contaminations and the yield was sufficient for PCR amplification. The primer developed in this study was amplifying ∼230 bp in all infected samples while not in healthy soil. The specificity and sensitivity of primer were tested on several Fusarium spp. and found that this primer was amplifying 10⁻⁶ dilution of the fungal DNA. The present study facilitates the rapid detection of Fusarium spp. from infected soil samples of guava collected from different agroclimatic regions in India. A rapid detection method for pathogens and a diagnostic assay for disease would facilitate an early detection of pathogen and lead to more effective control strategies.     

Soil and root populations of fluorescent Pseudomonas spp. associated with seedlings and field-grown plants are affected by sorghum genotype

Sorghum [Sorghum bicolor (L.) Moench] is valued for bioenergy, feed and food. Potential of sorghum genotypes to support differing populations of root- and soil-associated fluorescent Pseudomonas spp. or Fusarium spp., in two soils, was assessed. Culturable pseudomonads were enumerated from roots and soil of sorghum (Redlan and RTx433) and wheat (Lewjain) seedlings repeatedly grown in cycled soils in the growth chamber. Pseudomonads and Fusarium spp. were assessed from roots and soil of field-grown sorghum along with biological control traits hydrogen cyanide (HCN) and 2,4-diacetylphlorogluconol (phl) production. After four 4-week cycles, soil associated with Redlan seedlings had greater numbers of fluorescent pseudomonads than Lewjain. In dryland field conditions, RTx433 roots had greater numbers of pseudomonads than Redlan before anthesis but similar numbers after. There were no differences in numbers of pseudomonads from dryland soil or roots or soil of irrigated plants. Percentages of HCN-producing root isolates and phl soil isolates declined on irrigated Redlan plants, but percentages of HCN-producers increased in dryland conditions. Redlan roots had greater percentages of Fusarium isolates in the Gibberella fujikuroi complex. Results indicated that sorghum genotype affected root-associated populations of fluorescent Pseudomonas spp. and Fusarium spp. across soil environments.     

Interaction of Population Levels of Fusarium oxysporum f. sp. vasinfectum and Meloidogyne incognita on Cotton

In autoclaved greenhouse soil without Fusarium oxysporum f. sp. vasinfectum, Meloidogyne incognita did not cause leaf or vascular discoloration of 59-day-old cotton plants. Plants had root galls with as few as 50 Meloidogyne larvae per plant. Root galling was directly proportional to the initial nematode population level. Fusarium wilt symptoms occurred without nematodes with 77,000 fungus propagules or more per gram of soil. As few as 50 Meloidogyne larvae accompanying 650 fungus propagules caused Fusarium wilt. With few exceptions, leaf symptoms appeared sooner as numbers of either or both organisms increased. In soils infested with both organisms, the extent of fungal invasion and colonization was well correlated with the extent of nematode galling and other indications of the Fusarium wilt syndrome.     

Initial Copper Stress Strengthens the Resistance of Soil Microorganisms to a Subsequent Copper Stress

To improve the prediction of essential ecosystem functioning under future environmental disturbances, it is of significance to identify responses of soil microorganisms to environmental stresses. In this study, we collected polluted soil samples from field plots with eight copper levels ranging from 0 to 3,200 mg Cu kg^?1 soil. Then, the soils with 0 and 3,200 mg Cu kg^?1 were selected to construct a microcosm experiment. Four treatments were set up including Cu0-C and Cu3200-C without further Cu addition, and Cu0-A and Cu3200-A with addition of 57.5 mg Cu kg^?1 soil. We measured substrate-induced respiration (SIR) and potential nitrification rate (PNR). Furthermore, the abundance of bacterial, archaeal 16S rRNA genes, ammonia-oxidizing bacteria and archaea amoA genes were determined through quantitative PCR. The soil microbial communities were investigated by terminal restriction fragment length polymorphism (T-RFLP). For the field samples, the SIR and PNR as well as the abundance of soil microorganisms varied significantly between eight copper levels. Soil microbial communities highly differed between the low and high copper stress. In the microcosm experiment, the PNR and SIR both recovered while the abundance of soil microorganisms varied irregularly during the 90-day incubation. The differences of microbial communities measured by pairwise Bray–Curtis dissimilarities between Cu0-A and Cu0-C on day 0 were significantly higher after subsequent stress than before. However, the differences of microbial communities between Cu3200-A and Cu3200-C on day 0 changed little between after subsequent stress and before. Therefore, initial copper stress could increase the resistance of soil microorganisms to subsequent copper stress.