Denaturing gradient gel electrophoretic analysis of dominant 2,4-diacetylphloroglucinol biosynthetic phlD alleles in fluorescent Pseudomonas from soils suppressive or conducive to black root rot of tobacco

In Switzerland, similar types of rhizosphere pseudomonads producing the biocontrol compound 2,4-diacetylphloroglucinol (Phl) have been found in soils suppressive to Thielaviopsis basicola-mediated black root rot of tobacco as well as in conducive soils. However, most findings were based on the analysis of a limited number of Pseudomonas isolates, obtained from a single experiment and only from T. basicola-inoculated plants. Here, an approach based on denaturing gradient gel electrophoresis (DGGE) of dominant phlD alleles from tobacco rhizosphere provided different phlD migration patterns. Sequencing of phlD-DGGE bands revealed a novel phylogenetic cluster of phlD sequences found in both suppressive and conducive soils in addition to previously-documented phlD alleles. phlD-DGGE bands and alleles differed little from one plant to the next but more extensively from one sampling to the next during the three-year study. Three of the 13 bands and 12 of the 31 alleles were only found in suppressive soil, whereas five bands and 13 alleles were found exclusively in conducive soil. The population structure of phlD⁺ pseudomonads depended more on the individual soil considered and its suppressiveness status than on inoculation of tobacco with T. basicola. In conclusion, phlD-DGGE revealed additional phlD diversity compared with earlier analyses of individual Pseudomonas isolates, and showed differences in phlD⁺Pseudomonas population structure in relation to disease suppressiveness.     

Changes in soil hydraulic conductivity, runoff, and soil loss due to irrigation with different types of saline–sodic water

Irrigation with saline–sodic water causes sodic conditions in the soil which reduces soil productivity. We evaluated the changes in a number of important indices related to soil structural stability when treated wastewater (TWW), albeit with higher loads of organic matter and suspended solids, was used instead of more saline–sodic irrigation water, under different degrees of aggregate slaking. We studied soil saturated hydraulic conductivity (HC) using disturbed samples packed in columns, and soil infiltration rate, runoff and erosion under simulated rainfall. Aggregate slaking was manipulated by wetting the samples prior to all tests at either a slow (1–2 mm h^− 1) or a fast (50 mm h^− 1) rate. Samples of a calcareous silty clay (Typic Calciorthids) from the Bet She'an Valley, Israel, were taken from plots irrigated for three years with either TWW, saline–sodic Jordan River water (JRW), or moderately saline–sodic spring water (SPW), and also from a non-cultivated area (control). With little or no aggregate slaking (use of slow wetting), higher HC values and lower amounts of total runoff and soil loss were measured compared to when more severe aggregate slaking was induced (use of fast wetting). The HC values for the TWW treatment were similar to, or lower than, those for the control and significantly higher than those for the JRW treatment. For the runoff and soil loss data, differences among the water quality treatments were, generally, more pronounced when aggregate slaking was substantially reduced, and were related to soil sodicity. Under the latter condition, runoff and soil loss from the TWW treatment were comparable with those from the control and significantly lower than those from the JRW treatment. Our results suggested that replacing saline–sodic irrigation water with TWW could have favorable effects on soil structural stability, especially under conditions where aggregate slaking can be reduced (e.g., in regions with low to moderate rain intensities; and/or use of low intensity irrigation systems).     

Effect of successive cauliflower plantings and Rhizoctonia solani AG 2-1 inoculations on disease suppressiveness of a suppressive and a conducive soil

Disease suppressiveness against Rhizoctonia solani AG 2-1 in cauliflower was studied in two marine clay soils with a sandy loam texture. The soils had a different cropping history. One soil had a long-term (40 years) cauliflower history and was suppressive, the other soil was conducive and came from a pear orchard not having a cauliflower crop for at least 40 years. These two soils were subjected to five successive cropping cycles with cauliflower or remaining fallow in a greenhouse experiment. Soils were inoculated with R. solani AG 2-1 only once or before every crop. Disease decline occurred in all treatments cropped with cauliflower, either because of a decreased pathogen population or increased suppressiveness of the soil. Disease suppressiveness tests indicated that the conducive soil became suppressive after five subsequent cauliflower crops inoculated each cycle with R. solani AG 2-1. Suppressiveness of all treatments was measured in a seed germination test (pre-emergence damping-off) as well as by measuring the spread of R. solani symptoms in young plants (post-emergence damping-off). Results showed that suppressiveness was significantly stimulated by the successive R. solani inoculations; presence of the cauliflower crop had less effect. Suppressiveness was of biological origin, since it disappeared after sterilization of the soil. Moreover, suppressiveness could be translocated by adding 10% suppressive soil into sterilized soil. The suppressive soil contained higher numbers of culturable filamentous actinomycetes than the conducive soil, but treatments enhancing suppressiveness did not show an increased actinomycetes population. The suppressiveness of the soil samples did also not correlate with the number of pseudomonads. Moreover, no correlation was found with the presence of different mycoparasitic fungi, i.e. Volutella spp., Gliocladium roseum, Verticillium biguttatum and Trichoderma spp. The suppressive soil contained a high percentage of bacteria with a strong in vitro inhibition of R. solani. These bacteria were identified as Lysobacter (56%), Streptomyces (23%) and Pseudomonas (21%) spp. A potential role of Lysobacter in soil suppressiveness was confirmed by quantitative PCR detection (TaqMan), since a larger Lysobacter population was present in suppressive cauliflower soil than in conducive pear orchard soil. Our experiments showed that successive cauliflower plantings can cause a decline of the damage caused by R. solani AG 2-1, and that natural disease suppressiveness was most pronounced after subsequent inoculations with the pathogen. The mode of action of the decline is not yet understood, but antagonistic Lysobacter spp. are potential key organisms.     

Distribution of Pseudomonas populations harboring phlD or hcnAB biocontrol genes is related to depth in vineyard soils

The abundance and population structure of pseudomonads in soils collected from long-(1006 years) and short-(54 years) term grapevine monocultures in Switzerland were examined across five soil horizons within the 1.20-1.35 m range. Soil samples were baited with grapevine, and rhizosphere pseudomonads containing the biocontrol genes phlD (2,4-diacetylphloroglucinol synthesis) and/or hcnAB (hydrogen cyanide synthesis) were analyzed by MPN-PCR. The numbers of total, phlD⁺ and hcnAB⁺ pseudomonads decreased with depth by 1.5-2 log (short-term monoculture) and 3-3.5 log (long-term monoculture). In addition, the percentages of phlD⁺ (except in short-term monoculture) and hcnAB⁺ pseudomonads were also lower in deeper horizons. RFLP-profiling of phlD⁺ and hcnAB⁺ pseudomonads revealed three phlD and twelve hcnAB alleles overall, but the number of alleles for both decreased in relation to depth. The only phlD allele found in deeper horizons was also found in topsoil, whereas one hcnAB allele (k) found in deeper horizons in long-term monoculture was absent in the topsoil. This suggests that certain Pseudomonas ecotypes are adapted to specific depths. Four hcnAB alleles enabled discrimination between monocultures. We conclude that soil depth is a factor selecting phlD and hcnAB genotypes, and that the allelic diversity of the two biocontrol genes decreases with depth.     

Earthworms promote the reduction of Fusarium biomass and deoxynivalenol content in wheat straw under field conditions

A field experiment was conducted to elucidate ecosystem services provided by earthworms on the repression of phytopathogenic and toxinogenic fungi. The study focussed on decomposing Fusarium culmorum-infected and deoxynivalenol (DON)-contaminated wheat straw remaining on the soil surface as part in conservation tillage. Mesocosms were established in the topsoil of a winter wheat field located in Northern Germany, where conservation tillage has been practised for 20 years. Besides a non-earthworm treatment, two earthworm species were inoculated in the mesocosms either separately or combined: Lumbricus terrestris (anecic, detritivorous) and Aporrectodea caliginosa (endogeic, geophagous). The earthworms were exposed either to artificially Fusarium-infected wheat straw highly contaminated with DON or to non-infected straw serving as a control. The experiment was conducted during an eight week period after harvest from mid August to mid October. For both species, the artificially Fusarium-infected and DON-contaminated wheat straw was a more attractive food source than the non-infected control. In contrast to A. caliginosa, L. terrestris incorporated infected straw faster into the soil compared to control straw. Furthermore, the reduction of Fusarium biomass and DON concentration in wheat straw was significantly higher in the presence of L. terrestris than in treatments with A. caliginosa and without earthworms. Here, no significant differences could be measured between the Fusarium biomass and DON concentration in wheat straw. A. caliginosa seems not to be relevant for the reduction of Fusarium biomass and DON concentration. We concluded that amongst earthworms, anecic detritivorous species are the drivers to compensate possible negative consequences (like crop infection) of conservation tillage. They take an important role in the control of phytopathogenic and toxinogenic fungi surviving on plant residues and in the degradation of their mycotoxins.     

Dispersal patterns and behaviour of the nematode Phasmarhabditis hermaphrodita in mineral soils and organic media

The commercially available parasitic nematode Phasmarhabditis hermaphrodita is an effective biocontrol agent for slugs and particularly Deroceras reticulatum, a widespread pest species. Use of the nematode is currently limited by cost and it may be that by developing a fuller understanding of the ecology and behaviour of this nematode, more cost effective application strategies can be developed. We investigated the ability of two strains of P. hermaphrodita (one newly isolated and one that had been maintained in vitro for >15 years) to move through mineral soils and organic media. Active dispersal of both strains was found to be greatest in organic media (bark chips and leaf litter, and to a lesser extent peat) and the nematode was capable of growth and reproduction in leaf litter. Conversely, active dispersal was poor in mineral soils. Nematodes moved further in a clay loam compared with a sandy loam, and moved more at a bulk density of 1.0 vs. 1.2 Mg m⁻³. However, P. hermaphrodita was capable of moving greater distances in mineral soils by using the earthworm Lumbricus terrestris as a phoretic host. Our data suggest that P. hermaphrodita is a facultative parasite that is adapted to living in leaf litter and organic material where slugs frequently rest. The implications of these findings for using the nematode as a biological control agent for slugs are discussed.     

Host-parasite soil communities and environmental constraints: Modelling of soil functions involved in interactions between plant-parasitic nematodes and Pasteuria penetrans

Nematodes belonging to the genus Meloidogyne are the most ubiquitous and widespread plant-parasitic nematodes. They occur worldwide, are polyphagous and can parasitize most cultivated plants leading to reduced crop yields. They are especially harmful in developing countries because of the lack of suitable and feasible management strategies. Among all the control practices (chemicals, physical techniques, cultural practices, resistance), the use of natural enemies as biological control agents is the most recently developed. Pasteuria penetrans which is an obligate Gram-positive, endospore-forming bacterium, is perhaps the most promising plant-parasitic nematode biocontrol agent. Despite much research conducted on prey-predator interactions (host-parasite specificity, mechanisms of attachment, field efficacy), the influence of the soil environment on host-parasite interactions is poorly understood even when the soil appears to be the key factor. Beyond common studies on the influence of climatic conditions on the attachment of endospores of P. penetrans to nematodes, more knowledge about the systemic interactions between plants, soil water dynamics, soil texture and structure, and other biota on the parasitism of nematodes by P. penetrans would improve their utilization as biological control agents. The aim of this review is to analyze the literature dealing with the influence of the soil on nematode - P. penetrans interactions in order to suggest a helpful conceptual model based on partitioning the Pasteuria population in sub-populations according to their soil habitat (dispersible and non-dispersible aggregates, microporosity, macroporosity), not all of them being available for attachment and infection on nematodes. Such concerns should be taken into account by epidemiologists for improving biological management strategies based on the use of this bacterium.     

Persistence of toxins and cells of Bacillus thuringiensis subsp. kurstaki introduced in sprays to Sardinia soils

Sprays of commercial insecticidal preparations of the bacterium, Bacillus thuringiensis subsp. kurstaki (Btk), usually a mixture of cells, spores and parasporal crystals, have been used for the last 10 yr in Sardinia (Italy) to protect cork oak forests against the gypsy moth (Lymantria dispar L.). Until now, the protective antilepidopteran efficacies of each of the various spray treatments rather than their effects on the environment have been evaluated. Consequently, the persistence of Btk and its toxin, released in sprays (FORAY 48B^®), in soils of cork oak stands, located in Orotelli, Tempio Pausania and Calangianus (Sardinia), were investigated. In the Calangianus soil, the numbers of Btk remained essentially constant for 28 months (the longest time studied) after spraying, indicating that Btk was able to compete with the indigenous microbial community; the toxin was detected 28 months after spraying by immunological assay, but at a reduced concentration; and the larvicidal activity decreased essentially linearly to 14 months and then decreased markedly between 14 and 28 months. In the Tempio Pausania and Orotelli soils, cells of Btk were detected, whereas the toxin was not detected by immunological and larvicidal assays, 52 and 88 months (the longest times studied) after spraying, respectively. The numbers of Btk cells detected were probably too low to account for the presence of the toxin in all of the soils studied, as there was no correlation between numbers of Btk and toxin detected by immunological assays (correlation coefficient of −0.66) in the Calangianus soil. Our results indicated that Btk and its toxin introduced into soils in sprays can persist for long periods (at least 88 months for Btk and at least 28 months for its toxin).     

Transgenic Bt plants decompose less in soil than non-Bt plants

Bt plants are plants that have been genetically modified to express the insecticidal proteins (e.g. Cry1Ab, Cry1Ac, Cry3A) from subspecies of the bacterium, Bacillus thuringiensis (Bt), to kill lepidopteran pests that feed on corn, rice, tobacco, canola, and cotton and coleopteran pests that feed on potato. The biomass of these transgenic Bt plants (Bt+) was decomposed less in soil than the biomass of their near-isogenic non-Bt plant counterparts (Bt−). Soil was amended with 0.5, 1, or 2% (wt wt⁻¹) ground, dried (50 °C) leaves or stems of Bt corn plants; with 0.5% (wt wt⁻¹) ground, dried biomass of Bt rice, tobacco, canola, cotton, and potato plants; with biomass of the near-isogenic plants without the respective cry genes; or not amended. The gross metabolic activity of the soil was determined by CO₂ evolution. The amounts of C evolved as CO₂ were significantly lower from soil microcosms amended with biomass of Bt plants than of non-Bt plants. This difference occurred with stems and leaves from two hybrids of Bt corn, one of which had a higher C:N ratio than its near-isogenic non-Bt counterpart and the other which had essentially the same C:N ratio, even when glucose, nitrogen (NH₄NO₃), or glucose plus nitrogen were added with the biomass. The C:N ratios of the other Bt plants (including two other hybrids of Bt corn) and their near-isogenic non-Bt counterparts were also not related to their relative biodegradation. Bt corn had a significantly higher lignin content than near-isogenic non-Bt corn. However, the lignin content of the other Bt plants, which was significantly lower than that of both Bt and non-Bt corn, was generally not statistically significantly different, although 10-66% higher, from that of their respective non-Bt near-isolines. The numbers of culturable bacteria and fungi and the activity of representative enzymes involved in the degradation of plant biomass were not significantly different between soil amended with biomass of Bt or non-Bt corn. The degradation of the biomass of all Bt plants in the absence of soil but inoculated with a microbial suspension from the same soil was also significantly less than that of their respective inoculated non-Bt plants. The addition of streptomycin, cycloheximide, or both to the soil suspension did not alter the relative degradation of Bt+ and Bt− biomass, suggesting that differences in the soil microbiota were not responsible for the differential decomposition of Bt+ and Bt− biomass. All samples of soil amended with biomass of Bt plants were immunologically positive for the respective Cry proteins and toxic to the larvae of the tobacco hornworm (Manduca sexta), which was used as a representative lepidopteran in insect bioassays (no insecticidal assay was done for the Cry3A protein from potato). The ecological and environmental relevance of these findings is not clear.     

Symbiotic effectiveness of Bradyrhizobium japonicum in acid soils can be predicted from their sensitivity to acid soil stress factors in acidic agar media

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.     

Variation of Pythium-induced cocoyam root rot severity in response to soil type

In Cameroon, andosols are suspected to be suppressive to cocoyam (Xanthosoma sagittifolium) root rot disease (CRRD) caused by the Oomycete pathogen Pythium myriotylum. To determine factors involved in disease suppressiveness, andosols were studied in comparison to ferralsols known to be disease-conducive. Soil samples were collected from six sites of which three were in andosols around Mount Cameroon (Boteva, Njonji, and Ekona) and the three others in ferralsols (Bakoa, Lapkwang, and Nko’o canane). Greenhouse plant experiments were used to assess soil suppressiveness. Soils were artificially infested with two levels of P. myriotylum inoculum (100 and 300 mycelia strands g⁻¹ soil) prior to planting cocoyam. Disease severity was significantly higher in ferralsols than in andosols. Andosols partly lost their suppressiveness as a result of autoclaving and could recover suppressiveness following recolonisation by their original microflora. Soil microbial groups implicated in the disease suppression were investigated by assessing the effect of fungicide, bactericide, and pasteurisation on andosol suppressiveness. Andosols suppressiveness was significantly reduced following pasteurisation and treatment with fungicide and bactericide. The possible influence of microbial biomass on andosol suppressiveness was investigated by comparing microbial populations of suppressive andosols to those in andosols that had lost suppressiveness. A comparative analysis of suppressive and conducive soil properties was performed to identify soil variables, which may contribute to soil suppressiveness. Soil chemical analysis results showed that organic matter content was higher in andosols than in ferralsols. In addition, the content of mineral nutrients such as Ca, K, Mg and N, was higher in andosols than in ferralsols. These soil variables negatively correlated with disease severity. By contrast, sand and clay, which were higher in ferralsols than in andosols, were positively related to disease severity. This study has confirmed the suppressive nature of andosols from Mount Cameroon to CRRD. The results suggest that high organic matter content is likely mediating P. myriotylum suppression in andosols by improving soil structure, increasing soil nutrient content and microbial biomass, and sustaining microbial activity.     

To be or not to be—the effect of nature conservation management on flowering of Paeonia mascula (L.) Miller in Israel

Mount Meiron Nature Reserve is the largest Mediterranean reserve in Israel. Since its establishment, goat grazing and woodcutting have been prohibited and dense oak maquis has developed by succession. The succession of vegetation poses the risk of losing species. The peony (Paeonia masculla) is a locally endangered species in Israel, growing only in a small population in the Mount Meiron Nature Reserve with only 5% of plants flowering. We sought a management practice that would increase the flowering percentage of the peonies and secure the future of its population. We found that flowering peonies grew mainly in locations with about 47% direct sun radiation, while most plants grew under heavy shade with only 27% direct radiation. Creation of small gaps increased the flowering to 15-20%. To ensure the future of the peonies in the Mount Meiron Nature Reserve, small gaps must be created—even if this conflicts with broader current management policies.     

Field survival of the phytostimulator Azospirillum lipoferum CRT1 and functional impact on maize crop, biodegradation of crop residues, and soil faunal indicators in a context of decreasing nitrogen fertilisation

Heavy nitrogen fertilisation is often implemented in maize cropping systems, but it can have negative environmental effects. Nitrogen-fixing, phytohormone-producing Azospirillum plant growth-promoting rhizobacteria (PGPR) have been proposed as crop inoculants to maintain high yield when decreasing nitrogen fertilisation. In this context, agronomic and ecological effects of the inoculation of maize seeds with the PGPR Azospirillum lipoferum CRT1 were studied in two consecutive years. The inoculant was recovered from maize at 10⁵ CFU g⁻¹ root or higher. Inoculation enhanced root growth and development based on results of root biomass, rooting depth and/or parameters describing root system architecture, and a transient positive effect on shoot height was observed in the first year. Inoculation did not increase yield, but reducing mineral nitrogen fertilisation had only a minor effect on yield. This suggests that the lack of positive effect of the PGPR on yield was due to the fact that the whole field was heavily fertilised in years prior to the start of the experiment. Soil nitrogen levels decreased during the 2 years of the study, and the inoculant had no effect on residual soil nitrogen levels at harvest. Inoculation had no impact on Fusarium symptoms and concentration of the mycotoxin deoxynivalenol in maize kernels, but both were influenced by the interaction between inoculation and nitrogen fertilisation level. Inoculation did not influence meso/macrofaunal soil populations, but had a small but significant effect (smaller than the effect of added nitrogen) on decomposition, nitrogen mineralisation and mesofaunal colonisation of maize leaves (in litter bags). Overall, the ecological impact of seed inoculation with the PGPR A. lipoferum CRT1 was small, and its magnitude was smaller than that of chemical nitrogen fertilisation.     

Accumulation of trehalose in the thermophilic fungus Chaetomium thermophilum var. coprophilum in response to heat or salt stress

The disaccharide trehalose, known to be an effective protectant against various kinds of stress, was observed to accumulate in the cytosol of Chaetomium thermophilum var. coprophilum during heat stress. Trehalose was apparently neither involved in the defence of C. thermophilum var. coprophilum against high concentrations of sodium chloride nor directly linked to thermophily.In C. thermophilum var. coprophilum three different trehalose hydrolyzing activities were eluted from a mono Q anion exchange column by sodium chloride concentrations of 0.10, 0.15 and 0.24 M, respectively.     

Net nitrification rate and presence of Nitrosospira cluster 2 in acid coniferous forest soils appear to be tree species specific

The impact of four coniferous tree species and their corresponding soil factors on N transformation rates and presence of ammonia-oxidising bacteria (AOB) was studied in an acid pine forest soil (Appelscha, The Netherlands). Pine soil had a relatively low net nitrification rate, while spruce, fir and larch soils showed high net nitrification rates. 16S rRNA and amoA sequences were only found in soils with high nitrification rates and belonged solely to Nitrosospira cluster 2. We conclude that tree species, possibly through their effects on soil C/N ratios, determines the presence of Nitrosospira cluster 2. Whenever AOB are present, however, the AOB community composition appears to be similar.     

Novel strains of nodulating Burkholderia have a role in nitrogen fixation with papilionoid herbaceous legumes adapted to acid, infertile soils

We investigated the taxonomic position and symbiotic capabilities of two root-nodule bacterial strains isolated from the South African herbaceous, papilionoid legume Rhynchosia ferulifolia. The 16S rRNA gene sequence of the two strains was determined along with intragenic sequences of nodA and nifH, together with their symbiotic capabilities when inoculated onto the papilionoid legumes R. ferulifolia, Rhynchosia caribaea, Rhynchosia minima and Macroptilium atropurpureum (Siratro). Burkholderia phymatum STM815^T, Cupriavidus taiwanensis LMG 19424^T and root-nodule bacteria isolated from R. minima and Rhynchosia totta were included in the study. Root-nodule bacteria isolated from R. ferulifolia, WSM3937 and WSM3930, belong to the genus Burkholderia and are most closely related to Burkholderia terricola (98.8% similarity). The phylogenetic analysis of nodA and nifH revealed substantial similarity of the novel strains with Burkholderia tuberum STM678^T, a β-rhizobium also originated from South Africa, and only a distant relationship with South American Mimosa-nodulating β-rhizobia. R. ferulifolia was effectively nodulated only by Burkholderia sp. WSM3937 and WSM3930 and not by bradyrhizobia isolated from Rhynchosia minima and Rhynchosia totta or STM815 and LMG 19924. Nodules induced by the novel strains were determinate and hosted well organized symbiosomes within infected cells. In this study we describe a new symbiotic N-fixing relationship between Burkholderia sp. and the South African legume R. ferulifolia. This is the first report of N-fixation between β-rhizobia and an herbaceous, papilionoid legume from which the strains were originally isolated. The level of N-fixation in this symbiosis approached that achieved by effectively nodulated Medicago sativa and suggests that the β-rhizobia may have a role in N-fixation in agricultural systems.     

Using arbuscular mycorrhiza to reduce the stressful effects of soil compaction on corn (Zea mays L.) growth

Soil compaction is of great importance in agriculture, because its high levels may adversely affect plant growth and the environment. Since mechanical methods are not very efficient and economical, using biological methods to alleviate the stress of soil compaction on plant growth may be beneficial. The objectives of this study were to: (1) evaluate the effects of soil compaction on corn (Zea mays L.) growth, and (2) test the hypothesis that applying arbuscular mycorrhiza (AM) with different origins can partially or completely overcome the stressful effects of soil compaction on corn growth under unsterilized and sterilized conditions. Corn was planted in unsterilized and sterilized compacted soils, while treated with three species of AM including, Iranian Glomus mosseae, Iranian Glomus etunicatum, and Canadian Glomus mosseae, received from GINCO (Glomales in vitro Collection), Canada. Plant growth variables and soil resistance parameters were determined. AM significantly increased root fresh (maximum of 94% increase) and dry (maximum of 100% increase) weights in the compacted soil. AM with different origins may improve corn growth in compacted soils, though its effectiveness is related to the level of compaction and also to the interaction with other soil microorganisms.     

Escherichia coli O157 survival following the surface and sub-surface application of human pathogen contaminated organic waste to soil

Escherichia coli O157:H7 contaminated wastes such as animal manures and abattoir wastes, may be applied directly onto vegetation, the soil surface, or injected deep into the soil. Our aim was to determine the influence of method of waste application to land on E. coli O157:H7 survival. Bovine slurry and ovine stomach contents containing E. coli O157:H7 were applied to the surface vegetation or sub-surface injected at 25 cm below the soil surface. E. coli O157 survived but did not proliferate on grassland vegetation for up to 6 weeks and in the underlying soil for 8 weeks. Our results suggest that sub-surface injection of organic wastes into soil may reduce the risk of pathogen persistence in the environment.     

Control of apothecia of Sclerotinia sclerotiorum by soil amendment with S-H mixture or Perlka in bean, canola and wheat fields

Ascospores of Sclerotinia sclerotiorum produced from apothecia are the primary source of inoculum for causing diseases such as white mold of common bean, pod rot of pea, stem blight of canola and head rot of sunflower and safflower in the Canadian prairies. A field study was conducted for 4 years to determine efficacy of control of production of apothecia from carpogenically germinated sclerotia of S. sclerotiorum by soil amendment with Perlka^® (calcium cyanamide) and S-H mixture (a formulated compound). Results of the 4-year experiments showed that amendment of soil with Perlka^® at low (30 g/m²) or high (60 g/m²) rate was effective in reducing carpogenic germination of sclerotia and production of apothecia under the canopy of host crops (common bean and canola) and a non-host crop (wheat). In the experiments of 1988, for example, the numbers of apothecia produced in the treatments of Perlka^®-low rate (30 g/m²), Perlka^®-high rate (60 g/m²) and untreated control were 42, 46, and 182 apothecia/plot (m²), respectively, for bean; 89, 42, and 318 apothecia/plot (m²), respectively, for canola; and 146, 143, and 412 apothecia/plot (m²), respectively, for wheat. However, soil amendment of S-H mixture at low (30 g/m²) or high (60 g/m²) rate was ineffective in reducing carpogenic germination of sclerotia and production of apothecia for all the 4 years of testing in all three crops. The ineffectiveness of S-H mixture and the practicality of Perlka^® for control of Sclerotinia diseases of crops grown under Canadian prairie conditions are discussed.     

水保措施对红壤坡地柑橘林土壤呼吸及其水热因子的影响

[目的]探讨不同水土保持措施对红壤坡地柑橘林土壤呼吸的影响,为科学评价水土保持生态建设在应对气候变化方面的作用提供基础数据。[方法]采用试验观测和对比分析的方法研究横坡间作+等高草带、横坡间作、顺坡间作和清耕对照4种处理对柑橘林土壤呼吸速率的动态变化及其对土壤温度、土壤水分的响应。[结果]4个不同处理的柑橘林土壤呼吸速率均呈明显的单峰曲线变化,峰值出现在7月;各处理的土壤呼吸速率季节动态变化一致,采取横坡间作+等高草带和横坡间作两种水保措施均一定程度上增加了土壤呼吸速率;土壤呼吸速率与土壤温度呈显著正相关;横坡间作+等高草带和横坡间作处理的土壤呼吸的温度敏感性指数Q10较柑橘清耕和顺坡间作处理Q10值略有增加。[结论]红壤柑橘林采取水土保持措施后,土壤呼吸速率增强,且其对温度的响应增加。