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.     

Brassica napus seed meal soil amendment modifies microbial community structure, nitric oxide production and incidence of Rhizoctonia root rot

A low glucosinolate content (21.8 μmol g⁻¹) Brassica napus seed meal (RSM) applied to orchard soils altered communities of both pathogenic and saprophytic soil micro-organisms. RSM amendment reduced infection by native and introduced isolates of Rhizoctonia spp. and recovery of Pratylenchus spp. from apple roots. Root infection by Rhizoctonia solani AG-5 was also suppressed in split-root assays where a portion of the root system was cultivated in RSM-amended soils and the remainder grown in the presence of the pathogen but lacking RSM. R. solani hyphal growth was not inhibited by RSM amendment. Suppression of Pratylenchus was attained to an equivalent extent by amending soils with either RSM or soybean meal (SM) when applied to provide a similar N content. Thus, glucosinolate hydrolysis products did not appear to have a significant role in the suppression of Rhizoctonia spp. or Pratylenchus spp. obtained via RSM amendment. RSM amendment elevated populations of Pythium spp. and of ammonia-oxidizing bacteria that release nitric oxide but suppressed fluorescent pseudomonad numbers. Streptomyces spp. soil populations increased significantly in response to RSM but not SM amendment. The vast majority of Streptomyces spp. recovered from the apple rhizosphere produced nitric oxide and possessed a nitric oxide synthase homolog. We propose that transformations in the bacterial community structure are associated with the observed control of Rhizoctonia root rot, with NO production by soil bacteria potentially having a role in the induction of plant systemic resistance.     

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.     

Identifying the characteristics of organic soil amendments that suppress soilborne plant diseases

Application of organic amendments has been proposed as a strategy for the management of diseases caused by soilborne pathogens. However, inconsistent results seriously hinder their practical use. In this work we use an extensive data set of 2423 studies derived from 252 papers to explore this strategy. First, we assess the capability of a specific organic amendment to control different diseases; second, we investigate the influence of organic matter (OM) decomposition on disease suppressiveness; and third, we search for physical, chemical and biological parameters able to identify suppressive OM. OM was found to be consistently suppressive to different pathogens in only a few studies where a limited number of pathogens were tested. In the majority of studies a material suppressive to a pathogen was ineffective or even conducive to other pathogens, suggesting that OM suppressiveness is often pathogen-specific. OM decomposition in many studies (73%, n = 426) emerged as a crucial process affecting suppressiveness. During decomposition, disease suppression either increased, decreased, was unchanged or showed more complex responses, such as ‘hump-shaped’ dynamics. Peat suppressiveness generally decreased during decomposition, while responses of composts and crop residues were more complex. However, due to the many interactions of contributing factors (OM quality, microbial community composition, pathosystem tested and decomposition time), it was difficult to identify specific predictors of disease suppression. Among the 81 parameters analysed, only some of the 643 correlations showed a consistent relationship with disease suppression. The response of pathogen populations to OM amendments was a reliable feature only for some organic matter types (e.g. crop residues and organic wastes with C-to-N ratio lower than ∼15) and for pathogens with a limited saprophytic ability (e.g., Thielaviopsis basicola and Verticillium dahliae). Instead, population responses of the pathogenic fungi Phytophthora spp., Rhizoctonia solani and Pythium spp. appeared unrelated to disease suppression. Overall, enzymatic and microbiological parameters, rather than chemical ones, were much more informative for predicting suppressiveness. The most useful features were FDA activity, substrate respiration, microbial biomass, total culturable bacteria, fluorescent pseudomonads and Trichoderma populations. We conclude that the integration of different parameters (e.g. FDA hydrolysis and chemical composition by ¹³C NMR) may be a promising approach for identification of suppressive amendments.     

Response of soil microbial communities to compost amendments

Soil organic matter is considered as a major component of soil quality because it contributes directly or indirectly to many physical, chemical and biological properties. Thus, soil amendment with composts is an agricultural practice commonly used to improve soil quality and also to manage organic wastes. We evaluated in laboratory scale experiments the response of the soilborne microflora to the newly created soil environments resulting from the addition of three different composts in two different agricultural soils under controlled conditions. At a global level, total microbial densities were determined by classical plate count methods and global microbial activities were assessed by measuring basal respiration and substrate induced respiration (SIR). Soil suppressiveness to Rhizoctonia solani diseases was measured through bioassays performed in greenhouses. At a community level, the modifications of the metabolic and molecular structures of bacterial and fungal communities were assessed. Bacterial community level physiological profiles (CLPP) were determined using Biolog™ GN microtiter plates. Bacterial and fungal community structures were investigated using terminal restriction fragment length polymorphism (T-RFLP) fingerprinting. Data sets were analyzed using analysis of variance and ordination methods of multivariate data. The impact of organic amendments on soil characteristics differed with the nature of the composts and the soil types. French and English spent mushroom composts altered all the biological parameters evaluated in the clayey soil and/or in the sandy silty clay soil, while green waste compost did not modify either bacterial and fungal densities, SIR values nor soil suppressiveness in any of the soils. The changes in bacterial T-RFLP fingerprints caused by compost amendments were not related to the changes in CLPP, suggesting the functional redundancy of soil microorganisms. Assessing the density, the activity and the structure of the soil microflora allowed us not only to detect the impact of compost amendment on soil microorganisms, but also to evaluate its effect at a functional level through the variation of soil disease suppressiveness. Differences in disease suppressiveness were related to differences in chemical composition, in availability of nutrients at short term and in microbial composition due to both incorporation and stimulation of microorganisms by the compost amendments.     

Ecology of soil microflora and mycorrhizal symbionts in degraded forests at two altitudes

Summary Microbial populations were estimated in four different forest stands at different regenerational stages, two each at higher and lower altitudes. The fungal and bacterial populations showed marked seasonal variations at both altitudes. Quantitatively, the bacterial population was higher than the fungal population. Although 25 fungal species were isolated at the lower altitude, only 15 were obtained at the higher altitude. Penicillium chrysogenum and Trichoderma viride were dominant at the lower and higher altitudes, respectively. In the more degraded forest stand at the lower altitude both the fungal and the bacterial population showed a significant positive correlation with organic C (r=0.658 and 0.735, respectively), whereas in the less degraded forest stand there was a significant correlation only between the fungal population and organic C (r=0.835). At the higher altitude, however, a highly significant correlation (P<0.05) was observed between the fungal population, soil moisture and organic C in both the forest stands. Disturbance to the soil and vegetation adversely affected the microbial population, and also affected endogonaceous spores. At the lower altitude, plants in the more degraded forest stand were more mycotrophic compared to those in the less degraded stand. The level of mycorrhizal infection showed a highly positive correlation with soil moisture, organic C, total N, and available P. The spore population, however, was correlated negatively with these parameters. Three different endogonaceous genera, Glomus, Gigaspora, and Acaulospora, were identified during the course of investigation. Glomus, however, was dominant.     

土壤质地对柱花草生长发育、生物量及土壤肥力变化的影响

采用大田试验的方法,在云南省元谋县小雷宰流域内壤土、砂壤土和重壤土3种质地土壤上,以热研5号柱花草为材料,研究土壤质地对柱花草生长发育、生物量及土壤有机质、有机碳、全氮和全磷的影响。试验结果表明:3种土壤质地上种植柱花草,柱花草地上部和地下部生长量和生物量表现幼苗期增加缓慢,而分枝期后增加快的趋势。壤土耕性好,兼有砂土和重壤土的优点,有利柱花草地上部分的生长发育,柱花草地上部生长量、生物量及改善土壤肥力方面显著高于重壤土。砂壤土有利于柱花草根系向深层土壤生长,柱花草地下部生长量、生物量及根瘤显著高于种植在重壤土。在3种土壤质地种植柱花草后,土壤有机质、有机碳、全氮和全磷均有上升趋势。综合而言,通气性和保肥保水能力居中的壤土更适合柱花草的生长发育及干物质的积累。     

Effect of fungal to bacterial biomass ratio on the relationship between CO2 evolution and total soil microbial biomass

The relationship between the fungal: bacterial biomass ratio and the metabolic quotient (qCO₂) was studied in three different soils. In addition, the effect of the fungal: bacterial biomass ratio on the relationship between CO₂ evolution and the size of the soil microbial biomass was examined. Soil samples were collected from three experimental fields amended with various organic materials (Yatsugatake, Ibaraki, and Tochigi fields). The range of the fungal:bacterial biomass ratio in the Yatsugatake and Ibaraki fields was small (1.54–2.24 and 1.11–1.71, respectively), but it was large in the Tochigi field (1.18–3.75). We found a high negative correlation between this ratio and the metabolic quotient (qCO₂=2.10–0.361 (fungal:bacterial biomass ratio), R=–0.851, P<0.01) in the Tochigi field. Therefore, we suggest tha qCO₂ decreases with an increase in the fungal:bacterial biomass ratio, which may be due to a higher efficiency of substrate C use by fungal flora in comparison with bacterial flora. In the Yatsugatake and Ibaraki fields, there was a high positive correlation between CO₂ evolution and total microbial biomass. In contrast, no correlation was observed between these two parameters in the Tochigi field, probably reflecting the wide range of values for the fungal:bacterial biomass ratio. From the results obtained, we suggest that the fungal: bacterial biomass ratio is an important factor regulating the relationship between CO₂ evolution and the size of the microbial biomass.     

Re-establishment of suppressiveness to soil- and air-borne diseases by re-inoculation of soil microbial communities

The aim of this study was to investigate the potentials and limitations in restoring soil suppressiveness in disturbed soils. Soils from three sites in UK and Switzerland (STC, REC, THE) differing in their level of suppressiveness to soil-borne and air-borne diseases were γ-irradiated and this soil matrix was re-inoculated with 1% (w/w) of either parent native soil or native soil from the other sites (‘soil inoculum’). Suppressiveness to air-borne and soil-borne diseases was quantified by means of the host-pathogen systems Lepidium sativum (cress)-Pythium ultimum, an oomycete causing root rot and seedling damping-off, and Arabidopsis thaliana-Hyaloperonospora parasitica, an oomycete causing downy mildew. Soil microbial biomass, activity and community structure, as determined by phospholipid fatty acid (PLFA) profiles, were measured in native, γ-irradiated, and re-inoculated soils. Both, L. sativum and A. thaliana were highly susceptible to the pathogens if grown on γ-irradiated soils. Re-inoculation completely restored suppressiveness of soils to the foliar pathogen H. parasitica, independently of soil matrix or soil inoculum, whereas suppressiveness to P. ultimum depended on the soil matrix and, to a lesser extent, on the soil inoculum. However, the soil with the highest inherent suppressiveness did not reach the initial level of suppressiveness after re-inoculation. In addition, native microbial populations as defined by microbial biomass, activity and community structure, could not be fully restored in re-inoculated soils. As for suppressiveness to P. ultimum, the soil matrix, rather than the source of soil inoculum was identified as the key factor for re-establishing the microbial community structure. Our data show that soils do not or only slowly fully recover from sterilisation by γ-irradiation, indicating that agricultural soil management practices such as soil fumigation or heat treatments frequently used in vegetable cropping should be avoided.     

Chitinolytic and proteolytic activity of streptomycetes isolated from root-free soil,rhizosphere and mycorrhizosphere of pine (Pinus sylvestris L.)

Summary We tested 75 strains of Streptomyces spp. (25 taken from each environment of soil, rhizosphere, and mycorrhizosphere of pine, Pinus sylvestris L.) and all exhibited chitinolytic activity and hydrolysed gelatine and sodium caseinate in agar media. Enrichment of these media with glucose and NH₄NO₃ caused induction or stimulation of proteolytic Streptomyces spp. strains (80%) derived from root-free soil; inhibition of this activity was observed in most strains (92%) isolated from the root zone. The post-culture liquids of the rhizosphere strains cultured in the absence of glucose revealed a significantly higher proteolytic activity than those obtained from the root-free soil. The addition of glucose to the medium stimulated proteolytic activity in the post-culture broth of Streptomyces strains derived from soil and the mycorrhizosphere.     

Use of soil solarization to control root rots in gerberas (Gerbera jamesonii)

Summary An investigation was conducted during the summer months of 1986–1987 and 1987–1988 in Western Australia to evaluate the effect of soil solarization on the control of root rot of gerbera an also on the microbial and nutrient status of the soil. Infested soil cores were sampled from a site where root-rot was a severe problem and were removed to a non-infested site where they were subjected to soil solarization or fumigation. Soil solarization resulted in reduced root rot (root disease index 28.6%) in comparison to the untreated control (52.0%) 8 months after planting. Plants in the fumigated plots had 15.8% less disease than those in solarized plots. Solarization increased the total numbers of bacteria and actinomycetes, and the proportion of bacteria and fungi antogonistic to Fusarium oxysporum, F. solani and Rhizoctonia solani. The proportion of actinomycetes antagonistic to these fungi, however, did not differ between solarized and control soil treatments. There was a significant reduction in disease in plants grown in infested fumigated soil to which a 10% concentration of solarized soil had been added, suggesting the development of microbial suppression in solarized soil. Phytophthora cryptogea was eradicated to 30 cm by solarization as well as by fumigation. Solarization eliminated R. solani but not F. oxysporum to a soil depth of 10 cm. Solarization increased the levels of NO _n3 ^– -N and NH₄ ⁺-N in soil, but did not affect the concentrations of PO₄ ^3–, K⁺, Fe²⁺, organic C and pH. Yield (as number of flowers per plant) was increased by soil solarization and by fumigation.Increased yields and decreased disease severity in the solarized plots could have been caused by (1) a reduction in the infectivity of the infested soils, (2) an increase in the suppressiveness of the soil, and (3) an increased available of plant nutrients.     

Dual inoculation of pretransplant stage Oryza sativa L. plants with indigenous vesicular-arbuscular mycorrhizal fungi and fluorescent Pseudomonas spp.

Summary This study examined the response of rice (Oryza sativa L.) plants at the pretransplant/nursery stage to inoculation with vesicular-arbuscular mycorrhizal (VAM) fungi and fluorescent Pseudomonas spp., singly or in combination. The VAM fungi and fluorescent Pseudomonas spp. were isolated from the rhizosphere of rice plants. In the plants grown in soil inoculated with fluorescent Pseudomonas spp. alone, I found increases in shoot growth, and in root length and fine roots, and decreases in root growth, and P and N concentrations. In contrast, in the plants colonized by VAM fungi alone, the results were the reverse of those of the pseudomonad treatment. Dual inoculation of soil with VAM fungi and fluorescent Pseudomonas spp. yielded plants with the highest biomass and nutrient acquisition. In contrast, the plants of the control treatment had the lowest biomass and nutrient levels. The dual-inoculated plants had intermediate root and specific root lengths. The precentages of mycorrhizal colonization and colonized root lengths were significantly lower in the dual-inoculated treatment than the VAM fungal treatment. Inoculation of plants with fluorescent Pseudomonas spp. suppressed VAM fungal colonization and apparently reduced photosynthate loss to the mycorrhizal associates, which led to greater biomass and nutrient levels in dual-inoculated plants compared with plants inoculated with VAM fungi alone. Dual inoculation of seedlings with fluorescent Pseudomonas spp. and VAM fungi may be preferable to inoculation with VAM alone and may contribute to the successful establishment of these plants in the field.     

Screening of bacterial isolates from various European soils for in vitro antagonistic activity towards Rhizoctonia solani and Fusarium oxysporum: Site-dependent composition and diversity revealed

A cultivation-based approach was used to determine the in vitro antagonistic potential of soil bacteria towards Rhizoctonia solani AG3 and Fusarium oxysporum f. sp. lini (Foln3). Four composite soil samples were collected from four agricultural sites with previous documentation of disease suppression, located in France (FR), the Netherlands (NL), Sweden (SE) and the United Kingdom (UK). Similarly, two sites from Germany (Berlin, G-BR; and Braunschweig, G-BS) without documentation of disease suppression were sampled. Total bacterial counts were determined by plating serial dilutions from the composite soil samples onto R2A, AGS and King's B media. A total of 1,788 isolates (approximately 100 isolates per medium and site) was screened for antifungal activity, and in vitro antagonists (327 isolates) were found amongst the dominant culturable bacteria isolated from all six soils. The overall proportion of antagonists and the number of isolates with inhibitory activity against F. oxysporum were highest in three of the suppressive soils (FR, NL and SE). Characterization of antagonistic bacteria revealed a high phenotypic and genotypic diversity. Siderophore and protease activity were the most prominent phenotypic traits amongst the antagonists. The composition and diversity of antagonists in each soil was site-specific. Nevertheless, none of the antimicrobial traits of bacteria potentially contributing to soil suppressiveness analyzed in this study could be regarded as specific to a given site.     

Plant growth and microbial activity in a tropical soil amended with faecal pellets from millipedes and woodlice

The effect of soil fauna-mediated leaf litter (faecal pellets) versus mechanically fragmented (finely ground) leaf litter on biomass production of rice (Oryza sativa, var. Primavera) was assessed in pot tests. Rice seedlings were either grown in soil samples amended with faecal pellets of diplopods and isopods fed on leaf litter of a legume cover crop (Pueraria phaseoloides (Roxb.) Benth) and a peach palm (Bactris gasipaes) or in soil amended with finely ground leaf litter. The addition of faecal pellets caused a significant and dose-related increase in plant biomass compared to pure soil. Ground leaf litter induced a significantly smaller positive effect on plant biomass development with Pueraria litter > Bactris litter > mixed primary forest litter. In contrast, soil microbial biomass development during the 4 weeks plant test was higher in the soil amended with ground litter as compared to soil amended with feacal pellets. The results show a clear positive effect of the soil fauna on soil fertility and indicate differences in the availability of nutrients from the organic substrates to higher plants and soil microorganisms.     

Defoliation effects on Plantago lanceolata resource allocation and soil decomposers in relation to AM symbiosis and fertilization

Plants can mediate interactions between aboveground herbivores and belowground decomposers as both groups depend on plant-provided organic carbon. Most vascular plants also form symbiosis with arbuscular mycorrhizal fungi (AMF), which compete for plant carbon too. Our aim was to reveal how defoliation (trimming of plant leaves twice to 6 cm above the soil surface) and mycorrhizal infection (inoculation of the fungus Glomus claroideum BEG31), in nutrient poor and fertilized conditions, affect plant growth and resource allocation. We also tested how these effects can influence the abundance of microbial-feeding animals and nitrogen availability in the soil. We established a 12-wk microcosm study of Plantago lanceolata plants growing in autoclaved soil, into which we constructed a simplified microfood-web including saprotrophic bacteria and fungi and their nematode feeders. We found that fertilization, defoliation and inoculation of the mycorrhizal fungus all decreased P. lanceolata root growth and that fertilization increased leaf production. Plant inflorescence growth was decreased by defoliation and increased by fertilization and AMF inoculation. These results suggest a negative influence of the treatments on P. lanceolata belowground biomass allocation. Of the soil organisms, AMF root colonization decreased with fertilization and increased with defoliation. Fertilization decreased numbers of bacterial-feeding nematodes, probably because fertilized plants produced less root mass. On the other hand, bacterial feeders were more abundant when associated with defoliated than non-defoliated plants despite defoliated plants having less root mass. The AMF inoculation per se increased the abundance of fungal feeders, but the reduced and increased root AM colonization rates of fertilized and defoliated plants, respectively, were not reflected in the numbers of fungal feeders. We found no evidence of plant-mediated effects of the AM fungus on bacterial feeders, and against our prediction, soil inorganic nitrogen concentrations were not positively associated with the concomitant abundances of microbial-feeding animals. Altogether, our results suggest that (1) while defoliation, fertilization and AMF inoculation all affect plant resource allocation, (2) they do not greatly interact with each other. Moreover, it appears that (3) while changes in plant resource allocation due to fertilization and defoliation can influence numbers of bacterial feeders in the soil, (4) these effects may not significantly alter mineral N concentrations in the soil.     

Fitting plants to soil through mycorrhizal fungi: Mycorrhiza effects on plant growth and soil organic matter

Summary Vesicular-arbuscular mycorrhizal (VAM) fungi affect diverse aspects of plant form and function. Since mycorrhiza-mediated changes in host-plant responses to root colonization by different VAM fungi vary widely, it is important to assess each endophyte for each specific effect it can elicit from its host as part of the screening process for effectiveness. Three species of VAM fungi and a mixture of species were compared with non-VAM controls for their effects on soil organic matter contents and on nutrition and morphology in two varieties (native and hybrid) of corn (Zea mays L.) and one of sunflower (Helianthus annuus L.) in P-sufficient and N-deficient soil in pot cultures. Differences in soil organic matter due to the fungal applications were highly significant with all host plants. Native corn responded more to VAM colonization than the hybrid did; differences in treatments were significant in leaf area, plant biomass, and root: shoot ratio in the former, but not in the latter. Responses in the sunflower were similar to those in the native corn. Significant VAM treatment-related differences in shoot N and P contents were not reflected in shoot biomass, which was invariant. Correlations between plant or soil parameters and the intensity of VAM colonization were found only in soil organic matter with the native corn, in specific leaf area in the hybrid corn, and in plant biomass in the sunflower. The presence of the different endophytes and not the intensity of colonization apparently elicited different host responses.     

Mixed-species legume fallows affect faunal abundance and richness and N cycling compared to single species in maize-fallow rotations

Rotation of nitrogen-fixing woody legumes with maize has been widely promoted to reduce the loss of soil organic matter and decline in soil biological fertility in maize cropping systems in Africa. The objective of this study was to determine the effect of maize-fallow rotations with pure stands, two-species legume mixtures and mixed vegetation fallows on the richness and abundance of soil macrofauna and mineral nitrogen (N) dynamics. Pure stands of sesbania (Sesbania sesban), pigeon pea (Cajanus cajan), tephrosia (Tephrosia vogelii), 1:1 mixtures of sesbania + pigeon pea and sesbania + tephrosia, and a mixed vegetation fallow were compared with a continuously cropped monoculture maize receiving the recommended fertilizer rate, which was used as the control. The legume mixtures did not differ from the respective pure stands in leaf, litter and recycled biomass, soil Ca, Mg and K. Sesbania + pigeon pea mixtures consistently increased richness in soil macrofauna, and abundance of earthworms and millipedes compared with the maize monoculture (control). The nitrate-N, ammonium-N and total mineral N concentration of the till layer soil (upper 20 cm) of pure stands and mixed-species legume plots were comparable with the control plots. Sesbania + pigeon pea mixtures also gave higher maize grain yield compared with the pure stands of legume species and mixed vegetation fallows. It is concluded that maize-legume rotations increase soil macrofaunal richness and abundance compared with continuously cropped maize, and that further research is needed to better understand the interaction effect of macrofauna and mixtures of organic resources from legumes on soil microbial communities and nutrient fluxes in such agro-ecosystems.     

Influence of irrigation on the distribution and control of the nematode Meloidogyne javanica by the biocontrol bacterium Pasteuria penetrans in the field

Pasteuria penetrans, a bacterial parasite of plant-parasitic nematodes, is used to control root-knot nematode Meloidogyne spp. populations in vegetable crops. But its efficiency is variable, mostly because of the patchy distribution of the bacteria in arable fields. As the infective P. penetrans are non-motile bacteria in soil, abiotic soil factors can affect the bacteria–nematode relationships. An epidemiological study, conducted in a vegetable field, showed that abiotic factors such as irrigation, soil water holding capacity and texture, affected the efficiency of P. penetrans. A correspondence analysis between these abiotic factors and the density of P. penetrans spores in the soil, and the proportion of Meloidogyne javanica juveniles infected by the bacteria, revealed that irrigation affected directly the distribution of the spores in soil pores related to their passive transport by water flow. Laboratory experiments conducted on the passive transport of spores confirmed that intensive irrigation leached the spores down the soil profile and decreased the percentage of infected Meloidogyne juveniles.     

羟基磷灰石–植物联合修复对Cu/Cd污染植物根际 土壤微生物群落的影响

针对江西贵溪Cu、Cd重金属污染土壤,通过田间试验,比较无机生物材料羟基磷灰石及3种植物(海州香薷、巨菌草、伴矿景天)与羟基磷灰石联合修复对土壤总Cu、Cd的吸收及对活性Cu、Cd的钝化吸收能力差异。采用磷脂脂肪酸(PLFA)分析法,比较不同修复模式对土壤微生物群落结构的影响,以评估土壤微生态环境对不同修复措施的响应。研究结果表明:羟基磷灰石的施加可显著提高土壤pH,并有效钝化土壤活性Cu、Cd含量,但对土壤总Cu、Cd的含量影响较小。植物与羟基磷灰石的联合修复在显著降低土壤活性Cu、Cd(P0.05)的同时,减少了植物根际土壤总Cu、Cd的含量(P0.05)。不同修复措施对土壤微生物群落组成影响差异明显。单独施加羟基磷灰石与土壤真菌群落呈显著正相关,使土壤真菌生物量提高,从而引起真菌/细菌(F/B)的升高。植物与羟基磷灰石的联合修复可有效缓解土壤真菌化的趋势,其中巨菌草与羟基磷灰石的联合修复可有效提高土壤革兰氏阳性、革兰氏阴性细菌生物量及多样性,降低F/B值,从而降低土壤真菌病害的风险。不同植物根系活性代谢引起有机质的积累促进植物与羟基磷灰石处理中根际有机碳含量显著提高。聚类增强树(Aggregated boosted tree,ABT)分析结果表明:不同修复模式是影响土壤微生物群落的重要因素,其次土壤pH和Cu的含量及活性也是改变重金属污染区域微生物群落的因子。该研究从微生物群落结构角度解释了植物与羟基磷灰石联合修复对土壤微生态体系的作用,为开展Cu、Cd等重金属污染地植物与无机生物材料的联合修复方式的筛选及实施提供可靠的理论依据。     

Estimate of the source of soil protease in upland fields

Selective inhibition of bacterial or fungal growth in remoistened, oven-dried, inoculated Andosols indicated that bacteria were a more important source of benzyloxycarbonyl-l-phenylalanyl-l-leucine hydrolyzing activity (z-FLase) and casein-hydrolyzing activity (caseinase) than fungi. The same test indicated that bacteria were also a more important source of soil caseinase under upland conditions in a Gray Lowland soil. Most of the proteolytic bacteria isolated from the three upland fields by azocoll agar plates (Andosol upland field, 100%; Andosol uncultivated field, 96.4%; Gray Lowland upland field, 70.0%) wereBacillus spp. Most (100%, 97.1%, and 84.0%, respectively) of the gelatin liquefiers selected from the azocoll degraders, as those with high extracellular z-FLase and caseinase, were alsoBacillus spp. We conclude thatBacillus spp. are the major source of soil protease in the three upland fields studied.