Seasonal population changes of myxomycetes and associated organisms in five non-woodland soils, and correlations between their numbers and soil characteristics

Abstract Microbial populations and other characteristics of five non-woodland soils from sites in the West of England were studied quantitatively over the course of a year, and special attention was paid to the changing abundance of myxomycetes. The latter, expressed as numbers of plasmodium-forming units (PFUs), were on average more abundant than in three representative woodland soils. During the year, instances were recorded in which rapidly increasing populations of bacteria were accompanied by increasing numbers of phagotrophic microorganisms, including myxomycetes, and others in which rapidly increasingly populations of phagotrophs were accompanied by falls in the numbers of bacteria. Oscillating predator-prey relationships were suggested. Myxomycetes were generally a significant component of the phagotrophic populations, even exceeding 50% of the total soil naked amoebae. Data on the biological and abiotic characteristics of woodland [3] and non-woodland soils were combined and statistically analysed for correlations. Myxomycete abundance was positively correlated with numbers of soil amoebae, ciliates, total bacteria, myxobacteria and nematodes, but inversely correlated with numbers of dictyostelid cellular slime moulds. It was positively correlated with soil pH and level of potassium, and negatively correlated with moisture content, organic matter content, and nitrogen/phosphorus ratio. Calculations suggest that the biomass of myxomycetes in temperate soils is commonly 6.4–64 mg dry weight · m⁻².     

Seasonal population changes of myxomycetes and associated organisms in four woodland soils

Abstract The number of myxomycete plasmodium-forming units (PFUs) and associated microorganisms in soils from four woodland sites in the West of England were estimated at approximately monthly intervals over the course of a year, and other characteristics of the soils were determined. The numbers of PFUs were low except in an unusual high-phosphate woodland soil. This soil contained the fewest dictyostelid slime moulds. Conversely, the woodland soil with most dictyostelids contained the fewest PFUs. Naked soil amoebae were the most abundant phagotrophs. Myxomycetes were a comparatively small component of the phagotrophic community of the three typical woodland soils. Changes in the populations of naked amoebae, ciliates, myxomycetes and dictyostelids were sometimes synchronized with changes in the bacterial populations, which showed 9–35-fold seasonal variations in abundance at the four sites.     

Relationships between take-all,soil conduciveness to the disease,populations of fluorescent pseudomonads and nitrogen fertilizers

Take-all of wheat, caused by Gaeumannomyces graminis var tritici (Ggt), is reduced by ammoniacal fertilizers as compared to nitrate sources. This influence of nitrogen on the disease is only observed on nodal roots at flowering. But soil conduciveness to take-all, as measured in a soil bioassay, is modified earlier. Forty days after nitrogen application at early tillering, the NH₄-treated soil became less conducive than the NO₃-treated one. When nitrogen applications are done at sowing and at tillering, differences in disease propagation between the two soils are enhanced. Results from four years of experimentation show that when the level of natural soil inoculum is high, disease severity is reduced by ammonium, showing an effect on the parasitic phase of Ggt. At a low level of natural inoculum the effect of the source of nitrogen is mainly observed on the percent of infected plants, indicating that the saprophytic and preparasitic phases are affected. Rhizospheric bacterial populations increase from sowing to tillering, but differences on take-all conduciveness after tillering are not correlated with differences in the amounts of aerobic bacteria or fluorescent pseudomonads isolated from soils treated with different sources of nitrogen. Qualitative changes in fluorescent Pseudomonas spp. populations, like in vitro antagonism, are more likely to explain differences in soil conduciveness to take-all than are quantitative changes in this group. Nevertheless, the introduction of Ggt in a cropped soil leads to a greater increase in fluorescent pseudomonads populations than in total aerobic bacteria.The delay between reducing soil conduciveness and reducing disease in the field with ammonium nitrogen fertilization, the qualitative change of fluorescent pseudomonads populations and the role of necroses in rhizobacteria multiplication, provide information leading to our representation of a dynamic model based on the differentiation of the wheat root system into seminal and nodal roots.     

Population Dynamics of Active and Total Ciliate Populations in Arable Soil Amended with Wheat

Soil protozoa are characterized by their ability to produce cysts, which allows them to survive unfavorable conditions (e.g., desiccation) for extended periods. Under favorable conditions, they may rapidly excyst and begin feeding, but even under optimal conditions, a large proportion of the population may be encysted. The factors governing the dynamics of active and encysted cells in the soil are not well understood. Our objective was to determine the dynamics of active and encysted populations of ciliates during the decomposition of freshly added organic material. We monitored, in soil microcosms, the active and total populations of ciliates, their potential prey (bacteria and small protozoa), their potential competitors (amoebae, flagellates, and nematodes), and their potential predators (nematodes). We sampled with short time intervals (2 to 6 days) and generated a data set, suitable for mathematical modeling. Following the addition of fresh organic material, bacterial numbers increased more than 1,400-fold. There was a temporary increase in the number of active ciliates, followed by a rapid decline, although the size of the bacterial prey populations remained high. During this initial burst of ciliate growth, the population of cystic ciliates increased 100-fold. We suggest that internal population regulation is the major factor governing ciliate encystment and that the rate of encystment depends on ciliate density. This model provides a quantitative explanation of ciliatostasis and can explain why protozoan growth in soil is less than that in aquatic systems. Internally governed encystment may be an essential adaptation to an unpredictable environment in which individual protozoa cannot predict when the soil will dry out and will survive desiccation only if they have encysted in time.     

Population dynamics of active and total ciliate populations in arable soil amended with wheat

Soil protozoa are characterized by their ability to produce cysts, which allows them to survive unfavorable conditions (e.g., desiccation) for extended periods. Under favorable conditions, they may rapidly excyst and begin feeding, but even under optimal conditions, a large proportion of the population may be encysted. The factors governing the dynamics of active and encysted cells in the soil are not well understood. Our objective was to determine the dynamics of active and encysted populations of ciliates during the decomposition of freshly added organic material. We monitored, in soil microcosms, the active and total populations of ciliates, their potential prey (bacteria and small protozoa), their potential competitors (amoebae, flagellates, and nematodes), and their potential predators (nematodes). We sampled with short time intervals (2 to 6 days) and generated a data set, suitable for mathematical modeling. Following the addition of fresh organic material, bacterial numbers increased more than 1,400-fold. There was a temporary increase in the number of active ciliates, followed by a rapid decline, although the size of the bacterial prey populations remained high. During this initial burst of ciliate growth, the population of cystic ciliates increased 100-fold. We suggest that internal population regulation is the major factor governing ciliate encystment and that the rate of encystment depends on ciliate density. This model provides a quantitative explanation of ciliatostasis and can explain why protozoan growth in soil is less than that in aquatic systems. Internally governed encystment may be an essential adaptation to an unpredictable environment in which individual protozoa cannot predict when the soil will dry out and will survive desiccation only if they have encysted in time.     

Intermittent wetting of soils at high temperature reduces survival of the take-all fungus

Two pot experiments using naturally infested soil and a range of watering regimes were conducted to study the possible effect of level and frequency of wetting of hot soil (to simulate the period between growing seasons in Western Australia) on inoculum of the take-all fungus (Gaeumannomyces graminis var.tritici). In combination with the high soil temperatures, all watering regimes reduced infectivity and propagule number of the take-all fungus, this reduction being absent in dry soils.     

Soil conduciveness to take-all of wheat: Influence of the nitrogen fertilizers on the structure of populations of fluorescent pseudomonads

In a field cropped with wheat, a high and low level of soil conduciveness to take-all were induced by applying a nitrogen fertilizer with either calcium nitrate or ammonium sulphate. From these two soils, two representative populations of fluorescent pseudomonads were tested for their in situ behaviour. Take-all index and root dry weight were assessed on plants cropped in soils infested with Gaeumannomyces graminis var tritici (Ggt) and each bacterized with one of the isolates of fluorescent pseudomonads. The bacteria tested can be split into three groups: antagonists which reduce take-all, deleterious isolates which aggravate the disease and neutral without evident effect on the disease. The predominance of antagonistic fluorescent pseudomonads in the NH₄-treated soil and the predominance of deleterious ones in the NO₃-treated soil was confirmed after statistical analysis. The microbial impact on take-all must be more considered as the resulting effect of divergent activities of both rhizobacteria types than the only consequences of the presence of antagonistic pseudomonads. All the high cyanogenic pseudomonads were antagonists in situ and were more numerous in the NH₄-treated soil than in the NO₃-treated soil.     

Competition between soil bacteria and fusarium

Summary The phenomenon of competition has been characterized in liquid medium and sterile soil systems using a variety of soil bacteria andFusarium oxysporum f.cubense as test organisms. For most of the bacteria, suppression of the fungus was the result of a biologically induced nitrogen deficiency, this effect being reversed by the addition of excess inorganic nitrogen. High populations of competitors were found in two soils of neutral pH, but no isolates competed in the acid San Alejo loam.Agrobacterium radiobacter was able to compete when San Alejo loam was limed to about pH 6.6. Inhibition of the fungus by a number of gram-positive, spore-forming rods could not be accounted for in terms of competition for nutrients or by antibiotic production in artificial media.The competitive ability ofA. radiobacter when tested in twelve Central American soils was found to be related to pH in acid and neutral environments but was correlated with texture, organic-matter content and total nitrogen in soils of intermediate pH. In all soils where inhibition occurred, the competitive effect was overcome by additions of inorganic nitrogen. Excluding the group ofBacillus spp., the competitive ability of soil bacteria was related to the ability to develop in the absence of amino acids and growth factors but could not be correlated with growth rates of the bacteria in soil or liquid medium.It is suggested that competition for nutrients is a significant means of ecological control among members of the soil microflora and, together with competitive interactions for space and oxygen, may be the major factors governing the biological control of soil-borne fungi.The investigation was supported in part by a grant from the United Fruit Company. Agronomy Paper No. 471.     

Protozoan grazing of bacteria in soil—impact and importance

Interactions between bacteria and protozoa in soil were studied over 2-week periods in the field and in a pot experiment. Under natural conditions the total biological activity was temporarily synchronized by a large rainfall, and in the laboratory by the addition of water to dried-out soil, with or without plants. In the field, peaks in numbers and biomass of bacteria appeared after the rain, and a peak of naked amoebae quickly followed. Of the three investigated groups—flagellates, ciliates, and amoebae—only populations of the latter were large enough and fluctuated in a way that indicated a role as bacterial regulators. The bacterial increase was transient, and the amoebae alone were calculated to be able to cause 60% of the bacterial decrease. The same development of bacteria and protozoa was observed in the pot experiment: in the presence of roots, amoebic numbers increased 20 times and became 5 times higher than in the unplanted soil. In the planted pots, the amoebic increase was large enough to cause the whole bacterial decrease observed; but in the unplanted soil, consumption by the amoebae caused only one-third of the bacterial decrease.     

Interactions of Bacterial and Amoebal Populations in Soil Microcosms with Fluctuating Moisture Content

Sterilized soil samples (20 g of soil per 50-ml flask), amended with 600 μg of glucose-carbon and 60 μg of NH₄-N · g of dry soil⁻¹, were inoculated with bacteria (Pseudomonas paucimobilis) alone or with bacteria and amoebae (Acanthamoeba polyphaga). We used wet-dry treatments, which involved air drying the samples to a moisture content of approximately 2% and remoistening the samples three times during the 83-day experiment. Control treatments were kept moist. In the absence of amoebae, bacterial populations were reduced by the first drying to about 60% of the moist control populations, but the third drying had no such effect. With amoebae present, bacterial numbers were not significantly affected by the dryings. Amoebal grazing reduced bacterial populations to 20 to 25% of the ungrazed bacterial populations in both moisture treatments. Encystment was an efficient survival mechanism for amoebae subjected to wet-dry cycles. The amoebal population was entirely encysted in dry soil, but the total number of amoebae was not affected by the three dryings. Growth efficiencies for amoebae feeding on bacteria were 0.33 and 0.39 for wet-dry and constantly moist treatments, respectively, results that compared well with those previously reported for Acanthamoeba spp.     

Some microbiological aspects of surface and buried soils on Mt. Kenya,East Africa

When numbers of microorganisms in profiles of surface and buried horizons on Mt. Kenya were estimated by dilution plate counting they were found to be consistently lower than those from other soils in different geographical regions as determined from the literature. The lower numbers are probably characteristic of the poorly weathered Inceptisols and Entisols usually found in the alpine zone.The A horizons of the soils studied contain proportionately fewer of the total numbers of organisms in the A, B and C horizons than observed in most soils. Estimates of organic matter were positively correlated with numbers of fungi and bacteria in the A horizons. However, other factors such as severe drought, high light intensity, low temperatures, diurnal frost heaving, low pH and paucity of clay minerals may be significant factors in suppressing the more luxuriant growth of microbial populations.Organic and inorganic horizons of buried soils sometimes exhibit higher counts of microorganisms than adjacent horizons of surface soils. However, the bacteria and fungi even in deeply buried paleosols exhibit characteristics of an unspecialized heterotrophic population. Among fungi the species were obviously the same as those isolated from one or more of the overlying horizons. Taken in conjunction with other evidence from the profiles it is concluded that the microorganisms were introduced and represent a transient or non-active population. Contamination of buried organic horizons may influence the estimated age as assessed by radiocarbon dating.     

新疆阿克苏地区盐碱地粘细菌分离鉴定

【目的】采用传统的纯培养技术,分离新疆阿克苏地区典型的盐碱地中的粘细菌,并初步分析盐碱地土壤中可培养粘细菌资源的多样性。【方法】采用传统的水琼脂法、滤纸法和改良的土壤浸出液法分离新疆阿克苏地区25份盐碱地的粘细菌。结合分析土样的酸碱度、含盐量、地理位置及其植被分布情况分析新疆阿克苏地区盐碱地粘细菌资源多样性。【结果】共分离到58株粘细菌,它们被鉴定为:粘球菌属(Myxococcus)33株;珊瑚球菌属(Corallococcus)14株;孢囊杆菌属(Cystobacter)6株;堆囊菌属(Sorangium)2株;侏囊菌属(Nannocystis)2株;多囊菌属(Polyangium)1株。其中粘球菌抗逆性强,分离的菌株数最多,在pH值7.5-8.5范围的盐碱地中普遍存在;其次为珊瑚球菌属;而侏囊菌属、多囊菌属的菌株较少见。【结论】新疆阿克苏地区盐碱地粘细菌多样性不高,可能受分离纯化方法、含盐量以及土壤性质影响较大。     

Harvester ant nests, soil biota and soil chemistry

Many ant species accumulate organic debris in the vicinity of their nests. These organic materials should provide a rich resource base for the soil biota. We examined the effect of harvester ant nests (Pogonomyrmex barbatus) on the soil community and soil chemistry. Ant nest soils supported 30-fold higher densities of microarthropods and 5-fold higher densities of protozoa than surrounding, control soils. The relative abundances of the major groups of protozoa differed as well: amoebae and ciliates were relatively overrepresented, and flagellates underrepresented, in ant nest versus control soils. Densities of bacteria and fungi were similar in the two soil types. Concentrations of nitrate, ammonium, phosphorus, and potassium were significantly higher in ant nest soils, while concentrations of magnesium, calcium, and water were similar in nest and control soils. Ant nest soils were marginally more acidic than controls. The results demonstrate that P. barbatus nests constitute a significant source of spatial heterogeneity in soil biota and soil chemistry in arid grasslands. Received: 17 March 1997 / Accepted: 10 June 1997     

Occurrence and activity of phosphate-solubilizing fungi from coconut plantation soils

Summary The occurrence of phosphate-solubilizing fungi in coconut plantation soil types was investigated. The laterite, alluvial and clayey soils harboured more of the P-solubilizing fungi than the sandy soils. The isolated P-solubilizing fungi solubilized 26 to 74 per cent of the tricalcium phosphate in 5 to 15 days. The competitive saprophytic ability of the active P-solubilizing fungi in soil varied between the isolates. Eight fungi with high P-solubilizing capacity and high competitive saprophytic ability were recognised. They have better capacity to survive in soil and express their role in P-solubilization.Contribution No. 291 of the Central Plantation Crops Research Institute, Kasaragod, India     

Nitrification in some tropical soils

Summary Nitrification of soil N in 8 mineral and 2 histosols having a wide range in pH (3.4 to 8.6), organic C (1.22 to 22.70%) and total N (0.09 to 1.20%) was studied by measuring nitrate fromation under aerobic incubation of the soil samples at 30°C for 4 weeks. The amounts of NO₃-N produced in the soils varied from 0 to 123 μg/g of soil. Soil N in the two acid sulfate soils and one other acid soil did not nitrify under conditions that stimulate nitrification. Soils having pH more than 6.0 nitrified at a rapid rate and released NO₃-N ranging from 98 to 123 μg/g. The two organic soils differed considerably in their capacity to nitrify though the total amounts of mineral N released were similar in these soils. The amounts of NO₃-N formed in the soils was highly positively correlated with the soil pH but was not significantly correlated with the organic C of total N content of the soils. Statistical analysis also showed that nitrate formation was not significantly correlated with soil pH in soils having pH higher than 6.0.     

Natural suppression of take-all disease of wheat in Montana soils

This research was initiated to determine whether soils suppressive to take-all of wheat caused by Gaeumannomyces graminis var. tritici (Ggt) occur in Montana, and to identify the organisms most likely involved in this suppression. From an initial screening of eight soils collected from different wheat growing areas of Montana, two were highly suppressive to take-all. Microbial characterization of these soils indicated that different mechanisms were involved in the suppression. In Larslan soil, mycoparasitism appeared to be the main mechanism. Two different fungi with exceptional ability to reduce the severity of take-all were isolated from this soil. One of these fungi could parasitize the hyphae of Ggt. Field tests with these fungi in Ggt infested soil showed increases of over 100% in both harvestble tillers and grain yield as compared to treatments without these two fungi. In tests with 48 different bacteria and 10 actinomycetes from Larslan soil, none were able to consistently reduce severity of take-all alone, or in mixtures. In Toston soil, antibiosis by actinomycetes and perhaps the involvement of Pseudomonas spp. in production of antibiotics and/or siderophores appeared to be the most likely mechanisms involved in take-all suppression. Increases in shoot dry weight over that in the Ggt infested control using mixtures of pseudomonads and actinomycetes ranged from 25% to 87%. Actinomycetes added individually or in mixtures to soil infested with Ggt consistently reduced the severity of the disease to a greater extent than did mixtures of Pseudomonas spp.     

Wheat Cultivar-Specific Selection of 2,4-Diacetylphloroglucinol-Producing Fluorescent <Emphasis Type="Italic">Pseudomonas</Emphasis> Species from Resident Soil Populations

An emerging body of evidence indicates a role for plant genotype as a determinant of the species and genetic composition of the saprophytic microbial community resident to the rhizosphere. In this study, experiments were conducted to determine the capacity of five different wheat cultivars to enhance resident populations and support introduced strains of 2,4-diacetylphloroglucinol (2,4-DAPG)-producing fluorescent pseudomonads, a group of bacteria known to provide biological control of several soilborne diseases. When soils were cropped with three successive 28-day growth cycles of wheat, the 2,4-DAPG-producing strains were consistently recovered from the rhizosphere of the cultivar Lewjain, and commonly were present at populations higher than those recovered from other wheat cultivars. Based on restriction fragment length polymorphism and sequence analyses of phlD, a key gene involved in 2,4-DAPG production, two previously undefined phlD+ genotypes, referred to as genotypes PfZ and PfY, were discovered. Wheat cultivar Lewjain was the primary source of genotype PfY while cultivar Penawawa yielded the majority of genotype PfZ. Based on 16S rDNA sequence analysis, both new phlD genotypes were classified as P. fluorescens. Comparison of the rhizosphere competence of 2,4-DAPG-producing P. fluorescens Q2-87 (genotype B) and P. fluorescens LR3-A28 (genotype PfY) showed that both strains persisted at similar populations in the rhizosphere of all cultivars tested over a 30 day period when introduced as a seed inoculant. However, when strain LR3-A28 was applied as a soil inoculant, this strain was recovered at higher populations from the rhizosphere of wheat cultivar Lewjain than from the rhizospheres of two other cultivars. No cultivar effects were shown for strain Q2-87. Collectively, these results add further to evidence indicating a degree of specificity in interactions between plant cultivars and specific members of the saprophytic microbial community. Furthermore, as 2,4-DAPG-producing fluorescent Pseudomonas spp. have a central role in the spontaneous reduction in severity of take-all disease of wheat in response to continuous wheat monoculture, we postulate that the use of specific cultivars, such as Lewjain, which possess a superior capacity to enhance resident soil populations of these bacteria may have potential to reduce the length of the monoculture period required to induce natural suppressiveness of soils toward this disease.     

Density and diversity of protozoa in some arid Australian soils

This is the first extensive study of soil protozoa of arid lands. Twenty-six samples from litters, soils, termitaria, and a cyanobacterial crust, collected from central and south Australian arid lands, were analyzed for numbers and species of gymnamoebae, ciliates, and testacea. Amoebae ranged from 1,000-5,000/g of material, and were two orders of magnitude more abundant than ciliates. Both groups increased in abundance and species richness from bare soils through spinifex to mulga to chenopod vegetations. Testacea ranged 900-5,000/g with similar species richness throughout vegetations, but reached 11,900/g with a doubling of species in a refugium in Kings Canyon. The most prevalent species of amoebae, ciliates, and testacea were taxa associated with ephemeral and disturbed habitats (r-selection). The cyanobacterial crust might be considered a micro-refugium because it contained a number of non-encysting protozoa, including Thecamoeba sp. and Nassula picta, feeding on cyanobacterial filaments. The numbers and species richness of protozoa under shrubs were greater than in bare soils, supporting the resource island hypothesis that desert plants create soil heterogeneity by localizing soil fertility under their canopies.     

Regulation of Fusarium oxysporum population introduced into soil: the amoebal predation hypothesis

Abstract Inoculation of fungi into soil has been suggested for biological control of plant diseases. The aim of our work was to test the ability of protozoa to reduce the density of introduced fungal populations. The survival of Fusarium oxysporum in non-sterile soil was studied after introduction at densities of: 1 × 10⁴, 1 × 10⁶ and 5 × 10⁷ cfu/g soil. The dynamics of protozoa were also followed. The fungal populations remained close to the initial inoculation densities and did not induce the growth of indigenous protozoa. A bacterial population ( Enterobacter aerogenes ) was used to promote and stimulate the predatory activity of amoebae. Then, after simultaneous inoculation with bacteria and fungi, the density of protozoa increased but this had no effect on the fungal population, although some amoebae are able to feed on small fungal propagules such as conidia. The physiological state of Fusarium in soil and intraspecific competition seem to be more important in regulating introduced fungal populations than amoebal predation. We conclude that the regulation of bacterial and fungal populations in soil depend on different mechanisms.     

Influence of plant species on population dynamics, genotypic diversity and antibiotic production in the rhizosphere by indigenous Pseudomonas spp

The population dynamics, genotypic diversity and activity of naturally-occurring 2,4-diacetylphloroglucinol (DAPG)-producing Pseudomonas spp. was investigated for four plant species (wheat, sugar beet, potato, lily) grown in two different soils. All four plant species tested, except lily and in some cases wheat, supported relatively high rhizosphere populations (5 x 10(4) to 1 x 10(6) CFU/g root) of indigenous DAPG-producing Pseudomonas spp. during successive cultivation in both a take-all suppressive and a take-all conducive soil. Although lily supported on average the highest population densities of fluorescent Pseudomonas spp., it was the least supportive of DAPG-producing Pseudomonas spp. of all four plant species. The genotypic diversity of 492 DAPG-producing Pseudomonas isolates, assessed by Denaturing Gradient Gel Electrophoresis (DGGE) analysis of the phlD gene, revealed a total of 7 genotypes. Some of the genotypes were found only in the rhizosphere of a specific plant, whereas the predominant genotypes were found at significantly higher frequencies in the rhizosphere of three plant species (wheat, sugar beet and potato). Statistical analysis of the phlD(+) genotype frequencies showed that the diversity of the phlD(+) isolates from lily was significantly lower than the diversity of phlD(+) isolates found on wheat, sugar beet or potato. Additionally, soil type had a significant effect on both the phlD(+) population density and the phlD(+) genotype frequencies, with the take-all suppressive soil being the most supportive. HPLC analysis further showed that the plant species had a significant effect on DAPG-production by the indigenous phlD(+) population: the wheat and potato rhizospheres supported significantly higher amounts of DAPG produced per cell basis than the rhizospheres of sugar beet and lily. Collectively, the results of this study showed that the host plant species has a significant influence on the dynamics, composition and activity of specific indigenous antagonistic Pseudomonas spp.