A comparison of microbial-feeding nematodes and protozoa in the rhizosphere of different plants

Summary The biomass of microbial-feeding nematodes and protozoa was measured in the rhizospheres of peas, barley, grass and turnips grown for 10 weeks in pots containing a clay-loam soil; in the rhizospheres of peas and barley grown for 3 weeks in a sandy soil; and in the rhizosphere of barley grown for 11 weeks in an unfertilised and a fertilised clay-loam soil. The nematode biomass was consistently larger in the rhizosphere of all plants in both soils than in the bulk soil, but the protozoa biomass showed a rhizosphere effect only under pea and fertilised barley. The biomass of nematodes in the rhizosphere (1.2–22.3 g dry weight g^-1 dry soil) was greater than the biomass of protozoa (0.1–3.2 g g^-1), and greater under pea>barley>grass>turnip. It is suggested that nematodes are more able to exploit low bacterial densities than protozoa and that they initially migrate into the rhizosphere from the bulk soil. In samples of potato rhizosphere from field-grown plants, the nematode biomass was also greater than the active and total protozoan biomass. It is argued that in the rhizosphere the biomass of microbially feeding nematodes exceeds that of protozoa and that nematodes are more important in terms of nutrient cycling.     

Seasonal dynamics of amoebae in the root canopy of Zygophyllum dumosum in the Negev Desert, Israel

Water availability and nutrient limitation are the most important driving forces in hot desert ecosystems, determining vegetation cover and biological activity. In this study the amoeba population was determined in the upper soil layer (0-10 and 10-20 cm) under and between 4 individual Zygophyllum dumosum shrubs. The soil samples were taken from the different layer beneath the shrub and between the shrubs (control) along four seasons. Total number of amoeba was significantly higher in the Z. dumosum root canopy (2935 individual/g soil) in comparison to control samples (625 individual/g soil). Amoebal density was highest at 10-20 cm during the winter, spring and summer. In autumn, during heavy dewfall amoebal density was greatest in the top 10 cm of soil.     

Trophic interactions between rhizosphere bacteria and bacterial feeders influenced by phosphate and aphids in barley

The aim was to study the effects of P fertilization and leaf aphid attack on the trophic interactions of bacteria and bacterial feeders in the rhizospheres of barley plants. The density of protozoa peaked in the rhizospheres of plants fertilized with N and P, whereas nematodes peaked in the rhizospheres of plants to which only N had been added. Fingerprinting of bacterial communities by length heterogeneity polymerase chain reaction revealed differences in community structure between NP rhizospheres and N rhizospheres as well as aphid-related differences within N rhizospheres. Specifically, α-proteobacteria increased with P addition. To evaluate if differences in bacteria in terms of their quality as food could partly explain the observed differences in protozoan and nematode abundances, growth of the flagellate Cercomonas sp. was assessed with 935 bacteria isolated from the different treatments. This assay indicated that bacterial isolates were of higher food quality to Cercomonas sp. in NP than in N rhizospheres when plants were subjected to aphid attack. Bacteria of high and low food quality for Cercomonas sp., respectively, were fed to the nematode Caenorhabditis elegans and larval production examined. α-Proteobacteria supported the growth of Cercomonas sp. well, whereas Actinobacteria did not. In contrast, C. elegans reproduced poorly on most α-proteobacteria but were able to reproduce well on some Actinobacteria. These results suggest that the different response of protozoa and nematodes to P addition could be mediated through a food quality-related change in community composition of bacteria and that leaf aphid attack may interfere with nutrient effects on bacterial assemblages of rhizospheres.     

Localisation of nitrate in the rhizosphere of biochar-amended soils

A wheat seedling rhizobox approach was used to differentiate between the rhizosphere and non-rhizosphere (bulk) soil amended with low and high rates of biochar (20 and 60 t ha⁻¹ vs. control). Nitrate (NO₃⁻) was added as the main nitrogen (N) source because emerging biochar research points to reduced NO₃⁻ loss through leaching and gaseous loss as nitrous oxide. The rhizosphere under the different treatments were distinct (P = 0.021), with greater soil-NO₃⁻ and biochar-NO₃⁻ contents in the high biochar treatment. Biochar addition increased wheat root length ratio (P = 0.053) and lowered root N uptake (P = 0.017), yet plant biomass and N content were similar between treatments. The results indicate localisation of NO₃⁻ within the rhizosphere of biochar-amended soils which has implications for NO₃⁻ loss and improved nitrogen use efficiency.     

Protease and deaminase activities in wheat rhizosphere and their relation to bacterial and protozoan populations

Protease and deaminase activities and population dynamics of bacteria and protozoa were measured in the rhizosphere of wheat to study their interactions with the mineralization of nitrogen. The experimental design allowed the separation of roots and soil material by means of a gauze. The most pronounced rhizosphere effect was detected for all the measured variables in the soil closest to the gauze. The number of bacteria was significantly higher in the presence than in the absence of plants up to 4 mm away from the soil-root interface and the closer to this interface the higher the number. Protozoan and bacterial population dynamics were positively correlated; generally, populations of flagellates and amoebae were comparable and their sum accounted for the population of total protozoa. For both enzyme activities the rhizosphere effect extended up to 2 mm away from the soil-root interface. The histidinase activity was of bacterial origin, while it is likely that bacteria, protozoa and root hair all contributed to the overall caseinase activity. Decomposition of root exudates and native organic matter in the rhizosphere, reflected by a growing microbial population, is associated with nitrogen mineralization through increases in caseinhydrolysing and L-histidine-deaminating activities. The adopted soil-plant microcosm is suitable for the study of the rhizosphere effect over time of incubation and distance gradient from the soil-root interface.     

Survival and cell culturability of biocontrol Pseudomonas fluorescens CHA0 in the rhizosphere of cucumber grown in two soils of contrasting fertility status

 The effect of cucumber roots on survival patterns of the biocontrol soil inoculant Pseudomonas fluorescens CHA0-Rif was assessed for 22 days in two non-sterile soils, using a combination of total immunofluorescence cell counts, Kogure's direct viable counts and colony counts on plates containing rifampicin. In Eschikon soil (high fertility status for cucumber), CHA0-Rif persisted as culturable cells in bulk soil and in the rhizosphere, but colony counts were lower than viable counts and total cell counts inside root tissues. The occurrence of viable but non-culturable (VBNC) cells inside root tissues (5 log cells g^–1 root) was unlikely to have resulted from the hydrogen peroxide treatment used to disinfect the root surface, as hydrogen peroxide caused the death of CHA0-Rif cells in vitro. In Siglistorf soil (low fertility status for cucumber), the inoculant was found mostly as non-culturable cells. Colony counts and viable counts of CHA0-Rif were similar, both in bulk soil and inside root tissues, whereas in the rhizosphere viable counts exceeded colony counts at the last two samplings (giving about 7 log VBNC cells g^–1). In conclusion, soil type had a significant influence on the occurrence of VBNC cells of CHA0-Rif, although these cells were found in root-associated habitats (i.e. rhizosphere and root tissues) and not in bulk soil. Received: 12 November 1999     

Diversity and antagonistic potential of Pseudomonas spp. associated to the rhizosphere of maize grown in a subtropical organic farm

Pseudomonas spp. are one of the most important bacteria inhabiting the rhizosphere of diverse crop plants and have been frequently reported as biological control agents (BCAs). In this work, the diversity and antagonistic potential of Pseudomonas spp. in the rhizosphere of maize cultivars Nitroflint and Nitrodent grown at an organic farm in Brazil was studied by means of culture-dependent and -independent methods, respectively. Sampling of rhizosphere soil took place at three different stages of plant development: 20, 40 and 106 days after sowing. A PCR-DGGE strategy was used to generate specific Pseudomonas spp. fingerprints of 16S rRNA genes amplified from total community rhizosphere DNA. Shifts in the relative abundance of dominant populations (i.e. PCR-DGGE ribotypes) along plant development were detected. A few PCR-DGGE ribotypes were shown to display cultivar-dependent relative abundance. No significant differences in diversity measures of DGGE fingerprints were observed for different maize cultivars and sampling times. The characterisation and assessment of the antagonistic potential of a group of 142 fluorescent Pseudomonas isolated from the rhizosphere of both maize cultivars were carried out. Isolates were phenotypically and genotypically characterised and screened for in vitro antagonism towards three phytopathogenic fungi and the phytopathogenic bacterium Ralstonia solanacearum. Anti-fungal activity was displayed by 13 fluorescent isolates while 40 isolates were antagonistic towards R. solanacearum. High genotypic and phenotypic diversity was estimated for antagonistic fluorescent Pseudomonas spp. PCR-DGGE ribotypes displayed by antagonists matched dominant ribotypes of Pseudomonas DGGE fingerprints, suggesting that antagonists may belong to major Pseudomonas populations in the maize rhizosphere. Antagonists differing in their genotypic and phenotypic characteristics shared the same DGGE electrophoretic mobility, indicating that an enormous genotypic and functional diversity might be hidden behind one single DGGE band. Cloning and sequencing was performed for a DGGE double-band which had no corresponding PCR-DGGE ribotypes among the antagonists. Sequences derived from this band were affiliated to Pseudomonas stutzeri and P. alcaligenes 16S rRNA gene sequences. As used in this study, the combination of culture-dependent and -independent methods has proven to be a powerful tool to relate functional and structural diversity of Pseudomonas spp. in the rhizosphere.     

Magnitude,direction, and drivers of rhizosphere effect on soil nitrogen and phosphorus in global agroecosystem

The rhizosphere is the most active soil area for material transformation and energy flow of soil, root, and microorganism, which plays an important role in soil biochemical cycling. Although the rhizospheric nitrogen (N) and phosphorous (P) were easily disturbed in the agroecosystem, the effects of rhizosphere on the dynamics of soil N and P cycling have not yet been systematically quantified globally. We summarized the magnitude, direction, and driving forces of rhizosphere effects on agroecosystem's N and P dynamics by 1063 observations and 15 variables from 122 literature. Rhizosphere effects increased available N (AN, 9%), available P (AP, 11%), and total P (TP, 5%), and decreased nitrate N (NO₃–N, 18%) and ammonia N (NH₄–N, 16%). The effect of rhizosphere on total N (TN) was not significant. These effects improved AN in tropical (12%) and subtropical (14%) regions. The effect of rhizosphere on TP was greater under subtropical conditions than in other climates. The most substantial effects of the rhizosphere on TP and AP were observed under humid conditions. Rhizosphere effects increased AN and AP in vegetables more than in other crop systems. Application of N > 300 kg ha⁻¹ had the most significant and positive rhizosphere effects on TN and AN. P application of 100–150 kg ha⁻¹ had the greatest rhizosphere effects on TP and AP. These effects also improved the microbial (biomass N and P) and enzymatic aspects (urease, acid phosphatase, and alkaline phosphatase) of soil P and N cycling. Structural equation modeling suggested that aridity indices, fertilizer application rate, soil pH, microbial biomass, and soil enzymes strongly influence the magnitude and direction of the rhizosphere's effect on the P and N cycles. Overall, these findings are critical for improving soil nutrient utilization efficiency and modeling nutrient cycling in the rhizosphere for agricultural systems.     

The mycorrhizal fungus (Glomus intraradices) affects microbial activity in the rhizosphere of pea plants (Pisum sativum)

Pea plants were grown in γ-irradiated soil in pots with and without addition of the AM fungus Glomus intraradices at sufficient N and limiting P. Depending on the growth phase of the plant presence of AM had negative or positive effect on rhizosphere activity. Before flowering during nutrient acquisition AM decreased rhizosphere respiration and number of protozoa but did not affect bacterial number suggesting top-down regulation of bacterial number by protozoan grazing. In contrast, during flowering and pod formation AM stimulated rhizosphere respiration and the negative effect on protozoa decreased. AM also affected the composition of the rhizosphere bacterial community as revealed from DNA analysis (DGGE). With or without mycorrhiza, rhizosphere respiration was P-limited on very young roots, not nutrient limited at more mature roots and C-limited at withering. This suggests changes in the rhizosphere community during plant growth also supported by changes in the bacteria (DGGE).     

Plant nitrogen uptake drives rhizosphere bacterial community assembly during plant growth

When plants establish in novel environments, they can modify soil microbial community structure and functional properties in ways that enhance their own success. Although soil microbial communities are influenced by abiotic environmental variability, rhizosphere microbial communities may also be affected by plant activities such as nutrient uptake during the growing season. We predicted that during the growing season, plant N uptake would explain much of the variation in rhizosphere microbial community assembly and functional traits. We grew the invasive C₃ grass Bromus tectorum and three commonly co-occurring native C₃ grasses in a controlled greenhouse environment, and examined rhizosphere bacterial community structural and functional characteristics at three different plant growth stages. We found that soil N availability and plant tissue N levels strongly correlated with shifts in rhizosphere bacterial community structure. It also appeared that the rapid drawdown of soil nutrients in the rhizosphere during the plant growing season triggered a selection event whereby only those microbes able to tolerate the changing nutrient conditions were able to persist. Plant N uptake rates inversely corresponded to microbial biomass N levels during periods of peak plant growth. Mechanisms which enable plants to influence rhizosphere bacterial community structure and function are likely to affect their competitive ability and fitness. Our study suggests that plants can alter their rhizosphere microbiomes through influencing nutrient availability. The ways in which plants establish their rhizosphere bacterial communities may now be viewed as a selection trait related to intrinsic plant species nutrient demands.     

不同茬口设施番茄栽培的根圈基质中酶活性与养分效应

研究了番茄设施有机基质栽培的正茬、迎茬、连茬根圈生物性状和理化性状的变化, 分析了不同茬口对番茄栽培基质中酶活性、有机质含量和主要养分含量的影响。结果表明, 番茄连茬栽培基质的蛋白酶、磷酸酶、脲酶和过氧化氢酶活性下降, 有机质、全氮、铵态氮、碱解氮、全磷、速效磷、全钾及速效钾等主要养分含量降低, 而正茬栽培对基质酶活性、有机质和主要养分含量的影响较小。基质酶活性与理化性质的相关性分析表明, 番茄有机基质栽培中, 连茬、迎茬和正茬条件下基质的化学性质与生物学性质密切相关, 从而导致了番茄连茬基质微生态环境的改变。所有指标的相关性中, 脲酶与有机质和主要养分含量的相关系数最大, 说明脲酶活性是影响基质有机质含量和主要养分的重要因子。各茬口中, 速效钾、速效磷、全钾和过氧化氢酶活性的可塑性指数均最大, 不同茬口基质各指标的可塑性指数表现为新基质的最大、正茬次之、连茬最小, 说明连茬基质不适宜于番茄生长, 而正茬基质的生物和理化性质与新基质差异不大, 适宜于番茄生长发育。     

Protozoa from aboveground and ground soils of a tropical rain forest in Puerto Rico

Decomposition occurs in the aboveground and ground litter and soils of tropical rain forests, but little is known about the protozoa that stimulate bacterial activity and turnover. I examined litter and ground soils, epiphytic bryophyte soils on tree trunks and branches, and adventitious roots of lianas attached to tree trunks, within 2 m above ground in the Luquillo Experimental Forest, within the Caribbean National Forest, Puerto Rico. Amoebae numbered 69,000–170,000, ciliates 1000–25,000, and testate amoebae 58,000–190,000 g⁻¹ dry wt. of litter, but were reduced by 0.25–0.5 of these abundances in the underlying soils. In the aboveground soils, amoebae numbered 64,000–145,000, ciliates 1000–8000, and testate amoebae 84,000–367,000 g⁻¹ dry wt. of soil. Eighty species of ciliates and 104 species of testate amoebae were found. About 50% of the individuals in ciliate and 33% in testate amoebae populations were small r-selected species, illustrating that functional differences between species determine community composition. Although protozoan numbers are best described as “protozoan potential” because many individuals may be dormant, the high moisture content of tropical rain forest litter and soils suggest almost continually connected soil water films (necessary for protozoan transport), and together with the large numbers and biodiversity of protozoa, suggest that a major proportion of these protozoa contribute to the bacterial decomposition channel of organic matter.     

农牧复合生态系统氮磷营养循环与提高养分利用率的研究

本项实验研究(1988-1991)通过田间试验、饲喂试验和农牧结合的生态农业实体运营，使农田亚系统的氮肥利用率达到56.9%,磷36.3%，整个农牧复合系统的养分回收率氮49.9%，磷37.6%，比单一种植业的利用效率提高10-20个百分点，这在太行山前平原资源节约型生态农业建设中，是一种高效益的生态农业示范模式，生产上已开始推广应用。     

Evidence for gluconic acid production by Enterobacter intermedium as an efficient strategy to avoid protozoan grazing

The effect of pure gluconic acid and of gluconic-acid-producing bacteria on the activity of three protozoan species, Colpoda steinii (a ciliate), Vahlkampfia sp. (an amoeba) and Neobodo designis (a flagellate), was determined in vitro and in soil microcosms. Pure gluconic acid was shown to mediate disappearance of active cells, due to encystment and/or death of protozoa, at 0.15 mM in saline medium. Similarly, the presence of gluconic acid inhibited excystment of the three protozoa tested. Enterobacter intermedium 60-2G (Wt), a gluconic acid-producing rhizobacterium, elicited the same effects on protozoa when co-cultured in the presence of 5 g L⁻¹ glucose. However, the effect was not observed when glucose was omitted from the medium. Similarly, a pqqA isogenic mutant strain, unable to produce gluconic acid from glucose, exhibited a reduced effect on protozoan activity. Rhizosphere-microcosm studies performed with wheat (Triticum aestivum L.) confirmed the reduced ability of the pqqA mutant to limit protozoa reproduction compared to the Wt strain. Since the sodium salt of gluconic acid did not cause any significant stress to protozoa and considering that addition of 50 mM Tris-Cl (pH 7.2) abolished the deleterious effect of gluconic acid, acidification of the medium appeared as the key factor that induced encystment/death of protozoa. We propose that production and excretion of gluconic acid should be considered an efficient mechanism evolved by bacteria to escape, tolerate or defend themselves against protozoan grazing in rhizosphere environments.     

The model predator Acanthamoeba castellanii induces the production of 2,4, DAPG by the biocontrol strain Pseudomonas fluorescens Q2-87

Fluorescent pseudomonads show great potential as biological control agents due to their capability to produce antifungal toxins such as 2,4-diacetylphloroglucinol (DAPG). DAPG is synthesized from the precursor monoacetyl-phloroglucinol (MAPG) and its production depends on the metabolic state of the bacteria as well as on their interaction with other organisms. In this study we show that Pseudomonas fluorescens responds to the bacterivorous amoeba Acanthamoeba castellanii by upregulating the production of phloroglucinol derivates in a density-dependent manner. Living amoebae caused moreover a distortion of the MAPG to DAPG balance in favor of the latter, suggesting that amoebae may interfere with the first steps of DAPG synthesis. Predator-prey interactions appear thus to be an important factor for the regulation of antibiotics production in biocontrol microorganisms.     

土壤原生动物 ——研究方法及其在土传病害防控中的作用

原生动物是原生生物的一种,是土壤食物网中的消费者,能捕食细菌和真菌等其他微生物.除了对土壤微生物群落和物质循环产生重要影响外,根际原生动物与细菌、真菌等土壤微生物共同组成生物网络屏障,并在抵御土传病原菌入侵作物根系的过程中发挥着重要作用.然而,相对于根际有益细菌和有益真菌,国内外关于原生动物防控土传病害的效果及作用机制...     

Techniques for observing phosphorus mobilization in the rhizosphere

Summary The techniques described here were developed to visualize the dissolution of sparingly soluble calcium phosphate and the presence of root-borne phosphatase in the rhizosphere. Newly formed calcium phosphate precipitate was suspended in agar containing other essential nutrients. The agar was poured into Petri dishes and acrylglass boxes and was used as a growth medium for seedlings of wheat, rape, buckwheat, and rice. With NH ₄ ⁺ applied as the N source, the precipitate dissolved in the root vicinity and this was attributed to acidification. No dissolution occurred with NO ₃ ^– as the N source. The release of a neutral phosphatase from roots was verified by embedding the roots of young seedlings in agar at pH 7 containing phenolphthalein phosphate. After pH was raised to the alkaline range by adding sodium hydroxide, the agar around the roots turned purple, especially around the roots of P-deprived plants. The most intensive phosphatase activity was found in apical root regions.     

Grassland plants affect dissolved organic carbon and nitrogen dynamics in soil

Dissolved organic carbon (DOC) and nitrogen (DON) are central in many nutrient cycles within soil and they play an important role in many pedogenic processes. Plants provide a primary input of DOC and DON into soil via root turnover and exudation. Under controlled conditions we investigated the influence of 11 grass species alongside an unplanted control on the amount and nature of DOC and DON in soil. Our results showed that while the presence of plants significantly increases the size of a number of dissolved nutrient pools in comparison to the unplanted soil (e.g. DOC, total phenolics in solution) it has little affect on other pools (e.g. free amino acids). Grass species, however, had little effect on the composition of the DOC, DON or inorganic N pools. While the concentration of free amino acids was the same in the planted and unplanted soil, the flux through this pool was significantly faster in the presence of plants. The presence of plants also affected the biodegradability of the DOC pool. We conclude that while the presence of plants significantly affects the quantity and cycling of DOC and DON in soil, comparatively, individual grass species exerts less influence.     

Testate amoebae (protista) of an elevational gradient in the tropical mountain rain forest of Ecuador

We investigated the species composition of testate amoebae at three altitudes (1000, 2000 and 3000 m) and two horizons (L and F/H/Ah) of a tropical mountain rain forest in southern Ecuador. A total of 135 species and intraspecific taxa of testate amoebae were found (36 samples). Rarefaction plots suggest that only few more species are to be expected. Species number in the L horizon was at a maximum at 2000 m (109 species) and similar at 1000 and 3000 m (75 and 71 species, respectively). Species numbers in the F/H/Ah horizon were 29, 67 and 48 at 1000, 2000 and 3000 m, respectively. The density of testate amoebae in the L horizon increased significantly in the order 1000<3000<2000 m with 3147±129, 9198±144 and 12,497±1317 ind. g⁻¹ dry matter and in the F/H/Ah horizon with 176±25, 3118±97 and 4986±102 ind. g⁻¹ dry matter, respectively. The significant Horizon×Elevation interaction reflects the exceptionally low abundance of testate amoebae in the Ah horizon at 1000 m. The results suggest that species richness of testate amoebae does not decrease continuously with elevation; rather, it peaks at an intermediate elevation. Further, the data suggest that diversity, but not density of testate amoebae in tropical forests exceeds that in temperate forests. Morphological features of testate amoebae reflected semiaquatic habitat conditions. The great majority of testate amoebae species of the studied tropical mountain rain forests are geographically widespread, including temperate regions; however 9 of the species (i.e. 6.7%) are considered tropical, some of these species likely represent Gondwana relicts.