Suppression of Meloidogyne javanica by Pseudomonas aeruginosa IE-6S in tomato: the influence of NaCl, oxygen and iron levels

Understanding the environmental factors that influence the suppression of disease-suppressive strains of Pseudomonas fluorescens is an essential step toward improving the level and reliability of their biocontrol activity. A 0.8 M NaCl concentration was optimal for in vitro survival and growth of IE-6S⁺ while, nematicidal activity by IE-6S⁺ was maximal when the bacterium was exposed to 0.4 M NaCl. The bacterium was highly sensitive to high (1.6 M) NaCl concentration. Culture filtrate of the bacterium resulting from the medium supplemented with 0.2 or 0.4 M NaCl showed the presence of secondary metabolite, hydrogen cyanide (HCN). Soil amendment with IE-6S⁺ alone or in conjunction with up to 0.8 M NaCl enhanced bacterial efficacy towards Meloidogyne javanica, the root-knot nematode. Soil amendment with NaCl up to 0.8 M also resulted in enhanced bacterial rhizosphere colonization and growth of tomato seedlings. Protein content of the shoot was reduced when soil was amended with 1.6 M NaCl. Inner root establishment of the bacterium was greatly affected in the soils treated with 1.6 M NaCl. Under in vitro conditions, IE-6S⁺ showed enhanced growth when kept at ambient oxygen conditions while the growth of bacterium affected when incubated at low oxygen conditions. Culture filtrate of the bacterium resulting from low oxygen level caused greater mortality of M. javanica juveniles in vitro compared with the filtrates obtained from ambient oxygen conditions. Culture filtrate from low oxygen conditions also showed the presence of hydrogen cyanide while those from ambient oxygen condition did not. Under glasshouse conditions, regardless of bacterial application, nematode penetration rate was greater when the pots were watered from the top; nematode penetration was lowered in bacterized pots compared with non-bacterized controls. IE-6S⁺ applied in the pots either watered from the top or bottom had no significant impact on growth of tomato but protein contents of the leaves increased after treatment with the bacterium. Rhizosphere and inner root colonization of the bacterium increased when the pots were watered from the top. Under in vitro conditions, with an increased iron concentration in the form of FeEDDHA, growth of IE-6S⁺ and its nematicidal activity increased. Culture filtrate of IE-6S⁺ obtained from liquid King's B medium supplemented with 0.5 or 1.0 mM FeEDDHA showed the presence of HCN. Under glasshouse conditions, soil treated with FeEDDHA alone did not reduce nematode penetration rates but did reduce greatly when applied in conjunction with IE-6S⁺. FeEDDHA applied at 0.5 mg/kg of soil in combination with IE-6S⁺ significantly enhanced plant growth and leaf protein contents. FeEDDHA at 1 mg/kg of soil increased bacterial populations both in the rhizosphere and inner root tissues of tomato.     

Effects of Pseudomonas aeruginosa on the diversity of culturable microfungi and nematodes associated with tomato: impact on root-knot disease and plant growth

The available information on Pseudomonas biocontrol inoculants on the non-target fungal and nematode community is scant. The current paper addresses this issue and investigates the effects of biocontrol agents Pseudomonas aeruginosa IE-6 and IE-6S⁺ (previously shown to suppress several soil-borne plant pathogens) on soil microfungi and plant-parasitic nematodes as well as on the root-knot development and growth of tomato (Lycopersicon esculentum). Furadan, a granular nematicide was included as a treatment for comparative purposes. Treatments were applied to soil at the start of each 52-day-long tomato growth cycle, and their effects on the composition and diversity of rhizosphere and endophytic microfungi and plant-parasitic nematodes were examined at the end of first and fourth growth cycle. Several diversity indices were employed to assess community diversity. A total of 16 genera comprising 23 microfungal species were isolated from the tomato rhizosphere. The most abundant fungal species belonged to the genera Aspergillus, Fusarium, and Penicillium. With a few exceptions, fungi were neither exclusively inhibited nor specifically promoted by the application of treatments at any of the growth cycles studied. However, Paecilomyces lilacinus, an egg and female parasite of root-knot nematode, though exclusively absent in the controls was isolated from the treatments. Both general diversity and equitability of rhizosphere microfungi were greater at first compared to the fourth growth cycle while species richness remained uninfluenced across the growth cycles and treatments. However, Furadan and IE-6S⁺ treatments considerably abated general diversity and equitability. Of the microfungal species isolated from the rhizosphere seven were also recovered from surface-sterilized root tissue of tomato suggesting that all the endophytes are primarily rhizosphere organisms. Diversity of endophytic fungi was consistently lower compared with that of the rhizosphere. Both general diversity and equitability declined in all three treatments relative to controls in the root tissue but species richness remained unaltered. Diversity and equitability of plant-parasitic nematodes in soil were reduced by all three treatments over the controls at fourth growth cycle whilst species richness did not change at either growth cycle. The biocontrol agents significantly reduced root-knot development and enhanced shoot growth of tomato over the controls. The possible implications of fungal composition and abundance because of biocontrol by Pseudomonas application are discussed.     

Suppression of root-knot disease by Pseudomonas fluorescens CHA0 in tomato: importance of bacterial secondary metabolite, 2,4-diacetylpholoroglucinol

The antimicrobial metabolites 2,4-diacetylphloroglucinol (2,4-DAPG) and pyoluteorin contribute to the ability of Pseudomonas fluorescens strain CHA0 to control plant diseases caused by soil-borne pathogens. P. fluorescens strain CHA0 and its derivatives CHA89 (antibiotics-deficient) and CHA0/pME3424 (antibiotics overproducing) were investigated as potential biocontrol agents against Meloidogyne javanica the root-knot nematode. Exposure of root-knot nematode to culture filtrates of P. fluorescens under in vitro conditions significantly reduced egg hatch and caused substantial mortality of M. javanica juveniles. Nutrient broth yeast extract (NBY) medium amended with 2% (w/v) glucose or 1 mM EDTA markedly repressed hatch inhibition activity of the strain CHA0 but not that of CHA0/pME3424 or CHA89. On the other hand, NBY medium amended with glucose significantly enhanced nematicidal activity of the strain CHA0/pME3424. Neither glucose nor EDTA had an influence on the nematicidal activity of the strains CHA0 and CHA89. Under in vitro conditions, antibiotic overproducing strain CHA0/pME3424 and CHA0 expressed phl‘-’lacZ reporter gene but strain CHA89 did not. Expression of the reporter gene reflects actual production of DAPG. In general, CHA0/pME3424 expressed reporter gene to a greater extent compared to its wild type counterpart CHA0. Regardless of the bacterial strains, reporter gene expression was markedly enhanced when NBY medium was amended with glucose but EDTA had no such effect. A positive correlation between the degree of juvenile mortality and extent of phl‘-’lacZ reporter gene expression was also observed in vitro. Strain CHA0 produced zones of 4-6 mm on MM medium containing gelatin while strain CHA0/pME3424 and CHA89 did not. When MM medium containing gelatin was amended with 2% glucose of 1 mM EDTA size of haloes produced by the strain CHA0 reduced to 2 mm. Under glasshouse conditions aqueous cell suspension of the strains CHA0 or CHA0/pME3424 at various inoculum levels (10⁷, 10⁸ or 10⁹ cfu ml⁻¹) significantly reduced root-knot development. CHA89 caused significant reduction in galling when applied at 10⁹ cfu ml⁻¹. To better understand the mechanism of nematode suppression, split root bioassay was performed. Split-root experiments, that guarantee a spatial separation of inducing agent and a challenging pathogen, showed that soil treatment of one half of the root system with cell suspension of CHA0 or CHA0/pME3424 resulted in a significant systemic induced resistance leading to reduction of M. javanica infection of tomato roots in the non-baterized nematode treated half. The results clearly suggest that the antibiotic 2,4-DAPG from P. fluorescens CHA0 act as the inducing agents of systemic resistance in tomato roots. Populations of CHA0 and its derivatives declined progressively by 10-fold between first and fourth harvests (0-21 days after inoculation). However, bacterial populations increased at final harvest (28 days after application).     

Relationship between in vitro production of auxins by rhizobacteria and their growth-promoting activities in Brassica juncea L.

Rhizobacteria were isolated from the rhizosphere of different Brassica species and assayed for their ability to produce auxins in vitro. The isolates varied greatly in their potential for auxin production (ranging from 0.33 to 11.40 µg ml^-1). L-Tryptophan (an auxin precursor) addition to the media increased the auxin production by several fold. Based upon in vitro auxin production and growth promotion of B. juncea seedlings caused by various isolates under gnotobiotic conditions, promising isolates were selected and tested in pot trial to observe their effects on growth, yield and oil content of the same Brassica species. Results showed that seed inoculation with different isolates of rhizobacteria significantly increased plant height (up to 56.5%), stem diameter (up to 11.0%), number of branches (up to 35.7%), number of pods per plant (up to 26.7%), 1,000-grain weight (up to 33.9%), grain yield (up to 45.4%) and oil content (up to 5.6%) over the uninoculated control. Isolate S54 gave the most promising and consistent results. Highly significant correlations between L-TRP-derived auxin production by plant growth-promoting rhizobacteria (PGPR) in vitro and grain yield (r =0.77^**), number of pods (r =0.78^**) and number of branches per plant (r =0.77^**) of B. juncea were found. It was hypothesized that these PGPR may influence the growth and yield of inoculated plants by production of auxins in the rhizosphere of inoculated plants from the L-TRP present in the root exudates, although other mechanisms of action might have also contributed.     

Nitrous oxide emissions from the wheat-growing season in eighteen Chinese paddy soils: an outdoor pot experiment

To identify the key soil parameters influencing N₂O emission from the wheat-growing season, an outdoor pot experiment with a total of 18 fertilized Chinese soils planted with wheat was conducted in Nanjing, China during the 2000/2001 wheat-growing season. Average seasonal N₂O-N emission for all 18 soils was 610 mg m^-2, ranging from 193 to 1,204 mg m^-2, approximately a 6.2-fold difference between the maximum and the minimum. Correlation analysis indicated that the seasonal N₂O emission was negatively correlated with soil organic C (r²=0.5567, P<0.001), soil total N (r²=0.4684, P<0.01) and the C:N ratio (r²=0.4530, P<0.01), respectively. A positive dependence of N₂O emission on the soil pH (r²=0.3525, P<0.01) was also observed. No clear relationships existed between N₂O emission and soil texture, soil trace elements of Fe, Cu and Mg, and above-ground biomass of the wheat crop at harvest. A further investigation suggested that the seasonal N₂O-N emission (E, mg m^-2) can be quantitatively explained by E=1005-34.2SOC+4.1S_a (R²=0.7703, n=18, P=0.0000). SOC and S_a represent the soil organic C (g kg^-1) and available S (mg kg^-1), respectively.     

Plant growth enhancement and suppression of Macrophomina phaseolina causing charcoal rot of peanut by fluorescent Pseudomonas

A plant growth-promoting bacterial strain, GRC₂, was isolated from potato rhizosphere and characterized as fluorescent Pseudomonas. It produced a hydroxamate-type siderophore in iron-deficient tryptic soy medium. The production of hydrocyanic acid and indole acetic acid was also recorded under normal growth conditions. The strain showed a strong antagonistic effect against Macrophomina phaseolina, a charcoal rot pathogen of peanut, when co-cultured on tryptic soy agar medium. The growth inhibition of M. phaseolina was 74% after 5 days of incubation. Bacterization of peanut seeds with fluorescent Pseudomonas GRC₂ resulted in increased seed germination, early seedling growth, fresh nodule weight, grain yield and reduced charcoal rot disease of peanut in M. phaseolina-infested soil as compared with control. A streptomycin-resistant marker of the bacterium GRC₂^strep+ was used to monitor root colonization, which positively confirmed the efficient colonization of peanut root. Seed bacterization reduced charcoal rot disease in M. phaseolina-infested soil as compared with the control by 99%, making the organism a potential biocontrol agent against charcoal rot of peanut.     

Influence of soil parameters on the effect of 3,4-dimethylpyrazole-phosphate as a nitrification inhibitor

Nitrification inhibitors specifically retard the oxidation of NH₄⁺ to NO₂^- during the nitrification process in soil. In this study, the influence of soil properties on the nitrification-inhibiting effect of 3,4-dimethylpyrazole-phosphate (DMPP), a newly developed nitrification inhibitor, has been investigated. Based on short-term incubation experiments, where the degradation of DMPP could be largely disregarded, the oxidation of the applied NH₄⁺ was more inhibited in sandy soils compared with loamy soils. The influence of soil parameters on the relative NO₂^- formation could be described by a multiple regression model including the sand fraction, soil H⁺ concentration and soil catalase activity (R²=0.62). Adsorption studies showed that the binding behaviour of DMPP was influenced markedly by soil textural properties, viz. the clay fraction (r²=0.61). The adsorption of DMPP was found to be an important factor for the inhibitory effect on NH₄⁺ oxidation in a short-term incubation (r²=0.57). It is concluded that the evaluated soil properties can be used to predict the short-term inhibitory effect of DMPP in different soils. The significance of these results for long-term experiments under laboratory and field conditions needs further investigation.     

Effect of spore inoculum and agricultural practices on the vertical distribution of the biocontrol plant-growth-promoting bacterium <Emphasis Type="Italic">Pasteuria penetrans</Emphasis> and growth of <Emphasis Type="Italic">Meloidogyne incognita</Emphasis>-infected tomato

Three concentrations of Pasteuria spores applied to soil and some agricultural practices were evaluated for their effects on spore attachment to nematodes and biocontrol of Meloidogyne incognita on tomato in a microplot experiment. Applications of Pasteuria at concentrations of 5᎒¹⁰ spores/m² increased tomato fruit yield per plant by 46% compared to non-Pasteuria treatments but also increased nematode densities in soil at harvest time. M. incognita juveniles recovered from plots where Pasteuria was applied at 5᎒¹⁰ spores/m² showed greater spore attachment than those with application rates of 2.5᎒⁹ spores/m² or 5᎒⁹ spores/m². Pasteuria spores penetrated to 30-40 cm soil depth in a volcanic ash sandy soil after application of spore suspensions to the soil surface. Densities of over 2.5᎒⁴ spores/g of soil were reached at 0-30 cm soil depth only when the application rate was 5᎒¹⁰ spores/m², but at harvest and after fallow densities of about 2.5᎒⁴ spores/g of soil were also reached in the top 10 cm of soil at 2.5᎒⁹ and 5᎒⁹ spores/m² application rates. Spore densities in soil decreased after 6 months of fallow when densities at harvest time were higher than 10⁵ spores/g of soil. Tillage and additional watering 2 days after spore application increased spore densities in soil at harvest throughout the soil depth (0-40 cm).     

Nitrogen mineralization potential of dairy manures and its relationship to composition

The objectives of this study were to determine the variability in mineralization of dairy manure N, to determine if N mineralization can be predicted by compositional factors or by near- or mid-infrared reflectance spectroscopy. Dairy manures (n =107) were collected from farms in Maryland, Virginia, Pennsylvania, New York, and Connecticut. The composition of these manures ranged from 14 to 386 g dry matter kg^-1, 0.9 to 9.5 kg total N/m³, and 0.3 to 4.7 kg NH₄⁺-N/m³. Manure-amended soil was aerobically incubated at 25°C and concentrations of NH₄⁺-N and NO₃^--N were determined at day 2 and day 56. The manures were highly variable in their N mineralization characteristics, ranging from a net mineralization of 54.9% to a net immobilization of 29.2% of the organic N. When compositional parameters were individually regressed against percentage mineralized organic N, the highest correlation coefficient (r) was 0.164. A stepwise regression of all 11 variables yielded a maximal r of 0.486. These results suggest that the availability of dairy manure organic N is highly variable and that the availability cannot be predicted from simple compositional parameters. No relationship was found between near-infrared spectral characteristics and N mineralization suggesting that no simple relationship exists between N mineralization and compositional characteristics. There appears to be some potential for the use of mid-infrared for determining the mineralization potential of manures.     

Determination of acid phosphatase and dehydrogenase activities in the rhizosphere of nodulated legume species native to two contrasting savanna sites in Venezuela

Rhizospheric soils from 13 nodulated legume species were collected at two contrasting savanna sites. Site 1 (S-1) was characterized by 2.6 µg available P g^-1 soil, 2.5% Al saturation and a pH of 5.9. Site 2 (S-2) presented a pH of 4.0, 1.0 µg available P g^-1 soil and 40% Al saturation. For all legume species and at both sites, acid phosphatase (AP) and dehydrogenase (Dh) activities were significantly higher in rhizospheric than in non-rhizospheric soils. Between sites, the highest significant rhizospheric and non-rhizospheric AP and Dh activities were recorded at S-1, indicating a negative effect of S-2 properties on soil enzymatic activity. Among species, the highest significant AP activity was detected in the rhizosphere of Aeschynomene hystrix at S-1 and Galactia jussieuana and Mimosa pudica at S-2. The Dh activity was significantly higher in the rhizosphere of Calopogonium mucunoides at S-1 and Chamaecrista flexuosa and G. jussieuana in S-2. Dh activity was below detectable values in the rhizosphere of Chamaecrista rotundifolia, C. mucunoides and Phaseolus gracilis at S-2. Glucose amendment (GA) of soils did not enhance AP activity in all rhizospheric and non-rhizospheric S-1 and S-2 soils. In contrast, GA increased the Dh activity in all soils, regardless of the site and legume species. Nevertheless, Dh activity in GA S-2 rhizospheric and non-rhizospheric soils did not reach the values detected in non-amended rhizospheric and non-rhizospheric S-1 soils, respectively. The non-response of AP to glucose amendment can be considered as an indicator of its plant origin.     

The effects of organic amendments on the interactions between a nematophagous fungus Arthrobotrys oligospora and the root-knot nematode Meloidogyne mayaguensis parasitizing tomato plants

The aim of this study was to examine the effects of amendments with leaf biomass on the development of tomato plants in a soil where root-knot nematodes (Meloidogyne mayaguensis) and/or a nematophagous fungus (Arthrobotrys oligospora, strain ORS 18697) had been inoculated. Six origins of leaf biomass were chosen: Acacia mangium, Acacia holosericea, Eucalyptus camaldulensis, Casuarina equisetifolia, Azadirachta indica and Sorghum vulgare. These leaf biomass types inhibited the development of the aerial parts of the tomato plants. This negative effect was not observed when the fungus was inoculated. On the contrary, plant growth was stimulated. Moreover, the antagonistic activity of Arthrobotrys oligospora was strengthened in the presence of ground leaf powder, especially that from Acacia holosericea. The effects of phenolic compounds on fungal growth and predatory activity and on plant growth are discussed.     

元素硫和双氰胺对菜地土壤铵态氮硝化抑制协同效应研究

采用好气培养法,研究了双氰胺（DCD）、元素硫（S0）和元素硫分解中间物（S2O32-）及其组合对蔬菜地土壤氮素硝化抑制作用。结果表明,在培养试验72 d内,DCD+S0、DCD、DCD+ Na2S2O3处理土壤NH4+-N总量分别是N处理的5. 8、5.1、5.9倍;S0、Na2S2O3处理分别是N处理的1.8、1.4倍;而所有硝化抑制剂（DCD、S0、S2O32-）处理土壤NO3--N含量显著低于N处理,表明DCD、S0和S2O32-均能抑制菜地土壤铵态氮硝化。培养试验开始8 d后,Na2S2O3和DCD对铵态氮硝化抑制产生协同效应,16 d后S0和DCD对铵态氮硝化抑制也产生协同效应,这可能是由于S0 氧化中间体S2O32-、S4O62-具有抑制DCD降解作用,延长了DCD硝化抑制作用时间。建议蔬菜生产上推荐使用DCD+S0组合,以提高氮素利用率。     

Nitrogen fixation in biological soil crusts from southeast Utah,USA

Biological soil crusts can be the dominant source of N for arid land ecosystems. We measured potential N fixation rates biweekly for 2 years, using three types of soil crusts: (1) crusts whose directly counted cells were >98% Microcoleus vaginatus (light crusts); (2) crusts dominated by M. vaginatus, but with 20% or more of the directly counted cells represented by Nostoc commune and Scytonema myochrous (dark crusts); and (3) the soil lichen Collema sp. At all observation times, Collema had higher nitrogenase activity (NA) than dark crusts, which had higher NA than light crusts, indicating that species composition is critical when estimating N inputs. In addition, all three types of crusts generally responded in a similar fashion to climate conditions. Without precipitation within a week of collection, no NA was recorded, regardless of other conditions being favorable. Low (<1°C) and high (>26°C) temperatures precluded NA, even if soils were moist. If rain or snow melt had occurred 3 or less days before collection, NA levels were highly correlated with daily average temperatures of the previous 3 days (r²=0.93 for Collema crusts; r²=0.86 for dark crusts and r²=0.83 for light crusts) for temperatures between 1°C and 26°C. If a precipitation event followed a long dry period, NA levels were lower than if collection followed a time when soils were wet for extended periods (e.g., winter). Using a combination of data from a recording weather datalogger, time-domain reflectometry, manual dry-down curves, and N fixation rates at different temperatures, annual N input from the different crust types was estimated. Annual N input from dark crusts found at relatively undisturbed sites was estimated at 9 kg ha^-1 year^-1. With 20% cover of the N-fixing soil lichen Collema, inputs are estimated at 13 kg ha^-1 year^-1. N input from light crusts, generally indicating soil surface disturbance, was estimated at 1.4 kg ha^-1 year^-1. The rates in light crusts are expected to be highly variable, as disturbance history will determine cyanobacterial biomass and therefore N fixation rates.     

Use of principal component analysis to assess factors controlling net N mineralization in deciduous and coniferous forest soils

Net N mineralization was studied in three different forest sites (Belgium): a mixed deciduous forest with oak (Quercus robur L. and Quercus rubra L.) and birch (Betula pendula Roth) as dominant species, a deciduous stand of silver birch (Betula pendula) and a coniferous stand of Corsican pine (Pinus nigra ssp. Laricio). The organic (F + H) layer and mineral soil at different depths (0-10, 10-20 and 20-30 cm) were sampled at three locations in the mixed deciduous forest (GE, GF1, GF2), at one location in the silver birch stand (SB) and one in the Corsican pine stand (CP). All samples were incubated over 10 weeks under controlled temperature and moisture conditions. The net N mineralization rates in the organic and upper mineral layer (0-10 cm) were found to be significantly different from the other layers and accounted for 66-95% of the total mineralization over the first 30 cm. Net N mineralization rates in the organic layer ranged from 4.2 to 27.3 mg N m^-2 day^-1. Net N mineralization and nitrification rates were positively correlated. For the mineral soil, net N mineralization rates decreased with depth and the upper 10 cm showed significantly higher rates, ranging from 8.9 to 33.5 mg N m^-2 day^-1. The rates of the 10-20 cm and 20-30 cm sublayers were similar, ranging from 1.2 to 7.4 mg N m^-2 day^-1. The net N mineralization rates for the total mineral layer (0-30 cm) ranged from 17.4 mg N m^-2 day^-1 (SB) to 36.1 mg N m^-2 day^-1 (CP). Both from PCA and multiple regression analysis, we could conclude that net N mineralization rates were closely related to the initial mineral N content (N_initial). Furthermore, significant correlations were observed between the net N mineralization rate, the total carbon (TC) and NH₄⁺-N content for the mineral layers and between net N mineralization rate, total nitrogen (TN), hemicellulose content and C/N for the organic layers.     

Naidid worms (Oligochaeta, Naididae) in paddy soils as affected by the application of legume mulch and/or tillage practice

Reproduction, intrinsic rate of natural increase and population density of naidid worms were investigated in submerged paddy fields and the laboratory. No tillage plus legume-mulching increased the population density of naidid worms. Soil treatments with neither tillage nor legume mulch, and tillage practice alone, did not increase the number of worms. Dero dorsalis Ferronnière was dominant in soil of the no-tillage treatment. In laboratory experiments, legume-mulching with the proper amount of dissolved O₂ accelerated asexual reproduction of D. dorsalis through zooid budding. For the legume and aeration treatment, (N_i+1-N_i) N_i^-1 values (where N_i and N_i+1 are the populations at times t=i and t=i+1) were plotted against N_i+1. Utilizing this linear relation, this data fitted the logistic curve (r²=0.885, P<0.05). Based on the linear relation, the intrinsic rate of natural increase (r), carrying capacity (K), and doubling time (T) were calculated as 0.2125 day^-1, 12,666 m^-2, and 3.26 days, respectively. The amounts of legumes applied were highly correlated with the population of D. dorsalis, indicating that the weight of legume is a limiting factor with respect to carrying capacity. A literature review indicated a significant correlation (P<0.01) between intrinsic rate of natural increase and maximum body length of naidids with temperature conversion of the growth rate. Sexually mature worms were rarely found in submerged paddy fields. Sexual reproduction seems to be adopted in response to soil desiccation after paddy field drainage.     

Arbuscular-mycorrhizal inoculation of five tropical fodder crops and inoculum production in marginal soil amended with organic matter

Five fodder crops, Zea mays, Medicago sativa, Trifolium alexandrinum, Avena sativa, and Sorghum vulgare were inoculated with a consortia of indigenous arbuscular mycorrhizal (AM) fungi in non-sterile PO₄^3- deficient sandy loam soil amended with organic matter under field conditions. Shoot and root dry weights and total uptake of P and N of all the test plants were significantly increased by AM inoculation. Mycorrhizal inoculation increased yield in terms of shoot dry weight by 257% in T. alexandrinum followed by 50% in A. sativa, 28% in Z. mays, 20% in M. sativa and 6% in S. vulgare. Variation in dependence on mycorrhiza was observed among the fodder crops. T. alexandrinum showed a maximum dependence of 72% in contrast to 5.7% dependency in S. vulgare. Plant species showed differences in percentage AM colonization, with a high root infection recorded in Z. mays (76%). Spore production and infectious propagules (IP) were as high as 78 spores/IP g^-1 and 103 spores/IP g^-1 in S. vulgare. This study clearly indicates the potential of using indigenous AM inoculations in fodder crops grown in marginal soils along with in situ large-scale production of AM inocula.     

Arbuscular mycorrhizal fungi influence tomato competition with bahiagrass

A strip-tillage production system for tomatoes (Lycopersicon esculentum Mill.) is impacted by nutrient competition from bahiagrass (Paspalum notatum Flügge). Tomato and bahiagrass differ in mycorrhizal responsiveness and our objective was to evaluate the influence of arbuscular mycorrhizal (AM) fungi on the competitive pressure of bahiagrass on growth of tomato. The first experiment evaluated the effect of bahiagrass competition, soil pasteurization, and AM fungal inoculation on tomato growth, P content, and root colonization in a low-P soil. Tomato grown alone was very responsive to mycorrhizal colonization - shoot dry mass of inoculated plants was up to 243% greater than that of noninoculated plants. Tomato grown with bahiagrass had reduced root and shoot growth across all treatments compared with tomato grown alone, but there was an increase in shoot mass following AM fungal inoculation across both pasteurized and nonpasteurized treatments resulting in a >50% increase in shoot dry mass of tomato compared to noninoculated controls. A second experiment was conducted to test bahiagrass competition, soil pasteurization, AM fungal inoculation, and P amendment on tomato growth in a moderate-P soil. With bahiagrass competition and no P addition, inoculation increased root mass by 115% and shoot mass by 133% in pasteurized soil; however, with the application of 32 mg P kg^-1 the trend was reversed and inoculated plants were smaller than noninoculated controls. We conclude that the role of mycorrhizae in plant competition for nutrients is markedly impacted by soil nutrient status and reduced P application may allow tomatoes to take advantage of their inherent responsiveness to mycorrhizae in a low to moderate soil-P environment.     

Effects of deficit irrigation on tomato and eggplant and their infection with the root-knot nematode under controlled environmental conditions

Deficit irrigation has been widely investigated as a valuable strategy for dry regions where water is the limiting factor in crop cultivation. Soil moisture can be one of the important factors that influence root-knot nematode (RKN) disease development. To determine how different levels of irrigation can affect disease development, irrigation regimes ranging from 20% to full field capacity (FC) were applied to tomato and eggplant plants inoculated with Meloidogyne javanica (M. javanica) under controlled conditions. In addition, in vitro bioassays were done to evaluate the effect of water potential and soil moisture content on M. javanica viability in the soil and reproduction on plant hosts. The relative egg hatching percentage decreased significantly with decreasing water potential from –0.1 to –1 MPa. The use of 80% irrigation level caused minor reductions in growth but significantly reduced nematode infection load. Nematode infection was reduced even further at lower levels of irrigation, however this also led to marked reductions in fresh and dry weights of the tomato and eggplants. Therefore, deficit irrigation could be used at a rate of 80% or 60% of FC to increase water use efficiency and reduce the level of RKN (M. javanica) infection without greatly reducing the growth performance of tomato and eggplant crops.     

Oxidation of elemental sulfur in paddy soils as influenced by flooded condition and plant growth in pot experiment

A pot experiment was conducted to study the effects of rice cropping on elemental sulfur (S⁰) oxidation in three paddy soils under flooded conditions. Unaltered wet samples of paddy soils derived from cinnamon soil, black soil and red soil, respectively, were used. Repacked soils were incubated for 14, 28 and 42 days with or without addition of 2,000 mg kg^-1 S⁰ (particle size of 0.075 cm) and with or without cropping. Substantial amounts of oxidized S⁰ were found to be immobilized into organic S or taken up by plants. The apparent mass of oxidized S was calculated as the sum of the net increase of the sulfate-S and organic-S pools, and S taken up by plants. Under flooded conditions and in the absence of rice, the oxidation rates in all three soils were the greatest during the initial 14 days and then declined with progressive incubation. Incubation for a 42-day period resulted in oxidation of 5.58-10.15% of applied S⁰. Meanwhile 3.23-5.60% of the applied S⁰ was converted into organic S. Cropping with rice could significantly increase S oxidation and conversion of S⁰ into organic S in soils. Incubation for 42 days resulted in the oxidation of 10.28-14.87% of the applied S⁰, and conversion of 5.74-8.06% of applied S⁰ into organic S in the three soils. The results suggest that S⁰ fertilizers should be applied to the soil horizon rich with rice roots (i.e. the rhizosphere) in flooded paddy soils in order to increase the efficiency of S⁰ oxidation and plant availability.     

Carbon partitioning in plant and soil,carbon dioxide fluxes and enzyme activities as affected by cutting ryegrass

The effect of a single cut (simulated grazing) and regrowth of Lolium perenne on CO₂ efflux from soil (loamy Haplic Luvisol), on below-ground C translocation and on the distribution of plant C among different soil particle size fractions was investigated under controlled conditions with and without N fertilization by pulse labelling with ¹⁴C 7 times (four before and three after the cutting). The amount of ¹⁴C respired from the rhizosphere of Lolium decreased by a factor of about 3 during 1 month of growth. At the same time the amount of ¹⁴C stored in soil increased. Cut and non-fertilized plants respired less C in the rhizosphere compared to the uncut plants and cut fertilized plants. About 80% of the root-derived CO₂ efflux originated from the C assimilated after defoliation, and 20% originated from the C assimilated before cutting. N fertilization decreased the below-ground C losses (root respiration and exudation) during regrowth. The shoot is the main sink of assimilated C before and after the defoliation. N fertilization led to higher C incorporation into the shoot parts growing after defoliation compared to unfertilized plants. A lower incorporation of ¹⁴C was observed in the roots of N fertilized plants. The relative growth rates (expressed as ¹⁴C specific activity) of roots and stubble were minimal and that of shoot parts growing after defoliation was maximal. Twelve percent of ¹⁴C was found in the newly grown leaves after regrowth; nevertheless, 4.7% and 2.4% of ¹⁴C in the new shoot parts were translocated from the root and shoot reserves of unfertilized and fertilized plants, respectively. Most of the C retranslocated into the new Lolium leaves originates from the stubble and not from the roots. Between 0.5% and 1.7% of ¹⁴C recovered in shoots and below-ground C pools was found in the soil microbial biomass. Cutting and fertilization did not change ¹⁴C incorporation into the microbial biomass and did not affect xylanase, invertase, and protease activities. Tracing the assimilated C in particle size fractions revealed maximal incorporation for the sand and clay fraction.