Effect of continuous application of manures and fertilizers on rhizosphere microflora in arecanut palm

Summary The rhizosphere microflora of arecanut palm under continuous application of organic manures and inorganic fertilizers was studied. The nutrients applied are 100 g N, 40 g P₂O₅ and 140 g K₂O/palm/year in the form of organics and inorganics. The application of organic manure increased the microbial population. The increase in microbial population was observed between the rhizosphere samples collected at 0–30cm and 30–60 cm depths. The surface cultivation of soil increased the microbial population.Trichoderma sp. andAspergillus sp. dominated in therhizosphere of arecanut palm. Contribution No. 208. Central Plantation Crops Research Institute, Vittal-574243, Karnataka, India.     

Enrichment of diazotrophic bacteria from rice soil in continuous culture

Summary A chemostat was used as a model system to study competitive interactions of diazotrophic microorganisms. Enrichment experiments were carried out under microaerobic conditions (8.7 mol O₂/l) with malate as the sole carbon source. The starting material was a Korean rice soil including intact root pieces. The enrichment process was governed by the dilution rate. High dilution rates resulted in the enrichment ofAzospirillum lipoferum, whereas low dilution rates led to the predominance of an unidentified organism, named Isolate R. Dilution rates were set in the range from D=0.005 to D=0.1 h^–1. The growth kinetics of both organisms followed Monod's model in the enrichment culture. From the experiments, the maximum specific growth rate ofA. lipoferum and Isolate R were 0.069 h^–1 and 0.025 h^–1, respectively. The corresponding K_s-values were 8.4 and 0.9 (mg. 1^–1). The point of theoretical coexistence of both organisms was calculated to occur at a substrate concentration of s=3.0 (mg.l^–1) with a growth of rate =0.018 h^–1. Hence the preset nutritional niches occupied by at least two organisms.Azospirillum lipoferum seems to represent the copiotroph microflora and Isolate R is of the oligotroph type. In addition to its high substrate affinity Isolate R liberatedca. 75% of the fixed nitrogen into the medium, which indicates its potential role for mutualistic interactions in the rhizosphere.     

Ecology of nematophagous fungi: vertical distribution in a deciduous woodland

Summary The distribution of nematophagous fungi in soil collected from a deciduous woodland is compared to various biotic and abiotic soil factors. The microfungi are isolated at all depths down to a maximum of 35 cm. Predators forming constricting rings, adhesive branches and adhesive knobs are restricted to the upper litter and humus layers. The net forming predators and endoparasites are isolated at all depths, although they are significantly more abundant in the lower mineral rich soils. A much greater species diversity of nematophagous fungi is recorded in the upper organic zones.Preliminary soil analysis indicates thatCephalosporium balanoides is independent of all soil variables, while predators able to form traps spontaneously are restricted to the organic soils which are rich in nematodes. Non-spontaneous trap forming predators, which are excellent saprophytes, are isolated from the deeper soils which are low in nutrients. The ecological significance of these results is discussed.     

Nitrogen fixation (C2H2 reduction) in soil samples from rhizosphere of rice grown under alternate flooded and nonflooded conditions

Summary In a greenhouse study the influence of alternate flooded and nonflooded conditions on the N₂-ase activity of rice rhizosphere soil was investigated by C₂H₂ reduction assay. The soil fraction attached to roots represent the rhizosphere soil. Soil submergence always accelerated N₂-ase and this effect was more pronounced in planted system. Moreover, rice plant exhibited phase-dependent N₂-ase with a maximum activity at 60 days after transplanting. The alternate flooded and nonflooded regimes resulted in alterations of the N₂-ase activity. Thus, the N₂-ase activity increased following a shift from nonflooded to flooded conditions, but the activity decreased when the flooded soil was returned to nonflooded condition by draining. However, the differential influence of the water regime on N₂-ase was not marked in prolonged flooded-nonflooded cycles. Microbial analysis indicated the stimulation of different groups of free-living and associative N₂-fixing microorganisms depending on the water regime.     

Effects of inorganic amendments (N,P and K) to soil on the rhizosphere microflora of antirrhinum plants infected withVerticillium dahliae Kleb

Summary A study of the inorganic amendments (N, P and K) to soil, and their effect on the rhizosphere microflora, as well as their relation to the control of wilt of antirrhinum plants caused byVerticillium dahliae Kleb. was done. Ammonium sulphate was the only chemical found to be significantly inhibitory toV. dahliae in vitro. Soil amendments (NPK) affected the rhizosphere microorganisms of the antirrhinum plants. Higher concentration of the chemicals were phytotoxic. It was further observed that ammonium sulphate, and the combined chemicals (NPK 25%) in soil delayed the senescence in healthy plants, suggests that chemical fertilisers affected the host plants directly. Addition of ammonium sulphate (0.25%), calcium nitrate (0.25%, 0.5%) combined NPK (0.25%) to soil caused considerable reduction in disease severity. It is assumed that this reduction may be caused by the (1) fungitoxic nature of the chemicali.e. ammonium sulphate, (2) antagonistic environment for the pathogen in the rhizosphere was boostedi.e. where calcium nitrate was added as soil amendments and (3) reduction in disease severity in soil-amended with combined NPK, may be due to the fact that antagonistic actinomycete population was boosted in the rhizosphere.     

The effect of nitrate-nitrogen supply on bacteria and bacterial-feeding fauna in the rhizosphere of different grass species

Summary Microbial growth in the rhizosphere is affected by the release of organic material from roots, so differences in carbon budgets between plants may affect their rhizosphere biology. This was tested by sampling populations of bacteria and bacteriophagous fauna from the rhizosphere of Lolium perenne, Festuca arundinacea, Poa annua, and Poa pratensis, under conditions of high and low nitrate availability. Concentrations of soluble phenolics and lignin varied considerably between the species but were not related to differences in rhizosphere biology. L. perenne and F. arundinacea supported fewer bacteria than the Poa species. There was no significant rhizosphere effect on the groups of protozoa. The major indicators of rhizosphere productivity were the bacterial-feeding nematodes (mainly Acrobeloides spp.), and there was a large positive effect of added nitrate. Nematode biomass was significantly lower in the rhizosphere of the slow-growing P. pratensis compared with the fast-growing P. annua, indicating that the differential allocation of carbon has affects on rhizosphere biology. A large rhizosphere effect on enchytraeid worms was also observed, and their potential importance in the rhizosphere is discussed.     

Effect of the chemical and physical condition of the soil on Verticillium wilt of antirrhinum

Summary In vitro study showed thatVerticillium dahliae Kleb. grew well in a wide range of acid and alkaline media (viz. pH 3.5 to 10.5). The best growth of the fungus was observed in pH 5.5. Soil pH 3.5 was toxic for growing antirrhinum seedlings. Development of Verticillium wilt of antirrhinum was affected by soil pH. The severity of the disease was greater in alkaline soil conditions compared with acid conditions. Soil of pH 3.5 gave very good control of the symptom expression by the infected plants. Rhizosphere analysis results showed that fungal population with the exception ofPenicillium spp., was drastically reduced in the rhizosphere of the plants grown in acid soil. Although the overall population of fungi was reduced in theV. dahliae infected antirrhinum rhizosphere in acid soil, the population ofPenicillium spp. markedly increased. The antagonistic activity of thePenicillium spp. in the rhizosphere might also have reduced the disease severity. Since the seedlings did not grow properly in very dry and very wet soil, rhizosphere analysis of these soils was not possible. Disease severity was much less in wet soil compared with plants grown in medium moisture level and dry soil, but the plant growth was very poor. Dedicated to the memory of the late Prof. H. K. Baruah from whom I had the inspiration for research