Amal Chandra Das

Effect of the herbicides oxadiazon and oxyfluorfen on phosphates solubilizing microorganisms and their persistence in rice fields.

A field experiment has been conducted with two herbicides viz. oxadiazon [5-terbutyl-3-(2,4-dichloro-5-isopropoxyphenyl)-1,3,4-oxadiazol-2-one] and oxyfluorfen [2-chloro-1-(3-ethoxy-4-nitrophenyl)-4-(trifluoromethyl) benzene] at rates of 0.4 and 0.12 kg a.i. ha(-1), respectively, to investigate their effect on the growth and activities of phosphate solubilizing microorganisms in relation to availability of phosphorus as well as persistence of the herbicides in the rhizosphere soil of wetland rice (Oryza sativa L. variety IR-36). Application of herbicides stimulated the population and activities of phosphate solubilizing microorganisms and also the availability of phosphorus in the rhizosphere soil. Oxyfluorfen provided greater microbial stimulation than oxadiazon. Dissipation of oxyfluorfen and oxadiazon followed first order reaction kinetics with half-life (T(1/2)) of 8.8 and 12 days, respectively. Sixty days after application 0.5% and 3% of the applied oxadiazon and oxyfluorfen residues persisted, respectively, in the rhizosphere soil of rice.

Influence and persistence of phorate and carbofuran insecticides on microorganisms in rice field.

An experiment was conducted in microplots (4 m x 4 m) with two insecticides, phorate and carbofuran at rates of 1.5 and 1.0 kga.i.ha(-1) respectively, to investigate its effect on the population and distribution of bacteria, actinomycetes and fungi as well as the persistence of the insecticidal residues in rhizosphere soils of rice (Oryza sativa L., variety IR-50). Application of the insecticides stimulated the population of bacteria, actinomycetes and fungi in the rhizosphere soils, and the stimulation was more pronounced with phorate as compared to carbofuran. Both the insecticides did not have marked effect on the numbers of Streptomyces and Nocardia in the rhizosphere soils. However, the growth of Bacillus, Escherichia, Flavobacterium, Micromonospora, Penicillium, Aspergillus and Trichoderma with phorate and that of Bacillus, Corynebacterium, Flavobacterium, Aspergillus and Phytophthora with carbofuran were increased. On the other hand, the numbers of Staphylococcus, Micrococcus, Fusarium, Humicola and Rhizopus under phorate and Pseudomonas, Staphylococcus, Micrococcus, Klebsiella, Fusarium, Humicola and Rhizopus under carbofuran were inhibited. Both the insecticides persisted in the rhizosphere soil for a short period of time and the rate of dissipation of carbofuran was higher than that of phorate in the soil depicting the half-life (T1/2) 9.1 and 10.4 days, respectively.

Insecticides: their effect on microorganisms and persistence in rice soil.

A field experiment was conducted to investigate the effect of four insecticides, HCH, phorate, carbofuran and fenvalerate, at recommended doses on the preponderance of bacteria, actinomycetes and fungi. We also measured the persistence of the insecticides in the rhizosphere soil of rice. HCH and fenvalerate stimulated the proliferation of all of the microorganisms significantly. Phorate increased the population of bacteria and actinomycetes. Carbofuran accentuated the preponderance of actinomycetes in soil. Insecticides, in general, did not have marked influence on the proliferation of Bacillus, Streptomyces, Aspergillus and Fusarium in soil. However, we observed a stimulation of growth of Staphylococcus, Proteus and Sarcina with HCH, Pseudomonas, Corynebacterium, Erysipelothrix and Rhizopus with phorate, Serratia, Corynebacterium, Klebsiella, Escherichia, Rhizopus and Humicola with carbofuran, and Staphylococcus, Sarcina, Klebsiella and Nocardia with fenvalerate. On the other hand, there was an inhibition in growth of Pseudomonas, Micrococcus, Nocardia and Penicillium with HCH, of Pseudomonas, Micrococcus and Penicillium with carbofuran, and of Pseudomonas, Micrococcus and Micromonospora with fenvalerate. Different types of insecticides exhibited differential patterns of dissipation in soil. HCH had the highest persistence followed by phorate, carbofuran and fenvalerate, respectively.

Studies on the decomposition of non-conventional organic wastes in soil

Abstract An experiment was conducted to investigate the effect of four non-conventional organic wastes viz., Justicea simplex, Jatropa gossypifolia, Ipomoea cornea and Cestrum diurnum at a concentration of 0.5% (on soil weight basis) on the rate of decomposition and the microbial dynamics in soil. Justicea simplex liberated the maximum amount of carbon dioxide from soil on the 2nd day. Other organic additives liberated the highest amount of carbon dioxide on the 3rd day. During 84 days of decomposition, the cumulative amount of CO 2 evolution was in the order: Justicea simplex > Cestrum diurnum > Ipomoea cornea > Jatropa gossypifolia . Different organic residues exerted differential stimulation of microbial proliferation in the soil. Jatropa gossypifolia gave rise to the maximum number of total bacteria in soil. Cestrum diurnum and Jatropa gossypifolia induced a similar effect on the actinomycete population. The augmentation of fungal colonies in the presence of different wastes was similar. Cestrum diurnum harboured the maximum number of non-symbiotic nitrogen-fixing bacteria and phosphate-solubilizing microorganisms. Justicea simplex and Jatropa gossypifolia were superior to Cestrum diurnum and Ipomoea cornea with respect to nitrogen fixation. None of the organic additives stimulated the phosphate-solubilizing capacity of the soil.

Insecticidal effects on the activity and numbers of non-symbiotic N2-fixing bacteria and phosphate solubilizing microorganisms and on some chemical properties of rice soil

Abstract Four insecticides, viz. HCH, phorate, carbofuran and fenvalerate @ 7.5, 1.5, 1.0 and 0.35 kg a. i. ha- 1 respectively, were applied to the rice fields to investigate their effects on the availability of nutrients, N 2 -fixation and phosphate solubilization in the rhizospere soil of rice in relation to the yield of the crop. HCH highly stimulated the mineralization of organic C. Fenvalerate and HCH reduced the total N content to the highest extent. HCH and phorate liberated more available N (NH 4 + and NO 3 - ) as compared to carbofuran and fenvalerate. Phorate, however, released maximum amount of available P. Application of different insecticides accentuated the proliferation of aerobic non-symbiotic N 2 -fixing bacteria and phosphate solubilizing microorganisms and also their biochemical activities such as, non-symbiotic N 2 -fixing and phosphate solubilizing capacities of the rhizosphere soil resulting in greater yield of crop.

Phosphate-solubility and phosphatase activity in Gangetic alluvial soil as influenced by organophosphate insecticide residues

An experiment was conducted under laboratory conditions to investigate the effect of four organophosphate insecticides, viz. monocrotophos, profenophos, quinalphos and triazophos at their field application rates (0.75, 1.0, 0.5 and 0.6 kg a.i.ha(-1), respectively), on the growth and activities of phosphate solubilizing microorganisms in relation to availability of insoluble phosphates in the Gangetic alluvial soil of West Bengal, India. The proliferation of phosphate solubilizing microorganisms was highly induced with profenophos (38.3%), while monocrotophos exerted maximum stimulation (20.8%) towards the solubility of insoluble phosphates in soil. The phosphatase activities of the soil (both acid phosphatase and alkaline phosphatase) were significantly increased due to the incorporation of the insecticides in general, and the augmentation was more pronounced with quinalphos (43.1%) followed by profenophos (27.6%) for acid phosphatase, and with monocrotophos (25.2%) followed by profenophos (16.1%) for alkaline phosphatase activity in soil. The total phosphorus was highly retained by triazophos (19.9%) followed by monocrotophos (16.5%), while incorporation of triazophos and quinalphos manifested greater availability of water soluble phosphorus in soil.

Non-symbiotic N2-fixation and phosphate-solubility in Gangetic alluvial soil as influenced by pre-emergence herbicide residues.

An experiment has been conducted under laboratory conditions to investigate the effect of two pre-emergence herbicides viz., thiobencarb (at 1.5 and 4.5 kg a.i. ha(-1)) and pretilachlor (at 0.5 and 1.5 kg a.i. ha(-1)), on the changes of growth and activities of aerobic non-symbiotic N2-fixing bacteria and phosphate-solubilizing microorganisms in relation to availability of mineral nitrogen and soluble phosphorus in the Gangetic alluvial soil (Typic Haplustept) of West Bengal, India. Application of herbicides, in general, significantly increased growth and activities of microorganisms, resulting in greater release of available nitrogen and soluble phosphorus in soil; and the stimulation was more pronounced when the herbicides were applied at their lower concentrations (recommended field application rates), more so with thiobencarb, as compared to pretilachlor. As compared to untreated control, application of thiobencarb at lower concentration increased the proliferation of aerobic non-symbiotic N2-fixing bacteria, phosphate-solubilizing microorganisms and non-symbiotic N2-fixing capacity of soil to the extent of 54.0, 44.6 and 31.7%, respectively; and accumulated the highest amount of available nitrogen (37.8%) and phosphorus (54.5%) in soil, while pretilachlor at field application rate highly induced (37.2%) phosphate-solubilizing capacity of soil. At higher concentration, pretilachlor was superior to thiobencarb in augmenting the growth and activities of phosphate-solubilizers. The results of the present study also indicated that gradual increase in concentration of the herbicides over their recommended field application rates was not much conducive for growth and activities of microorganisms, and subsequent release of nutrients in soil.