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Pesticides are an important agricultural input, and the introduction of new active ingredients with increased efficiencies drives their higher production and consumption worldwide. Inappropriate application and storage of these chemicals often contaminate plant tissues, air, water, or soil environments. The presence of pesticides can lead to developing tolerance, resistance or persistence and even the capabilities to degrade them by the microbiomes of theses environments. The pesticide-degrading microorganisms gain and employ several mechanisms for attraction (chemotaxis), membrane transport systems, efflux pumps, enzymes and genetical make-up with plasmid and chromosome encoded catabolic genes for degradation. Even the evolution and the mechanisms of inheritance for pesticide-degradation as a functional trait in several microorganisms are beginning to be understood. Because of the commonalities in the microbial responses of sensing and uptake, and adaptation due to the selection pressures of pesticides and antimicrobial substances including antibiotics, the pesticide-degraders have higher chances of possessing antimicrobial resistance as a surplus functional trait. This review critically examines the probabilities of pesticide contamination of soil and foliage, the knowledge gaps in the regulation and storage of pesticide chemicals, and the human implications of pesticide-degrading microorganisms with antimicrobial resistance in the global strategy of 'One Health'.

Due to human activities to a greater extent and natural processes to some extent, a large number of organic chemical substances such as petroleum hydrocarbons, halogenated and nitroaromatic compounds, phthalate esters, solvents and pesticides pollute the soil and aquatic environments. Remediation of these polluted sites following the conventional engineering approaches based on physicochemical methods is both technically and economically challenging. Bioremediation that involves the capabilities of microorganisms in the removal of pollutants is the most promising, relatively efficient and cost-effective technology. However, the current bioremediation approaches suffer from a number of limitations which include the poor capabilities of microbial communities in the field, lesser bioavailability of contaminants on spatial and temporal scales, and absence of bench-mark values for efficacy testing of bioremediation for their widespread application in the field. The restoration of all natural functions of some polluted soils remains impractical and, hence, the application of the principle of function-directed remediation may be sufficient to minimize the risks of persistence and spreading of pollutants. This review selectively examines and provides a critical view on the knowledge gaps and limitations in field application strategies, approaches such as composting, electrobioremediation and microbe-assisted phytoremediation, and the use of probes and assays for monitoring and testing the efficacy of bioremediation of polluted sites. Keywords: Bioremediation approaches, Organic pollutants, Electrobioremediation, GEMs, Rhizoremediation, Limitations

Rice-straw amendment increased methane production by 3-fold over that of unamended control. Application of P as single superphosphate at 100 μg (g soil)-1 inhibited methane (CH 4 ) production distinctly in flooded alluvial rice soil, in the absence more than in the presence of rice straw. CH 4 emission from rice plants (cv. IR72) from alluvial soil treated with single superphosphate as basal application, in the presence and absence of rice straw, and held under non-flooded and flooded conditions showed distinct variations. CH 4 emission from non-flooded soil amended with rice straw was high and almost similar to that of flooded soil without rice-straw amendment. The cumulative CH 4 efflux was highest (1041 mg pot -1 ) in rice-straw-amended flooded soil. Appreciable methanogenic reactions in rice-straw-amended soils were evident under both flooded and non-flooded conditions. Rice-straw application substantially altered the balance between total aerobic and anaerobic microorganisms even in non-flooded soil. The mitigating effects of single-superphosphate application or low-moisture regime on CH 4 production and emission were almost nullified due to enhanced activities of methanogenic archaea in the presence of rice straw.

Bioremediation of Cr(VI) in long-term contaminated soil environment has not been very successful due to the known reoxidation of Cr(III) to Cr(VI). Microbial role on the presence of Cr(VI) in soil samples from a long-term tannery waste contaminated site, even 25 years after cessation of waste disposal was examined. The contaminated soil, unamended or amended with 6.6% (w/w) cow manure, was incubated under 70% water holding capacity and saturated conditions. Water-soluble and exchangeable Cr(VI) declined to undetectable levels within 20 days only in saturated soil, irrespective of the presence or absence of cow manure. When the saturated soil with no Cr(VI) present was subjected to drying, Cr(VI) reappeared in cow manure-amended and unamended soil within 10 days of drying, but at levels far less than that present in the soil before saturation. When the saturated soil was autoclaved and then dried, Cr(VI) was not formed. Evidently, microorganisms are involved in the reoxidation of chromium and the resulting negligible attenuation of Cr(VI) at the long-term tannery waste contaminated site.

In laboratory incubation studies with three soils of varying physicochemical characteristics, phorate was more persistent in nonflooded (60% water holding capacity) soils than in flooded soils. Phorate sulphoxide was recovered as the only metabolite of phorate in nonflooded soils while three metabolites (diethyl dithiophosphate, triethyl dithiophosphate and an unidentified metabolite) were formed in flooded soils. Study indicates that in nonflooded soils phorate is degraded via oxidation while in flooded soils hydrolysis is the major degradation process. Degradation of phorate was accelerated by an increase in incubation temperature. Preexposure or repeated application of soils to phorate slightly decreased the persistence of phorate or its metabolites. Decreased persistence of phorate and its metabolites formed in nonsterile soils compared to sterile soils suggested the role of microorganisms in their transformation.

In a laboratory incubation study, effects of amendment with sodium salts of SO4 2−, Cl− and HCO3 − either singly or as a mixture on CH4 production in a nonsaline alluvial soil under flooded condition were investigated. Methane production was considerable in the unamended alluvial soil, but was significantly inhibited following amendment with salts of different anions to raise the pore water EC to 8 dS·m−1. SO4 2− was the most inhibitory to CH4 production and the degree of inhibition followed the order SO4 2− > salt mixture > HCO3 − > Cl−. Salt amendment did not adversely affect soil microbial activities as expressed in terms of soil redox potential (Eh) and soil pH. However, readily mineralizable carbon content, an indicator of substrate availability for methanogenic bacteria, differed significantly among the treatments. Most probable number estimates indicated that acetotrophic methanogenic bacterial population was lowest in Cl−-amended soils followed by SO4 2−-amendment with little or no changes in HCO3...

Significant quantities of cadmium (Cd) have been added to the soils globally due to various anthropogenic activities, raising concerns for environmental health. Microorganisms play a unique role in the soil ecosystem, because of their contributions to soil fertility. Contrasting trends, reported on the toxic effects of heavy metals including Cd on soil microorganisms and their activities, are attributable to short-term studies often limited to a single soil type and conducted under controlled laboratory conditions. There is a paucity of reliable field data on Cd alone, since most field studies on Cd-microorganism interactions in soils are based on sewage sludge containing multimetals and organic substances. No single parameter can be used to generalize Cd toxicity and different parameters can provide contrasting results. A battery of relevant tests, rather than just one single assay, involving important microbial activities should therefore be included in ecotoxicity studies. The bioavailability of Cd and associated toxicity to soil biota vary with time, soil type, speciation, ageing, Cd-source, organisms and the environmental factors. The available fraction or soil solution Cd, and not the total concentration of Cd, seems to correlate well with the toxicity parameters.

Although CH 4 production is sensitive to temperature, it is not clear how temperature controls CH 4 production directly versus the production of organic substrates that methanogens convert into CH 4 . Therefore, this study was done to better understand how CH 4 production in rice paddy soil responded to temperature when the process was not limited by the availability of substrates. In a laboratory-incubation study using three Indian rice soils under flooded conditions, the effect of temperature on CH 4 production was examined. CH 4 production in acid sulphate, laterite, and alluvial soil samples under flooded conditions distinctly increased with increase in temperature from 15°C to 35°C. Laterite and acid sulphate soils produced distinctly less CH 4 than alluvial soils. CO 2 production increased with increase in temperature in all the soils. The readily mineralizable carbon C and Fe 2+ contents in soils were least at 15°C and highest at 35°C, irrespective of soil type. Likewise, a significant correlation ...

Agricultural sources of atmospheric methane include flooded rice ( Oryza sativa L.) paddies. However, certain soil nutrient management and cultural practices offer opportunities to reduce methane emissions. The effect of application of ammonium thiosulphate, a potential source of nitrogen and sulphur and also an inhibitor of nitrification and urease on methane production and emission from flooded alluvial (Typic Haplaquept) rice soil in India, was examined. Methane production and emission from control and urea-amended soil samples were almost identical. Application of ammonium thiosulphate to laboratory-incubated flooded soil (30 and 60 μg N g −1 soil) and flooded rice fields (45.6 and 60 kg N ha −1 ) effected a distinct inhibition of methane production and emission. Ammonium thiosulphate stimulated the population of sulphate-reducing bacteria (SRB) to a greater extent at 60 μg N g −1 soil than at 30 μg N g −1 soil. In ammonium thiosulphate-applied rice field plots, mean methane efflux decreased by about 38 and 60% at 45.6 and 60 kg N ha −1 , respectively, over that of control. Inhibition of methane production by ammonium thiosulphate is, at least in part, due to the stimulation of SRB. Results suggest the mitigation potential of ammonium thiosulphate on methane emission from flooded rice paddies.

With advances in biotechnology, bioremediation has become one of the most rapidly developing fields of environmental restoration, utilizing microorganisms to reduce the concentration and toxicity of various chemical pollutants, such as petroleum hydrocarbons, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, phthalate esters, nitroaromatic compounds, industrial solvents, pesticides and metals. A number of bioremediation strategies have been developed to treat contaminated wastes and sites. Selecting the most appropriate strategy to treat a specific site can be guided by considering three basic principles: the amenability of the pollutant to biological transformation to less toxic products (biochemistry), the accessibility of the contaminant to microorganisms (bioavailability) and the opportunity for optimization of biological activity (bioactivity). Recent advances in the molecular genetics of biodegradation and studies on enzyme-tailoring and DNA-shuffling are discussed in this paper.

The effects of addition of inorganic redox substances (species of NO - 3 , Mn 4+ , Fe 3+ , and SO 2- 4 ) on methane production and oxidation in anoxic rice (Oryza saliva L.) soil samples were examined. Sulfate was the most inhibitory for methane production followed by Fe 3+ , NO - 3 , and Mn 4+ , in that order. Addition of rice straw at a rate of 1% (w/w) as a carbon source to increase the electron donor to the electron acceptor ratio did not completely alleviate the inhibitory effects of redox species on methane production. Interestingly, laboratory incubation studies showed that addition of MnO 2 and K 2 SO 4 enhanced aerobic methane oxidation in soil samples held at 60% water holding capacity. The suspensions of pretreated soil samples with different redox species, when tested for their ability to oxidize methane in soil solution equivalent medium supplemented with respective redox species under aerobic and anaerobic conditions showed differential effects of redox species. Nitrate and Fe 3+ stimulated methane oxidation under anaerobic conditions and retarded it under aerobic conditions. Manganese(IV) ion retarded methane oxidation under anaerobic conditions, but enhanced it under aerobic conditions. However, SO 2- 4 stimulated methane oxidation in soil solution equivalent medium under both aerobic and anaerobic conditions.

Application of a commercial formulation of the herbicide butachlor (N-butoxymethyl-2-chloro-2′,6′-diethyl acetanilide) at 1 kg a.i. ha–1 to an alluvial soil planted with direct-seeded flooded rice (cv. Annada), significantly inhibited both crop-mediated emission and ebullition fluxes of methane (CH4). Over a cropping period of 110 days, the crop-mediated cumulative emission flux of CH4 was lowered by ∼20% in butachlor-treated field plots compared with that of an untreated control. Concurrently, ebollition flux of CH4 was also retarded in butachlor-treated field plots by about 81% compared with that of control plots. Significant relationships existed between CH4 emission and redox potential (Eh) and Fe2+ content of the flooded soil. Application of butachlor retarded a drop in soil redox potential as well as accumulation of Fe2+ in treated field plots. Methanogenic bacterial population, counted at the maturity stage of the crop, was also low in butachlor-treated plots, indicating both direct and indirect inhibitory effects of butachlor on methanogenic bacterial populations and their activity. Results indicate that butachlor, even at field-application level, can effectively abate CH4 emission and ebollition from flooded soils planted to rice whilst maintaining grain yield.

1 Division of Agricultural Chemicals, Indian Agricultural Research Institute, New Delhi 110 012, India 2 Division of Environmental Sciences, Indian Agricultural Research Institute, New Delhi 110 012, India 3 Division of Microbiology, Indian Agricultural Research Institute, New Delhi 110 012, India 4 National Centre for Conservation and Utilization of Blue Green Algae, Indian Agricultural Research Institute, New Delhi 110 012, India 5 Division of Plant Physiology, Indian Agricultural Research Institute, New Delhi 110 012, India

Rice paddies are an important human-made ecosystem for the global CH4 budget. CH4, which is produced in the predominantly anaerobic bulk soil layers, is oxidized significantly before it reaches the atmosphere. Roots of rice, in addition to supporting the consumption of CH4, contribute to the total CH4 production in the soil. The various controls of CH4 emission from this ecosystem depend on the structure of plant and microbial communities and their interactions. Availability of organic substrates, electron acceptors and other soil- and plant-related factors influence the activities of microbial communities. Agronomic practices including fertilization and application of pesticides have effects on CH4 emission. Recent studies using molecular retrieval approaches with small subunit rRNA-encoding gene (rDNA) sequences and functional genes, showed the richness of diversity of the microbial community in rice paddy soils, which includes members of the Archaea and methanotrophs. There is need for further research to know the consequences, at the ecosystem level, of changes in microbial diversity and microbial communities in paddy soils. This will aid in understanding the mechanisms involved in the mitigating effects of certain agricultural practices.

In a laboratory incubation study, methane (CH4) production in two saline soils and a nonsaline soil sample was investigated under flooded conditions. Mean CH4 production was remarkable (630.86 ng CH4/g) in nonsaline alluvial soil, but low (12.97 ng CH4/g) in acid sulfate saline (Pokkali) soil which was attributed to the high sulfate content of the later. CH4 production was also low in the coastal saline (Canning) soil (142.36 ng CH4/g) but increased upon leaching the soil of its salt content. Addition of salts to the nonsaline alluvial soil at 4, 8, 16 and 20 dS/m progressively decreased CH4 production. The inhibition of CH4 production was related to low microbial activities as reflected by decreased microbial biomass C and low soil microbial population including that of methanogens.

Tropical rice paddy is considered to be one of the major anthropogenic source of atmospheric methane (CH 4 ). In a field study spread over the dry and wet seasons of a calendar year, the CH 4 emission from upland (oilseed and pulse) crops in the dry season and a succeeding lowland rice ( Oryza sativa L.) crop in the wet season was compared with rice–rice rotation in both seasons under flooded conditions. Cumulative CH 4 flux from the upland crop followed by lowland rice crop was low (12.52–13.09 g CH 4 m 2 ) compared to that of the rice–rice rotation (39.96 g CH 4 m 2 ). What was particularly interesting is that the seasonal mean CH 4 emission from the lowland rice in wet season preceded by an upland crop in dry season was low when compared to that of lowland rice in wet season preceding a dry season flooded rice. Results indicate that the cumulative CH 4 emission from tropical rice ecosystem can be lowered by growing suitable upland crops to reduce the period that rice paddies are submerged during an annual cropping cycle.

Methane (CH4) emission from rice fields at Cuttack (State of Orissa, eastern India) has been recorded using an automatic measurement system (closed chamber method) from 1995-1998. Experiments were laid out to test the impact of water regime, organic amendment, inorganic amendment and rice cultivars. Organic amendments in conjunction with chemical N (urea) effected higher CH4 flux over that of chemical N alone. Application of Sesbania, Azolla and compost resulted in 132, 65 and 68 kg CH4 ha-1in the wet season of 1996 when pure urea application resulted in 42 kg CH4 ha-1. Intermittent irrigation reduced emissions by 15% as compared to continuous flooding in the dry season of 1996. In the wet season of 1995, four cultivars were tested under rainfed conditions resulting in a range of emissions from 20 to 44 kg CH4 ha-1. Application of nitrification inhibitor dicyandiamide (DCD) inhibited while Nimin stimulated CH4 flux from flooded rice compared to that of urea N alone.Wide variation in CH4 production and oxidation potentials was observed in rice soils tested. Methane oxidation decreased with soil depth, fertilizer-N and nitrification inhibitors while organic amendment stimulated it. The results indicate that CH4 emission from the representative rainfed ecosystem at the experimental site averaged to 32 kg CH4 ha-1yr-1.

Experiments were conducted to determine methane emission from a rainfed lowland rice field (water depth about 3-30 cm) and an irrigated shallow rice field (4-6 cm), both planted to the same cultivar, cv. 'Gayatri,' as influenced by fertilizer management practices. Methane emission peaked from 100 to 125 days after transplanting followed by a decline in rainfed lowland field plots. Application of prilled urea did not enhance methane emission significantly over that of the untreated control. Subsurface application of urea supergranules was, however, effective in reducing the methane flux over that of the control. Methane emission was lowest in plots treated with the mixture of prilled urea and Nimin (a nitrification inhibitor). Under irrigated shallow conditions, the application of prilled urea and green manure (Sesbania rostrata), singly and in combination, significantly increased methane emission over that of the control. Cumulative methane efflux from control and prilled urea treated lowland rice field was about 4-10 times higher than that in irrigated shallow fields. These results suggest that by virtue of their readily mineralizable carbon and ninhydrin reactive nitrogen, these substrates can serve as positive indicators of methane emission potential of rice fields.

In a laboratory incubation study, application of a commercial formulation of tridemorph to a tropical rice soil under flooded condition significantly stimulated CH 4 production at low levels (5, 10 and 20 μg.g -1 ) but inhibited the process at higher levels of 50 and 100 μg.g -1 . Oxidation of CH 4 , however, was progressively inhibited with increasing concentrations of the fungicide. Increase in CH 4 production in tridemorph-amended soil was attributed to a hastening in the soil reduction, increase in the K 2 SO 4 -extractable C content and an increase in the population of methanogenic bacteria. Inhibitory effect of the fungicide on the oxidation of CH 4 , on the other hand, was related to the inhibition in the population of methanotrophic bacteria.

Hydrolysis of urea associated with different component parts (surface and subsurface soil from unplanted and planted fields and standing water) of tropical flooded rice soil was investigated. Urea hydrolysis followed a first-order kinetics with surface soil having the highest urease activity followed by subsurface soil and flood water. Urea hydrolysis varied with the crop growth stage and was more pronounced at tillering and panicle stages under greenhouse conditions and at maturity stage under field conditions. A certain degree of cultivar variation on urease activity at both surface and subsurface soil was also noticed. Urea hydrolysis was considerably higher in soils from green manure amended field plots than that of control and urea amended plots. Results indicate that addition of green manure to a flooded soil, while increasing microbial activity, could also enhance urea hydrolysis. © 1999 Society of Chemical Industry

Although a wide range of microorganisms have been discovered that are able to degrade highly stable, toxic xenobiotics, still many pollutants persist in the environment. Recent advances in the field of r-DNA technology has provided solutions to these problems. One important factor limiting the bioremediation of sites contaminated with certain hazardous wastes is the slow rate of degradation. This slow rate limits the practicality of using bacteria in remediating contaminated sites. It is possible to extend the range of substrates that an organism can utilize. It is even possible to endow an organism with the ability to degrade a predetermined range of xenobiotics. Because biotechnological processes are based on natural activities of microorganisms and constitute variations in classic domestic waste treatment processes, they are publicly more accepted. This is an area where genetic engineering can make a marked improvement by manipulating catabolic genes of microorganisms. Advances in r-DNA technology have opened up new avenues to move toward the goal of genetically engineered microorganisms to function as "designer biocatalysts" in which certain desirable biodegradation pathways or enzymes from different organisms are brought together in a single host with the aim of performing specific detoxification. In the last 2 decades much progress has been made in this direction, and as a result catabolic genes have been cloned and characterized for organochlorines, polychlorinated biphenyls, chlorobenzoates, naphthalene etc. The aim of this review is to provide an insight in the recent advances made on characterization and expression of catabolic genes that encode the degradation/detoxification of these persistent and toxic xenobiotic compounds.

CH4 production in an alluvial soil, unamended or amended with rice straw (1% w/w), was examined under nonflooded [–1.5 MPa, –0.01 MPa and 0 MPa (saturated) and flooded (1 : 1.25 soil to water ratio)] conditions during a 40-day incubation in closed Vacutainer tubes. CH4 production was negligible at –1.5 MPa, but increased with an increase in the moisture level. Addition of rice straw distinctly increased CH4 production in the soil at all moisture levels including –1.5 MPa. Evidence, in terms of the drop in redox potential and Fe2+ accumulated, suggested that the addition of rice straw hastened the reduction of the soil, even under nonflooded conditions; thus its addition stimulated even the nonflooded soil to produce CH4 in substantial amounts. Our results indicate that many currently unidentified sources of CH4, possibly including organic-amended nonflooded soils, may make a significant contribution to the global CH4 budget.

Application of 2,4–13 and its analog 2,4,5-T at 0.75 μg g −1 soil led to a distinct increase in microbial biomass carbon content over that of untreated soil samples both under flooded and nonflooded conditions. 2,4-D was inhibitory to microorganisms at 7.5 and 15.0 μg g −1 soil. Repeated applications of a commercial formulation (an isomeric mixture) of hexachlorocyclohexane (HCH) to flooded soil caused a marked increase in microbial biomass content. Technical grade γ-HCH was also stimulatory to microbial biomass content. Suspensions of soil treated even once with commercial HCH formulation effected rapid aerobic degradation of α- and γ-HCH, but not β- and δ-HCH in a mineral salts medium supplemented individually with these isomers as a single carbon source.

The role of microorganisms in the degradation of carbosulfan (2,3-dihydro-2,2-dimethyl-7-benzofuranyl-(di-n-butyl)-aminosulfenyl++ + methyl carbamate), an analogue of carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl-N-methyl carbamate) was studied by selective enrichment of microorganisms degrading either or both insecticides by repeated application of the insecticides, individually or in combination to flooded soil. Soil suspension from the pots treated with carbosulfan and carbofuran, individually or in combination collected after two applications, effected distinctly more rapid hydrolysis of carbosulfan than did the suspension from untreated pots or the uninoculated medium. The rate of hydrolysis was further accelerated by soil suspensions collected after six applications of the insecticides in the order carbosulfan treatedcarbofuran treatedcarbosulfan + carbofuran treated. The ability of treated and untreated suspension to degrade carbofuran was also studied. The rate of degradation of carbofuran by the suspension was in the order carbofuran retreatedcarbosulfan + carbofuran retreatedcarbosulfan retreated soil. Further involvement of microorganism in the rapid degradation of carbosulfan was confirmed by testing the degradation in sterilized and nonsterilized enrichment culture.

Applications of a commercial formulation of carbofuran, a carbamate insecticide, at rates of 2kg and 12kg active ingredient ha–1 to flooded fields planted to rice led to significant inhibition of methane emission. Likewise, laboratory incubation studies showed that carbofuran applied at low rates (5 and 10μgg–1soil) inhibited the net methane production relative to that of the control, but stimulated it when applied at a rate of 100μgg–1soil. Interestingly, carbofuran increased the oxidation of methane when applied at low rates and inhibited it when applied at a rate of 100μgg–1soil.

In a field study, phosphorus applied as single superphosphate (SSP) distinctly inhibited CH 4 emission from a flooded field plot planted to rice. In subsequent laboratory studies, application of P at 50 and 100 μ g g −1 dry soil as K 2 HPO 4 stimulated CH 4 production in a P-deficient soil, while there was no such stimulation in P-normal alluvial soil. CH 4 production in soils depended on the source of P applied as evidenced by stimulation with K 2 HPO 4 or Jordan rock phosphate, while Mussorie rock phosphate and SSP were inhibitory. The results indicate that the S content of the P sources determines the extent of stimulation or inhibition of CH 4 production. Moreover, supplementary addition of K 2 SO 4 , even with K 2 HPO 4 , mimicked the inhibitory effect of SSP on CH 4 production. In practice, use of SSP in rice cultivation, in addition to supplying P to the growing crop, could mitigate CH 4 production and emission through its S content.

In a laboratory incubation study, effect of various anions on net methane production in two rice soils (alluvial and acid sulphate) under flooded conditions was examined. Methane production was considerable in alluvial soil and almost negligible in acid sulphate soil, albeit with a higher density of viable methanogens, during 30-day incubation without salts. Sodium salts of hydroxide and phosphate further stimulated methane production in alluvial soil and marginally in acid sulphate soil. But, addition of sodium molybdate, a selective inhibitor of sulphate-reducing bacteria, increased the production of methane in acid sulphate soil. In contrast, nitrite, nitrate, sulphite and sulphate suppressed the production of methane in both soils. Acetate served as an excellent substrate for methanogenesis in alluvial soil, but not in acid sulphate soil. Succinate and citrate also stimulated methane production especially in alluvial soil, but after a longer lag. In acid sulphate soil, most of the added carbon in the form of sodium salts of carboxylic acids was converted to CO2 and not methane, which is consistent with their preferential use by the sulphate-reducing bacteria. In general, none of the amendments could increase production of methane in acid sulphate soil to the same level as in alluvial soil.

Wide variation in CH 4 flux was noticed among the ten rice cultivars grown under uniform field conditions. Cumulative CH 4 flux ranged from 4.61 g m -2 to 20.25 g m -2 . The rice cultivars could be ranked into three groups based on their CH 4 flux potential. Rice cultivars could also be arranged based on their peak CH 4 emission occurring either at vegetative, reproductive or at both growth stages. Of the several variables studied (root region redox potential, above- and underground biomass, grain and straw yield, duration of the crop, percent area occupied by the air space and root oxidase activity), only oxidase activity of the root tip exhibited a significant (negative) correlation with CH 4 flux indicating an indirect effect of root oxidation potential on CH 4 flux. Data presented in this study, demonstrate inherent variation in CH 4 flux among different rice cultivars that can be used for developing future mitigation options.

Application of commercial formulation (an isomeric mixture) of hmachlorocyelohexane (HCH) to flooded rise fields planted to cv. Ran at 1 kg active ingredient.ha−1 significantly inhibited mute emission. Negative P oxafed between methane emission from HCH-treated fields and root oxidise activity (in tents of α-naphthylamine oxidised). Under laboratory condaition, the addition of technical grade α-, β-, γ- and δ-isomers (99.1% purity) of HM individually at 20 μg.g−1 soil at flooding, effected a distinct inhibition of methane production. Evidence suggests that the inhibitory effect of all the four isomers on methane production only until 20–25 days after their application was related to their persistence in flooded soil.

In a greenhouse study, the effect of moisture regimes (continuously flooded, continuously nonflooded, alternately flooded) on methane efflux from an alluvial soil planted to rice was studied using the closed chamber method. Methane efflux was almost 10 times more pronounced under continuously flooded conditions than under continuously nonflooded conditions. Intermittently flooded regimes (alternately flooded and nonflooded cycles of 40 or 20 days each) emitted distinctly less methane than the continuously flooded system. A significant negative correlation was found between methane emission under different water regimes and rhizosphere redox potential. Extractable Fe2+, readily mineralizable carbon (RMC) and root biomass presented a significant positive correlation with cumulative methane emission. The correlation of methane emission with other plant parameters and microbial biomass was not significant. Our results further suggest the possibility of reduced methane emissions through appropriate water management in a rainfed rice ecosystem.

Diurnal variation in CH4 efflux from continuously flooded fields planted to rice (Oryza sativa L. cv. IR-36) was examined at different crop growth stages using a closed chamber method during the wet season. CH4 emission showed a distinct diurnal pattern especially at tillering, panicle initiation and maturity stages of a field-grown rice crop, with maximum emission in the early afternoon (12.00 to 15.00) followed by a decline to a minimum around midnight. Among several variables (ambient temperature, flood water temperature, redox potential, soil pH, and root oxidase activity), a significant negative correlation existed between oxidase activity of the root base and diurnal fluctuations in CH4 efflux at tillering stage. Evidence also suggested that redox status in the rhizosphere region and atmospheric, soil, and water temperatures influenced CH4 emission from rice fields probably by their contrasting effects on CH4 production and oxidation.

In a study on spatial distribution of methane oxidation in an unplanted flooded field, methane-oxidizing activity, analysed in soil samples under laboratory conditions, decreased with increasing depth (25 cm and beyond). In a flooded field planted to rice, rates of methane oxidation followed the order : rhizosphere (collected from roots at 10-20 cm depth) > surface soil at (0-1 cm) > subsurface soil at 10-20 cm depth, diagonally 10-15 cm away from the centre of hill. Application of ammonium sulfate and, to a lesser extent, urea to surface, rhizosphere and subsurface soil samples from flooded field planted to rice effected a distinct inhibition of methane oxidation. Nitrification inhibitors (thiourea, sodium thiosulfate and dicyandiamide) were also effective in inhibiting methane oxidation. Both surface and rhizosphere soil samples harbored higher populations of methane-oxidizing bacteria than the subsurface soil. Inhibition of methane oxidation in surface and rhizosphere soil samples concomitant with the suppression of autotrophic ammonium oxidizers by nitrification inhibitors implicates an active involvement of autotrophic ammonium oxidizers in methane oxidation.

Nitrification associated with the various components [subsurface soil from unplanted and planted (rhizosphere) fields, standing water and surface soil from planted and unplanted fields and leaf sheath suspensions] of submerged rice paddies was examined in incubation experiments with solutions inoculated with soil or water samples. Substantial nitrification occurred in all samples, standing water and surface soil samples in particular, during their 40-day incubation with NH 4 + −N. Almost all the NH 4 + −N, disappeared during incubation with standing water, was recovered as NO inf3 sup- −N. This, compared to 70–80% from all soil samples and only 29% from leaf sheath suspensions. Significant loss of nitrogen, especially from leaf sheath suspensions, is probably due to nitrification-denitrification as evidenced by its complete recovery in the presence of N-Serve. Nitrification potential of the soil and water samples varied with the crop growth stage and was more pronounced at tillering and panicle inititation stages than at other stages. Nitrification potential of samples from green-manure-amended plots was distinctly less than that of samples from control and urea-amended plots. Most probable number (MPN) estimates of ammonium-oxidizing bacteria were always higher in surface soil in both planted and unplanted plots at all stages of crop growth.

Results of national methane campaign launched in 1991 to assess methane budget from Indian paddy fields are reported. The campaign involved a number of scientific institutions and universities with National Physical Laboratory at Delhi operating as a nodal agency and covered most of the major rice growing regions of India. Methane emission rates ranged between −0.64 and 84.1 mg −2 h −1 . The methane budget from Indian paddies has been estimated to be around 4.0 TgY −1 with a range between 2.7 to 5.4 TgY −1 .

In a study on methane production from two rice soils (alluvial and acid‐sulfate) differing in physicochemical characteristics, methane production in both soils was negligible under nonflooded conditions. Submergence enhanced the production of methane in alluvial soil, especially upon its incubation after leaching. However, methane production was not extensive in organic‐matter‐rich, acid‐sulfate (Pokkali) soil even under submergence and despite favorable conditions of near neutral pH and low redox potential. Addition of calcium sulfate or leachate from acid‐sulfate soil inhibited the production of methane in flooded alluvial soil. Addition of molybdate, a known inhibitor of sulfate‐reducing bacteria, marginally enhanced the production of methane in acid‐sulfate soil but retarded it in alluvial soil. The findings suggest the involvement of not only sulfide but also other factors such as salinity in inhibiting methane production in acid‐sulfate soil.

The degradation of uniformly 14 C-labeled isomers (α, γ, and β) of hexachlorocyclohexane (HCH), added separately to a mineral salts medium as a sole source of carbon, by a soil bacterium, Pseudomonas sp., under aerobic conditions was studied. During aerobic degradation, about 10-12% of the 14 C in α- and γ-HCH was accounted for as 14 CO 2 as compared to 5% from β-HCH. Most of the 14 C in α- and γ-HCH accumulated as water-soluble products, while formation of water-soluble products from β-HCH was negligible. Almost all of the 14 C in the three isomers of HCH was accounted for in different fractions (CO 2 , chloroform-diethyl ether, water phase) following bacterial degradation under aerobic conditions. GC-MS analysis of β-HCH residues in the chloroformdiethyl ether extract suggested the formation of pentachlorocyclohexanol and tetrachlorocyclohexanediol as products of aerobic metabolism of β-HCH

In a study on CH4 emission from flooded rice fields under irrigated conditions, fields planted with rice emitted more methane than unplanted fields. The CH4 efflux in planted plots varied with the rice variety and growth stage and ranged from 4 to 26 mg h-1m-2. During the reproductive stage of the rice plants, CH4 emission was high and the oxidation power of rice roots, in terms of α-naphthylamine oxidation, was very low. The CH4 emission reached a maximum at midday and declined to minimum levels at midnight, irrespective of the rice variety. The peak CH4 emission at midday was associated with higher solar radiation and higher soil/water temperature.

p-Nitrophenol was mineralized more rapidly in the rice rhizosphere than in unplanted soil under non-flooded and flooded conditions. The effect of the rhizosphere on the mineralization of p-nitrophenol was more pronounced under flooded conditions and was influenced by the pre-exposure of the rice rhizosphere to p-nitrophenol. The rice rhizosphere pretreated with p-nitrophenol generated more 14 CO 2 from 14 C-p-nitrophenol than that without pretreatment.

The presence of unwanted substances in the environment becomes “pollution” when damage or deleterious effects caused by those substances are detected. Pollution is essentially a process by which a natural or a man-made resource becomes unfit for beneficial use (Enger and Smith 2006). Many man-made substances, in addition to certain naturally occurring organic and inorganic compounds, different forms of energy (e.g., heat, light and noise), and other things, add to the diversity of pollutants. Pollutants, which can occur at the local or the global level, render varying influences on soil and water that depend on the nature and spreading rate of the pollutants themselves, and produce short- or long-term effects on elements of an ecosystem. Undesirable changes in air, water, and soil may induce diverse responses from living organisms that are similar to those induced by the presence of pollutants. The effects on organisms caused by pollutants may range from innocuous to toxic. Toxicity, which has been defined as an inherent property of a substance to cause an adverse biological effect, is the result of a chemical disturbance that affects complex and interrelated systems involving cells, tissues, organs, or metabolic processes (ECETOC 1985).

Fenitrothion or its hydrolysis product, 3-methyl-4-nitrophenol was applied thrice to three soils under flooded or nonflooded conditions at 15-day intervals. After third application, suspensions of treated and untreated soils were tested for their ability to degrade fenitrothion or 3-methyl-4-nitrophenol added to a mineral salts medium. Repeated additions of fenitrothion or 3-methyl-4-nitrophenol conditioned the three soils for accelerated hydrolysis of fenitrothion, but depending on soil type, moisture regime and the applied substrate (fenitrothion or 3-methyl-4-nitrophenol). Some enrichment cultures degraded fenitrothion past 3-methyl-4-nitrophenol to nitrite.

A Pseudomonas sp., that readily degraded α-,β- and γ-isomers of hexachlorocyclohexane (HCH) in a mineral salts medium under aerobic conditions, was tested for its ability to degrade these isomers after their application to two soils under flooded or nonflooded conditions. Interestingly, the concentration of soil-applied α- and γ-HCH decreased from the original concentration of 5 μg g −1 soil to undetectable amounts within 10–20 days in soils inoculated with Pseudomonas sp., and the loss of both isomers was more pronounced under nonflooded conditions than under flooded conditions. During the corresponding period, there was no appreciable loss of either isomer from uninoculated soils. The degradation of β-HCH was not accelerated by bacterial inoculation of the soils under either water regime despite the ability of the bacterium to degrade this isomer when added to the mineral salts medium. The rapid degradation of α- and γ-HCH, and not β-HCH, in inoculated soils is attributed to the ability of the bacterium to utilize α- and γ-HCH, and not β-HCH, for growth. Addition of sodium acetate to the mineral salts medium enhanced the bacterial degradation of β-HCH implicating cometabolism and retarded the degradation of γ-HCH.

The dominant bacteriaPseudomonas sp. andArthrobacter sp. were isolated from the standing water of carbofuran-retreatedAzolla plot.Arthrobacter sp. hydrolysed carbofuran added to the mineral salts medium as a sole source of carbon and nitrogen while no degradation occurred withPseudomonas sp. Interestingly, when the medium containing carbofuran was inoculated with bothArthrobacter sp. andPseudomonas sp., a synergistic increase in its hydrolysis and subsequent release of CO2 from the side chain was noticed. This synergistic interaction was better expressed at 25° C than at 35° C. Likewise, related carbamates, carbaryl, bendiocarb and carbosulfan were more rapidly degraded in the combined presence of both bacterial isolates.

Moist surface soil samples from a flooded soil, pretreated twice with a commercial formulation of an isomeric mixture of hexachlorocyclohexane (commercial HCH) to induce accelerated aerobic biodegradation of γ-HCH, were incubated at 20, 25 and 35°C with and without replenishment of water lost by evaporation. Soil samples, kept continuously moist, retained the capacity for accelerated aerobic biodegradation of added γ-HCH even after 40 days incubation at all three temperatures, but the enhancement factor (causing accelerated degradation of γ-HCH) was inactivated at 25 and 35°C, and not at 20°C, within 20 days when commercial HCH-acclimatized soil samples were subjected to moisture stress. Evidence suggested that temperature-induced moisture stress caused the inactivation of the enhancement factor, especially at 35°C. The enhancement factor developed almost with equal ease in both rice-straw-amended and unamended soil irrespective of water regime (flooded or non-flooded) after multiple applications of a commercial formulation of HCH. The population of γ-HCH-degrading aerobic, but not anaerobic, micro-organisms increased distinctly after each successive application of commercial HCH, interestingly at identical rates in both flooded and non-flooded soils. Sphingomonas paucimobilis isolated from flooded soil degraded α-,β-,δ- and γ-isomers of HCH under aerobic conditions.

Efforts are continuously being made to understand the non-target effects of environmental pollutants toward microalgae and cyanobacteria because of their ubiquity in aquatic and terrestrial environments and their highly adaptive survival abilities under environmental and evolutionary pressure over geological time. Depending on the toxicity criteria employed for these ecologically beneficial organisms, the impact of low and high doses of pollutants can range from stimulation to total inhibition. All of the investigations carried out so far have been predominantly concerned with individual chemicals despite the occurrence of pollutants in mixtures. In addition, only individual isolates have been primarily used to gather scientific information on the toxicity of pollutants. The risk assessment of pollutants toward these organisms necessitates further investigations, combining innovative molecular ecological methods and those for in situ analysis at the community level. The present review highlights the toxic influences of organic and inorganic pollutants and the response in terms of detoxification and resistance by these organisms. Refereed/Peer-reviewed

Soils (an alluvial and a laterite) contained in pots were treated with methyl parathion, fenitrothion or p-nitrophenol at 15 day intervals under flooded or non-flooded (60% WHC) conditions. Suspensions (non-sterile and sterile) of these treated and untreated soils were tested for their ability to degrade methyl parathion, parathion, fenitrothion and diazinon in a mineral salts medium. Only non-sterile suspensions of methyl parathion- and p-nitrophenol-enriched soils (flooded or non-flooded) distinctly effected accelerated hydrolysis of methyl parathion, parathion and fenitrothion; in contrast, diazinon was not hydrolysed by the suspension of methyl parathion-enriched soils. p-Nitrophenol, formed from methyl parathion or parathion, was eventually metabolized to nitrite by the enrichment cultures while 3-methyl-4-nitrophenol, formed from fenitrothion, resisted further degradation. As in soil enrichment cultures, two bacterial isolates, one each from methyl parathion-enriched alluvial and laterite soils (flooded), effected rapid hydrolysis of methyl parathion, parathion and fenitrothion and then metabolized p-nitrophenol and not 3-methyl-4-nitrophenol, to nitrite. Fenitrothion-retreated soils (flooded or non-flooded) were not conditioned for accelerated hydrolysis of fenitrothion and related phosphorothioate insecticides.

The degradation of carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl N-methylcarbamate), sorbed to three soils of varying physicochemical characteristics, by an enrichment culture from a carbofuran-retreated Azolla plot was studied. The concentration of carbofuran, sorbed to the alluvial soil, decreased to undetectable levels (less than 2 μg) within 3 days of inoculation with the enrichment culture; in contrast, 68-83% of the insecticide, sorbed to Pokkali and Kari soils, was recovered even 6 days after inoculation []

A commercial wettable-powder formulation (50% a. i.) of hexachlorocyclohexane (HCH) was applied to unplanted and planted (to rice) soils under flooded and non-flooded conditions at 1 kg a. i. ha−1 at 15-day intervals. A mineral salts medium supplemented with γ-HCH as a sole source of C was inoculated with suspensions from HCH-treated and untreated soils (unplanted or planted to rice) and incubated under aerobic conditions. γ-HCH disappeared completely within 10 days from the aerobically incubated medium inoculated with the suspension from the treated soil compared to less than 30% loss from the uninoculated medium or from the medium inoculated with the suspension from the untreated soil, during the corresponding period. Soil samples from HCH-treated flooded pots lost their capacity for accelerated degradation of γ-HCH after autoclaving. The addition of HCH clearly stimulated aerobic degradation of γ-HCH, even in predominantly anaerobic flooded soil. The factor responsible for the accelerated degradation of γ-HCH in pretreated soil was not detected in the deeper layers (>10 cm) of flooded soil and in fields that had dried following the rice harvest.