B. Ramakrishnan

Methane emission from rice fields at Cuttack, India

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−1 in 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−1 yr−1.

Diurnal variation in methane efflux at different growth stages of tropical rice

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.

Wetland rice soils as sources and sinks of methane: a review and prospects for research.

Abstract 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.

Cultivar variation in methane efflux from tropical rice

Wide variation in CH4 flux was noticed among the ten rice cultivars grown under uniform field conditions. Cumulative CH4 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 CH4 flux potential. Rice cultivars could also be arranged based on their peak CH4 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 CH4 flux indicating an indirect effect of root oxidation potential on CH4 flux. Data presented in this study, demonstrate inherent variation in CH4 flux among different rice cultivars that can be used for developing future mitigation options.

Review: Current status and future prospects of associative nitrogen fixation in rice

Biological sources of nutrients are gaining importance over the chemical and organic sources from the standpoint of environmental safety and quality, and sustainable agriculture. The nutrient input for a growing rice crop can largely be met by promoting the activities of physiologically diverse microorganisms in the aerobic, anaerobic and interface zones in the ecologically important flooded soils. Associative bacteria contribute from 10 to 80 kg N per hectare per cropping season depending upon the ecosystem, cultural practices and rice variety grown. In addition to N contribution, these bacterial associations can improve the nutrient transformations and contribute to plant growth-promoting effects. Current improved agronomic and crop production management systems greatly affect the contributions of biological sources to the overall soil nutrient status. Azospirillum and other associative bacterial systems have been intensively researched using various evaluation techniques to understand the diazotrophic rhizocoenosis. Researches clearly indicate that these associations are governed by several soil, water, nutrient, agrochemical, plant genotype and other biological factors. Considerable efforts have been made so far in selecting efficient bacterial strains as inoculants and identifying host genotypes which support maximum nitrogenase activity in addition to other beneficial traits of effective associative relationships. Knowledge gained so far on how the N2-fixing system in rice functions suggests the need for providing optimum management practices to ensure greater contribution from the plant-microbe associations. Holistic approaches integrating technological developments and achievements in biological sciences could lead to crop improvement. Research on extending nitrogen-fixing symbiosis to rice using molecular and genetic approaches is underway, albeit at a slow pace. The need for further fine-tuning and developing management practices, innovative approaches to improve rice-bacterial systems and the strategies to sustain the benefits from associative diazotrophy are discussed.

Methane production in unamended and rice-straw-amended soil at different moisture levels

Abstract 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.

Effect of the herbicide butachlor on methane emission and ebullition flux from a direct-seeded flooded rice field

Abstract 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.

Influence of the insecticide carbofuran on the production and oxidation of methane in a flooded rice soil

Abstract 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.

Methane production in two Indian rice soils

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.

Temperature Dependence of Methane Production in Tropical Rice Soils

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 existed between microbial population (methanogens and sulphate reducers) and CH 4 production. Comparing the temperature coefficients ( Q 10 ) for methane production within each soil type at low (15°C-25°C) and medium (25°C-35°C) temperature intervals revealed that these values were not uniform for both alluvial and laterite soils. But acid sulphate soil had Q 10 values that were near 2 at both temperature intervals. When these soil samples were amended with substrates (acetate, H 2 -CO 2 , and rice straw), there were stimulatory effects on methane production rates and consequently on the Q 10 values. The pattern of temperature coefficients was characteristic of the soil type and the nature of substrates used for amendment.