Plant and Soil201: 157–164, 1998.© 1998Kluwer Academic Publishers. Printed in the Netherlands.

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Methane flux in relation to growth and phenology of a high yielding ricevariety as affected by fertilization

Smita Singh, A.K. Kashyap and J.S. SinghDepartment of Botany, Banaras Hindu University, Varanasi 221 005, India∗

Received 21 May 1997. Accepted in revised form 6 March 1998

Key words:growth parameters, methane flux, N-fertilization,Oryza sativa, root porosity, urea

Abstract

Influence of urea application on growth parameters (shoot height, and weight, root volume, weight and porosity;number of tillers; grain yield) and their relationship with methane (CH4) flux was investigated inOryza sativa(var. Pant Dhan-4) under flooded soil condition. The study design consisted of (a) fertilized vegetated, (b) controlvegetated, (c) fertilized bare, and (d) control bare plots. Crop growth and CH4 flux measurements were conductedfrom 9 to 115 days of rice transplanting at regular intervals of 10 days. Results showed that there were significantdifferences due to days (dates of measurement) and fertilization in all growth parameters except shoot height. Day× fertilization interaction was significant for all growth parameters. CH4 fluxes ranged from 0.4 to 20.2, 0.1 to11.9, 0.09 to 2.2 and 0.004 to 1.5 mg m−2 h−1 under treatments (a), (b), (c) and (d), respectively. Maximum CH4flux was recorded at the flowering stage. All the growth parameters, including number of tillers, showed strongpositive relationship with total methane flux. Root porosity was also strongly correlated with total CH4 emission.It was concluded that CH4 emission was substantially influenced by crop phenology and growth, and fertilization.The study emphasizes the substrate production and conduit effects of rice plants on CH4 flux.

Introduction

Methane is a chemically reactive trace gas and permolecule is some 25–35 times more effective as agreenhouse gas than CO2 (Kiehl and Dickinson 1987).The mixing ratio of methane has been increasing andhas reached a level of 1.7µmol/mol of air (Slaninaand Warneck, 1994). Over the past 20 yr, CH4 growthrate declined; in the late 1970s the concentration wasincreasing by about 20 ppbv yr−1, during the 1980sthe growth rate dropped to 9–13 ppbv yr−1. Aroundthe middle of 1992, CH4 concentration briefly stoppedgrowing but since 1993 the global growth rate hasreturned to about 8 ppbv yr−1 (IPCC, 1995). Steeleet al. (1992) and Dlugokencky et al. (1994) also re-ported that the rate of increase has slowed down in thelast decade. Rice paddy fields are important anthro-pogenic source of methane and this source strength hasincreased in recent decades due to expansion of rice

∗ FAX No.: 91 0542 317074. E-mail: jssingh@banaras.ernet.in

cultivation. There is strong evidence that more than90% of CH4 emission from the anoxic paddy soils isthrough the rice plant (Inubushi et al. 1989; Schutz etal., 1989). The aerenchyma tissue of rice plants servesas a conduit to transport CH4 from the anoxic soil tothe atmosphere (Mariko et al., 1991).

Rice fields exhibit complex interaction betweenplants, soil and the atmosphere (Husin et al., 1995).There are many factors that could affect methaneproduction and emission from rice fields includingclimate, soil type, water management, rice cultivars,organic and inorganic fertilizers, and season. In thisstudy, we measured CH4 emission from flooded agri-cultural fields planted to a high-yielding rice variety,with and without urea fertilization, and have attemptedto correlate the fluxes with shoot and root biomass,shoot height, root volume, root porosity, number oftillers, grain yield and phenological stages of the ricecrop.

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Materials and methods

Experimental field and cultivation of rice plant

The experimental field was located in the Instituteof Agricultural Sciences, Banaras Hindu University,Varanasi (25◦18′ N lat. and 83◦3′ E long., 129 m abovemsl). The region has a seasonally dry tropical climatewith a typical monsoonal character. The year is divis-ible into a cold winter (November–February), a hotsummer (April–June), and a warm rainy season (July–September). The annual rainfall averages 1100 mm,of which 85% falls during the rainy season from thesouth-west monsoon. The soil is well drained, siltyloam, Inceptisol with 8–8.4 pH, 0.68–0.76% organicC, 0.07–0.08% total N, and 304–340µg g−1 total P.

The experimental field consisted of 12 plots, each15 m2, in completely randomized block design. A0.5 m strip separated each plot. Basal treatment of KCl+ P2O5 + farmyard manure was applied at a rate of60:60:1000 kg ha−1, at the time of plowing. Six plotswere fertilized with urea while the other six plots werenot fertilized and served as control. In fertilized plots,urea was broadcasted at the time of transplanting ata rate of 40 kg N ha−1. Additional application of ureawas in the form of two split doses (30 kg N ha−1 each),one at the time of active tillering and grain fillingstage of rice crop. Out of the 12-plots, six plots (withand without urea) were transplanted with rice (vege-tated plots) while the other six plots (with and withouturea) were kept as bare (unvegetated plots). Thus theexperiment consisted of 3 plots each for fertilized veg-etated, control vegetated, fertilized bare, and controlbare treatments.

Twenty-one day old seedlings of rice (var. PantDhan-4) were transplanted to vegetated plots at a spac-ing of 15 cm on 17 July 1995. Row to row distance was20 cm. Field was submerged before transplanting, andthe water level varied between 6.9–23.9 cm over a pe-riod of 89 DAT (days after transplantation) in responseto rainfall events. Thereafter there was no standingwater but the soil remained saturated up to 115 DAT.Pant Dhan-4 (IR262X Ramadja), a high yielding ricevariety, was released in 1983 for cultivation under irri-gated condition in this region (eastern Uttar Pradesh).It is an early variety of medium height plants, havingvegetative phase, 45–55 DAT, reproductive phase, 55–75 DAT, and ripening phase, 80–121 DAT. Seeds wereobtained from the Institute of Agricultural Sciences,Banaras Hindu University. The fertilizer dose and thewater level mimiced the regional practices.

Plant growth measurements

Growth of rice plant was monitored at regular inter-vals of 10 days up to the harvest. Rice hills identi-cal to those present within the area covered by thealuminium base of the CH4 flux measurement cham-ber were marked for growth measurements. Thesemarked hills were randomly selected for growth mea-surements. One rice hill was harvested from eachexperimental plot on each sampling date, and rootswere collected together with soil as a block (15× 20× 15 cm depth) using a rectangular open-top plasticcylinder. Soil was completely washed out with tap wa-ter. Shoot height was measured with a meter scale.The tip of the longest leaf of the hill was consideredas the end point of rice plant. Number of tillers werealso counted. Root volume was measured by waterdisplacement (in mL) method. After the volume mea-surement, half of the root material was dried at 60◦Cfor estimation of root weight and the other half wasused for determination of root porosity. Shoots werealso dried at 60◦C to constant weight for biomassdetermination.

Measurement of root porosity

Root porosity was measured following Jensen et al.(1969). In brief, a 15 mL pycnometer was filled withwater and weighed. About 0.2–0.5 g of washed freshroots were cut (2–2.5 cm from the root apex) and gen-tly blotted dry. The roots were then carefully insertedinto the water-filled pycnometer and reweighed. Theroots were retrieved, ground with mortar and pestle,and returned quantitatively into the pycnometer forreweighing. The porosity of the roots was determined,using the following relationship

POR = [(p&gr)− (p&r)][(r + p)+ (p&r)] × 100

where POR = root air space (porosity) in %; r = massof roots in g; p = mass of water-filled pycnometer ing; p & r = mass of pycnometer with roots and water ing; and p & gr = mass of pycnometer with ground rootsand water in g.

Measurement of methane flux

Methane flux was measured in each plot at regularintervals (10 days) after transplanting throughout therice growth period although we do recognise that CH4evolution may fluctuate tremendously even over a 1-

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or 2-day period. Sampling for CH4 was accomplishedby using 2 piece transparent perspex chamber as de-scribed by Parashar et al. (1993). One aluminiumbase was installed manually in each plot before thecommencement of sampling for CH4. The base en-closed 6 rice hills. A perspex chamber (52.5 L× 33W × 63 H cm) which fitted into the groove of thealuminium base was put in place at the time of sam-pling, covering an area of 0.168 m2. The air insidethe chamber was isolated from the outside atmosphereand the system was made air-tight by filling the groovein the aluminium base with water. A battery-operatedair-circulation pump connected to polyethylene tubingwas used to mix the air inside the chamber and to drawthe air samples from the headspace into Tedlar bagsat 0, 15 and 30 min. Sampling of methane gas wasconducted between 9.00–11.00 AM. The temperatureinside the perspex chamber was recorded using a ther-mometer inserted through a rubber septum installedat the top of the chamber. On an average, tempera-ture inside headspace increased by 3◦C during the 30min enclosure. Soil temperature was also measuredwith the help of soil thermometer inserted into thesoil near the chamber at the time of sampling. Soiltemperature did not vary during the 30-min enclo-sure. Temperature inside the chamber varied between45 (August) and 35◦C (November), and the corre-sponding soil temperature varied from 33 to 27◦C.Air samples were analysed for CH4 on a gas chro-matograph equipped with a flame ionization detectorand porapak Q column. Column, injector, and detec-tor temperatures were maintained at 27, 110 and 180◦C, respectively, and nitrogen served as a carrier gas(flow rate 30 mL min−1). Fluxes were calculated bythe difference in CH4 concentration between sampleintervals, using the following expression (Parashar etal., 1996):

CH4 flux (F) (mg m−2h−1) =BVSTP × CCH4 × 16× 1000× 60

106 × 22400× A× t

where BVST P (chamber air volume in cc at STP) =BV × BP × 273

(273+ T )+ 760

BV = chamber air volume[(H-h) LW-biomass volume inside box]

where H = chamber height; h = water level abovechannel (cm); L = chamber length (cm); W = chamberwidth (cm); BP = Barometric pressure (mm Hg); T =Chamber air temperature at the time of sampling (◦C);

CCH4 = Change in CH4 concentration in ppmv fromzero minute sampling to the t minute sampling; A =paddy area covered by the box (m2); and t = time inminutes.

Results and discussion

Root and shoot dry weights, shoot height, root volume,and number of tillers for control and fertilizer treatedplots across the growth period of rice are plotted inFigure 1. ANOVA indicated that all growth parameterswere significantly correlated with growth period andfertilization.

Growth of rice was divisible into three main phasesas described by Vergara (1970); vegetative phase(from seed germination to panicle initiation), repro-ductive phase (from panicle initiation to flowering)and ripening phase (from flowering to full maturity).There was no influence of fertilizer treatment on theduration of phenophases. Tanaka (1976) describedthree types of phasal development in rice on the ba-sis of tiller number and phenophase. These are: TypeA, maximum tillering between panicle initiation andflowering stage; Type B, maximum tillering at the timeof panicle initiation; and Type C, maximum tilleringbefore panicle initiation stage. In the present studythe maximum tiller number stage coincided with pan-icle initiation and, therefore, Pant Dhan-4 falls underType B. Maximum tiller numbers averaged 310 and350 m−2 for control and fertilized plots, respectively.After the maximum tiller number stage, some tillersdied and the number of tillers declined.

Shoot height increased gradually upto the ripeningstage of rice plant (Figure 1a). Shoot weight exhib-ited a sigmoid growth pattern. The biomass increasedexponentially up to the flowering stage (65–77 DAT)and subsequently remained constant (Figure 1b). Rootweight peaked at the flowering stage and was 171 and213 g m−2 for control and fertilized plots, respectively.After the flowering stage, root weight declined slowly(Figure 1c). Results on changes in root volume to adepth of 15 cm across the growth period showed thatroot volume also peaked at flowering stage and was1713 mL m−2 and 2466 mL m−2 for control and fer-tilized plot, respectively (Figure 1d). The grain yieldaveraged 5100 kg ha−1 for the control plots and 6400kg ha−1 for the fertilized plots.

Observations on methane flux were initiated on9th day after transplanting and continued till 115DAT (Figure 2). In the vegetated control plot, CH4

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Figure 1. Changes in (a) shoot height and number of tillers (b) shoot weight, (c) root weight, and (d) root volume of rice plants during thecultivation period. Bars indicate 1 SE.

flux ranged from 0.11 to 11.93 mg m−2 h−1. Thesevalues compare with those reported for Delhi (0.39–2.90 mg m−2 h−1), Dehradun (0.07–0.13 mg m−2

h−1), Karnal (1.47–57.98 mg m−2 h−1), Faizabad(0.1–3.5 mg m−2 h−1), Allahabad (0.4 mg m−2 h−1)and Cuttack (4.0–26 mg m−2 h−1) for different ricevarieties (Adhya et al., 1994; Parashar et al., 1991,1996). The above sites are a part of northern Indiaexcept for Cuttack which is located in the easternpart of India, and have sandy loam soils as in thepresent investigation. Experiments conducted in thewestern part of India (Ahmedabad, Gujarat) which isrelatively dry, the flux rates ranged between−0.36and 27.43 mg m−2 h−1 although soil was sandy loam(Gupta et al., 1994). Methane flux was found to belower (2.9–8.6 mg m−2 h−1) in rice fields of Kalyani(West Bengal), a high rainfall area (Saha et al., 1989).Experiments with non-vegetated plots confirmed pre-vious experiments by Holzapfel-Pschorn et al. (1986)

and resulted in 0.004–2.173 mg CH4 m−2 h−1 whichis about 3.6–10.75% of the vegetated plots. The re-sults thus indicated that rice plants play a major role inmethane emission from the soil (Holzapfel-Pschorn etal., 1986; Schutz et al., 1989).

Methane emission from the rice plots increasedup to flowering stage (Figure 2) and covaried withshoot and root growth parameters. Regression analysisconducted for the rate of CH4 flux and plant growthparameters up to the flowering stage, separately forfertilized and control plots, indicated that there was asignificant positive relationship with shoot height andweight; number of tillers, and root weight and volume(r2 ranged from 0.582 to 0.947,P< 0.001). These re-lationships may not be causal but they do indicate thatthe CH4 emission covaried with crop growth. Seiler(1984) reported maximum CH4 emission at the endof heading and flowering of rice plants in Spain. Potexperiments with or without13C-labelled rice straw

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Table 1. Relationships between mean growth parameters of rice plant (X)and total methane emission for the crop period (Y, g m−2), according to Y =a + bX. For this analysis each replicate plot was considered an independentplot. Values in parenthesis are 1 SE

Growth parameters a b r2 p

No. of tillers (m−2) −63.547 0.363 0.858 0.0079

(±17.16) (±0.07)

Root volume (mL m−2) −0.406 0.018 0.726 0.0311

(±6.671) (±0.005)

Grain yield (g m−2) −9.733 0.052 0.809 0.0145

(±7.497) (±0.012)

Root porosity (%) −1.046 1.094 0.914 0.0028

(±3.445) (±0.160)

Shoot weight (g m−2) −29.957 0.109 0.752 0.0252

(±14.623) ±0.031)

Root weight (g m−2) −25.555 0.378 0.881 0.0055

(±8.567) (±0.069)

Figure 2. Temporal variation in CH4 emission rate from irrigatedbare and planted plots of rice field. The planted rice variety wasPant Dhan-4. Each data point is an average of measurements fromthree replicate plots. Bars indicate 1 SE.

and rice plants revealed that the peak CH4 emissionat the heading stage was mainly due to the release oforganic materials from rice plants (Chidthaisong andWatanabe, 1997).

The rate of methane emission declined after flow-ering, both in control (after 65 DAT) and fertilizeramended plots (after 75 DAT) and reached a neg-ligible level by 115 DAT (Figure 2). A decline inCH4 emission from rice field at the end of season has

been also demonstrated by Nouchi et al. (1994). Itwas suggested that this decline was due to a declinein conductance of the rice body for CH4 transport.The decline in conductance was possibly due to re-duced permeability of the root epidermal layer as aconsequence of aging (Nouchi et al., 1994).

In this study CH4 emission was greater from plotsamended with urea (Figure 2). The methane emissionpeaked 10 days later than control indicating the impor-tance of nitrogenous substrate. Significant increases inCH4 emission due to application of urea has been re-ported in several investigations (Lindau, 1994; Schutzet al., 1989; Singh et al., 1996). It has been arguedthat application of N-fertilizer decreases soil C:N ra-tio and promotes activity of soil microorganisms, thuscontributing to increased CH4 production (Wang etal., 1992). Since fertilization enhances plant growthmarkedly, a larger input of organic material into soilas root exudates in the fertilized plots might causehigher CH4 production rates as argued by Schutz etal. (1989).

Total CH4 emission from vegetated plots for theentire crop growth period averaged 15.43 and 26.12g m−2 for control and fertilizer amended plots, respec-tively.

Root and shoot weight averaged across growthstages, and total grain yield showed strong positiverelationship with total methane flux (Table 1). Theseparameters are perhaps associated with the supply ofresources for methanogenesis. A strong predictive re-lationship has been shown between CH4 flux and net

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Table 2. Root porosity of Pant Dhan-4during different growth phases. Values inparentheses are Number of days after trans-planting (%± 1 SE)

Stage Control Fertilized

Pre-flowering 14.54 19.25

(40) (±0.98) (±1.62)

Flowering 17.73 36.36

(60) (±2.14) (±2.63)

Post-flowering 15.47 21.41

(75) (±2.91) (±1.76)

ecosystem production across wetlands from Florida toAlaska (Whiting and Chanton, 1993), demonstratingthe importance of substrate production for methano-genesis.

There was a positive relationship between themean number of tillers and total CH4 emission indi-cating the significance of availability of aerenchyma(channels) through which CH4 emission is effected. Apositive influence of number of tillers on CH4 emis-sion has been demonstrated by Neue and Sass (1994).However, Watanabe et al. (1995), while investigatingthe influence of rice cultivar on CH4 emission frompaddy fields, concluded that neither the number oftillers, nor the shoot length, shoot weight and rootweight were correlated with CH4 emission rates at thetillering or reproductive growth stages.

Regression analysis showed direct relationship be-tween total CH4 (g m−2) flux, and mean root volumeand mean root porosity (Table 1). In the present study,root porosity varied from 14.54 to 17.73% in the con-trol plants and from 19.25 to 36.36% in the fertilizedplants (Table 2). ANOVA indicated significant differ-ences in root porosity due to treatment (F1,12 = 32.22,P = 0.0001) and crop growth stage (F2,12 = 13.49,P= 0.0009). The interaction growth stage× treatmentwas also significant (F2,12 = 6.70, P = 0.011). Theroot porosity expresses the magnitude of root air spaceand is a consequence of anaerobiosis (Kludze et al.,1993). It would appear that nitrogen availability leadsto larger cells hence greater root porosity. The signif-icant relationship between CH4 flux and root porosityindicated that the formation of root air space is an im-portant determinant for methane flux, and influencesefficient conductance of CH4 through the aerenchymaof the rice plant. Nevertheless it is also likely that therecould be efficient conduction of atmospheric O2 to the

Table 3. Analysis of variance for seasonal CH4 emission, bykeeping plant characteristics as covariates and treatment asmain effect

Source of d.f. Mean square F ratio P

variation

Covariate 1 142.1379 21.08 0.019

(root volume)

Main effect 1 33.3584 4.95 0.113

(treatment)

Residual 3 6.7426

Covariate 1 167.9908 21.12 0.019

(tiller no.)

Main effect 1 3.8676 0.49 0.543

(treatment)

Residual 3 7.9553

Covariate 1 172.4462 22.23 0.018

(root weight)

Main effect 1 0.0087 0.001 0.976

(treatment)

Residual 3 7.7564

Covariate 1 147.1981 20.03 0.021

(shoot weight)

Main effect 1 26.4855 3.60 0.154

(treatment)

Residual 3 7.3469

Covariate 1 178.9318 35.76 0.009

(root porosity)

Main effect 1 1.7803 0.36 0.599

(treatment)

Residual 3 5.0040

Covariate 1 158.4830 648.39 0.000

(grain yield)

Main effect 1 36.5079 149.36 0.001

(treatment)

Residual 3 0.2444

roots leading to increased rate of methane oxidation.Thus CH4-production could have been much larger.

The analysis presented in Table 1 is based on datapooled from control and fertilized plots, and thereforemay be a reflection of fertilizer effect only. We testedthe relative importance of plant variables and of thedirect effect of fertilization on seasonal CH4 emissionthrough ANOVA of CH4 flux against treatment (con-trol, fertilization) by keeping the plant variables ascovariates. We asked the question whether differencesin CH4 flux remain or whether they shrink towards in-significance after adjusting for the regression of fluxon plant variables. When plant variables were used

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as covariates, the main effect (treatment) became in-significant except for yield; covariates were significantin all cases. Even in the case of yield, although bothyield and treatment accounted for significant increasein flux, yield was the more effective (Table 3). Fromthis analysis it is apparent that effect of plant variablespredominated over direct fertilization effect.

In conclusion, fertilizer amendment of soil in-creased the source strength of CH4, both in vegetatedand unvegetated conditions. The source strength wasfurther enhanced in the presence of rice plants. Thestudy showed a clear positive relationship betweenplant growth and CH4 emission but separation be-tween the various growth parameters was not possible.The root porosity of rice plant was significantly in-creased by fertilizer application and was strongly cor-related with CH4 emission. This study emphasizes thesubstrate production and conduit effects of rice plants.

Acknowledgements

Funding support was received from Department ofBiotechnology. One of us (SS) was supported by theUniversity Grants Commission through GATE Fel-lowship.

References

Adhya T K, Rath A K, Gupta P K, Rao V R, Das S N, Parida K M,Parashar D C and Sethunathan N 1994 Methane emission fromflooded rice fields under irrigated conditions. Biol. Fert. Soils 18,245–248.

Chidthaisong A and Watanabe I 1997 Methane formation and emis-sion from flooded rice soil incorporated with13C-labelled ricestraw. Soil Biol. Biochem. 29, 1173–1181.

Dlugokencky E G, Maisaire K A, Lang P M, Tans P P, Steele L Pand Nisbet E G 1994 A dramatic decrease in the growth rate ofatmospheric methane in the northern hemisphere during 1992.Geophys. Res. Lett. 21, 45–48.

Gupta M, Verma S D, Parashar D C and Gupta P K 1994 Temporalvariation of methane emission from rice paddy fields of Gujarat.Ind. J. Radio Space Phys. 23, 265–268.

Holzapfel-Schorn A, Conrad R and Seiler W 1986 Effects of veg-etation on the emission of methane from submerged paddy soil.Plant Soil 92, 223–233.

Husin Y A, Murdiyarso D, Khalil M A K, Rasmussen R A,Shearer M J, Sabiham S, Sunar A and Adijuwana H 1995Methane flux from Indonesian wetland rice: The effects of watermanagement and rice variety. Chemosphere 31, 3153–3180.

Inubushi K, Hori K, Matsumoto S, Umebayashi M and Wada H1989 Methane emission from the flooded paddy soil to the at-mosphere through rice plant. Jap. J. Soil Sci. Plant Nutr. 60,318–324. (In Japanese with English summary).

IPCC 1995 Climate Change 1995. The Science of Climate Change.Eds. L G Meira Filho, B A Callander, N Harris, A Kattenberg andMaskell. Cambridge University Press, Cambridge, New York.

Jensen C R, Luxmoore R J, Van Gundy S D and Stolzy L H 1969Root air-space measurements by a pycnometer method. Agron.J. 61, 474–475.

Kiehl J T and Dickinson R E 1987 A study of radiative effectsof enhanced atmospheric CO2 and CH4 on early earth surfacetemperatures. J. Geophys. Res. 92, 2991–2998.

Kludze H K, DeLaune R D and Patrick W H Jr 1993 Arenchymaformation and methane and oxygen exchange in rice. Soil Sci.Soc. Am. J. 57, 387–391.

Lindau C W 1994 Methane emissions from Louisiana rice fieldsamended with nitrogen fertilizers. Soil Biol. Biochem. 26, 353–359.

Mariko S, Harazono Y, Owa N and Nouchi I 1991 Methane inflooded soil water and the emission through rice plants to theatmosphere. Environ. Exp. Bot. 31, 343–350.

Neue H-U and Sass R L 1994 Trace gas emissions from rice fields.Environ. Sci. Res. 48, 119–147.

Nouchi I, Hosono T, Aoki K and Minami K 1994 Seasonal varia-tion in methane flux from rice paddies associated with methaneconcentration in soil water, rice biomass and temperature, and itsmodelling. Plant Soil 161, 195–208.

Parashar D C, Gupta P K, Rai J, Sharma R C and Singh N 1993Effect of soil temperature on methane emission from paddy field.Chemosphere 26, 247–250.

Parashar D C, Mitra A P, Gupta P K, Rai J, Sharma R C, SinghN, Koul S, Ray H S, Das S N, Parida K M, Rao S B, Kanungo,Ramasami T, Nair B U, Swamy M, Singh G, Gupta S K, Singh AR, Saikia B K, Barua A K S, Pathak m g, Iyer C S P, Gopalkrish-nan M, Sane P V, Singh S N, Banerjee R, Sethunathan N, AdhyaT K, Rao V R, Palit P, Saha A K, Purukait N N, Chaturvedi GS, Sen S P, Sen M, Sarkar B, Banik A, Subbaraya B H, LalS, Venkatramani S, Lal G, Chaudhary A and Sinha S K 1996Methane budget from paddy fields in India. Chemosphere 33,737–757.

Parashar D C, Rai J, Gupta P K and Singh N 1991 Parameters affect-ing methane emission from paddy fields. Indian J. Radio SpacePhys. 20, 12–17.

Saha A K, Rai J, Raman V, Sharma R C, Parashar D C, Sen S Pand Sarkar B 1989 Methane emission from inundated fields in amonsoon region. India J. Radio Space Phys. 18, 215–217.

Slanina J and Warneck P 1994 Introduction: Assessment of un-certainties in the projected concentrations of methane in theatmosphere (Technical Report), Chap. 1. Pure Appl. Chem. 66,139–142.

Schutz H, Holzapfel-Pschorn A, Conrad R, Rennenberg H andSeiler W 1989 A 3-year continuous record on the influenceof daytime, season and fertilizer treatment on methane emis-sion rates from an Italian rice paddy. J. Geophys. Res. 94,16405–16416.

Seiler W 1984 Contribution of biological processes to the globalbudget of CH4 in atmosphere.In Current Perspectives in Mi-crobial Ecology. Eds M J Klug and C A Reddy. pp 468–477.American Society for Microbiology, Washington.

Singh J S, Singh Smita, Raghubanshi A S, Singh Saranath andKashyap A K. 1996 Methane flux from rice/wheat agroecosys-tems as affected by crop phenology, fertilization and water level.Plant Soil 183, 323–327.

Steele L P, Dlugokencky E G, Lang P M, Tans P P, Martin R C andMaiaire K A 1992 Slowing down of the global accumulation ofatmospheric methane during the 1980’s. Nature 358, 313–316.

164

Tanaka A 1976 Comparisons of rice growth in different environ-ments. In International Rice Research Institute “Climate andRice”. pp 429–448. Los Banos, Philippines.

Vergara B S 1970 Plant growth and development.In Rice ProductionManual. pp 17–37. University of Philippines, Languna.

Wang Z P, Lindau C W, Delaune R D and Patrick W H Jr 1992Methane production from anaerobic soil amended with rice strawand nitrogen fertilizers. Fert. Res. 33, 115–121.

Watanabe A, Majiwara M, Tashiro R and Kimura M 1995 Influenceof rice cultivar on methane emission from paddy fields. Plant Soil176, 51–56.

Whiting G J and Chanton J P 1993 Primary production control ofmethane emission from wetlands. Nature 364, 794–795.

Section editor: S Recous