Effect of the herbicide butachlor on methane emission ... - Springer Link

Biol Fertil Soils (2001) 33 : 175–180

Q Springer-Verlag 2001

ORIGINAL PAPER

S.R. Mohanty 7 K. Bharati 7 B.T.S. Moorthy B. Ramakrishnan 7 V.R. Rao 7 N. Sethunathan T.K. Adhya

Effect of the herbicide butachlor on methane emission and ebullition flux from a direct-seeded flooded rice field Received: 15 March 2000

Abstract Application of a commercial formulation of the herbicide butachlor (N-butoxymethyl-2-chloro2b,6b-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 F20% 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 Fe 2c content of the flooded soil. Application of butachlor retarded a drop in soil redox potential as well as accumulation of Fe 2c 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. Key words Methane emission flux 7 Ebollition flux 7 Direct-seeded flooded rice 7 Butachlor 7 Methanogenic bacteria

S.R. Mohanty 7 K. Bharati 7 B. Ramakrishnan 7 V.R. Rao N. Sethunathan 7 T.K. Adhya (Y) Laboratory of Soil Microbiology, Division of Soil Science and Microbiology, Central Rice Research Institute, Cuttack 753006, India e-mail: tkadhya6dte.vsnl.net.in Fax: c91-671-641744 B.T.S. Moorthy Division of Agronomy, Central Rice Research Institute, Cuttack 753006, India

Introduction In recent years, concern over global warming and consequent climate change has led to a worldwide interest in the study of greenhouse gas emissions, including those of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). Predominantly anaerobic flooded rice fields are considered as the major anthropogenic source of atmospheric CH4 (Houghton et al. 1996). With the intensification of rice cultivation to meet the increased demand of the twenty-first century (International Rice Research Institute 1999), CH4 emission from rice paddies is expected to increase (Anastasi et al. 1992). CH4, produced in the reduced subsurface layer of a flooded rice soil, is emitted to the atmosphere through either plant-mediated transport, ebullition as gas bubbles or diffusion through the soil-water column (Holzapfel-Pschorn et al. 1985; Rennenberg et al. 1992). While diffusion is a very slow process, plant-mediated transport is the most important method of release of CH4 to the atmosphere followed by ebollition. Ebollition contributes to 20% of the total CH4 emission from rice fields and is the only major pathway for CH4 release in unplanted rice fields or marshy lowlands (Denier van der Gon et al. 1992). However, a growthstage dependent variation in ebollition of CH4 in flooded rice fields has been reported with increased rates found in the early growing season and late maturity stages (Neue et al. 1994). Current research is focused on identifying various management practices that could potentially reduce CH4 emissions from rice cultivation whilst maintaining or increasing rice yield. The use of pesticides has now become an integral part of intensive rice growing systems, protecting modern high-yielding rice varieties from pests in order to achieve high yields. Many of the agrochemicals effect qualitative and quantitative alterations in the activities and populations of different groups of soil microorganisms (Rao et al. 1993). While some insecticides, such as DDT (McBride and Wolfe 1971) and HCH (isomeric mixture) (Satpathy et al. 1997) are known to inhibit

176

CH4 production and emission, the effects of herbicides on CH4 production and emission are not clearly known. Currently, herbicides are being increasingly used in Indian agriculture, in general, and rice culture in particular. Butachlor (N-butoxymethyl-2-chloro-2b,6b-diethyl acetanilide) belongs to the chloroacetanilide group of herbicides which inhibit protein synthesis in developing plant tissue, and is largely used for pre-emergence and/ or early post-emergence control of a variety of undesirable grasses and selected broad-leaved weeds in transplanted and direct-seeded rice (Hackett 1998). Biodegradability and the short half-life period (t1/2) of butachlor argue for its increased use in intensive rice cultivation (Chen 1981; Chen and Wu 1978). Butachlor is a widely used herbicide in rice culture in India and ranks first among the different herbicides used with a total consumption of approximately 2,600 m t year –1 (Agnihotri 1999). We studied the effect of butachlor on CH4 emission and ebollition fluxes from a tropical flooded field planted to rice and grown under irrigated condition.

Materials and methods Field experiment The field experiment was conducted during the dry-cropping season (January–May 1997) under irrigated conditions, in the experimental plots of the Central Rice Research Institute, Cuttack, India. The farm is situated at 20726bN latitude and 85756bE longitude. Mean rainfall during the dry season was 85 cm (average of last 10 years) and the monthly mean maximum and minimum temperatures were 26.5 7C and 12.7 7C, respectively. The soil was a typic Haplaquept (deltaic alluvium; FAO, Gleysol) with a sandy clayloam texture (clay 259 g kg –1, silt 216 g kg –1, sand 525 g kg –1, pH 6.2, cation exchange capacity 150 mmol kg –1 soil, electrical conductivity 0.6 dS m –1, organic matter 16.1 g kg –1, total N 0.9 g kg –1, Olsen-P 8 mg kg –1). In the field plot used in the experiment, a rice crop was grown during the preceding wet season (July–November) without the application of any herbicide and was left fallow after harvest, before the present dry season rice crop. One week before sowing, the field was dry-ploughed to break the clods. Two days after initial ploughing, the fields were floodirrigated, puddled thoroughly to 10 cm depth and levelled. Pregerminated seeds of rice (cv. Annada) were broadcast-sown at the rate of 75 kg ha –1 in 6!3 m plots, well separated by leaves. A common basal dose of 40 kg ha –1 each of P2O5 and K2O in the form of single superphosphate and muriate of potash was applied to the crop at the time of sowing. Fertilizer N as urea (80 kg N ha –1) was applied – half at 20 days after sowing (DAS) and the rest in two equal halves at 40 and 60 DAS, respectively. A commercial formulation of butachlor (Machete 50% EC; Monsanto Enterprises, India) was mixed with sand and broadcast-applied to the field plots at a rate of 1.0 kg a.i. ha –1 at 7 DAS. Field plots with no butachlor application and left unweeded, served as control. Both treatments were replicated three times in a randomized block design. The field plots were initially irrigated at 7 DAS and after the crop established were maintained continuously flooded (5B2 cm) for the rest of the period of crop growth. Standard crop management practices were adopted for raising the crop to maturity. Plant-mediated CH4 flux measurements Plant-mediated CH4 emission flux from the rice fields was measured by a closed chamber method (Adhya et al. 1994, 1998) at

5 day intervals from sowing until maturity. Samplings for CH4 flux measurements were made at 0900–0930 hours and 1500–1530 hours, and the average of the morning and evening fluxes was used as the flux value for the day. For measuring CH4 emission, aluminium bases (57 cm length ! 37 cm width ! 10 cm height) with a channel to accommodate perspex chambers were installed manually in the field plots at the measurement sites within 1 week of sowing. Plant hills enclosed inside the aluminium base were covered with a locally fabricated perspex chamber (53 cm length ! 37 cm width ! 71 cm height). A battery-operated air circulation pump with air displacement of 1.5 l min –1 (M/s Aerovironment, Monrovia, Calif.), connected to polyethylene tubing was used to mix the air inside the chamber and draw the air samples into Tedlar air-sampling bags (M/s Aerovironment) at fixed intervals of 0, 15 and 30 min. The air samples from the sampling bags were analysed for CH4. CH4 ebullition flux measurements To measure CH4 ebollition flux, perspex boxes (length ! width ! height: 40!15!20 cm) were installed between rice hills with two boxes per replicate in the field. PVC pegs sunk into the paddy field held the boxes, which remained at the same position during the entire season. Samples (6 ml) of headspace gas were collected through the septum port of each box with a gas-tight syringe at 0 and 24 h into freshly evacuated 5 ml draw vacutainer tubes (15 ml capacity, obtained from Becton, Dickinson, New Jersey). The flood water level inside each box was measured at 0 and 24 h of sampling. The gas samples from the vacutainer tubes were analysed by gas chromatography for quantification of CH4. Gas chromatography The CH4 was estimated in a Shimadzu GC-8A gas chromatograph equipped with FID and a 0.5 nm molecular sieve. The column and detector were maintained at 70 7C and 110 7C respectively. The gas samples were injected through a sample loop (3 ml) with the help of an on-column injector using a multiport valve. The GC was calibrated before and after each set of measurement using 5.38, 9.03 and 10.8 ml CH4 ml –1 in N2 (Scotty II Analyzed gases; M/s Altech Associates) as primary standard and 2.14 ml CH4 ml –1 in air as secondary standard. Under these conditions, the retention time of CH4 was 0.65 min and the minimum detectable limit was 0.5 ml ml –1. CH4 flux was expressed as mg m –2 h –1. The data were subjected to statistical analysis.

Soil analyses Measurements for redox potential and pH were done with each set of CH4 flux measurement. The redox potential of the soil of the root region was measured by inserting a combined platinumcalomel electrode (Barnant, Ill.) to the root region and measuring the potential difference in mV (Pal et al. 1979). The electrode was calibrated earlier against standard redox buffer [0.0033 M K3Fe(CN)6 and 0.0033 M K4Fe(CN)6 in 0.1 M KCl, which has an Eh of c463 mV at 25 7C]. The observed redox values were subsequently corrected to that of a hydrogen electrode by adding c240 mV to the redox readings. The pH of the soil and water was monitored with a portable pH meter (Philips model PW 9424; Philips Analytical, Cambridge, UK). Soil chemical components were analysed from field soils sampled by inserting a tube auger (2 cm diameter) to a depth of 5–7 cm, in between two rice hills. The soil samples, after draining excess water, were immediately subsampled for measurement of Fe 2c and readily mineralizable carbon (RMC). Extractable Fe 2c was measured by agitating fresh soil samples with NH4OAC : HCl (pH 2.8) for 1 h and determining Fe 2c by colorimetry after reacting with orthophenanthroline (Pal et al. 1979) and expressed as mg Fe 2c g –1 soil. Readily mineralizable carbon (RMC) was meas-

177 ured by extracting soil samples with 0.5 M K2SO4 and titrating the extract with ferrous ammonium sulphate after wet digestion with chromic acid (Vance et al. 1987; Mishra et al. 1997).

Microbiological analyses The population of total aerobic bacteria in the soil samples was estimated by the standard dilution plate technique using tryptone yeast extract medium (Rand et al. 1975) and expressed as colony forming units (cfu) g –1 dry soil. Methanogenic bacterial population was enumerated following an anaerobic culture tube technique (Kaspar and Tiedje 1982). The medium used for enumeration of methanogenic bacteria consisted of mineral salt solution with yeast extract and resazurin (Kaspar and Tiedje 1982) and 20 ml l –1 each of vitamin and mineral solutions (Wolin et al. 1964). The anaerobic tubes containing the media prepared under continuous flow of N2, were stoppered with rubber septa following inoculation with serial dilutions of soil samples and incubated at 28B2 7C for 30 days. Detection of more than 2 ml ml –1 of CH4 in the headspace of culture tubes was considered as evidence for the presence of methanogens. The population of methanogenic bacteria was then counted by MPN (most probable number) assay (Alexander 1982).

Plant parameters Mean aerial biomass (fresh and dry weights) of rice plants was measured by harvesting above-ground portions on each day of CH4 sampling. Weed biomass (both fresh and dry weights) from replicated control plots was measured by harvesting the weed flora from 1 m 2 area and expressed as Mg ha –1. Grain and straw yields of rice from individual replicated treatments were measured at maturity and expressed as Mg ha –1.

Statistical analyses Individual character data sets were statistically analysed and the mean comparison between treatments was established by Duncan’s multiple range test using IRRISTAT (ver. 3.1: International Rice Research Institute, the Philippines) and differences are reported at P~0.05. Simple and multiple correlation analyses between CH4 flux and select soil parameters were established to find out the effect of the herbicide on select soil characters visà-vis CH4 emission.

Fig. 1 Methane efflux from a direct-seeded flooded rice field untreated or treated with the herbicide butachlor (means of four replicates plotted; bars/half-bars indicate the standard deviation)

sion value of 2.06 mg m –2 h –1 for the cropping period. The cumulative plant-mediated CH4 flux in butachlortreated plots was 53.50B7.16 kg ha –1 as compared with 66.64B11.21 kg ha –1 in untreated control plots amounting to a 20% inhibition over the untreated control for the entire cropping period. The ebullition flux of CH4 was monitored in butachlor-treated and untreated field plots. Like plant-mediated emission flux, butachlor also exhibited an inhibitory effect on cumulative ebullition flux of CH4 from soil. Thus, cumulative ebullition flux in butachlortreated plots was 25.21B18.54 g ha –1 as compared with 133.32B82.51 g ha –1 in untreated control plots, amounting to an inhibition of 81% (Fig. 2). However, unlike plant-mediated emission flux, ebullition flux of CH4 exhibited a higher degree of variation. The effect of butachlor on ebullition flux lasted for about 50 days after its application. Thereafter, the ebullition flux in both butachlor-treated and untreated field plots was statistically the same. Emission of CH4 through ebullition can be considerable when CH4 production rates in the soil are high (Byrnes et al. 1995) and a growth stage-dependent variation in ebullition of CH4 in flooded rice fields has also been reported (Neue et al.

Results and discussion In the present study, plant-mediated CH4 flux from the control (no butachlor) field plots sown to rice varied from 0.01 to 6.28 mg m –2 h –1 with a mean emission value of 2.52 mg m –2 h –1 for a cropping period of 110 days. CH4 emission increased with crop growth, with the lowest rate being observed at seedling stage, and increased gradually to reach the highest value during the reproductive stage. CH4 emission declined thereafter with another small peak at maturity. Application of butachlor at the rate of 1 kg a.i. ha –1 to flooded fields sown to rice significantly inhibited CH4 flux over that of the untreated control (Fig. 1), with CH4 flux remaining less in butachlor-treated plots albeit with a sharp peak at 60 DAS that was higher than the corresponding unamended control. CH4 flux in butachlor-treated plots ranged from 0.01 to 7.41 mg m –2 h –1 with a mean emis-

Fig. 2 Seasonal pattern of ebullition flux of CH4 from a directseeded flooded rice field untreated or treated with the herbicide butachlor (means of two replicates plotted; bars/half-bars indicate the standard deviation)

178 Table 1 Variation in the redox potential of the soil of the root region of rice plants (cv. Annada) grown in field plots untreated or treated with butachlor. Average of three replicate observations Treatment

Redox potential (mV) Days after sowing

Control cButachlor

10

20

30

40

50

60

70

80

90

P16 a P12 a

P29 a P18 b

P70 a P58 b

P104 a P103 a

P99 a P63 b

P134 b P147 a

P147 a P135 b

P140 a P130 b

P128 b P143 a

In a column, means followed by the same letter are not significantly different at P~0.05 level by DMRT

Table 2 Variation in pH in the standing water (W) and soil (S) of the root region of rice plants grown in field plots untreated or treated with butachlor. Average of three replicate observations Treatment

pH Days after sowing 10 W

S

20 W

S

30 W

40 S

W

S

50 W

60 S

W

70 S

W

S

80 W

90 S

W

S

Control 7.83 a 7.05 a 7.61 a 7.03 a 8.05 b 7.42 a 8.18 a 6.56 a 7.11 b 6.76 a 6.89 b 6.14 b 7.28 a 6.86 a 7.09 a 6.64 b 7.15 a 7.06 a cButachlor 7.79 b 7.02 a 7.61 a 7.03 a 8.32 a 6.57 b 7.88 b 6.54 a 7.15 a 6.64 b 6.98 a 6.84 a 6.85 b 6.84 a 6.87 b 6.96 a 6.94 b 6.74 b In a column, means followed by the same letter are not significantly different at P~0.05 level by DMRT

1994). However, in the present study, there was no correlation between the plant-mediated emission and ebullition fluxes of CH4 in both butachlor-treated and untreated field plots. It is interesting that CH4 flux by both plant-mediated emission and ebullition was significantly inhibited by butachlor even at a realistic field application rate of 1 kg a.i. ha –1. Acetylene reduction activity in a flooded alluvial soil was inhibited by butachlor at a field application level of 1.8 kg ha –1 (Jena et al. 1987) indicating that herbicides can affect important soil microbial processes even at recommended field application rates. We also evaluated the effect of butachlor on some important soil processes with regard to CH4 flux. Redox potential, an important parameter governing CH4 emission, was higher in butachlor-treated plots (Table 1) except at 60 and 90 DAS. This indicates that the application of butachlor probably retards the soil reduction process, thereby maintaining a comparatively higher redox status than in untreated field plots. There is evidence that hexachlorocyclohexane (HCH), an organochlorine insecticide, retards soil reduction (Pal et al. 1979) and also CH4 production and emission from flooded rice soils (Satpathy et al. 1997). It is possible that butachlor, a chloroacetanilide herbicide, might have an effect similar to that of HCH. A simple correlation analysis showed a significant negative relationship (rpP0.792**) between redox potential and CH4 emission, indicating higher CH4 emission under the more reduced conditions of untreated plots compared with butachlor-treated plots.

Application of butachlor affected pH of both soil and water (Table 2). While pH changes in butachlortreated soil were marginal, changes in pH of standing water were highly variable. Methanogenic bacteria have their optimum activity at near neutral to slightly alkaline soil pH (Neue and Roger 1993; Oremland 1988). Possibly, higher Eh coupled with a differential pH adversely affected methanogenic activity in butachlor-treated plots. Methanogenesis is the penultimate step of the thermodynamic sequence of reduction of a flooded soil and the iron redox system plays a key role in this sequence (Ponnamperuna 1972). Variation in the Fe 2c content of soils as affected by butachlor application was therefore studied (Table 3). A highly significant positive correlation (rp0.566**) existed between CH4 emission and accumulation of Fe 2c in flooded soils. Fe 2c content of the soil from butachlor-treated plots was significantly lower that of the untreated control except at 70 DAS. This suggested significant inhibition of Fe 2c accumulation in butachlor-treated plots. Accumulation of Fe 2c in a flooded soil is a good indicator of the reducing environment, a condition which favours methanogenesis. CH4 emission from soils is governed by the decomposition of organic matter, which provides substrates for methanogenesis (Yagi and Minami 1990) as well as for other heterotrophs. Readily mineralizable carbon (RMC) content is a useful indicator of methanogenic potential of a soil (Mishra et al. 1997). RMC, measured as K2SO4-extractable carbon, was significantly higher

179 Table 3 Variation in Fe 2c content of a flooded alluvial soil planted to rice untreated or treated with butachlor. Average of two replicate observations mg Fe 2c recovered g P1 dry soil

Treatment

Days after sowing

Control cButachlor

10

20

30

40

50

60

70

80

75 a 52 a

95 a 80 a

155 a 175 a

399 a 179 b

887 a 806 b

2925 a 2502 b

3518 b 3626 a

4316 a 4187 b

In a column, means followed by the same letter are not significantly different at P~0.05 level by DMRT

Table 4 Variation in the readily mineralizable carbon (RMC) content of a flooded alluvial soil planted to rice untreated or treated with butachlor. Average of two replicate observations Treatment

Readily mineralizable carbon (mg g P1 dry soil) Days after sowing

Control cButachlor

10

20

30

40

50

60

70

80

133 a 241 a

145 a 321 a

1281 a 1483 a

1770 b 2473 a

1552 a 1682 a

1079 b 1431 a

933 a 590 b

768 a 750 a

In a column, means followed by the same letter are not significantly different at P~0.05 level by DMRT

Table 5 Changes in total aerobic bacteria (cfu, colony-forming units) and methanogenic bacterial population (MPN, most probable number) (!10 6 g P1 soil) in a flooded alluvial soil untreated or treated with butachlor and planted to rice. Sampling was done at the maturity stage (95 days after sowing) of the crop Treatment

Aerobic bacteria (cfu)

Methanogens (MPN)

Control cButachlor

36 15

0.45 0.39

(Table 4) in butachlor-treated fields than in the untreated control, indicating that the application of butachlor did not inhibit normal biochemical activity, including decomposition of organic matter and the accumulation of RMC. However, what was interesting is the inhibition of CH4 emission in spite of a high RMC content. Probably, butachlor directly inhibited methanogenic bacterial activity per se, resulting in lower production of CH4 and its subsequent emission. Butachlor application had a significant inhibitory effect on soil aerobic bacteria and methanogenic bacterial populations as well (Table 5), the reasons for which is not clear. Application of butachlor has been shown Table 6 Grain and straw yields of rice (cv. Annada) and cumulative plant-mediated CH4 flux in untreated and butachlor-treated plots. Average of three replicate observations

to inhibit the populations of anaerobic and microaerophilic nitrogen fixers and Azospirillum in a nonflooded alluvial soil (Jena et al. 1987). Methanogenesis being mediated by strictly anaerobic methanogenic bacteria, the results show that butachlor had a direct inhibitory effect on the population of methanogenic bacteria, resulting in low CH4 flux from soils treated with butachlor. Grain and straw yields of paddy increased by 13.5 and 23.4% in butachlor-treated plots over that of untreated plots (Table 6). Computation of plant-mediated CH4 emission (kg) per tonne of rice yield indicated that butachlor application led to a significant decrease in CH4 flux vis-à-vis grain yield (Table 6). Butachlor application resulted in an increase in the rice grain yield due to elimination of competition from weeds. The weed flora in the unweeded control plots used in this study produced a biomass yield of 1.81B0.23 t ha –1 on a fresh weight basis, suggesting the extent of competition. Aquatic weeds, especially grasses, are also known to act as transporter of CH4 formed in submerged soil, although at a much slower rate (Holzapfel-Pschorn et al. 1986). However, the weed flora in the dry-season rice fields of the experimental plots comprised mainly

Treatments

Grain yield (Mg ha P1)

Straw yield (Mg ha P1)

Cumulative CH4 (kg ha P1)

kg CH4 Mg P1 grain yield

Control cButachlor

4.30 a 4.88 b

4.70 a 5.80 b

66.64 a 53.50 b

15.52 a 10.98 b

In a column, means followed by the same letter are not significantly different at P~0.05 level by DMRT

180

sedges (80%) dominated by Cyperus difformis L., Fimbristylis miliacea (L.) Vahl., and Scirpus articulatus L. and dicots (20%), but almost no grasses (Moorthy 1995). Sedges, unlike grasses, do not have aerenchyma and are less likely to transport CH4 than rice plants and aquatic grasses. What was interesting is that both plantmediated and ebullition flux of CH4 was reduced following application of butachlor to the paddy fields. Herbicides are being regularly applied to control weeds in field crops including rice. Our study shows that the application of butachlor had the additional advantage of retarding both plant-mediated and ebullition flux of CH4 in a rice ecosystem. This inhibition could be both direct – through the inhibition of methanogenic bacterial populations and their activity, or indirect – through inhibition of soil reduction and Fe 2c accumulation in flooded soil. Application of a xenobiotic chemical exclusively to mitigate greenhouse gas emission is not an eco-friendly approach. Admittedly, the inhibition of CH4 emission by the herbicide butachlor, even at the recommended application rate of 1 kg a.i. ha –1, might have an additional advantage in regulating CH4 emission from rice paddies. Acknowledgments The authors thank the Director of the Institute for permission to publish this work. This work was supported, in part, by the IRRI-UNDP Interregional Research Program on Methane Emission in Rice Fields (GLO/91/G31).

References Adhya TK, Rath AK, Gupta PK, Rao VR, Das SN, Parida K, Parashar DC, Sethunathan N (1994) Methane emission from flooded rice fields under irrigated conditions. Biol Fertil Soils 18 : 245–248 Adhya TK, Pattnaik P, Satpathy SN, Kumaraswamy S, Sethunathan N (1998) Influence of phosphorus application on methane emission and production in flooded paddy soil. Soil Biol Biochem 30 : 177–181 Agnihotri NP (1999) Pesticides: safety evaluation and monitoring. AICRP on Pesticide Residues, Division of Agricultural Chemicals, Indian Agricultural Research Institute, New Delhi, pp 1–173 Alexander M (1982) Most probable number method for microbial population. In: Page AL, Miller RH, Keeney DR (eds) Methods of soil analysis, part 2. American Society of Agronomy, Madison, Wis., pp 815–820 Anastasi C, Dowding M, Simpson VJ (1992) Future CH4 emissions from rice production. J Geophys Res 97 : 7521–7525 Byrnes BH, Austin ER, Tays BK (1995) Methane emission from flooded rice soils and plants under controlled conditions. Soil Biol Biochem 27 : 331–339 Chen YL (1981) Degradation of butachlor in paddy fields. Technical Bulletin, Food and Fertilizer Center, Taiwan, pp 1–22 Chen YL, Wu TC (1978) Degradation of herbicide butachlor by soil microbes. Pestic Sci 3 : 411–417 Denier van der Gon HAC, Neue HU, Lantin RS, Wassmann R, Alberto MCR, Aduna JB, Tan MJP (1992) Controlling factors of methane emission from rice fields. In: Bates NH, Bridges EM (eds) World inventory of soil emission potentials. (WISE report 2) International Soil Reference and Information Centre, Wageningen, pp 81–92 Hackett AG (1998) Butachlor. In: Roberts T (ed) Metabolic pathways of agrochemicals: herbicides and plant growth regulators, part I. Royal Chemical Society, London, pp 194–198

Holzapfel-Pschorn A, Conrad R, Seiler W (1985) Production, oxidation and emission of methane in rice paddies. FEMS Microbiol Lett 31 : 343–351 Holzapfel-Pschorn A, Conrad R, Seiler W (1986) Effects of vegetation on the emission of methane from submerged paddy soil. Plant Soil 92 : 223–233 Houghton JT, Meira Filho LG, Callander BA, Harris N, Katterberg A, Maskell K (1996) IPCC report on climate change: the science of climate change: WGI contribution to the IPCC second assessment report on methane emission from rice cultivation. Cambridge University Press, Cambridge International Rice Research Institute (1999) Sustaining food security beyond the year 2000 – a global partnership for rice research: medium-term plan 2000–2002. IRRI, Los Ban˜os, Philippines, pp 1–130 Jena PK, Adhya TK, Rao VR (1987) Influence of carbaryl on nitrogenase activity and combination of butachlor and carbofuran on nitrogen-fixing microorganisms in paddy soils. Pestic Sci 19 : 179–184 Kaspar HF, Tiedje JM (1982) Anaerobic bacteria and processes. In: Page AL, Miller RH, Keeney DR (eds) Methods of soil analysis, part 2. American Society of Agronomy, Madison, Wis., pp 989–1009 McBride BC, Wolfe RS (1971) Inhibition of methanogenesis by DDT. Nature 234 : 551 Mishra S, Rath AK, Adhya TK, Sethunathan N (1997) Effect of continuous and alternate water regimes on methane flux from rice under greenhouse conditions. Biol Fertil Soils 24 : 399–405 Moorthy BTS (1995) Weed control in puddled seeded rice in dry season in coastal Orissa. Indian J Weed Sci 27 : 194–196 Neue HU, Roger PA (1993) Rice agriculture: factors controlling emissions. In: Khalil MAK (ed) The global cycle of methane: sources, sinks, distribution and role in global change. (NATO advanced science series) Springer, Berlin Heidelberg New York, pp 254–298 Neue HU, Lantin RS, Wassmann R, Aduna JB, Alberto MCR, Andales MJF (1994) Methane emission from rice soils of the Philippines. In: Minami K, Mosier AR, Sass RL (eds) CH4 and N2O: global emissions and controls from rice fields and other agricultural and industrial sources. Yokendo, Tokyo, pp 55–63 Oremland RS (1988) The biogeochemistry of methanogenic bacteria. In: Zehnder AJB (ed) Biology of anaerobic microorganisms. Wiley, New York, pp 641–702 Pal SS, Barik S, Sethunathan N (1979) Effects of benomyl on iron and manganese reduction and redox potential in flooded soil. J Soil Sci 30 : 155–159 Ponnamperuna FN (1972) The chemistry of submerged soils. Adv Agron 24 : 29–96 Rand MC, Greenberg AE, Taras MJ, Franson MA (1975) Standard methods for the examination of water and wastewater. American Public Health Association, Washington, pp 1–1193 Rao VR, Adhya TK, Sethunathan N (1993) Effect of pesticides on soil health. In: Dhaliwal GS, Singh B (eds) Pesticides: their ecological impact in developing countries. Commonwealth Publishers, New Delhi, pp 112–130 Rennenberg H, Wassmann R, Papen H, Seiler W (1992) Trace gas exchange in rice cultivation. Ecol Bull 42 : 164–173 Satpathy SN, Rath AK, Mishra SR, Kumaraswamy S, Ramakrishnan B, Adhya TK, Sethunathan N (1997) Effect of hexachlorocyclohexane on methane production and emission from flooded rice soil. Chemosphere 34 : 2663–2671 Vance ED, Brookes PC, Jenkinson DS (1987) An extraction method for measuring soil microbial biomass. Soil Biol Biochem 19 : 703–707 Wolin EA, Wolfe RS, Wolin MJ (1964) Viologen dye inhibition of methane formation by Methanobacillus omelianskii. J Bacteriol 87 : 993–998 Yagi K, Minami K (1990) Effect of organic matter application on methane emission from some Japanese paddy fields. Soil Sci Plant Nutr 36 : 599–610