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Chapter 6

Methane emission from rice-cum-fish culture of Apatani’s

Introduction

Methane is the second most important greenhouse gas in the atmosphere. During

the last 200 years methane concentration in the atmosphere has more than doubled. Its

current atmospheric concentration of 1.7 ppm by volume, up from 0.7 ppm in

preindustrial times. It is to be noted here that one molecule of methane traps

approximately 30 times more heat than one molecule of carbon dioxide. The heating

effect of the atmospheric methane increase is approximately half that of the carbon

dioxide increase (Dickinson and Cicerone, 1986, Ramanathan et al., 1985). Presently

methane concentration in the atmosphere is reportedly increasing by 1% per year. It has

been predicted that with this rate of increase in concentration will bring about a drastic

change in the global climatic condition in future and unpredictable consequences for the

whole chemistry of the atmosphere.

Though it is an important greenhouse gas, methane also affects the chemistry of

the atmosphere by altering the concentrations of tropospheric ozone, hydroxyl radicals,

and carbon monoxide. In the stratosphere, it acts as a sink for chlorine but is also a

source for hydrogen and water vapour. The current accumulation of methane in the

atmosphere is approximately 4700 Tg (Wahlen et al., 1989), and the global annual

emission is estimated to be 500 Tg with an apparent net flux of 40 Tg/yr (Cicerone and

Oremland 1988).

The overall budget of atmospheric methane is fairly well established, but the

strength of individual sources remains uncertain. According to estimates, anthropogenic

sources contribute the largest (340 Tg/yr) followed by natural sources (160 Tg/yr). It is

interesting to note that 80% of the total methane emission is of modern biogenic origin

while only 20% is due to fossil carbon sources (Wahlen et al., 1989).

Methane is produced as the end product of the anaerobic breakdown of organic

matter in wetland rice soils. Methanogenic bacteria produce methane in a strict absence

of free oxygen and at redox potentials of less than -150 mV (Wang et al., in press). In

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wetlands where rice cultivation is done, methane is largely produced by trans-

methylation of acetic acid and, to some extent, by the reduction of carbon dioxide

(Takai, 1970). The formation of methane is preceded by the production of volatile acids.

On flooding, short-term evolution of hydrogen immediately follows the disappearance of

oxygen, carbon dioxide increases, and, with decreasing carbon dioxide, methane

formation increases (Neue and Scharpenseel 1984, Takei et al., 1956). The delay of

methane production depends on the pattern of soil, pH, availability of substrate and

temperature. In tropical flooded rice soils, where soil temperatures are 25-30deg.C,

methane production in alkaline and calcareous soils may start hours after flooding, in

neutral soils it is delayed two to three weeks, and in acid soils methane may only be

formed five or more weeks after flooding. Methane production is negatively correlated

with soil-redox potential and positively correlated with soil temperature, soil carbon

content, and rice growth (Neue and Roger in press). The rate and pattern of organic

matter addition and decomposition determine the rate and pattern of methane formation.

Methane production generally increases during the cropping season; although the

population density of methanogens remains fairly stable (Schutz et al., 1989). Easily

degradable crop residues, fallow weeds, and soil organic matter are the major source for

initial methane production. At later growth stages of rice, root exudates, decaying roots,

and aquatic biomass seem to be more important. Methane production is enhanced in the

rooted soil zones (Sass et al., 1991).

Rice fields with submerged water conditions have been identified recently as a

major source of atmospheric methane. Though rice fields has long been identified as

potential methane emission source earlier by Harrison and Aiyer in 1913, the first

comprehensive measurements of emission from rice fields were reported only in the

early 1980s (Holzapfel–Pschorn et al., 1985, Seiler et al., 1984, Cicerone et al., 1983,

Cicerone and Shetter, 1981). In a natural wetland condition, flooding a rice field cuts off

the oxygen supply from the atmosphere to the soil, which results in anaerobic

fermentation of soil organic matter. Methane is a major end product of such an anaerobic

condition. It is released from submerged soils to the atmosphere by diffusion and

ebullition and through roots and stems of rice plants. Recent global estimates of

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emission rates from wetland rice fields range from 20 to 100 Tg/yr (IPCC 1992), which

corresponds to 6-29% of the total annual anthropogenic methane emission.

Rice (Genus Oryza), first gathered by human 12,000 years ago and first

cultivated 6000 years ago, is one of the world’s primary food crops (Chang 1976). About

90% of the world’s rice is grown in Asia, and almost a third and a fifth of the world’s

rice are consumed in China and India, respectively. In India, the largest cropped area is

devoted to rice (44.5 million ha). Rice is, preferably, grown under flooded conditions in

many agro–ecological zones. The demand for food has led to double or triple cropping

of rice annually as well as exploitation of problematic soils and marginal lands for rice

cultivation. Rice productivity is largely dependent on the fertility status. Improvement of

soil quality through soil organic carbon management has remained the major concern for

tropical rice soils.

Being a staple cereal crop, rice production will substantially increase during the

next decade (IRRI, 1989), it is important to design cultural practices that will improve

rice yield while reducing CH4 emission from rice environments. This chapter reports on

the estimation of methane from the paddy-cum-fish cultivation systems of Apatanis.

Methodology

Sample collection

For collecting gas samples from crop fields, acrylic chambers of 50 cm × 30 cm

× 100 cm made of 6 mm. Aluminum channels were inserted 10 cm inside the soil and

the channels were filled with water to make the system air tight. A battery operated fan

was fixed inside the chamber to homogenize air thoroughly. A thermometer was also

inserted to monitor the inside temperature. One 3-way stopcock was fitted at the top of

chamber to collect gas samples. The chamber was thoroughly flushed several times with

a syringe. Gas samples were drawn with the help of hypodermic needles. After drawing

the sample, syringes were made air-tight with a three-way stopcock. Samples of four

replications of each treatment were taken from the plots and the average was taken as

representative value for the treatment. Head space volume inside the box was recorded,

which was used to calculate flux of methane gas. The samples were analyzed in a 3600

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gas chromatograph equipped with flame ionization detection (FID) (column: molecular

sieve 5A (80/100 mesh); carrier gas argon; column temperature 90ºC.

Calculation of methane flux:

Cross-sectional area of the chamber (m2) = A

Headspace (m) = H

Volume of headspace (L) = 1000 × AH

CH4 concentration at 0 time (µL L-1) = Cₒ

CH4 concentration after time t (µL L-1) = Ct

Change in concentration in time t (µL L-1) = (Ct – Cₒ )

Volume of CH4 evolved in time t (µL) = (Ct – Cₒ) × 1000 AH

When t is in hours, then flux (mL m-2 h-1) = {(Ct – Cₒ) × AH} / (A × t)

Now 22.4 ml of CH4 is 16 mg at STP

Hence, Flux = {(Ct – Cₒ) / t} × H × 16 / 22.4 × 10000 × 24 mg ha-1 d-1

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Table 6.1: Methane emissions during different stages of paddy growth in the paddy fields of study site

SITESVEG REP MAT

NSY TNP ATG PAI HDG FLG MAT AFH

Hari 1.04 2.77 11.29 9.32 7.06 8.24 2.11 0.89

Hija 1.09 2.82 11.46 9.84 7.23 8.09 2.26 0.94

MT 1.13 2.39 11.31 9.47 7.72 8.61 2.54 0.83

DTA 1.05 2.56 11.27 9.25 7.46 8.13 2.41 0.91

BUA 1.17 2.93 11.45 9.59 7.24 8.37 2.19 0.80

BM 1.02 2.68 11.34 9.21 7.17 8.15 2.08 0.76

VEG: Vegetative phase, REP: Reproductive phase, MAT: Maturation phase, NSY: Nursery stage, TNP: Transplanting stage, ATG: Activetillering stage, PAI: Panicle Initiation stage, HDG: Heading stage, FLG: Flowering stage, Mat: Maturation stage, AFH: After Harvesting stage

Table 6.2: Two-way ANOVA of methane emission

Factor df F - Values P

Site 5 0.56ns -

Stages 7 178.81* < 0.05

Site X Stages 34 15.46* < 0.05

ns – not significant

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Figure 6.1: Diurinal variation of methane emission in all the villages under study site

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Fig 6.2: Temperature vs SMC vsCH4

Fig 6.3: WHC vs SMC vs CH4 Fig 6.4: SOC vs SR vs CH4

Fig 6.5: SOC vs MBC vS CH4 Fig 6.6: MBP vs Available P vsCH4

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Results

The processes involved in methane emission from flooded rice field to the

atmosphere include methane production in the soil by methane-producing bacteria

(methanogens), methane oxidation within oxic zones of the soil and flood water by

methane-oxidizing bacteria (methanotrophs), and later vertical transport of the gas from

soil to the atmosphere. Methane is produced in the terminal step of several anaerobic

microbial degradation chains. The amount of methane produced in flooded rice soils is

primarily determined by the availability of methanogenic substrates and the influence of

environmental factors. The sources of organic carbon for methanogenic substrates are

primarily rice plants via root exudation, root senescence and plant litter (Lu et al., 1999,

Schütz et al., 1991, Holzapfel–Pschorn and Seiler, 1986) or added organic matter for

fertilization and remains from previous crop (Denier van der Gon and Neue, 1995,

Wassmann et al., 1993, Cicerone et al., 1992, Sass et al., 1991, Yagi and Minami, 1990,

Schütz et al., 1989). The effect of added organic matter for fertilization depends on type

and amount. As also stated earlier, factors affecting methane production from soil

surface include texture (Neue et al., 1994, Sass et al., 1994), climate (Sass et al., 1991,

Schütz et al., 1990), and agricultural practices, such as water regime and management

(Wassmann et al., 2000, Yagi et al., 1996, Lewis, 1996, Wassmann et al., 1995, Sass et

al., 1992, Inubushi et al., 1990).

Methane released from the paddy during different stages is summarized in table

6.1. Not much difference was observed among the different sites under consideration

whereas significant difference was seen in different stages of paddy growth. It is seen

from table that highest amount of methane is released during active tillering stage

(11.29–11.45 mg m-2 h-1) and lowest after paddy harvesting (0.76 – 0.94 mg m-2 h-1).

Methane emission was low during the nursery stage (1.02–1.17 mg m-2 h-1), however. In

general, greater emission of methane was observed in the last stage of vegetative phase

and reproductive phase and emissions continued to be high during all the three stages

viz. panicle initiation stage, heading stage and flowering stage. Further a two-way

factorial ANOVA (Table 6.2) showed that methane emission varied significantly across

stages (F = 178.81, p < 0.05) and interaction between site and stage (F = 17.40, p <

0.05), whereas variation across sites was not insignificant.

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Methane emission (estimated in mg m-2 h-1) pattern from the fields was similar in

all the study sites under consideration (Figure 6.1). Methane was estimated to be highest

in active tillering stage and in the morning hours (before 10 AM). Lowest was observed

in after harvesting stage, during early morning. It is also evident from figures (Figure

6.1) that comparatively lower amounts of methane were released in the afternoon period.

Highest was released in the active tillering stage and lowest in after harvesting stage.

Early afternoon (12 noon) showed greater amounts of release pattern than in the later

part afternoon (Figure 6.1).

Methane emission had significant (p < 0.05) positive correlations (Figures 6.2,

6.3 & 6.4) with temperature (r =0.346), soil moisture content (r =0.289 soil organic

carbon (r =0.214, p = 0.05), water holding capacity (r =0.509, p = 0.081) and available

phosphorous (r =0.275, p = 0.053), and there was a negative correlation between

methane emission and soil respiration (r = –0.303, p = 0.041). Other parameters such as

pH, total Kjeldhal nitrogen, microbial biomass C, N and P did not show any significant

correlation with methane emission.

Discussion

An increasing trend in atmospheric CH4 during the past 300 years has been

observed (Blake and Rowland, 1988, Steele et al., 1987, Khalil and Rasmussen, 1985).

This may be making an important contribution to an increase in global temperature, due

to the relatively high absorption of infrared radiation by CH4 (Bouwman, 1990). The

main biotic source of atmospheric CH4 are wetland rice cultivation, natural wetlands,

ruminating animals, landfills, oceans and lakes, and biomass burning. CH4 formation in

flooded soils is a microbiological process affected by many environmental factors.

However, the interactions between soil chemical and physical properties and CH4

emission are not yet well understood. Information on the effects of different soil

parameters on CH4 emission and their quantitative relationship is necessary to provide a

theoretical basis for controlling CH4 emission in flooded rice soils.

Methanogenesis in soil is an energy transformation process, mediated by CH4–

generating bacteria present in soil. These microbes under suitable temperature and

anaerobic conditions enhance the emission of methane from soil and plant roots. A

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positive correlation between CH4 emission and soil moisture (Figure 6.3) could be be

attributed to the fact that an increase in soil moisture usually improve microbial activity.

The amount of soil organic matter and decomposable carbon (C) are generally

considered critical controlling factors of CH4 production. The organic matter content of

cultivated soils is considered an index of soil fertility. It is apparent from previous works

that addition of organic matter to wetland rice fields, whether or not in combination with

mineral fertilizers, causes elevated CH4 emissions. A cultivated soil with relatively high

amount of organic matter did implicate greater micro–organic activity. Therefore, a

positive relationship is seen between CH4 emission and the soil organic carbon (Wang et

al., 1993). In the present study also, CH4 emission had significant positive correlation

with soil organic carbon content (Figure 6.5). Again, it was also found that methane

production generally decreases when the C content and the C/N ratio of the soil

decreases (Mer and Roger, 2001). Reportedly, emission of methane is also influenced

by many factors such as temperature, solar radiation etc (Lindau et al., 1993, Yagi and

Minami, 1993). Temperature effects may also be reflected in regional differences caused

by climatic variation (Kimura et al., 1991, Parashar et al., 1991). In the present study,

soil temperature and solar radiation was found to be highest between 10 am to 12 pm

and during this period higher amounts of methane emission were estimated (Figure 6.1).

Many workers have observed that there is scarcely any relation between soil pH and

methane emission, however, near neutral pH (6.9–7.2) showed a tendency to act as the

optimum pH for methane formation (Wang et al., 1993, Conrad, 1989, Williams and

Crawford, 1984). Since the soil of the present study site was acidic (<6), hence no any

significant relationship could be found (r =0.111, p = 0.417).

The importance of the various redox systems varies from one soil to another,

which has been illustrated by Takai (1961) who showed for a range of Japanese soils that

the ratio of carbon dioxide (CO2) to CH4 released depends strongly on the ratio of

oxidizing capacity to reducing capacity. Any effect on the water content and distribution

of oxygen in the paddy soil, and on the availability of organic matter, electron acceptors

(nitrate, iron, manganese, sulphate) and nutrients (N, P, S, K and trace elements), also

influences CH4 production and emission (Conrad 1989). Since, a negative correlation

was seen between methane emission and soil respiration (Figure 6.4), which could be

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attributed to deceased soil respiration with increasing anaerobic condition when

production and emission of methane increased per se.

Methane emission showed a significant positive correlation with available

phosphorous in this study (Figure 6.6). Phosphate transformation in paddy soils has

been reviewed by Patrick and Mahapatra (1968). It has been reported that when a soil is

submerged, concentration of water soluble and available phosphorous increases and

reaches a peak in 20–30 days of flooding (Mohanty and Patnaik, 1976). The increase in

phosphorous availability due to submergence of acid soils is attributed to:

a) hydrolysis of Fe (III) – P and Al – P,

b) reduction of Fe (III) to Fe (II) – P,

c) release of P from anion exchange sites of clays and hydrous oxides of Fe and Al,

and

d) release of occluded P on reduction of reduction of hydrated ferric oxide coatings.

The seasonal variation of CH4 emission seems to depend on many factors, but

some patterns seem to be characteristic. For instance, shortly after inundation, the CH4

emission from rice fields is similar to emissions from unplanted fields (Sass et al., 1990,

Holzapfel–Pschorn and Seiler, 1986). In the present study, however, it was observed that

greater emission of methane was observed during active tillering and reproductive stage

(Table 6.1). These peaks may be attributed to increased efficiency of gas transport

through the root system of the rapidly growing rice plants during tillering and to the

supply of easily decomposable organic substrates in the form of root exudates (Sass et

al., 1990). These exudates consist mainly of carbohydrates, organic acids and amino

acids, and they are preferentially released during the vegetative and reproductive stage of

rice growth (Schutz et al., 1989, Boureau, 1977). The peak in methane emissions

observed in the period when plants reach their reproductive stage is related to the high

activity of the rice plants providing soil bacteria with organic root exudates or root litter

(Yagi and Minami 1990, Schiitz et al., 1989, Minami and Yagi, 1988, Holzapfel–

Pschorn and Seiler 1986). It is seen in the present study that some amount of CH4

emission also occurred in the latest stage (maturation and after harvest stage) of the rice

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crop that may be related to the decay of roots (Schiitz et al., 1989). Yagi and Minami

(1990) observed methane emission after the paddy field was left to dry out, possibly due

to release of captured methane.

Overall, this study clearly revealed that variability in CH4 emission took place

from the rice fields of Apatani valley, during different stages of paddy growth; highest

was observed during reproductive and last part of vegetative stage. Further, emission of

methane was differed physico–chemical properties of soil. While parameters such as soil

temperature, moisture content, water holding capacity, organic carbon and available

phosphorous show a positive significant relationship with emission of methane from

soil, others such as total Kjeldhal nitrogen, microbial biomass carbon, nitrogen and

phosphorous do not show any significant correlation with methane emission. pH also

was not significantly correlated with methane emission, as the pH of soil tended towards

acidic nature (the pH was found to be less than 6). Soil respiration on the other hand

showed a negative correlation with methane emission. It can be attributed to the fact that

soil respiration is favoured by aerobic condition while emission of methane takes place

in complete anaerobic condition.