Methane Flux from Indonesian Wetland Rice : the Effect of Water Management and Rice Variety
I. INTRODUCTION
Background and Justification of the Study
Methane (CH,) is a chemically reactive atmospheric trace gas, it is 25 to 35
times more effective as a greenhouse gas than CO, on per-molecular basis (Kiehl and
Dickinson, 1987). The presence of 1.3 ppmv of methane in the atmosphere causes the
globahy averaged surface temperature 1.3
O C
higher than it would be with no
methane (Domer and Ramanathan, 1980). It was discovered recently that the global
concentration of atmospheric methane is increasing (Rasmussen and Khalil, 1981;
Fraser et al., 1981; Blake et al., 1982; Khalil and Rasmussen, 1983; Ehhalt et al.,
1983; Blake and Rowland, 1988). The increase of methane was predicted to have a
significant impact on the world's climate (Ramanathan,
et al., 1985) and the
chemistry of the troposphere (Bolle et al., 1986).
Methane concentrations have almost doubled in the last 100 - 200
years
(Rasmussen and Khalil, 1984). Ice core measurements showed that the concentration
of methane from the interglacial period until 200 years ago (preindustrial period) was
between 650 ppbv and 750 ppbv (Khalil and Rasmussen, 1982; Craig and Chou,
1982; Khalil and Rasmussen, 1984). The present globally averaged concentration of
atmospheric methane is about 1700 ppbv (Steel et al., 1987; Khalil, 1992; Tyler,
1991). Its current annual rate of increase is about 0.7 % or 12
+ 1 ppbvlyr (Khalil et
al., 1992; Khalil and Rasmussen, 1993). This increase is due net excesses
sources over sinks, which amounts to about 36 Tg/yr (1 Tg
=
of
1012 g = 1 million
tons); every 1 ppbv of methane in the atmosphere corresponds to approximately 3 Tg
(Blake, 1984). Crutzen (1991) estimates that the total atmospheric sink of methane
is 460 f 100 Tglyr, therefore the total annual source of atmospheric methane is 500
f 100 Tg. Roughly 70% of the total global emission of atmospheric methane stems
from anthropogenic sources mainly from anaerobic decay of organic matter in rice
fields and enteric fermentation in ruminants and about 30% from natural sources,
mainly from wetlands.
Methane is released into the atmosphere principally in two ways (Cicerone and
Oremland, 1988): (1) anaerobic degradation of biological organic matter (biogenic)
and (2) direct release due to production and combustion of fossil fuel and natural gas
leakage (non-biogenic). Biogenic methane formation results from complex
biochemical reactions involving bacteria during the decomposition of organic matter.
These methanogenic bacteria are strictly anaerobic, requiring reducing environments
for growth. For this reason, rice
soils are the major habitat for methanogenic
bacteria.
Methane is produced in large quantities in the anoxic soil of rice fields and
emitted into the atmosphere (Cicerone and Shetter 1981; Cicerone et al., 1983; Seiler
et al., 1984). The interest of study on methane emissions from rice fields started to
intensify after it was discovered that methane concentration is increasing in the
atmosphere and could contribute to global climate change. During the last 10 years,
every global budget of methane has included the world's rice fields as a major source
(Khalil et al., 1991). Most of the methane emitted from rice fields is expected to be
from the Asian region as it has 90% of the total world rice harvest area, out of which
about 52 % is from China and India. Of the total harvested area in Asia, about 50 % is
irrigated and another 30 % is wetland rainfed rice (Bachelet and Neue, 1993).
There are many factors that could affect methane production and emission from
rice fields such as climate, soil type, water management, rice cultivars, organic and
inorganic fertilizer, time of the day, and season; and rice fields exhibit complex
interaction between plants, soil and atmosphere.
Accurate estimates of methane
emissions from rice fields are difficult to calculate due to the lack of experimental
data on the methane fluxes that include those factors.
Estimates of methane emission from rice fields show a wide range between 30
and 100 Tglyr. This wide range is caused by various measurements of methane flux
from different regions in the world. The average total methane emissions from rice
fields is estimated to about 60 Tglyr, constituting about 18% of emissions from all
sources (Shearer and Khalil, 1993). Until 1990, most of the methane flux studies
from rice fields were limited to temperate regions (mostly USA, Italy and Spain)
where rice cultivation is minimal (3 % of total world's rice fields) and insufficient or
no data were available from the main rice producing countries like India, China,
Bangladesh, Thailand, Indonesia, Vietnam and Myanmar. Only recent flux
measurements in Asia have been reported for Japan (Yagi and Minami, 1990), China
(Khalil et al., 1990a; Khalil et al., 1991, Wang, et al., 1992a), India (Parashar et
al., 1992) and Thailand (Yagi, et al., 1992).
Until 1993, no data of the methane flux from Indonesian rice fields have been
published. In March 1992, a pilot study was conducted by M.A.K. Khalil, D.
Murdiyars:, and Y. A. Husin to measure methane flux from wetland rice in Bogor.
Temporary result showed that the maximum methane flux was about 1.4 g/m2/day
(unpublished data).
Indonesian wetland rice in 1991 covered an area of 8.227 million ha (Central
Bureau of Statistics of Indonesia, 1992) or about 6.3 % of the total world's wetland
rice areas. The harvested areas of Indonesian wetland rice in 1991 were 9.17 million
ha of which 53% were located in Java Island. Rice is the staple food for more than
95 % of the Indonesian population. Indonesia, once as the largest rice importer in the
world, became self-sufficient in rice production in 1984. This goal has been achieved
through great effort by the Indonesian government in enhancing multiple cropping
and irrigation schemes, especially in Java, Sumatera and Sulawesi Islands.
In the absence of methane emission data from Indonesia, some studies estimated
the total methane emissions to be 2.9 - 3.7 Tglyr (Mathews et al., 1991), 3.7 - 4.8
Tglyr (Taylor et al., 1991), 5.8 - 9.8 Tglyr (Japan Environmental Agency and
Ministry of Population and Environment of Indonesia, 1992), 3.5 - 4.5 Tg/yr
(Bachelet and Neue, 1993), and 6.2 Tglyr (Shearer and Khalil, 1993). These
estimates were based on extrapolations of methane flux data from other countries
(temperate regions) or on an assumption that a fraction of net primary production
(NPP) of rice plant is converted to methane. These estimates of methane emissions
from Indonesian rice fields need to be verified through direct methane flux
measurements at the source by incorporating the main factors that can affect methane
flux. By having methane flux data, total methane emissions from Indonesian rice
fields can be estimated more accurately.
Definitions
Wetland Rice: is agricultural land, squared and separated by small dikes to contain
water, where the main crop is usually wetland paddy (without considering the
status of the land). It includes the land that is registered at land income tax
office, regional development contribution, 'bengkok' land, iUegal ownership,
swamps for rice cultivation, and newly opened lands. Wetland rice includes
irrigated rice fields, rainfed, and valleys. (Central Bureau of Statistics of
Indonesia).
Dryland Rice: is agricultural land where rice is planted on a garden, dry field or
shifting cultivation (Central Bureau of Statistics of Indonesia).
Saturated Soil Condition: (macak-macak in Indonesian) the rice field is irrigated
many times with water just enough to make rice soils wet in which the
moisture content is above the field capacity.
Diurnal: applied to events or cycles that repeat at daily intervals (MacMillan
Dictionary of the Environment)
Puddling: the trampling of wet soil by livestock; it causes the soil to become less
permeable (MacMillan Dictionary of the Environment)
Framework of the Study
Recognizing that no data are available on methane flux from rice fields in
Indonesia, therefore direct methane flux measurements at the source are very
important. This study is focused on the understanding of methane flux from irrigated
wetland rice in which its source strength is still largely uncertain, especially for the
tropical countries. This study is important because very little is known about
methanogenesis in submerged soils of rice fields in tropical regions and even less is
known on the influence of rice cultivars and water managements on methanogenesis
and methane emission. In addition, this study is the frrst of its kind on methane flux
measurements from rice fields in Indonesia.
General physiological conditions of the rice plant seems to be more important to
stimulate methanogenesis than the momentary rate of photosynthesis (Cicerone and
Shetter, 1981; Seiler et al., 1984; Nouchi, 1992). Beside plant factors, irrigation
water management is also one of the key factors that strongly affect methane
emission. Irrigated wetland rice has the highest potential to produce methane because
flooding and, consequently, anoxic conditions are assured and controlled. The
duration and pattern of flooding and saturation are important criteria for methane
formation. Aerating wetland rice soils to reduce fluxes without hampering rice
production is a tempting mitigation technology (Neue and Roger, 1993).
Based on the above mentioned facts, this study was conducted to explore the
differences of methane flux from two rice varieties (short and long growing period),
and three types of irrigation water management i.e. continuous flooding, intermittent
irrigation and saturated soil condition (macak-macak, in Indonesian) and the
interaction of these two factors.
Goals and Objectives
Based upon the above background and framework, this
achieving the following goals and objectives:
study is aimed at
a. To study methane flux differences from three types of irrigation water
management: (1) continuous flooding-flowing with 5-cm water depth, (2)
intermittent irrigation or multiple aeration, consists of flooding the rice field with
5 cm water depth and let the water to evaporate to achieve a saturated soil
conditi~nand reflooded again and (3) keeping the rice soils in saturated condition
(macak-macak) throughout the rice growing period.
b. To study methane fluxes from two different rice varieties, IR-64 and Cisadane.
Both rice varieties have different physiological and agronomic characteristics.
c. To study the diurnal variations of methane flux.
d. To estimate total methane emission from Indonesian wetland rice, based upon
distribution and harvested areas of wetland rice in Indonesia.
e. To explore an appropriate water management in rice cultivation which can reduce
methane emission without adversely affecting rice yield.
Hypotheses
The aforementioned goals and objectives generate the following hypotheses
which direct the methodology employed in the study:
a. Methane emission rates from wetland rice agriculture are dependent on the
irrigation water management applied during the entire growing period. Aerating
the rice soils will affect the amount of methane emitted into the atmosphere.
b. Each rice varieties has different ability in emitting methane from rice soils into the
atmosphere.
c. Time of the day influences methane emission rates from wetland rice.
11. WTE-
REVIEW
Atmospheric Methane
Methane Concentration in the Atmosphere
It is known that the global concentration of atmospheric methane is increasing
(Rasmussen and Khalil, 1981, 1984; Fraser et al., 1981; Blake et al., 1982; Khalil
and Rasmussen, 1983; Ehhalt et al., 1983; Blake and Rowland, 1988). Methane is
increasing sufficiently rapidly within the last two centuries that it is second only to
CO, in affecting global warming (Khalil, 1991).
Results of analyses of methane in air entrapped at polar ice cores conducted by
several researchers (Khalil and Rasmussen 1982, 1984, 1986; Craig and Chou, 1982,
Stauffer et al., 1985; Pearman et al., 1986) showed that the present increase of
atmospheric methane started about 100 - 200 years ago (see Figure 1). According to
Khalil et ~ 1 . (1990b) the pattern of the present rapid increase is likely resulted from
human activities which is linked to rapidly growing population. The record of
methane spanning over the last 150,000 years showed that methane concentrations
had varied naturally because of changing climatic conditions from ice ages to
interglacial periods. During ice ages methane drops to around 350 ppbv and during
warmer interglacial times the concentration were about 650 ppbv. The concentration
of methane from interglacial period until 200 years ago (preindustrial period) was
between 650 ppbv and 750 ppbv (Figure 1).
The global average of atmospheric methane concentrations over the last decade
is shown in Figure 2 (Khalil and Rasmussen, 1990a), which also shows the result of
three systematic global studies spanning between 1978 and 1988 by Steele et al.
(1987), Blake and Rowland (1988), and Khalil and Rasmussen (1990a). It can be seen
from Figure 2, that there is a remarkably good agreement between the studies even
though the methods and strategies were quite different.
A Camp Cenluy
O DE 08
+ 057
BHD
500
900
I
I
I 100
i
I
I
1300
I
1SO0
I
I
1700
I
I
1900
Air age (years A.D.)
Figure 1. Atn~osphericmethane variations over the past 1000 years from ice core data (Raynaud and
Chappellaz, 1993). Greenland ice cores: Dye 3 (Craig and Chou, 1982). Camp Century and
Crete (Rasmussen and Khalil. 1984). Antartic ice cores: Byrd (Rasmussen and Khalil, 1984);
DE 08 (Etheridge et al., 1992). Siple and South Pole (Stauffer et al., 1985), D 57 (Raynaud et
al., 1988), BHD (Etheridge et al., 1988).
0
Khalil &
Rasmussen
+
Steele
et al.
Blake &
Rowland
Year
Figure 2. Monthly global average concentration from three long-term systimatic
studies of methane in the atmosphere (Khalil and Rasmussen, 1990a).
Concentrations of methane are higher over continents and over the northern
hemisphere, generally are higher compared to that over the southern hemisphere
(Figure 3), this is because most of the sources are land-based and many are related to
human activities (Khalil et al. 1990b). In 1989, the average methane concentration in
the northern hemisphere was about 1750 ppbv, and was about 1670 ppbv in the
southern hemisphere.
The present global average concentration of atmospheric methane is about 1700
ppbv (Khalil, 1992; Tyler, 1991), and before being affected by human activities (200
years ago) it was about 650 - 750 ppbv. There is a clear evidence that atmospheric
methane has more than doubled in the last 200 years (Khalil and Rasmussen, 1982;
Craig and Chou, 1982; Rasmussen and Khalil, 1984; Perman et al., 1986). In any
case, contemporary methane amounts and their rate of increase are novel, human
activities are clearly involved in causes for the increase of atmospheric methane from
650 ppbv to 1700 ppbv (Cicerone and Orernland, 1988). The increase of methane
over the past 200 years is probably due to the increase of emissions (70%) (Khalil
and Rasmussen, 1985).
The first evidence for an increase of methane in the atmosphere was given by
Rasmussen and Khalil (1981) based on the results of a relatively short
methane
record at Cape Meares, Oregon. The results showed a 1-2%/yr rates of increase
between 1965 and 1980. Successive work of KhaLil and Rasmussen (1990a) showed
that during the last decade (1980 - 1988) the overall methane concentration had
increased at an average rate of 16.6
+ 0.4 ppbv/yr or 1.02 + 0.02%/yr. Blake and
Rowland (1988) reported an increase rate of 1%/yr, and Steele et al. (1987) reported
a value which was slightly less than 0.8 %/yr. Khalil et al. (1992) reported that the
rate of methane increase appeared to be slowing down with an average slowdown is
nearly -1 ppbv/yr (see Figures 4 and 5); the current rate of increase is estimated to
be 12
+ 1 ppbv/yr or 0.7 % /yr (Khalil et al., 1992).
0
NOAAICMDL
Adjusted
001 1988
Average
- Polynomial
Fit
Sine of Latitude
Figure 3. The latitudinal distribution of methane (Khalil et al., 1993a).
Alaska
Oregon
Hawaii
Samoa
Tasmania
Antarctic
Time (months)
Figure 4. The trends of methane over 3-year overlapping periods of time calculated by linear regression.
The rate of increase is slowing down in recent years. (Data of Khalil and Rasmussen in Raynaud
and Cliappellaz, 1993).
Time
Figure 5. Global average concentrations of methane (open circles). The solid line is the trend
during the first 2 years and the last 2 years of measurements. The dashed portions of
these lines are extrapolations. The rate of increase in the first 2 years is significantly
faster than that in the last 2 years (Khalil and Rasmussen, 1993)
The slowdown of methane increase may be caused by the decrease of sources
from ruminants and rice fields. Over the last decade, cattle population and rice field
areas have not increase much compared to the rapid rates of increase in 1950s
(Khalil and Shearer, 1993). Yet other, newer anthropogenic sources such as landfills,
natural gas leakages, low-temperature coal burning and the like may be increasing
and eventually may also cause methane to increase. This represents a shift from
agricultural sources to other sources which are related to energy and waste disposal
(Raynaud and Chappellaz, 1993).
Global Sources and Sinks of Methane
It is generally accepted that the most dominant methane sources are related to
land surface and soil which represent the most important sources of atmospheric
methane (Schiitz et al., 1990). The emissions of methane to the atmosphere come
from both 'anthropogenic and natural sources. The major sources of methane
controlled by human activities (anthropogenic sources) are from the raising of
ruminants, wetland rice cultivation, natural gas use, venting of gas from oil and gas
exploration and exploitation, landfills, and biomass burning. The wetlands are
considered to produce the largest natural emissions. Smaller natural sources include,
forest soil, lakes, tundra, wild animals and insects (Khalil, 1991).
There have been several efforts to characterize the sources of methane and to
balance the atmospheric methane budget. But up to now, there is still substantial
uncertainty in the overall budget, especially in relative contribution from a number
of sources, such as termite, rice fields, landfills, natural wetlands and permafrost. As
seen in Table 1, the many proposed budgets of methane are very different from each
other. Estimate of emissions from individual sources are less certain than estimate
from larger groups of sources as the total anthropogenic or natural emissions or the
total global emission (Khalil, 1991)
Khalil and Rasmussen (1990b) showed that the best estimate of total present
emissions should be around 420 - 620 Tglyr and the anthropogenic fraction (ratio of
anthropogenic to total emission rates) should be around 40-70%, by taking into
account the present lifetime of methane is 8-12 years.
The mixing ratio (concentration) of methane in earth's atmosphere is defined by
its source strengths and removal (sink) strengths. Concentration of methane can
increase only if the sources increase or the sinks decrease; both possibility are
plausible (Khalil, 1992). The main sink for atmospheric methane is its reaction with
tropospheric OH radicals:
0, + hv
O('D) + H20
CH, + OH
> O(lD) + 0,
> 20H
> H20 + CH,
Table 1. Estimate o f methane emission from various anthropogenic and n a t u r a l sources: 1978
Methane Source
Ehhalt &
Schmitt
(1978)
-
l W 1 (adapted from: K h a l i l and Rasmussen, 1990b)
Oonahue
Sheppard
Crutzen
e t at.
Khalil &
Rasmussen
Blake
Sei Ler
(1982)
(1983)
(1984)
(1984)
Crutzen
(1985)
Seiler
(1979)
120
95
25
70-160
149-189
25-110
70-100
30-75
50-100
60
120-200
20-70
70-100
70-170
55-100
(1983)
(1986)
Bimenger
Cicerone 8
& Crutzen Oremland
(1987)
(1988)
1
Bouwnan
Tyler
(1990)
(1991)
Anthropogenic:
Runinants
Rice paddy f i e l d s
Biomass burning
Landfills
Coal mining
Oil/gas system
Automobiles
Other Anthropogenic
100-200
280
100-2201
140-280
8-28
16-50
1
7-21
1
110-210
100
TotalAnthropogenic
396-550
366-760
339
Natural wetlands
Lakes
Oceans
190-300
1-25
1 17
200-300
Tropical f o r e s t s
Tundra
39
35
30
767
0-3
3-50
90
39
60
60
30-60
30-110
50
20
30
20-30
40
34
33
35
30-40
1-2
40
62- 100
140-250
320
297-560
200-335
267-397
261-447
30-220
150
10
13
120-190
13
5-21
60-400
15-60
1-7
70-90
25-70
15-35
70-80
18-91
30-100
30-70
35
0-35
65-100
60-170
50-100
30-70
25-45
25-50
66-90
60-140
55-100
30-70
35
30-40
1-2
183-411
80- 100
70- 170
10-40
5-70
10-35
25-50
0.5
5-25
255-535
277-477
205.5-490.5
100-200
1-25
5-20
40-160
15-35
120-200
0-30
10-100
2-42
1-5
27.5-154.5
26-167
116-345
57-237
149.5-379.5
Natural:
-
Other n a t u r a l
Total Natural
150
12
48
5-15
26-137
1-20
192-345
203-350
871
180-370
233
198-624
21-82
70-90
40-105
T O T A L
588-895
569-1110
1210
320-620
553
495-1184
221-417
337-487
301-552
209-578
371-880
334-714
355-870
Ratio Anthropogenic
Emission t o Total
67-61
64-68
44-40
58
60-47
90-80
79-82
87-81
87-81
69-61
83-67
58-56
Emission ( X )
28
The methyl radical undergoes further oxidation and produces CO and other products
(Khalil, 1991). It is estimated that 85% of the methane emitted into the atmosphere is
destroyed by OH radicals (R3) in the trophosphere. In stratosphere, almost all of the
r e r n i h g methane is destroyed by OH, C1 atoms and O('D) atoms. A small fraction
of methane goes through the stratosphere to the mesosphere where an additional
sink, very short UV light destroys methane photolytically (Cicerone and Oremland,
1988). Complete oxidation of methane produces CO, and H,O, which schematically
can be represented by the combustion of methane:
CH,
+ 2 0,
> CO,
+ 2 H,O
(R4)
Khalil and Rasmussen (1985) calculated that much of the increase of methane
over the past 200 years was clue to the increase of emissions (70%) and about 30%
due
to a depletion of OH radicals; the present abundance of OH radicals in
troposphere is estimated about 20% less than several hundred years ago.
The other process that remove methane is dry soils through oxidation by
methanotrophic bacteria. The oxidation (uptake) of methane in dry soils is estimated
with a net global sink of 32 f 16 Tg CH,lyr (Seiler and Conrad, 1987), of 6 - 58
Tglyr (Born et al., 1989), and of 35 f 15 Tglyr (Crutzen, 1991). Crutzen and Gidel
(1983) estimated that methane transport to the stratosphere is about 60 Tglyr. The
stratospheric reaction of CH, with C1 and O('D) is 10 f 5 Tglyr (Crutzen, 1991).
Recently, Vaghjiani and Ravishankara (1991) have found that the rate of removal of
methane with OH is 25 % less than currently used value. Based on this result Crutzen
(1991) estimates that the total atmospheric sink of methane is 460
With an annual increase in the atmospheric loading by 45
total methane sources is 505
+
100 Tglyr.
+ 5 Tglyr, therefore
the
+ 105 Tglyr, of which about 100 + 20 Tglyr comes
from fossil fuels and methane hydrates (fossil 'dead' methane), and about 405 Tglyr
comes from biogenic sources (fermentation by methanogenic bacteria).
It is clearly seen that sources and sinks are not balanced and leading to the
increasing concentrations, and the allocation among the individual sources and sinks
still not well known. The imbalance between aggregate sources and sinks drives the
growth in atmospheric methane.
Methane Production and Emission from Wetland Rice
Methane Production
Methane is an important product formed during the bacterial degradation of
organic matter in anoxic environments such as flooded soils, wetlands, estuaries,
marine and freshwater sediments, and the gastrointestinal track of animals (Whitman
et al., 1992). Methane producing bacteria (methanogens) are a specific group of
microbes that catabolize a small number of molecules and produce methane as the
major catabolic product (Boone, 1993). Recognized substrates metabolized by
methanogenic bacteria include hydrogen reduction of CO,,
acetate, formate,
methanol, methylated amines and CO (Cicerone and Oremland, 1988).
According to Boone (1993), decomposition process of a complex organic matter
in anoxic environments is divided into three major steps, namely fermentation (or
acidogenesis), synthropic acetigenesis, and methanogenesis (Figure 6). Each step is
catalyzed by a separate group of bacteria, however, all reaction occur simultaneously.
matter
Propionate, butyrate,
I CH4and CO, I
Figure 6 . Organic matter decomposition in methanogenic ecosystems (Boone, 1993).
Reviews of the biology, ecology and biochemistry of methanogens (Vogels et
al., 1988) indicate that all methanogens are very strictly anaerobic bacteria that all
form methane as a major terminal catabolic product. Methane formation results from
complex biochemical reaction as shown in Figure 7 (Boone, 1993).
Methane Emission
Rice is physiologically adapted to grow in water-inundated soils, creating ideal
condition for methane emissions. Rice soils are flooded for a long period so that
anoxic soil conditions arise and provide an optimal environment for methane
production. The biogenic mineralization of organic material in anoxic ecosystem of
rice soil, is one of the most important sources of atmospheric methane.
The great potential for methane release from wetland rice has long been noted
by Koyama (1963), based on the study of methane release from incubated soil in
laboratory. The first in situ
measurements of methane flux from wetland rice
over a complete growing period
were conducted in California by Cicerone and
Shetter (1981), who discovered the importance of methane transport through the
plants as opposed to diffusion and bubble transport across the water-air interface.
Since 1981, direct measurements of methane flux from wetland rice have been conducted by a number of researchers.
Based on a number of investigations, methane emission rates from wetland rice
show a wide range of values between
Background and Justification of the Study
Methane (CH,) is a chemically reactive atmospheric trace gas, it is 25 to 35
times more effective as a greenhouse gas than CO, on per-molecular basis (Kiehl and
Dickinson, 1987). The presence of 1.3 ppmv of methane in the atmosphere causes the
globahy averaged surface temperature 1.3
O C
higher than it would be with no
methane (Domer and Ramanathan, 1980). It was discovered recently that the global
concentration of atmospheric methane is increasing (Rasmussen and Khalil, 1981;
Fraser et al., 1981; Blake et al., 1982; Khalil and Rasmussen, 1983; Ehhalt et al.,
1983; Blake and Rowland, 1988). The increase of methane was predicted to have a
significant impact on the world's climate (Ramanathan,
et al., 1985) and the
chemistry of the troposphere (Bolle et al., 1986).
Methane concentrations have almost doubled in the last 100 - 200
years
(Rasmussen and Khalil, 1984). Ice core measurements showed that the concentration
of methane from the interglacial period until 200 years ago (preindustrial period) was
between 650 ppbv and 750 ppbv (Khalil and Rasmussen, 1982; Craig and Chou,
1982; Khalil and Rasmussen, 1984). The present globally averaged concentration of
atmospheric methane is about 1700 ppbv (Steel et al., 1987; Khalil, 1992; Tyler,
1991). Its current annual rate of increase is about 0.7 % or 12
+ 1 ppbvlyr (Khalil et
al., 1992; Khalil and Rasmussen, 1993). This increase is due net excesses
sources over sinks, which amounts to about 36 Tg/yr (1 Tg
=
of
1012 g = 1 million
tons); every 1 ppbv of methane in the atmosphere corresponds to approximately 3 Tg
(Blake, 1984). Crutzen (1991) estimates that the total atmospheric sink of methane
is 460 f 100 Tglyr, therefore the total annual source of atmospheric methane is 500
f 100 Tg. Roughly 70% of the total global emission of atmospheric methane stems
from anthropogenic sources mainly from anaerobic decay of organic matter in rice
fields and enteric fermentation in ruminants and about 30% from natural sources,
mainly from wetlands.
Methane is released into the atmosphere principally in two ways (Cicerone and
Oremland, 1988): (1) anaerobic degradation of biological organic matter (biogenic)
and (2) direct release due to production and combustion of fossil fuel and natural gas
leakage (non-biogenic). Biogenic methane formation results from complex
biochemical reactions involving bacteria during the decomposition of organic matter.
These methanogenic bacteria are strictly anaerobic, requiring reducing environments
for growth. For this reason, rice
soils are the major habitat for methanogenic
bacteria.
Methane is produced in large quantities in the anoxic soil of rice fields and
emitted into the atmosphere (Cicerone and Shetter 1981; Cicerone et al., 1983; Seiler
et al., 1984). The interest of study on methane emissions from rice fields started to
intensify after it was discovered that methane concentration is increasing in the
atmosphere and could contribute to global climate change. During the last 10 years,
every global budget of methane has included the world's rice fields as a major source
(Khalil et al., 1991). Most of the methane emitted from rice fields is expected to be
from the Asian region as it has 90% of the total world rice harvest area, out of which
about 52 % is from China and India. Of the total harvested area in Asia, about 50 % is
irrigated and another 30 % is wetland rainfed rice (Bachelet and Neue, 1993).
There are many factors that could affect methane production and emission from
rice fields such as climate, soil type, water management, rice cultivars, organic and
inorganic fertilizer, time of the day, and season; and rice fields exhibit complex
interaction between plants, soil and atmosphere.
Accurate estimates of methane
emissions from rice fields are difficult to calculate due to the lack of experimental
data on the methane fluxes that include those factors.
Estimates of methane emission from rice fields show a wide range between 30
and 100 Tglyr. This wide range is caused by various measurements of methane flux
from different regions in the world. The average total methane emissions from rice
fields is estimated to about 60 Tglyr, constituting about 18% of emissions from all
sources (Shearer and Khalil, 1993). Until 1990, most of the methane flux studies
from rice fields were limited to temperate regions (mostly USA, Italy and Spain)
where rice cultivation is minimal (3 % of total world's rice fields) and insufficient or
no data were available from the main rice producing countries like India, China,
Bangladesh, Thailand, Indonesia, Vietnam and Myanmar. Only recent flux
measurements in Asia have been reported for Japan (Yagi and Minami, 1990), China
(Khalil et al., 1990a; Khalil et al., 1991, Wang, et al., 1992a), India (Parashar et
al., 1992) and Thailand (Yagi, et al., 1992).
Until 1993, no data of the methane flux from Indonesian rice fields have been
published. In March 1992, a pilot study was conducted by M.A.K. Khalil, D.
Murdiyars:, and Y. A. Husin to measure methane flux from wetland rice in Bogor.
Temporary result showed that the maximum methane flux was about 1.4 g/m2/day
(unpublished data).
Indonesian wetland rice in 1991 covered an area of 8.227 million ha (Central
Bureau of Statistics of Indonesia, 1992) or about 6.3 % of the total world's wetland
rice areas. The harvested areas of Indonesian wetland rice in 1991 were 9.17 million
ha of which 53% were located in Java Island. Rice is the staple food for more than
95 % of the Indonesian population. Indonesia, once as the largest rice importer in the
world, became self-sufficient in rice production in 1984. This goal has been achieved
through great effort by the Indonesian government in enhancing multiple cropping
and irrigation schemes, especially in Java, Sumatera and Sulawesi Islands.
In the absence of methane emission data from Indonesia, some studies estimated
the total methane emissions to be 2.9 - 3.7 Tglyr (Mathews et al., 1991), 3.7 - 4.8
Tglyr (Taylor et al., 1991), 5.8 - 9.8 Tglyr (Japan Environmental Agency and
Ministry of Population and Environment of Indonesia, 1992), 3.5 - 4.5 Tg/yr
(Bachelet and Neue, 1993), and 6.2 Tglyr (Shearer and Khalil, 1993). These
estimates were based on extrapolations of methane flux data from other countries
(temperate regions) or on an assumption that a fraction of net primary production
(NPP) of rice plant is converted to methane. These estimates of methane emissions
from Indonesian rice fields need to be verified through direct methane flux
measurements at the source by incorporating the main factors that can affect methane
flux. By having methane flux data, total methane emissions from Indonesian rice
fields can be estimated more accurately.
Definitions
Wetland Rice: is agricultural land, squared and separated by small dikes to contain
water, where the main crop is usually wetland paddy (without considering the
status of the land). It includes the land that is registered at land income tax
office, regional development contribution, 'bengkok' land, iUegal ownership,
swamps for rice cultivation, and newly opened lands. Wetland rice includes
irrigated rice fields, rainfed, and valleys. (Central Bureau of Statistics of
Indonesia).
Dryland Rice: is agricultural land where rice is planted on a garden, dry field or
shifting cultivation (Central Bureau of Statistics of Indonesia).
Saturated Soil Condition: (macak-macak in Indonesian) the rice field is irrigated
many times with water just enough to make rice soils wet in which the
moisture content is above the field capacity.
Diurnal: applied to events or cycles that repeat at daily intervals (MacMillan
Dictionary of the Environment)
Puddling: the trampling of wet soil by livestock; it causes the soil to become less
permeable (MacMillan Dictionary of the Environment)
Framework of the Study
Recognizing that no data are available on methane flux from rice fields in
Indonesia, therefore direct methane flux measurements at the source are very
important. This study is focused on the understanding of methane flux from irrigated
wetland rice in which its source strength is still largely uncertain, especially for the
tropical countries. This study is important because very little is known about
methanogenesis in submerged soils of rice fields in tropical regions and even less is
known on the influence of rice cultivars and water managements on methanogenesis
and methane emission. In addition, this study is the frrst of its kind on methane flux
measurements from rice fields in Indonesia.
General physiological conditions of the rice plant seems to be more important to
stimulate methanogenesis than the momentary rate of photosynthesis (Cicerone and
Shetter, 1981; Seiler et al., 1984; Nouchi, 1992). Beside plant factors, irrigation
water management is also one of the key factors that strongly affect methane
emission. Irrigated wetland rice has the highest potential to produce methane because
flooding and, consequently, anoxic conditions are assured and controlled. The
duration and pattern of flooding and saturation are important criteria for methane
formation. Aerating wetland rice soils to reduce fluxes without hampering rice
production is a tempting mitigation technology (Neue and Roger, 1993).
Based on the above mentioned facts, this study was conducted to explore the
differences of methane flux from two rice varieties (short and long growing period),
and three types of irrigation water management i.e. continuous flooding, intermittent
irrigation and saturated soil condition (macak-macak, in Indonesian) and the
interaction of these two factors.
Goals and Objectives
Based upon the above background and framework, this
achieving the following goals and objectives:
study is aimed at
a. To study methane flux differences from three types of irrigation water
management: (1) continuous flooding-flowing with 5-cm water depth, (2)
intermittent irrigation or multiple aeration, consists of flooding the rice field with
5 cm water depth and let the water to evaporate to achieve a saturated soil
conditi~nand reflooded again and (3) keeping the rice soils in saturated condition
(macak-macak) throughout the rice growing period.
b. To study methane fluxes from two different rice varieties, IR-64 and Cisadane.
Both rice varieties have different physiological and agronomic characteristics.
c. To study the diurnal variations of methane flux.
d. To estimate total methane emission from Indonesian wetland rice, based upon
distribution and harvested areas of wetland rice in Indonesia.
e. To explore an appropriate water management in rice cultivation which can reduce
methane emission without adversely affecting rice yield.
Hypotheses
The aforementioned goals and objectives generate the following hypotheses
which direct the methodology employed in the study:
a. Methane emission rates from wetland rice agriculture are dependent on the
irrigation water management applied during the entire growing period. Aerating
the rice soils will affect the amount of methane emitted into the atmosphere.
b. Each rice varieties has different ability in emitting methane from rice soils into the
atmosphere.
c. Time of the day influences methane emission rates from wetland rice.
11. WTE-
REVIEW
Atmospheric Methane
Methane Concentration in the Atmosphere
It is known that the global concentration of atmospheric methane is increasing
(Rasmussen and Khalil, 1981, 1984; Fraser et al., 1981; Blake et al., 1982; Khalil
and Rasmussen, 1983; Ehhalt et al., 1983; Blake and Rowland, 1988). Methane is
increasing sufficiently rapidly within the last two centuries that it is second only to
CO, in affecting global warming (Khalil, 1991).
Results of analyses of methane in air entrapped at polar ice cores conducted by
several researchers (Khalil and Rasmussen 1982, 1984, 1986; Craig and Chou, 1982,
Stauffer et al., 1985; Pearman et al., 1986) showed that the present increase of
atmospheric methane started about 100 - 200 years ago (see Figure 1). According to
Khalil et ~ 1 . (1990b) the pattern of the present rapid increase is likely resulted from
human activities which is linked to rapidly growing population. The record of
methane spanning over the last 150,000 years showed that methane concentrations
had varied naturally because of changing climatic conditions from ice ages to
interglacial periods. During ice ages methane drops to around 350 ppbv and during
warmer interglacial times the concentration were about 650 ppbv. The concentration
of methane from interglacial period until 200 years ago (preindustrial period) was
between 650 ppbv and 750 ppbv (Figure 1).
The global average of atmospheric methane concentrations over the last decade
is shown in Figure 2 (Khalil and Rasmussen, 1990a), which also shows the result of
three systematic global studies spanning between 1978 and 1988 by Steele et al.
(1987), Blake and Rowland (1988), and Khalil and Rasmussen (1990a). It can be seen
from Figure 2, that there is a remarkably good agreement between the studies even
though the methods and strategies were quite different.
A Camp Cenluy
O DE 08
+ 057
BHD
500
900
I
I
I 100
i
I
I
1300
I
1SO0
I
I
1700
I
I
1900
Air age (years A.D.)
Figure 1. Atn~osphericmethane variations over the past 1000 years from ice core data (Raynaud and
Chappellaz, 1993). Greenland ice cores: Dye 3 (Craig and Chou, 1982). Camp Century and
Crete (Rasmussen and Khalil. 1984). Antartic ice cores: Byrd (Rasmussen and Khalil, 1984);
DE 08 (Etheridge et al., 1992). Siple and South Pole (Stauffer et al., 1985), D 57 (Raynaud et
al., 1988), BHD (Etheridge et al., 1988).
0
Khalil &
Rasmussen
+
Steele
et al.
Blake &
Rowland
Year
Figure 2. Monthly global average concentration from three long-term systimatic
studies of methane in the atmosphere (Khalil and Rasmussen, 1990a).
Concentrations of methane are higher over continents and over the northern
hemisphere, generally are higher compared to that over the southern hemisphere
(Figure 3), this is because most of the sources are land-based and many are related to
human activities (Khalil et al. 1990b). In 1989, the average methane concentration in
the northern hemisphere was about 1750 ppbv, and was about 1670 ppbv in the
southern hemisphere.
The present global average concentration of atmospheric methane is about 1700
ppbv (Khalil, 1992; Tyler, 1991), and before being affected by human activities (200
years ago) it was about 650 - 750 ppbv. There is a clear evidence that atmospheric
methane has more than doubled in the last 200 years (Khalil and Rasmussen, 1982;
Craig and Chou, 1982; Rasmussen and Khalil, 1984; Perman et al., 1986). In any
case, contemporary methane amounts and their rate of increase are novel, human
activities are clearly involved in causes for the increase of atmospheric methane from
650 ppbv to 1700 ppbv (Cicerone and Orernland, 1988). The increase of methane
over the past 200 years is probably due to the increase of emissions (70%) (Khalil
and Rasmussen, 1985).
The first evidence for an increase of methane in the atmosphere was given by
Rasmussen and Khalil (1981) based on the results of a relatively short
methane
record at Cape Meares, Oregon. The results showed a 1-2%/yr rates of increase
between 1965 and 1980. Successive work of KhaLil and Rasmussen (1990a) showed
that during the last decade (1980 - 1988) the overall methane concentration had
increased at an average rate of 16.6
+ 0.4 ppbv/yr or 1.02 + 0.02%/yr. Blake and
Rowland (1988) reported an increase rate of 1%/yr, and Steele et al. (1987) reported
a value which was slightly less than 0.8 %/yr. Khalil et al. (1992) reported that the
rate of methane increase appeared to be slowing down with an average slowdown is
nearly -1 ppbv/yr (see Figures 4 and 5); the current rate of increase is estimated to
be 12
+ 1 ppbv/yr or 0.7 % /yr (Khalil et al., 1992).
0
NOAAICMDL
Adjusted
001 1988
Average
- Polynomial
Fit
Sine of Latitude
Figure 3. The latitudinal distribution of methane (Khalil et al., 1993a).
Alaska
Oregon
Hawaii
Samoa
Tasmania
Antarctic
Time (months)
Figure 4. The trends of methane over 3-year overlapping periods of time calculated by linear regression.
The rate of increase is slowing down in recent years. (Data of Khalil and Rasmussen in Raynaud
and Cliappellaz, 1993).
Time
Figure 5. Global average concentrations of methane (open circles). The solid line is the trend
during the first 2 years and the last 2 years of measurements. The dashed portions of
these lines are extrapolations. The rate of increase in the first 2 years is significantly
faster than that in the last 2 years (Khalil and Rasmussen, 1993)
The slowdown of methane increase may be caused by the decrease of sources
from ruminants and rice fields. Over the last decade, cattle population and rice field
areas have not increase much compared to the rapid rates of increase in 1950s
(Khalil and Shearer, 1993). Yet other, newer anthropogenic sources such as landfills,
natural gas leakages, low-temperature coal burning and the like may be increasing
and eventually may also cause methane to increase. This represents a shift from
agricultural sources to other sources which are related to energy and waste disposal
(Raynaud and Chappellaz, 1993).
Global Sources and Sinks of Methane
It is generally accepted that the most dominant methane sources are related to
land surface and soil which represent the most important sources of atmospheric
methane (Schiitz et al., 1990). The emissions of methane to the atmosphere come
from both 'anthropogenic and natural sources. The major sources of methane
controlled by human activities (anthropogenic sources) are from the raising of
ruminants, wetland rice cultivation, natural gas use, venting of gas from oil and gas
exploration and exploitation, landfills, and biomass burning. The wetlands are
considered to produce the largest natural emissions. Smaller natural sources include,
forest soil, lakes, tundra, wild animals and insects (Khalil, 1991).
There have been several efforts to characterize the sources of methane and to
balance the atmospheric methane budget. But up to now, there is still substantial
uncertainty in the overall budget, especially in relative contribution from a number
of sources, such as termite, rice fields, landfills, natural wetlands and permafrost. As
seen in Table 1, the many proposed budgets of methane are very different from each
other. Estimate of emissions from individual sources are less certain than estimate
from larger groups of sources as the total anthropogenic or natural emissions or the
total global emission (Khalil, 1991)
Khalil and Rasmussen (1990b) showed that the best estimate of total present
emissions should be around 420 - 620 Tglyr and the anthropogenic fraction (ratio of
anthropogenic to total emission rates) should be around 40-70%, by taking into
account the present lifetime of methane is 8-12 years.
The mixing ratio (concentration) of methane in earth's atmosphere is defined by
its source strengths and removal (sink) strengths. Concentration of methane can
increase only if the sources increase or the sinks decrease; both possibility are
plausible (Khalil, 1992). The main sink for atmospheric methane is its reaction with
tropospheric OH radicals:
0, + hv
O('D) + H20
CH, + OH
> O(lD) + 0,
> 20H
> H20 + CH,
Table 1. Estimate o f methane emission from various anthropogenic and n a t u r a l sources: 1978
Methane Source
Ehhalt &
Schmitt
(1978)
-
l W 1 (adapted from: K h a l i l and Rasmussen, 1990b)
Oonahue
Sheppard
Crutzen
e t at.
Khalil &
Rasmussen
Blake
Sei Ler
(1982)
(1983)
(1984)
(1984)
Crutzen
(1985)
Seiler
(1979)
120
95
25
70-160
149-189
25-110
70-100
30-75
50-100
60
120-200
20-70
70-100
70-170
55-100
(1983)
(1986)
Bimenger
Cicerone 8
& Crutzen Oremland
(1987)
(1988)
1
Bouwnan
Tyler
(1990)
(1991)
Anthropogenic:
Runinants
Rice paddy f i e l d s
Biomass burning
Landfills
Coal mining
Oil/gas system
Automobiles
Other Anthropogenic
100-200
280
100-2201
140-280
8-28
16-50
1
7-21
1
110-210
100
TotalAnthropogenic
396-550
366-760
339
Natural wetlands
Lakes
Oceans
190-300
1-25
1 17
200-300
Tropical f o r e s t s
Tundra
39
35
30
767
0-3
3-50
90
39
60
60
30-60
30-110
50
20
30
20-30
40
34
33
35
30-40
1-2
40
62- 100
140-250
320
297-560
200-335
267-397
261-447
30-220
150
10
13
120-190
13
5-21
60-400
15-60
1-7
70-90
25-70
15-35
70-80
18-91
30-100
30-70
35
0-35
65-100
60-170
50-100
30-70
25-45
25-50
66-90
60-140
55-100
30-70
35
30-40
1-2
183-411
80- 100
70- 170
10-40
5-70
10-35
25-50
0.5
5-25
255-535
277-477
205.5-490.5
100-200
1-25
5-20
40-160
15-35
120-200
0-30
10-100
2-42
1-5
27.5-154.5
26-167
116-345
57-237
149.5-379.5
Natural:
-
Other n a t u r a l
Total Natural
150
12
48
5-15
26-137
1-20
192-345
203-350
871
180-370
233
198-624
21-82
70-90
40-105
T O T A L
588-895
569-1110
1210
320-620
553
495-1184
221-417
337-487
301-552
209-578
371-880
334-714
355-870
Ratio Anthropogenic
Emission t o Total
67-61
64-68
44-40
58
60-47
90-80
79-82
87-81
87-81
69-61
83-67
58-56
Emission ( X )
28
The methyl radical undergoes further oxidation and produces CO and other products
(Khalil, 1991). It is estimated that 85% of the methane emitted into the atmosphere is
destroyed by OH radicals (R3) in the trophosphere. In stratosphere, almost all of the
r e r n i h g methane is destroyed by OH, C1 atoms and O('D) atoms. A small fraction
of methane goes through the stratosphere to the mesosphere where an additional
sink, very short UV light destroys methane photolytically (Cicerone and Oremland,
1988). Complete oxidation of methane produces CO, and H,O, which schematically
can be represented by the combustion of methane:
CH,
+ 2 0,
> CO,
+ 2 H,O
(R4)
Khalil and Rasmussen (1985) calculated that much of the increase of methane
over the past 200 years was clue to the increase of emissions (70%) and about 30%
due
to a depletion of OH radicals; the present abundance of OH radicals in
troposphere is estimated about 20% less than several hundred years ago.
The other process that remove methane is dry soils through oxidation by
methanotrophic bacteria. The oxidation (uptake) of methane in dry soils is estimated
with a net global sink of 32 f 16 Tg CH,lyr (Seiler and Conrad, 1987), of 6 - 58
Tglyr (Born et al., 1989), and of 35 f 15 Tglyr (Crutzen, 1991). Crutzen and Gidel
(1983) estimated that methane transport to the stratosphere is about 60 Tglyr. The
stratospheric reaction of CH, with C1 and O('D) is 10 f 5 Tglyr (Crutzen, 1991).
Recently, Vaghjiani and Ravishankara (1991) have found that the rate of removal of
methane with OH is 25 % less than currently used value. Based on this result Crutzen
(1991) estimates that the total atmospheric sink of methane is 460
With an annual increase in the atmospheric loading by 45
total methane sources is 505
+
100 Tglyr.
+ 5 Tglyr, therefore
the
+ 105 Tglyr, of which about 100 + 20 Tglyr comes
from fossil fuels and methane hydrates (fossil 'dead' methane), and about 405 Tglyr
comes from biogenic sources (fermentation by methanogenic bacteria).
It is clearly seen that sources and sinks are not balanced and leading to the
increasing concentrations, and the allocation among the individual sources and sinks
still not well known. The imbalance between aggregate sources and sinks drives the
growth in atmospheric methane.
Methane Production and Emission from Wetland Rice
Methane Production
Methane is an important product formed during the bacterial degradation of
organic matter in anoxic environments such as flooded soils, wetlands, estuaries,
marine and freshwater sediments, and the gastrointestinal track of animals (Whitman
et al., 1992). Methane producing bacteria (methanogens) are a specific group of
microbes that catabolize a small number of molecules and produce methane as the
major catabolic product (Boone, 1993). Recognized substrates metabolized by
methanogenic bacteria include hydrogen reduction of CO,,
acetate, formate,
methanol, methylated amines and CO (Cicerone and Oremland, 1988).
According to Boone (1993), decomposition process of a complex organic matter
in anoxic environments is divided into three major steps, namely fermentation (or
acidogenesis), synthropic acetigenesis, and methanogenesis (Figure 6). Each step is
catalyzed by a separate group of bacteria, however, all reaction occur simultaneously.
matter
Propionate, butyrate,
I CH4and CO, I
Figure 6 . Organic matter decomposition in methanogenic ecosystems (Boone, 1993).
Reviews of the biology, ecology and biochemistry of methanogens (Vogels et
al., 1988) indicate that all methanogens are very strictly anaerobic bacteria that all
form methane as a major terminal catabolic product. Methane formation results from
complex biochemical reaction as shown in Figure 7 (Boone, 1993).
Methane Emission
Rice is physiologically adapted to grow in water-inundated soils, creating ideal
condition for methane emissions. Rice soils are flooded for a long period so that
anoxic soil conditions arise and provide an optimal environment for methane
production. The biogenic mineralization of organic material in anoxic ecosystem of
rice soil, is one of the most important sources of atmospheric methane.
The great potential for methane release from wetland rice has long been noted
by Koyama (1963), based on the study of methane release from incubated soil in
laboratory. The first in situ
measurements of methane flux from wetland rice
over a complete growing period
were conducted in California by Cicerone and
Shetter (1981), who discovered the importance of methane transport through the
plants as opposed to diffusion and bubble transport across the water-air interface.
Since 1981, direct measurements of methane flux from wetland rice have been conducted by a number of researchers.
Based on a number of investigations, methane emission rates from wetland rice
show a wide range of values between