Origins of methane emitted from paddy fields.
Possible carbon sources for CH4
emitted from rice paddies are 1) organic matter applied to the fields, such as
rice straw (RS), 2) soil organic matter (SOM), and 3) carbon supplied from rice
plants (RP), such as exudates and sloughed tissues.
１．Contribution of SOM
to CH4 emission from paddy fields
２．Effects of quality
of RS and incorporation site on CH4 emission from paddy fields
３．Prediction of the increase in CH4 emission from paddy soils by RS application
４．Contribution of
photosynthesized carbon to CH4 emitted from paddy fields
５．Contribution of RS
carbon to CH4 emitted from paddy fields
６．Evaluation of
origins of CH4 carbon emitted from rice paddies
Contribution
of SOM to CH4 emission from paddy fields (Ref. 2)
The
potential CH4 production originated from SOM in Japanese paddy
fields was estimated from chemical properties of paddy soils of respective soil
series, their acreage and thermal regimes during rice growing period. Total
carbon mineralization (CO2 plus CH4) was calculated from
the estimation of nitrogen mineralization of SOM under anaerobic incubation.
The assignment of mineralized carbon to CH4 portion was calculated
from the ratio of oxidation capacity, represented by free iron content of soil,
to reduction capacity, represented by estimated NH4 production
during rice growing period.
Yst
= 1.70 + 17.5x1 + 0.444x2 – 0.2333x3 – 1.58x4 (Eq.
1)
where
Yst : the amount of NH4-N mineralized during the
anaerobic incubation (mg/100g soil; 30 °C,
10 weeks)
x1
: the total nitrogen content in soil
x2
: the amount of CEC (meq/100g soil)
x3
: the amount of exchangeable Ca (meq/100g soil)
x4
: the free iron content (%)
Yoshino, T. and Dei, Y.
(1977) Prediction of nitrogen release in paddy soils by means of the concept
effective temperature. J. Cent. Agric.
Exp. Stn., 25:1-62
Y
= k[(T-15)D]n (Eq.
2)
where Y: the amount of NH4-N
mineralized (mg/100g soil)
K
: the coefficient relating to the potential of mineralized nitrogen
T
: incubation temperature (°C)
(T-15) : the effective temperature above 15°C
D
: the duration of incubation period (days)
n
: a constant relating to the pattern of ammonification (n ranges from 0.7 to
1.0)
(T-15)D : termed as ‘the summation of effective temperature’
Yoshino, T. and Dei, Y.
(1977) Prediction of nitrogen release in paddy soils by means of the concept
effective temperature. J. Cent. Agric.
Exp. Stn., 25:1-62
Dei, Y. and Yamazaki, S.
(1979) Effect of water and crop management on the nitrogen-supplying capacity
of paddy soils. In Nitrogen and Rice.
International Rice Research Institute, Los Banos, Laguna, Philippines, 451-463
From Equations 1 and 2, the amount of nitrogen mineralized during rice growing period at respective paddy field (Y) was estimated referring the mean transplanting and harvesting date of each prefecture, and the summation of effective temperature to the statistics (Statistic Information Department, Japan).
Inubushi, K. and Wada,
H. (1988) Mineralization of carbon and nitrogen in chloroform-fumigated paddy
soil under submerged conditions. Soil
Sci. Plant Nutr., 34:287-291
Estimation
of CH4/CO2 ratio of mineralized carbon: Takai
(1961) incubated the paddy soils under anaerobic conditions and found good
correlation (r = 0.973) between the
ratio of CH4 to CO2, and the ratio of ‘oxidation capacity’
to ‘reduction capacity’ of soils. He termed the ‘oxidation capacity’ as the sum
of the amounts of O2 and NO3 in the soil plus the
produced (Mn2+ + Fe2+) during incubation. NH4
produced during incubation was termed as the ‘reduction capacity’.
Better correlation was obtained when free iron content was used as the
oxidation capacity’, and the correlation equation was;
(CO2/CH4)
ratio = 289[free Fe content (%) / NH4 produced during incubation (mg
NH4-N/100g soil)] + 7.10 (Eq.
3)
Takai, Y. (1961) Reduction and microbial metabolism in
paddy soils (3). Nogyo Gijutsu
(Agricultural Technology). 16:122-126
(in Japanese)
The total potential amount of CH4
production from Japanese paddy fields was estimated to be 8.6 ´ 104 ton-C/one crop
season. CH4 production potential per the unit area ranged from 24 to
54 kg-C/ha. The CH4 production potential increased sharply as the
fields locate more south. The few
information on CH4 emission during the rice growing season from
paddy fields in the world ranged from 120 to 770 kg-C/ha per one crop season
(Bouwman 1990). Thus, the CH4
emission from paddy fields are several times larger in amount than those of CH4
production potential from soil organic matter. This finding seemed to suggest
that fresh plant residues and root exudates contribute significantly to CH4
emission in the paddy fields.
Amounts of CH4 production from Japanese paddy fields: As
the data source for the estimation of CH4 production potential from
Japanese paddy fields, the final reports of the National Project entitled ‘Soil
Survey for Maintenance of Farmland fertility in Japan’ were consulted, in which
data sets from 3343 paddy fields were available including the contents of total
nitrogen, free iron, exchangeable Ca, and CEC.
Bouwman, A.F. (1990)
Exchange of greenhouse gases between terrestrial ecosystems and the atmosphere.
In Bouwman, A.F. (ed.) Soils and the
Greenhouse Effect. John Wiley and Sons, Chichester, 61-127
Effects
of quality of RS and incorporation site on CH4 emission from paddy fields (Ref. 6-8)
Effect
of weathering of RS in the field during the off-crop season on the CH4
emission from paddy fields treated with RS was investigated in a pot
experiment. The same amounts of RS collected from a paddy field during the
off-crop season (in October, December, and February) as that of fresh RS were
applied, and seasonal variations of CH4 emission rates were
determined.
Differences
in the CH4 emission rates among the treatments were observed in the
first 48-d period after transplanting, and the treatments applied with RS
collected in December and February showed lower CH4 emission rates
than the treatment applied with fresh RS.
The
total amounts of CH4 emitted from the treatment applied with RS
collected in February decreased by 31 and 15% for the first 48-d period and the
whole growth period, respectively, compared with the treatment applied with
fresh RS.
Contribution
of organic constituents in RS to the increase of the CH4 emission
rates from paddy fields was estimated in a pot experiment. The same amounts of
RS in which lipid, water-soluble polysaccharide, and hemicellulose fractions
were successively removed were applied to the submerged paddy soil.
CH4
emission rates were markedly different among the treatments in the early period
of growth of the rice plant. The removal of lipids and lipids plus
water-soluble polysaccharides from RS increased the total CH4 emission
by 36 and 46% during the first 48-d period after transplanting, respectively.
The additional removal of hemicellulose decreased the total CH4
emission by 23% during the same period compared with the treatment applied with
original RS.
On
the other hand, the CH4 emission rates in the subsequent growth
stages were similar among the treatments.
Based
on the difference in the CH4 emission in RS, hemicellulose was
estimated to contribute most significantly to the CH4 emission from
paddy fields treated with RS in the early period of rice growth.
The
effect of the incorporation site of RS on CH4 emission rates from
flooded soils planted with rice was studied in pot experiments. The
incorporation of RS into the lower layers of soil increased the CH4
emission rates during the early period of rice growth compared with the
incorporation into the upper layers or the uniform incorporation into the whole
soil.
CH4
emission rates from a rice plant were also markedly different between tillers
whose associated roots grew close to RS and those away from it. CH4
emission rates from a rice plant varied among shoots. CH4 emission
rates through the older shoots were larger than those through the younger ones
at the tillering and milky stages, although the difference was not significant
at the harvesting stage. CH4 emission rates fluctuated 3-5 times
among shoots of a rice plant throughout the growth stages.
Prediction
of the increase in CH4 emission from paddy soils by RS application (Ref. 3, 9)
Although
it is well known that the application of RS increases the CH4
emission from paddy fields, the degree of increase in CH4 emission
by the increase in RS application and the inter-annual variation of CH4
emission have not been quantified. The following study showed the relationships
between the amount of CH4 emission to the atmosphere and the
application level of RS in a pot experiment, and proposed the general equation
for estimating the increase in CH4 emission by RS application from
any paddy field in any region and in any year.
The
relationship between the amount of CH4 emission to the atmosphere
from submerged paddy soils with rice plants and the application level (0-8 g/kg
soil) of RS in soil was investigated first in a pot experiment. Amounts of CH4
emitted from pots with respective RS levels differed between a clayey yellow
soil and a silty gray lowland soil. However, the increase in cumulative amounts
of CH4 emission with the increase in the application level of RS was
similar in pattern between the two soils, and the increase (Y) was formulated
with a logistic curve: Y = k[a/(1 + be-cx)]
(Eq. 4); x, application level of RS; k, a coefficient for relative CH4
emission specific to respective soils.
Since
the seasonal variations in coefficients, a, b, and c in the logistic equation
were also formulated as the function of the days after transplanting (t):
a(t) = 1.52 ´ 103/(1 + 24.4e-0.0539t) r = 0.999*** (Eq.
5)
b(t) = -8.48 + 2.25 ´ 103/(t – 10.1) r = 0.997*** (Eq.
6)
c(t) = 0.666 + 6.38/(t – 13.3) r
= 0.981*** (Eq. 7)
Therefore,
Equation 4 can be rewritten as follows using Equations (5) – (7):
Y(x, t) = [a(t)/(1 + b(t)e-c(t)x)] (Eq. 8)
The
mineralization of soil organic nitrogen is known to be temperature-dependent
and expressed by the equation of Y = k[(T-15)D]n as mentioned
before. CH4 emission rates fluctuate year to year and location to
location, even if the cultivation practices were the same, and the temperature is
considered to have a great influence on their fluctuation. Therefore, the
coefficients a, b, and c in the above equation were reformulated against the
sum of effective temperature (E, S(T-15); T, daily average temperature),
and the increase in cumulative amounts of CH4 emission from any
paddy soil by any level of RS application was estimated by the following
equation: Y = k[a(E)/(1 + b(E)e-c(E)x)].
a(E) = 1.58 ´ 103/(1 + 14.2e-0.00504E) r = 0.998*** (Eq.
9)
b(E) = -10.1 + 2.31 ´ 104/(E – 18.8) r = 0.997*** (Eq.
10)
c(E) = 0.666 + 60.5/(E – 18.8) r = 0.982*** (Eq.
11)
Equation
8 were obtained from the clayey yellow soil (Anjo soil), and it was available
to the other silty gray lowland soil (Yatomi soil) by assigning k = 1.35.
Y(x, t) = 1.35[a(t)/(1 + b(t)e-c(t)x)] (Eq. 12)
Contribution
of photosynthesized carbon to CH4 emitted from paddy fields (Ref. 1, 4, 5)
Emission
rates of CH4 from paddy soil with and without RS applications were
measured with pot experiments to estimate the contribution of rice straw to
total CH4 emission during the growing period of rice plants. The CH4
derived from RS was calculated to be 44% of the total emission. 13CO2
uptake experiments were carried out four times from June 30 to September 13,
1994, to estimate the contribution percentages of photosynthesized carbon to
the total CH4 emission. The contribution percentages of photosynthesized
carbon to the total CH4 emitted to the atmosphere were 3.8% around
June 39, 31% around July 25, 30% around August 19, and 14% around September 13
in the treatment with RS applications, and 52% around July 25, 28% around
August 19, and 15% around September 13 in the treatment without RS application.
They were calculated to be 22% and 29% for the entire growth period in the
treatments with and without RS applications, respectively.
In addition, it was known from this
experiment that within 3 hours photosynthesized carbon was translocated to the
roots, released into the rhizosphere, and then transformed to CH4.
This CH4 quickly entered the roots and was released back to the
atmosphere.
Contribution
of RS carbon to CH4 emitted from paddy fields (Ref. 11)
It is
generally recognized that the application of RS increases CH4
emission from rice paddies. To
estimate the contribution of RS carbon to CH4 emission, a pot
experiment was conducted using 13C-enriched RS. The percentage
contributions of RS carbon to CH4 emission
throughout the rice growth period were 10±1, 32±3, and 43±3% for the treatments with RS
applied at the rates of 2, 4, and 6 g/kg soil, respectively. The increase in
the rate of application of RS increased CH4 emission derived from
both RS carbon and other carbon sources. The percentage contribution of RS
carbon to CH4 emission was larger in the earlier period (maximum
96%) when the decomposition rate of RS was larger. After RS decomposition had
showed, CH4 emission derived from RS carbon decreased. However, the d 13C values of CH4 emitted
from the pots with 13C-enriched RS applied at the rates of 4 and 6
g/kg soil were significantly higher than those from the pots with natural RS
until the harvesting stage.
Evaluation
of origins of CH4 carbon emitted from rice paddies
From
the above experiments, all of SOM, RS, and photosynthates were known to be the
important origins of CH4 carbon emitted from rice paddies. To
estimate the contribution of each carbon source to CH4 emission, a
pot experiment was conducted using 13C-enriched soil sample and 13C-enriched
RS as tracers. The percentage contribution of RP was estimated by subtraction.
When RS was applied at a rate corresponding to 6 t/ha, the percentage
contributions of RS, SOM, and RP carbon to CH4 emission throughout
the period of rice growth were 42%, 18-21%, and 37-40%, respectively. The
values for SOM and RP carbon for the treatment in which RS was not applied were
15-20% and 80-85%, respectively.
Seasonal variations in the percentage
contribution of SOM carbon to CH4 emission were small in the range
between 13% and 30% for the pots with RS and between 15% and 24% for the pots
without RS. In the RS-applied treatment, RS and SOM carbon accounted for almost
100% of the CH4 carbon early in the period of rice growth, while
65-70% of the CH4 emission in the milky stage was derived from RP
carbon.
In conclusion, RP carbon was the main source of CH4
carbon emitted from rice paddies in the treatment without RS applications,
while RS carbon and RP carbon were the main sources of it in the early and late
stages of rice cultivation, respectively, in the treatment with RS
applications. In either treatment, the contribution of SOM carbon to CH4
emission was small only about 20% throughout the growth period.
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