ABSTRACT global rice production is responsible for 11%


warming and climate change has reached the alarming levels due to increase of
greenhouse gas emissions into the atmosphere which includes carbon dioxide,
methane and nitrous oxide. Flooded rice (Oryza
sativa L.) cultivation has been identified as one of the prominent global agricultural
sources of anthropogenic methane (CH4) emissions. Moreover, it has
been estimated that global rice production is responsible for 11% of total
anthropogenic CH4 emissions. The inventory of CH4
emission from rice cultivation in Malaysia was estimated from 1990 to 2014 and was
also used as basis for computing the projected emissions up to 2030 by using Auto-Regressive
Integrated Moving Average (ARIMA) model. Results showed that CH4
emissions is higher from granary area (continuously flooded) than non-granary
area (rain-fed) due to different water management practices. Continuously
flooded irrigation system which lead to anaerobic conditions emit almost (75%)
higher CH4 than rain-fed irrigation system. emissions forecasted will
continuously increase from 2015 to 2030 within the confidence limits. Emissions
were forecasted to increase up to 88 Gg by 2030 due to increase of country
population which will lead to expansion of cultivation area in order to fulfil
country needs.

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Keywords: Methane, greenhouse gas emissions,
rice cultivation



Global warming is the unusually
rapid increase in Earth’s average surface temperature over the past century
primarily due to the emissions greenhouse gases (GHG) into the atmosphere where
it has a significant impact on the world’s climate. The world is experiencing
an unprecedented rise in surface temperature 1. There is increase in global
surface temperature by 0.74 ± 0.18 °C between the beginning and the end of the
20th century and it is projected to increase by 1.1 to 6.4 °C in the
21st century 2. This rise is attributed to the increased rate of GHG
emissions into the atmosphere caused by the activities of human beings together
with the anthropogenic nature of the problem. Over for the past 20 years,
climate change which caused by human activities has emerged as a global concern
30, 28.

Human population has grown up from
3.5 billion to more than 7 billion since 1970 up to 2010 and projected to
increase up to 9.2 billion in 2040 9,18. Thus, it means that agricultural
production will be more than doubled due to higher consumption together with
the shift towards more animal-based products in the diet 8.  At present, agricultural production and land-use
change (LUC) are responsible for almost 1/4 of total GHG emissions from human
activities 32. This sector releases significant amounts of carbon dioxide (CO2),
methane (CH4), and nitrous oxide (N2O) to the atmosphere 33.
Compared to other sectors, agriculture contributes 10.3% out of world total GHG
emissions in 2011 8. Concentration of CH4 had been increasing
since last 20 years at average rate of 0.8% in a year causing it to be one of
the major concern in GHG emissions 34. Atmospheric methane (CH4)
is a potent greenhouse gas with high absorption potential for infrared
radiation. Methane is present at about 1.8 ppmV in the atmosphere 13. It was estimated
that 50% of CH4 emissions from global anthropogenic contribution
were from agricultural activities 34.

Agriculture is one of the important
sector in Malaysia. Throughout for many years, this sector has been the
backbone of country’s economy by the production of agricultural products for
domestic consumption, as the earner of foreign exchange. Moreover, agriculture sector
also contributes to the national Gross Domestic Products (GDP). Agriculture
sector continued to expand in 2015 with a contribution of 8.9% to Malaysia’s
GDP. Oil palm was a major contributor to the GDP of agriculture sector at 46.9%
followed by other agriculture (17.7%), livestock (10.7%), fishing (10.7%), and
rubber (7.2%) as well as forestry & logging (6.9%) in 2015 5. According
to Malaysia’s Second National Communication (NC2), agriculture sector
contributed 3% of the total GHG emissions for the year 2000 compared to
industrial processes sector (IP) with 6%, waste sector with 12%, land use,
land-use change and forestry (LULUCF) with 13% and energy sector with 66% 22.
Contribution of agriculture sector (Figure 1) as reported in the  Malaysia’s First Biennial Update Report
(BUR1) increased up to 5% of the total GHG emissions for the year 2011 23.

Rice (Oryza sativa L.) is the major food crop for people living in Asia, where
about 80% of rice is grown under irrigated wetland conditions 27.
Furthermore, the world’s annual rice production must increase from 518 million
tons in 1990 to 760 million tons by 2020 for the purpose of food security 11.
 Rice fields
have been one of the major concern by scientists worldwide as a part of the three
most persuasive and long-lived GHG in the
atmosphere, CO2, CH4, and N2O are emitted from
rice fields 21. Rice fields are one of the
major atmospheric sources of CH4 as most of the world’s rice is
grown on flooded fields 15, 35, 36, 37,. Flooded rice fields emit CH4
due to a methanogenesis process which occurs in anaerobic conditions, during the decomposition of organic matter 21,
12. Rice cultivation has been specifically identified as one of the leading
global agricultural sources of anthropogenic CH4 emissions,
accounting for approximately 22% of the total global agriculturally related CH4
emissions 34. Moreover, it has been estimated that global rice production is
responsible for 11% of total anthropogenic CH4 emissions 18, 35.

Rice is the third most widely
planted crop in Malaysia after oil palm and rubber. In 2014, approximately 679,239
ha were planted with rice (wetland and dryland paddy) including those that are
planted twice a year 4. There are 2 planting seasons in Malaysia, main season
where rice is grown without depending wholly on any irrigation system and
off-season (dry period) where normally rice cultivation depends on an
irrigation system. Total area of rice cultivation in Malaysia can be classified
into three categories which are granary areas (irrigated), non-granary areas
(rain-fed) and upland (Table 1). Granary areas refer to major irrigation schemes
(areas greater than 4,000 ha) and recognized by the government in the National
Agricultural Policy (NAP) as the main rice producing areas. Previously, there
are 8 granary areas in Malaysia, namely MADA, KADA, IADA KERIAN, IADA Barat
Laut Selangor, IADA Pulau Pinang, IADA Seberang Perak, IADA KETARA and IADA
Kemasin Semerak . Other rice cultivation areas which is less than 4,000 ha are
classified as non-granary area or rain-fed area 4.

Water management system under which
rice is grown is one of the most important factors affecting CH4
emissions besides fertilization practices, soil temperature, soil type, rice
variety, and cultivation practices. The main water management practice applied
in Malaysia is continuous flooding with 2 planting seasons in a year (double
cropping). When the fields are flooded, aerobic decomposition of organic matter
gradually depletes most of the oxygen (O2) present in the soil,
causing anaerobic soil conditions. Once the environment becomes anaerobic, CH4
is produced through anaerobic decomposition of soil organic matter by methanogenic
bacteria 36. Upland rice fields are not flooded, thus are believed not
emitting CH4.

Approximately 60-90% of the CH4
produced is oxidized by aerobic methanotrophic bacteria in the soil (some
oxygen remains at the interfaces of soil and water, and soil and root system) 13.
Some of the CH4 is also leached away as dissolved CH4 in
floodwater that percolates from the field. The remaining un-oxidized CH4
is transported from the submerged soil to the atmosphere primarily by diffusive
transport through the rice plants. Minor amounts of CH4 also escape
from the soil via diffusion and bubbling through floodwaters 7. The pathways
of CH4 emissions from a flooded rice field are shown schematically
in Figure 2. As the CH4 approaches the soil-water interface or the
plant roots, it enters a region of oxidising conditions where aerobic
micro-organisms convert some of the CH4 to CO2. This is
either released via the water to the atmosphere or is used by the plant.

Inventories in Malaysia from 1990-2014
were prepared to determine the contribution of CH4 emissions from
rice cultivation. An emission inventory is used to assess the impact of
specific human activities and the main sources responsible for such emissions besides
to develop and assess the results of specific mitigation strategies 37.



2.1       Activity Data Collection

data (1990-2014) were primarily based on planted area statistics, which are
available from a national statistics agency as well as complementary
information on cultivation period and agronomic practices.  Those activity data were broken down by regional
differences in rice cropping practices or water regime. Planted area estimates
corresponding to different conditions obtained on a countrywide basis through
accepted methods of reporting. Only planted area data were used to estimate the
emission. Activity data were obtained from Paddy Statistics of Malaysia by
Department of Agriculture (DOA).


2.2       Emission Calculations from Inventory

The emission calculations were
based on the guidelines given by the Intergovernmental Panel on Climate Change
(IPCC) on how to conduct national GHGs inventories. Methane emissions were
estimated by multiplying daily emission factor (EF) by cultivation period of
rice and annual harvested areas. In its most simple form, this equation, Eq. (1)
was implemented using national activity data such as national average
cultivation period of rice and area harvested and a single EF 14.


Rice            =          ?i,j,k (EFi,j,k *
ti,j,k * Ai,j,k * 10-6)                       (1)


Rice               =         annual methane emissions from rice
cultivations                                                                    (Gg CH4

                        EFi,j,k                =          a daily emission factor for i,j,k and
k conditions                                                                    (kg CH4
ha-1 day-1)

                        tijk                    =          cultivation period of rice for i,j and k conditions (day)

                        Aijk                  =          annual harvested area of rice i,j and k conditions                                                                   (ha yr-1)

                        i,j and k           =          represent
different ecosystem, water regimes, type and                                                         other
conditions under which CH4 emissions from rice                                                         may vary

Due to close proximity,
Thailand rice CH4 EF of 1.6 kg CH4 ha-1 day-1
was used as Malaysia not yet developed country specific EF at present 13.
Scaling factors water regimes during the cultivation period relative to
continuously flooded field and the default CH4 emissions scaling
factor for water regimes before the cultivation period (non-flooded pre-season
< 180 days) were based on IPCC default values as in Table 2. 2.3       Emission Forecasting The forecasting of methane emission from 2015 to 2030 was carried out using the Auto-Regressive Integrated Moving Average (ARIMA), which is one of the general classes of models used for forecasting time series. Differencing and lagging are the transformations used to fix the model. In the forecasting equation "auto-regressive" terms are used to describe lags of the differenced series that appear in the equation, the "moving average" terms describe lags of the forecast errors, and an "integrated" version of a stationary series is used for a time series requiring differencing to become stationary. The model classification of a non-seasonal ARIMA model is as an "ARIMA (p,d,q)", where p represents the autoregressive terms, d represents the non-seasonal differences, and q represents the lagged forecast errors that appear in the prediction equation (moving average). This time interval was chosen because the models give accurate forecast over short time periods 26, 37.   3.         RESULT AND DISCUSSION Results of the calculated CH4 emissions estimation from 1990-2014 are presented in Figure 3. Based on Figure 3, we can conclude that CH4 emissions is higher from granary area (continuously flooded) than non-granary area (rain-fed) due to different water management practices. Continuously flooded irrigation system which lead to anaerobic conditions emit almost (75%) higher CH4 than rain-fed irrigation system.  Methane is produced in the anaerobic layers of paddy soil by bacterial decomposition of organic matter 6. The organic matter converted to CH4 is derived mainly from plant-borne material 3. Moreover, statistical analysis using Statistical Analysis System (SAS) Version 9.3 showed that there is a positive correlation between annual rice planted in granary area (irrigated) and CH4 emissions (Table 3) with r =  0.9380, p < 0.01. Therefore, bigger granary area will lead to higher emissions of CH4. Moreover, Figure 3 showed a rapid increase of CH4 emissions from 83.06 Gg in 2013 to 88.08 Gg in 2014. This may due to new granary area gazetted by the government of Malaysia in 2014 which is IADA Pekan with 4,940 ha and IADA Rompin with 2,920 ha 4. Figure 4 showed the observed and forecasted methane emission from 1980-2014 and 2015-2030. Forecasted methane emissions had been done using ARIMA Model (0,2,2). Table 4 shows the model fit statistics where the value of the coefficient of determination (R2) was 0.8888 which indicated that less than 11% of the total variations were not explained by the model. These R2 values are indications of good model fits. Generally, it was noted that there were no seasonal patterns in the trends, hence there was no need to transform the data to stabilise the variance. The model was then used to fit the emission of methane from these sources. Table 5 also shows the Ljung-Box Q statistics for model validation. It indicated that the models were correctly specified because the p values above 0.05 which implied that all structures in the observed series had been accounted for. It has been used to validate the correctness of the fitted models. Based on the correctness of the fitted model, it was used to predict methane emissions from 2015 to 2030. Figure 4 showed that emissions forecasted will continuously increase from 2015 to 2030 within the confidence limits. Emissions were forecasted to increase up to 88 Gg by 2030. Department of Statistics Malaysia (DOSM) reported that Malaysia's population are projected to increase from 28.6 million in 2010 up to 41.5 million by 2040 5. Rice is one of the most important crops in Malaysia as rice is the staple food for the country 31. Hence, with the growth of population numbers, demand for rice will also increase. In order to fulfil those demand, rice production should be boosted to ensure food security. This can be done mostly through expansion of rice cultivation area besides producing high yield rice variety or five seasons of cultivation in two years' time interval. Both expansion of rice cultivation area and five seasons of cultivation in two years' time interval will increase CH4 emissions as there is a positive correlations between planted area and CH4 emissions. In 2014, two new granary area, IADA Pekan and IADA Rompin had been gazetted by the government causing addition of emissions by 6% in 2014. Moreover, under Bario Rice Industry Development project, two new granary areas which is 5,000 ha in Kota Belud, Sabah and 5,100 ha in Batang Lupar, Sarawak will be established by the year 2020 20. This is in line with the forecasted emissions which will continuously increase up to 2030. Water management has been recognized as one of the most important practices that affect CH4 emissions from rice fields 10. Irrigated areas had higher CH4 emissions than rain-fed areas, due to the longer period of flooding in rice fields 21. Development of efficient irrigation water management practices such as alternate cycles of wetting and drying could reduce more CH4 emissions than continuously flooded fields besides saving more water 21, 1. Moreover, due to the scarcity of freshwater resources available for irrigated agriculture and escalating food demand around the world in the future, it will be necessary to produce more food with minimum water usage 24.   4.         CONCLUSSION The investigation has provided an historical analysis of methane emission from 1990 to 2014 and made projections from 2015-2030. Methane emissions projected to increase up to 2030 due to the expansion of rice cultivation area in order to fulfil the demands as the country population also projected to increase by 41.5 million in 2040 5. Methane has a shorter atmospheric life span of 12-17 years and its reduction will have a much more immediate impact on climate 34. Results showed that CH4 emissions from granary area (irrigated field) was 74% higher than non-granary area (rain-fed field). Earlier study in China indicated that there was no significant difference on the rice yield between flooded rice field and water saving irrigation rice field which proven that non-flooded fields can also promise good harvest besides reducing CH4 emissions and water wastage  17, 25. Moreover, previous study done in Kedah, Malaysia showed that CH4 emission rate from non-flooded area with the water saving irrigation method was found relatively low compared to the flooded area 17. This calls for further investigation on the effects of water management practices towards rice yield as climate condition in Malaysia may differ due to geographical location.


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