2005.present & future methane emissions from rice filed in Đông ngạc - từ liêm - việt nam

7/31/2019 2005.Present & Future Methane Emissions From Rice Filed in ng Ngc - T Lim - Vit Nam

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EARTH SCIENCES CENTREGTEBORG UNIVERSITYB446 2005

PRESENT AND FUTURE METHANEEMISSIONS FROM RICE FIELDS IN NG

NGC COMMUNE, HANOI, VIETNAM.

Sara Sandin

Department of Physical GeographyGTEBORG 2005


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GTEBORGS UNIVERSITETInstitutionen fr geovetenskaperNaturgeografiGeovetarcentrum

PRESENT AND FUTURE METHANEEMISSIONS FROM RICE FIELDS IN NG

NGC COMMUNE, HANOI, VIETNAM.

Sara Sandin

ISSN 1400-3821 B446ProjektarbeteGteborg 2005

Postadress Besksadress Telefo Telfax Earth SciencesCentre Geovetarcentrum Geovetarcentrum 031-773 19 51 031-773 19 86 Gteborg UniversityS-405 30 Gteborg Guldhedsgatan 5A S-405 30 Gteborg

SWEDEN


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Summary

Sandin, S. (2005) Present and future methane emissions from rice fields in ng Ngc commune,Hanoi, Vietnam. Gteborg University, Department of Physical Geography, 41 pp.

Since the turn of the last century greenhouse gases, such as methane and nitrous oxide, have

increased due to extensive farming. Vietnam in South East Asia is the worlds biggest exporter ofrice. Vietnam expands its agricultural land to meet the needs of the global market and an increasingpopulation. As the rice growing areas expand the emissions of methane increase. There is a need toinvestigate how high the emissions are from Vietnamese paddy fields in order to establish itscontribution to the global greenhouse gas budget. The objectives of this project are to measuremethane emissions from three paddy fields in the outskirts of Hanoi in ng Ngc commune inNorthern Vietnam. The measurements were done during a short period, but estimations in methaneemission were made for a whole rice cycle. The methane emissions were measured in situ and inlaboratory. The highest methane emission was emitted from rice field with a high water level; 15cm, and the least methane was emitted from the paddy field with intermittent irrigation. Themethane emission were 228 mg CH

4/m2 and day from the field with high water level, 167 mg CH

4

/m2 and day from the field with medium water level (10 cm) and 88 mg CH 4 /m2 and day from the

field with intermittent irrigation. The emission of methane could be reduced in many ways. Betterwater management, a suitable selection of rice cultivars and fertilizer are some of the suggestionsmade in this report.

Sammanfattning

Sandin, S. (2005) Nutida och framtida metanemissioner frn risflt i ng Ngc-kommunen,Hanoi, Vietnam. Gteborgs Universitet, Naturgeografiska institutionen, 41 s.

Sedan 1900-talets brjan har vxthusgaser ssom metan och kvveoxid kat p grund av katjordbruk. Vietnam i sydstra Asien r vrldens strsta exportr av ris. Vietnam kar sinajordbruksomrden fr att mta vrldsmarknaden och en kande population. D omrden bevxtamed ris kar, kar ven metanutslppen. Det finns ett behov att underska hur hga utslppen rfrn vietnamesiska risflt fr att befsta dess bidrag till den globala budgeten av vxthusgaser.Syftet med detta projekt r att mta metanutslpp frn tre risflt i utkanten av Hanoi, i ng Ngc-kommunen i norra Vietnam. Mtningarna gjordes under en kort period, men uppskattningar imetanutslpp r gjorda fr en hel riscykel. Metanutslppen mttes p plats och i laboratorium. Dehgsta metanutslppen kom frn risfltet med hg vattenniv; 15 cm, och minst metan slpptes utfrn risfltet med periodisk bevattning. Metanutslppen var 228 mg CH4 /m

2 och dag frn fltetmed hg vattenniv, 167 mg CH4 /m

2 och dag frn fltet med mellanvattenniv (10 cm) och 88 mg

CH4 /m2

och dag frn fltet med periodisk bevattning. Metanutslppen kan minskas p mnga stt.Bttre bevattningssktsel, mer passande val av rissort och gdningsmedel r ngra av frslagen idenna rapport.

Keywords: Paddy field, rice, methane emission, Vietnam.


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PrefaceThis project is part of a 20 point course at the department of Physical Geography at GteborgUniversity.

This study was made possible by the funding from the Swedish International DevelopmentCooperation Agency (SIDA).

I would like to thank my supervisors professor Deliang Chen and Ph D student Elisabeth Simeltonat the department of Physical Geography at Gteborg University for their valuable input, supportand enthusiasm during the development of this paper.

I would also like to thank my supervisor in Vietnam, Msc Tran Duc Toan at the National Instituteof Soils and Fertilizers for support and help during my stay in Hanoi. Many thanks to Mr Phoungfor his help with many diverse things, from organizing measurements to helpful tips.

I am also grateful to my best friend Martin Andersson for useful (healthy) criticism, Mum for theEnglish corrections and Dad for help with scanning. I am also grateful to all who wrotecontributions to my travel diary on the internet and kept my spirits up when needed and made meeven happier during my stay in Vietnam.


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Table of contents

1.Introduction .................................................................................................................................. 11.1 Methane sources ....................................................................................................................... 11.2 The growth of rice .................................................................................................................... 21.3 Increasing demand for rice ....................................................................................................... 2

1.4 Rice production in Vietnam ..................................................................................................... 21.5 Presentation of problem ........................................................................................................... 21.6 Objectives ................................................................................................................................. 2

2. Emissions from paddy fields ....................................................................................................... 42.1 Emission of methane from rice paddies ................................................................................... 4

2.1.1 Factors affecting the emission of methane........................................................................ 62.2.2 Mitigation options for methane emission........................................................................ 10

2.3 Emission of nitrous oxide from rice paddies.......................................................................... 113. Study area - Vietnam ................................................................................................................. 12

3.1 Morphology............................................................................................................................ 123.2 Climate ................................................................................................................................... 123.3 Land use ................................................................................................................................. 123.4 Greenhouse gas emission ....................................................................................................... 133.5 Rice paddies ........................................................................................................................... 133.6 Hanoi province ....................................................................................................................... 13

3.6.1 Soil characteristics of Hanoi province ............................................................................ 133.6.2 ng Ngc commune...................................................................................................... 14

4. Methods....................................................................................................................................... 154.1 In situ measurements .............................................................................................................. 15

4.1.1 Temperature and methane ............................................................................................... 154.1.2 Rice variety, water and fertilizer management................................................................ 17

4.1.3 Mapping and GPS ........................................................................................................... 174.1.4 Interviews ........................................................................................................................ 174.2 Laboratory measurements ...................................................................................................... 17

4.2.1 Air samples...................................................................................................................... 174.2.2 Soil samples..................................................................................................................... 18

4.3 Modelling methane emissions................................................................................................ 184.3.1 The seasonal variations of methane ................................................................................ 184.3.2 The future methane emission .......................................................................................... 18

4.4 The measurement fields ......................................................................................................... 205. Results ......................................................................................................................................... 23

5.1 Soil properties ........................................................................................................................ 23

5.2 Interviews ............................................................................................................................... 245.2.1 Farming calendar ............................................................................................................. 245.2.2 Water management .......................................................................................................... 255.2.3 Fertilizer usage ................................................................................................................ 255.2.4 Rice variety and mitigation options ................................................................................ 26

5.3 Methane emission................................................................................................................... 265.3.1 Methane emission measured in situ................................................................................. 265.3.2 Methane emission measured in situ and in laboratory .................................................... 275.3.3 Diurnal methane emission............................................................................................... 275.3.4 Seasonal methane emission............................................................................................. 285.3.5. Future methane emission................................................................................................ 29

6. Discussion.................................................................................................................................... 306.1 Field studies............................................................................................................................ 30


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6.2 Calculations ............................................................................................................................ 316.3 Mitigation options for Vietnam.............................................................................................. 33

7. Conclusions ................................................................................................................................. 358. References ................................................................................................................................... 36

8.1 Articles and books .................................................................................................................. 368.2 Internet addresses ................................................................................................................... 38

8.3 Other references ..................................................................................................................... 399. Appendix 1 .................................................................................................................................. 40


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1. IntroductionThe global average surface temperature has increased with 0.6 0.2C over the last century.According to the Intergovernmental Panel on Climate Changes (IPCC) SRES scenarios the globalaverage surface temperature is predicted to increase 1.4 to 5.8C over the period 1990 to 2100(IPCC 2001). Methane has, among other greenhouse gases like carbon dioxide, nitrous oxide andchlorofluorocarbons, a strong infrared absorption band, which traps the outgoing long waveradiation from earths surface (Wuebbles & Hayhoe 2002). The IPCC (1996a) points out that thewarming potential of methane and nitrous oxide is 21 times and 280 times greater respectively thanthat of carbon dioxide in 100 years time scale. Goudie (2002) remarks that land changes are thelargest human induced emission of methane. In the year 2000 the methane concentration in theatmosphere was 1.75 parts per million volume (ppmv), which was an increase compared to thepreindustrial level of 0.7 ppmv (Weubbles & Hayhoe 2002). Schimel (2000) concludes that themethane concentration was continuing to increase at an annual rate of 1 %, see also figure 1.Figure 1 shows the methane emissions of different sources emitted globally from 1860 to 1994. Itis clear that the emissions of methane have increased during this period. The emissions from rice is

high but has not increased as much as livestock during the last decades. According to Goudie(2002) it is difficult to estimate the long term level of methane in the atmosphere due to the factthat methane reacts quickly with other substances, which decreases the amount of methane.

1.1 Methane sources

According to IPCC (1996a) methane is emitted through both biological and industrial processesand the atmospheric concentration of methane has increased approximately 246 % of itspreindustrial concentration. Methane is produced in an anaerobic environment such as rice paddies,swamps, sludge digesters, rumens and sediments (Banker et al. 1995). According to Sommer et al.(2004) the anthropogenic methane sources are of special interest, because they offer an opportunityto manipulate and reduce the emissions. Approximately 410-660 million tons methane are emittedglobally per year (IPCC 1996a) and between 25.4 and 54 million tons of this is due to irrigated rice

fields (Cole et al. 1995).

Figure 1. The change of different anthropogenic sources of methane

emission from 1850 to 1994 (CDIAC 2004).


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1.2 The growth of riceRice is grown in approximately one third of the worlds irrigated cropland. Rice represents one halfof the irrigated cropland in Asia. Rice is often grown in monsoonal climates due to its tolerance toflooding. Intensification of cultivation of irrigated rice has been registered, above all in Asia,especially that of wet rice, which is an exceptionally large source of methane. During the dry

stages of the rice large amounts of nitrous oxide is emitted from the paddy field. These areas alsohave problems with waterlogging and increased salinity in the soil. The intensification of wet andirrigated rice production is believed to increase application of fertilizer, herbicides, pesticides andthe use of genetically engineered crops (Goudie 2002).

1.3 Increasing demand for riceAccording to the International Rice Research Institute (IRRI 2004) the rice production has toincrease from 585 million tons in 2003 to 800 million tons to meet the demand in 2025. Thismeans that the yield must increase or the irrigated land used to grow rice must expand. Then thereis a risk that the rice paddy areas increase at the expense of low methane emitting areas.

1.4 Rice production in VietnamVietnam represents 12-15 % of the world market in production of rice and is the worlds biggestexporter. The population of Vietnam is continuously increasing with nearly 1.5 million people peryear. Since rice is the main feeding source in Asia, the rice yield must increase. The rice area inVietnam increased from 6302000 hectares (ha) in 1991 to 7468000 ha in 2002. This means anincrease of 18.5 % (VBF 2004). The production of rice comes mainly from irrigated rice fields(73.9 %) and from rainfed rice (18.1 %) (NC-1003 2005). According to Food and AgricultureOrganisation (FAO) the rice area in Vietnam has decreased to 7449300 ha in 2003. The rice yieldwas 4633 kg/ha in 2003 (FAO 2004). During 2002 the average Vietnamese ate 168.9 kg rice peryear (FAO 2005).

1.5 Presentation of problemSince the turn of the last century greenhouse gases, such as methane and nitrous oxide, haveincreased due to extensive farming. Vietnam expands its agricultural land to meet the needs of theglobal market and an increasing population. As the rice growing areas expand the emissions ofmethane increase. There is a need to investigate how high the emissions are from Vietnamese ricefields, both upland rice and paddy rice in order to establish its contribution to the globalgreenhouse gas budget. Vietnam may need international help to reduce greenhouse gas emissionsfrom the agricultural sector. There is therefore a need to know what factors affect the emissions ofmethane and what there is to be done against it. There is also a need to know what the presentconditions are in order to take the correct precautions.

1.6 Objectives

The objectives of this project are to measure methane emissions from three paddy fields in theoutskirts of Hanoi in ng Ngc commune in Northern Vietnam. The measurements were doneduring a short period, but estimations were made for a whole rice cycle. The diurnal variations ofmethane and air temperature were measured and also soil characteristics, fertilizer usage, ricevariety and water management were at the three paddy fields. The results were compared to othermeasurements in Asia of methane at a local level. Land use maps over the three fields were createdto show the land use in the area. Interviews were done to investigate the rice cultivation traditionand policies regarding rice cultivation in the area. The study was complemented with literature

studies of plausible countermeasures.


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Important questions relating to this are:

How high are the methane emissions from small rice fields during flowering in the Hanoiarea?

How high are these measurements in Vietnam in relation to other measurements in Asia?

Will the methane emissions increase in the future?Through literature studies the different factors affecting methane emission will be treated. Thepossibility to reduce methane emission and the possibility to implement these techniques will alsobe considered. An overview of the policies that exist today in Vietnam to reduce the methaneemissions from paddy fields will be discussed.


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2. Emissions from paddy fieldsThe emissions from paddy fields are put in relation to other emissions in table 1. The contributionfrom rice paddies to the methane budget is high, only second to natural wetlands.

Table 1. Major methane sources and sinks. The numbers are approximate. The unit is millions of

metric tons of carbon equivalent per year.Sources Approximate rate(Range)

Sinks Approximate rate(Range)

Natural wetlands(methanogenesis)

85 (75-150) Reactions with hydroxylradical and atmosphericaccumulation

355 (290-420)

Rice paddies(methanogenesis)

80 (45-130) Soil uptake 20 (15-25)

Bovines 60 (45-75)Natural gas leakage 75 (60-90)Landfill/waste 60 (60-80)Biomass burning 40 (35-75)Termites 15 (5-75)

Oceans and lakes 10 (5-15)Total: 425 (310-765) 375 (305-445)

Source: Goudie 2002.

There are uncertainties in the measurements and the values in parenthesis show the range of themethane emissions. The totals of sinks and sources do not add up, creating a gap in the annualbudget. The calculations have large uncertainties in the estimates and therefore it cannot beconcluded that this difference really exists. More accurate measurements have to be done.

About 55% of the worlds rice fields are irrigated and 75% of the worlds rice comes from irrigatedrice fields. The rainfed rice paddies in the lowlands represent 25 % of the worlds rice areas and 17

% of the worlds rice production (NC-1003 2005).

2.1 Emission of methane from rice paddiesFor a better understanding of the processes that drive the methane emission from rice paddies abrief introduction to plant and soil chemistry will follow.

Carbon as a base of which methanogenic microorganisms live is thought to come from threesources: the death of root tissue from crops, the decay of both freshly added organic matter andhumus and carbohydrate exudates from living root tissue. Depending on which of these threepathways used, production of H2 and CO2 or acetate (CH3COO

-) is done through breakdown of

organic matter (CH2O). Methanogenic bacteria can then produce methane either from the H2 orCO2 through the following equation (Wassmann et al. 2000):

CO2 + 4 H2 CH4 + 2 H2O (1)

Or methane could be emitted through acetate:

CH3COO- + H+ CO2 + CH4 (2)

And a summary could be written as:

2 (CH2O) CO2 + CH4 (3)


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This is also confirmed by research done by Fullen & Catt (2004). Bharati et al. (2001) concludesthat the methanogenesis in flooded soils is influenced by the availability of fermentable substrates.

According to Schtz et al. (1989) methane isemitted from rice paddies to the atmosphere viathree pathways; diffusion, ebullition and plant-

mediated transport (figure 2). Ebullition is whenbubbles due to fermentation are emitted throughthe water. Less than 1 % of the emissions fromrice fields come from diffusion from subsurfacesoil to flood water. During the transplanting stage,ebullition is the most important way of emittingmethane, but this is only about 10 % of the totalemission during a plants emitting cycle. Theremaining 90 % comes from the rice plant itself.The methane is emitted through the plantsaerenchyma in leaf blades, sheath, culm and roots.The rice plant is an effective gas exchange systembetween the anaerobic soils and the troposphere(Holzapfel-Pschorn et al. 1986). According tomeasurements by Wassmann et al. (2000) whenthe methane production in the soil was high, methanewas mostly emitted through the aerenchyma of riceplants rather than through ebullition. The plantsaerenchyma formation and root exudation are affectedby cultivar and soil parameters such as nutrient availability, physical impedance and redoxpotential (Wassmann et al. 2000). The soil oxidation-reduction (redox) potential is a measure of the

degree of aeration in a soil. A high redox potential indicates a high oxygen level. Low redox valuesmay be an indication that conditions are anaerobic. The soil oxidation-reduction (redox) potential(Eh, measured in millivolts) are important for the methane production, since the methanogens aredependent on anaerobic conditions (Bharati et al. 2001).

The rice goes through different growth periods and the transplanting stage is when the young riceis replanted in the paddy field. This treatment is not used everywhere, sometimes the rice is planteddirectly in the paddy field without transplanting.

Figure 2. The emission of methane

through the rice paddy and plant.

(Holzapfel-Pschorn et al. 1986)


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Next stage (figure 3) is the active tillering stage, when the rice develops leaves and shoots. Duringthe latter part of the panicle development stage (about 16 days after visual panicle initiation), thesheath of the flag leaf swells. This swelling is called booting. The heading stage is when thepanicle emerges out of the flag leaf sheath (panicle initiation). The flowering stage occurs when theanthers of the terminal spikelets protrude and shed pollen. It occurs 25 days after visual panicleinitiation regardless of the variety. When the grain colour in the panicles begins to change fromgreen to yellow, the rice is mature (IRRI 2004).

2.1.1 Factors affecting the emission of methane

Climate, water management, soil characteristics, fertilizer application and rice variety all affect themethane emission fluxes (Liping 2001). According to the IPCC (1996b, 2004) the amount ofmethane emitted is believed to be a function of the rice species, number and length of the harvest,soil type and temperature, irrigation practices and fertiliser use.

2.1.1.2 Climate

Methane emission rate is closely connected to temperature. High temperatures increase thedegradation of organic matter and enhance the production of methane (Yang & Chang 1999).According to Liou et al. (2003) and Singh et al. (2003) the soil temperature is an important factorfor methane production in paddy soils. Singh et al. (2003) found that the methane emission peakedin the afternoon (at 1400h) and the soil temperature are at its highest during these hours,concluding that methane emission is a soil temperature dependent process if all other factors areleft constant. Temperature is also a major factor affecting the interannual variation in methaneemission (Watanabe et al. 2005).

Yang & Chang (1999) found that the air temperature had the highest correlation coefficient withmethane emission among the test temperatures air, soil and water when measuring methaneemissions from rice fields. They also conclude that temperature and water management are themajor factors influencing methane emission from rice paddies. They also found that most methane

is emitted during the daytime hours. According to Liu & Wu (2004) the methane production wasvery active at high temperatures, during an evaluation of methane emissions of Taiwanese rice

Figure 3. The growth stages of the rice plant and the different parts of the rice plant (IRRI 2004).


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paddies. Watanabe et al. (2005) found that the relationship between seasonal methane emission andtemperature and rice straw application can be described as:

when measuring methane emission from rice plants planted in pots, where x is the rice straw

application rate (g/kg soil), y is the seasonal methane emission (mg C / pot), E = (T-15) the sumof the effective temperature (C) and T is the daily mean air temperature (C) during the growthseason. This relationship was derived when other parameters effecting methane emission were heldconstant. The effective temperature is a concept derived from the assumption that at 15C themethanogenesis is considered to be negligible and over this temperature the methane emissionstarts. This equation can only be used when temperatures are above 15C.

If global warming continues there would be an increase in methane production and emission. Butone has to take into account other changes induced by global warming, such as changedprecipitation and soil moisture (Fullen & Catt 2004).

2.1.1.3 Water management

Methane emission measurements made on pots with flooded and non-flooded rice showed that themethane emissions were always lower from the continuously non-flooded pots (Bharati et al.2001). Singh et al. (2003) found that the methane emission rates dropped drastically with completewater drainage, even if the drainage was only for a short period. When the water is drained, theoxic conditions impair the methane production (methanogenesis). According to Yang & Chang(1999) the methane emissions from soil with continuous flooding was 2.8 times higher than thatwith intermittent irrigation. Intermittent irrigation is the process when the paddy field is floodedwhen the rice is planted, then the soil is drained and not re-irrigated until the soil is perfectly dried(Yang & Chang 2001). Measurements in Los Baos, Philippines show that drying the paddy field

during midtillering resulted in reduced methane emissions by 15-80 % as compared with continousflooding (Wassmann et al. 2000). Wassmann et al. (2000) also concludes that the drainage of watershould be of short duration, only 7 to 10 days and timed so that the plants have used up thenitrogen (N) fertilizer applied at basal and vegetative stages. Reflooding should be done before theN fertilizer is applied at the panicle initiation. Measurements with the closed chamber method inJakenan, Indonesia showed that rainfed rice emits less methane than irrigated rice according toWassmann et al. (2000).

Intermittent irrigation in paddy fields reduces methane emissions significantly (Yang et al. 2003).Mishra et al. (1997) found that the accumulative methane emission was around 70% lower forintermittent irrigated paddies than continuously flooded paddies.

2.1.1.4 Soil Characteristics

Emission measurements made by Bharati et al. (2001) showed that the methane emissions variedduring the growth period of the plant. Methane emissions where low during the early growth stagesof the rice plant. This may be due to low levels of methanogenesis (Satpathy et al. 1997). Methanemaxima were measured during the reproductive and ripening stages (Baharati et al. 2001), and thisis probably due to higher availability of fatty acids and sugars.

Bharati et al. (2001) also investigated the relationship between flooded and non-flooded conditionsand soil redox potential (Eh) and found that the redox potential was high during non-floodedconditions. The redox potential is a measure of the degree of aeration in a soil. In the laterite soilthe redox potential was high through the whole crop season under non-flooded conditions. Inalluvial soil the redox potential was low during the reproductive state under flooded conditions. In

y (x, E) = (0.02x + 2.067)E (32x + 1866) (4)


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the laterite soil the redox potential dropped drastically, which could be due to high emission ofmethane. This may mean that even though the paddy field is well drained, it could emit high levelsof methane. Paddy fields sometimes contain anoxic microsites, such as aggregated soil particles orsand grains, even under non-flooded conditions with different redox gradients favouringmethanogenesis (Rath, Mohanty et al. 1999). Measurements in Prachinburi, Thailand in deepwaterrice show that low emissions of methane are related to acid sulphate soils (Wassmann et al. 2000).

According to Yang & Chang (1999) the soil redox potential and soil pH correlates badly withmethane emission. The soil redox potential had a correlation coefficient of less than 0.35 and soilpH had 0.30 and water pH had even less: 0.25. This is to be compared with air temperature, whichhad a mean correlation coefficient during the transplanting stage of 0.83. Their results also showedthat during high methane emission and flooded circumstances, the redox potential was low andduring intermittent irrigation the methane emission was low, with high redox potential. However,the fluctuation of soil pH during the day was not significant, except during the transplanting stage(Yang & Chang, 1999).

Another soil characteristic which affects the methane emission is methane oxidation.Approximately 95 % of the methane produced in flooded soils is oxidized to carbon dioxide beforeit is released to the environment. Therefore the methane oxidation is important for thebiogeochemical cycling of methane (Wassmann et al. 2000).

2.1.1.5 Fertilizer application

Fertilizers work differently when applied to the rice paddies. Some fertilizers decrease the methaneemissions and some stimulate them. According to Yang & Chang (2001) application of greenmanure (Sesbania) resulted in increased methane emission. This conclusion was also reached bySingh et al. (2003). Rice straw or green manure increased methane emission and humifiedsubstrates generated less methane. Rice-straw and bio-fertilizer treatment increased methaneemission by 16 % and 114 % respectively. This is also the result from measurements done in

Jakenan, Indonesia; according to Wassmann et al. (2000) rice straw application results in a 40 %increase in methane emissions. Paddies with fermented cow dung decreased methane emission by64 % and leaf manures generated 58 % less methane (Singh et al. 2003). Urea fertilizer, much usedin Asia, had the highest methane emission rate followed by ammonium chloride and ammoniumsulphate in decreasing order (Liou et al. 2003). In deepwater rice fields in Prachinburi, Thailandthe methane emission was highest from paddy fields using straw incorporation followed by strawcompost and least emission was received with straw ash incorporation (Wassmann et al. 2000).Liou et al. (2003) points out that the amount of fertilizer used, how and when you apply it alsogreatly effects the methane emissions. During the second crop season the green manure usage gave1.4-2.9 times higher methane emission than a paddy field with intermittent irrigation andconventional chemical fertilizer. A paddy field with rice straw application gave 3.3-12.0 times

higher methane emission than a paddy field with intermittent irrigation and conventional chemicalfertilizer during the second crop season (Yang et al. 2003). According to Rath & Swain et al.(1999) prilled urea did not enhance methane emission, urea supergranules applied under water andsoil was an effective methane inhibitor. A mix of prilled urea and Nimin (a nitrification inhibitor)was most efficient at reducing methane emission. Application of nitrification inhibitors, nitra pyrinand especially wax-coated calcium carbide retarded methane emission from paddy fields withoutlowering yield and could be an alternative to intermittent irrigation.

Ammonium thiosulphate is a suitable option to mitigate methane emission during flooded ricecultivation. Ammonium thiosulphate lowered the methane emissions by 38-60% depending on howmuch was applied (Rath et al. 2002). According to Wassmann et al. (2000), rice straw gave the

most methane emission followed by green manure, urea and ammonium sulphate. Sulphateapplication even reduced the seasonal emission of methane due to competition between methane


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producing and methanogenic bacteria. Plots receiving 6 t/ha of rice straw in addition to mineralfertilizer, increased annual methane emission rates approximately 2 to 3 fold as compared to theplots only treated with mineral fertilizer, regardless of soil type (Yagi & Minami 1990).

2.1.1.6 Rice variety

Singh et al. (2003) reached the conclusion that the Indian rice variety Saket had the highestmethane emission followed by Pant 4, Sarju 52 and Sundari in decreasing order. According to Liouet al. (2003), who made measurements on two rice varieties in Taiwan, Indica rice were emittingmore methane than Japonica rice. The emission of methane increased when potassium nitrate (afertilizer) was added to the Indica rice. According to Wassmann et al. (2000) the modern cultivarIR72 gave the highest seasonal methane emission. No special plant trait could be singled out as thecause for the high emissions.

2.1.1.7 Other remarks on methane emission

Some tropical areas may have two, three or even four crop seasons. The methane emitted from thepaddy field differs between the crop seasons. Yang & Chang (2001) found that the total methane

emission in the second crop season was two to five times higher than the first crop season, whenmeasuring from rice fields in Taiwan which have two crop seasons. They also found that duringthe second crop season the highest methane emissions were measured during the transplantingstage and for the first crop season the booting stage. Yang & Chang (1999) found that the methaneemission of the first crop season was only 19-41% of that in the second crop season. Liou et al.(2003) found that the methane emissions during the second crop season were high during thetransplanting to active tillering stage. They also found that the methane emissions were 1.7 to 2-fold higher at noon than at early morning during the second crop season. According to Liu & Wu(2004) the methane emission was 7-16 times higher on the second crop season than in the first cropseason. They also found that much methane is emitted during the transplanting stage for the secondcrop season and the effective tillering and booting stages for the first crop season. According to the

methane emission model of Liu & Wu (2004), temperature is the most important parameter formethane emission rates and that soil pH, soil water depth and soil organic content also areimportant parameters. Another reason for the different emissions during the growth stage is that therice morphology and size is changing and therefore also the way the rice plant emits methane(Schtz et al. 1989).

All research does not point in the same direction. According to Yang & Chang (2001) methaneemission measurements made on paddy soil (not planted with rice) and paddy field (planted withrice) were almost the same when the same climate, water management, fertilizer application rateand soil characteristics was used. However Holzapfel-Pschorn et al. (1986) concludes that thepresence of rice plants stimulated the methane emission both in laboratory and field measurements.

A compilation of the research discussed above regarding factors that increase or decrease methanefrom paddy soils is found in figure 4.


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2.2.2 Mitigation options for methane emission

One important factor for farmers is to ensure that grain yield is unaffected, even though mitigationoptions are implemented. According to Singh et al. (2003) measurements from rice paddies inIndia, the most important ways to lower methane emission is improved water management, the

right selection of rice cultivars, fertilizer usage. According to Wassmann et al. (2000) the ricecultivar should be selected with care since the methane emission potential ranges from 9% to 56%.Singh et al. (2003) also points out that drainage of the paddy field before the heading stage lowersthe methane emissions without lowering the yield. This makes better water management an eco-friendly alternative. One problem they also point out is that many tropical countries lack of watercertain months and abundance in water other months. This means that many farmers need to keepthe water on the fields in order to get a harvest. At times it can be difficult to drain the paddy fieldsbecause of heavy raining. Yang & Chang (1999) concludes that intermittent irrigation is a veryuseful strategy to lower methane emissions from paddy field. Intermittent irrigation is a goodmitigation option, but must be evaluated further and done in the way described above according toWassmann et al. (2000).

According to Liu & Wu (2004) some farmers in Taiwan choose to leave the rice straw from thefirst crop season in the paddy field. If the rice straw were removed after the first crop season, themethane emissions would decrease. Liu & Wu (2004) also suggest that the depth of the paddy fieldwater should be increased to 25 cm, which can reduce the methane emission rate by 18 %according to a methane emission model. Wassmann et al. (2000) presents another mitigation optionthrough finding that direct seeded rice reduced methane emission by 16-54 % compared withtransplanted rice. This may also make it easier for farmers to plant rice.

In the thin surface layer or rice rhizosphere methane is transformed to carbon dioxide. This process

is due to autotrophic bacteria, which utilize methane as sole carbon and energy source.

Increase methaneemission

Decrease methaneemission

ClimateIncreasing temperatureIncreasing precipitation

Water managementContinous floodingWater depth

Water managementIntermittent irrigationWater drainage

Soil characteristicsAnoxic micrositesSoil redox potential

pH - ?

Fertilizer application

Rice straw and greenmanureUrea fertilizer

Fertilizer applicationHumified substratesAmmonium sulphatePrilled ureaNitrification inhibitors

Rice varietySaketIndica riceIR72

Rice variety Japonica rice

Several cropseasons

AgriculturalmethodTransplanting

Figure 4. A summary of the factors affecting methane emissionThe rice variety Japonica does not decrease the methane emission, but it has lower emissionvalues compared to other rice varieties in the study.


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Accelerating methane oxidation could be a mitigation option, but there needs to be more studiesbefore it can be developed as a field-scale technology (Wassmann et al. 2000).

2.3 Emission of nitrous oxide from rice paddiesNitrous oxide is a greenhouse gas that also is emitted through the paddy field. Emission of nitrous

oxide is connected to the methane emission rates. Agriculture is the main source of most nitrousoxide emissions. Nitrous oxide is produced from soil processes as an intermediate product ofmicrobial nitrification and denitrification and could be enhanced by addition of organic manures orbiomass burning (Fullen & Catt 2004). According to Liping & Lin (2001) non-flooded fields emitnitrous oxide and flooded fields almost do not emit any nitrous oxide at all. Consideration must betaken to the fact that nitrous oxide is a more effective greenhouse gas than carbon dioxide andmethane, especially in the long term period. Results from Liping & Lin (2001) show thatmidseason aeration of paddy fields reduce methane emissions by 50 % and this decrease inmethane is larger than the increase in nitrous oxide on a 100-year scale. According to Yang et al.(2003) the nitrous oxide emissions also decrease with appropriate application of nitrogen fertilizerand irrigation.


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3. Study area - Vietnam

3.1 Morphology

Vietnam covers an area of 332560 km2. Vietnam borders to the South China Sea in the east, the

Gulf of Thailand in the south, Cambodia and Lao in the west and China in the north (figure 5).Two major rivers; the Red River in the north and the Mekong in the south, have their mouths inVietnam. The Song Hong (Red river) and its tributaries the Song Da (Black river) and Song Chaiflow south-east in deeply incised valleys through mountains which reach heights up to 3000 m.Southward to the Mekong delta, the country is divided into a mountainous interior and a narrowcoastal plain. Most of the population in Vietnam lives in the deltas of Red river and the Mekong(Moores & Fairbridge 1997).

3.2 ClimateNorthern Vietnam has four distinct seasons and twomonsoon regimes. The northeast monsoon (from

November to April) brings cold and dry air from theSiberian high pressure and the southwest monsoon(from May to October) brings hot, wet air from theBengal Gulf. This summer monsoon brings heavy rainsand typhoons. The average temperature in Hanoi is23C and the average annual rainfall in all of Vietnamis 1500 to 2000 mm (figure 6). The sunshine hours are1500 to 2000 per year and the average solar radiation is100 kcal cm-1 year-1 (NISF 2002).

3.3 Land use

Before 1943, Vietnam had a forest coverage exceeding43 %. The forest coverage has changed extensively due to

Figure 5. Location of Vietnam and Hanoi (Maps, 2004)

Hanoi province

0

5

10

15

20

2530

35

J F M A M J J A S O N D

Month

Tempera

ture

(C

)

0

50

100

150

200

250

300

350

400

Prec

ipita

tion

(mm

)

Figure 6. Temperature and precipitation

patterns in Hanoi province 2003 (Stat,2004).

The equator


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population growth and two wars, one against France and one against USA. In 1994 the forest landhad decreased to 28-29 %, however this figure has increased over the last years due to reforestationmeasures.

The crop yields are not high compared to the crop yields of an industrialized country, howeverVietnam agriculture growth rate is so high that it assures enough food for domestic consumption

and a portion for export. Agriculture, forestry and aquaculture products make 27.4 % of GDP(Vietnam INC 2003).

3.4 Greenhouse gas emissionAccording to the Vietnam greenhouse gas inventory made in 1994, the total greenhouse gasemission was 103.8 million tons of CO2 equivalents. Methane (CH4) contributed with most of theemissions; 52.5 million tons which is 50.6 % of the emissions. Agriculture represents the largestemitter of CO2 equivalents (50.5%) followed by emissions from the energy sector (24.7%). Ricecultivation represents 62.4 % of the methane emissions from agriculture. One of the mitigationoptions suggested in the Vietnam Initial National Communication (INC) is reduction of the

methane emissions by improvement of irrigation-drainage management in rice fields (VietnamINC 2003).

3.5 Rice paddiesThe total agricultural land in Vietnam is 9537000 ha (2002) and of this 7449300 ha (2003) is landgrown with rice. Rainfed paddies represent 3415000 ha and irrigated paddies stand for 2276000 ha(FAO 2004). Deep water paddies, the least used way of growing rice in Vietnam, are mainlylocated in the Mekong Delta. The typical rice field close to Hanoi is rainfed shallow water lowlandarea and after the harvest the fields are burnt. Different kinds offertilizers are used; pig manure, rice straw residue andmanufactured fertilizer. The water management differs, some

farmers have the ability to keep the fields filled with water, butsome have to be content with the periodic water from rainfall. Thefarmers use the rice varieties that are available for purchase in thearea (Personal communication, Tran Duc Toan).

3.6 Hanoi provinceHanoi province has 42500 ha of agricultural land and a major partof this is paddy rice (Stat 2004). ng Ngc commune, where themeasurements have been made, is located northwest of Hanoicentre (figure 7). ng Ngc commune is indicated by a black star

and the dotted rectangle represents an approximate location of thecity of Hanoi including surrounding communities. Also shown inthe map is the international airport, the railroads, Ho Tay Lake andthe Red River. The alluvial soil of the red river delta, which mostparts of Hanoi province comprise of, is very fertile.

3.6.1 Soil characteristics of Hanoi province

The alluvial soils of the Hanoi province are formed from different river alluvial deposits. The soilin the measurement areas is a fluvisol. The Red River has an irregular water regime, which givesrise to different texture in the profiles and plains. Soils of the Hanoi province often have layers ofclay or silt far from rivers or sand close to rivers. The soils of the Red River delta are very fertile.

The soils have medium texture, bright brown colour, neutral reaction, high base saturation, oxides,carbon and total nitrogen content are usually medium and phosphorus and potassium content are

Figure 7. Map of Hanoi and its surroundings

1.4 km14 km1.4 km14 km


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high. In intensive regions of the Red River delta rice crop yields can reach 14-15 tons hectare -1yield-1 (NISF 2002).

3.6.2 ng Ngc commune

ng Ngc commune is located west of the Ho Tay Lake (figure 7). It is a rural area with differentkind of agricultural fields (mostly rice), lowconstructed houses (2-3 floors) and gravelled roads.Along the main road there are many small cementfactories and one large factory (indicated by a yellowstar in figure 8). Measurements were made in threedifferent fields indicated by figure 8.

Figure 8. Map ofng Ngc and vicinities

Measurement points

Cement factory

Bridge

Big road

Small road

Railroad

River, lake

StreamN

Measurement points

Cement factory

Bridge

Big road

Small road

Railroad

River, lake

Stream

Measurement points

Cement factory

Bridge

Big road

Small road

Railroad

River, lake

StreamN

1.5 km1.5 km


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4. MethodsBecause of economic limitations measurements could only be made during three days. It wasdecided to further investigate the relationship between methane emission and air temperature, sinceit seems to have the best correlation (Yang & Chang 1999). The well-known way of measuringmethane i.e. the chamber method could not be used due to economic limitations.

Since the wind influences the measurements in this case the methane emissions need to be high inorder to be correctly measured. The measurements were therefore carried out during the secondcrop season, because the emissions during this period are higher than the first crop season (Yang &Chang 2001). The methane emissions were measured when the rice was flowering to pinpoint amaximum value due to the possible enhancement made by application of fertilizer.

The methane emissions of the rice field were measured directly with a particle counter and an airsample was also taken to analyse the methane content in laboratory.

4.1 In situ measurements

The measurements of soil properties, temperature, methane emission, GPS and mapping weremade in ng Ngc commune north of Hanoi. Hanoi is in the time zone GMT+07:00, hereafterreferred to as local time (LT).

4.1.1 Temperature and methane

The measurements were made on three different areas on three days: the 17th, 18th and 19th ofOctober 2004. The temperature was measured with an ordinary mercury thermometer. Noprofessional shelter from incoming radiation was used. The thermometer was kept in shade whenpossible. The wind speed was low during the measurements.

The methane emission was converted into CO and was calculated by a device branded MX-21 plusOldham, France (figure 9). Pictures of the measurement equipment can be seen in figure 10, takenby Sara Sandin 2004-10-17. A plastic tube is inserted to the pump and connected to a stick with arubber band. This is the air intake, arrow 3 in figure 10, picture 1. It took approximately 10 to 15minutes to get a reading from the MX-21 plus. The methane emission and temperaturemeasurements were done at 8.30, 10.30, 12.30 am and 2.30, 4.30 pm (LT). Firstly the in situmeasurements were done and secondly the plastic bag was filled with air from the pump.

The methane emission in the paddy field was converted to CO by the following reaction:

5CH4 + 6KMnO4 + 9H2SO4 5CO + 3K2SO4 + 6MnSO4 + 19H2O (5)

PumpChemical reaction MX-21plus

Air pressure controlFilter of CO

Figure 9. Measurement equipment in situ


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The methane emission was then calculated by the following two formulas (NISF, 2004):

a) Defining the methane content (in ppm)

Cm = 4/7Ck

Where: Cm is methane content in the airCkis value of CO received in MX-214/7: Conversion coefficient

b) Defining the methane content C (in mg/m2)

C= 1.29 Cm / [1 + (T / 273)]

Where: Cm is content of CH4 in the air (ppm)

T is the temperature at the measurement time (C)

4.1.1.1 The diurnal variations of methane

The methane content were measured 5 times per day during 10-15 minutes each and to get thediurnal emissions the measurements were plotted in a graph, with 10 minutes interval on the x-axisand methane emission on the y-axis. The least mean square method was used and a polynomialequation of the second degree was calculated from the graph. The polynomial equation was used tocalculate a mean value for each 10 minutes period. The emissions of the intervals are then addedbetween 6 am and 8 pm because this is when the most methane emission is emitted (Yang &Chang 1999). The emissions during the night have not been included in the calculation.

Picture 1

21

3

4

Figure 10. Picture 1 shows the in situ measurement. Arrow 1 indicates the particle measurement equipment,

2 the pump, 3 the intake of air and 4 the reaction tube. Picture 2 shows the plastic bag sampling.

Picture 2


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4.1.2 Rice variety, water and fertilizer management

Interviews have been made with the owners of the rice fields in order to find out what kind of ricevariety, water and fertilizer management have been used in the field. The questions asked are foundin appendix 1.

4.1.3 Mapping and GPSThe mapping of the three fields was made with a compass and a pedometer. The area wassubdivided into smaller areas by lines such as roads, paths and streams and the areas in-betweenwere mapped and classified. These maps were then scanned and converted into raster images inIdrisi32 version I32.11. The maps were then digitalized in CartaLinx version 1.2 and were madethematic with Map Info Professional 7.0.

The Global Positioning measurements were made with a Garmin GPS with an approximatelyaccuracy of 10-20 meters. There was no signal obstruction because the rice fields were open. Thetransverse Mercator projection WGS84 and the rotational ellipse GRS80 were used to get thecoordinates.

4.1.4 Interviews

The interviews were made 4th 5th and 7th of November. I was not present at the interviews due toa misunderstanding. Therefore follow up questions could not be asked and the expressions andbehaviour of the interviewee could not be observed first hand. It cannot be ruled out that the personinterviewing may have affected the answers in the interview. However, the person interviewingwere not involved in other parts of this research project. Many of the questions were not answeredand it cannot be known if it is a result of neglect or a mistake. The questions asked are found inappendix 1 and the questions not answered are bold.

4.2 Laboratory measurementsAir and soil samples were taken at the three fields. The air samples were taken in plastic containersvia a pump, see picture 2 in figure 10. The soil samples were taken at 5 cm depth in a plastic bag.

4.2.1 Air samples

The methane emission cannot be measured directly from the plastic bags. The methane needs to beconverted into another substance. The following reaction was used to transform the methane intocarbonoxide.

5CH4 + 6KMnO4 + 9H2SO4 5CO + 3K2SO4 + 6KMnO4 + 19H2O

The methane of the air sampling bags was mixed with liquid NaOH. Then H 2SO4 was added to themixture and the air was pumped into a reaction tube with KMnO4. The carbonoxide was the put inwith the liquid I2O5 to create iodide, which was measured.

CH4 liquid NaOH liquid H2SO4 Pump reaction tuble ( KMnO4) liquid I2O5

The carbonoxide reacted with iodideoxide to create iodide:

5CO + I2O5 I2 + 5CO2

The iodide (I2) was detectable in a Jasco V-530 (Japan).


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4.2.2 Soil samples

The three soil samples were taken on the three measurement sites on 5 cm depth. The samples weresent to the laboratory at NISF, where it was analysed according to FAO guidelines.

Granulometric analysis was carried out by sieving and sedimentation (Robinson pipette)

considering clay (


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increase during the same time period. The model estimations of increased temperature from Sandin(2005) are used to calculate the future methane emissions of North Vietnam during the autumn.

A relationship between temperature and methane emissions (see table 2) was found by Xu et al.(PU 2005) through modelling emission from rice paddy soil measurements in Chongqing (1999)and Sichuan (1989), China. The relationship is not firmly established but due to difficulties in

finding other sources, this relationship was used to create future emissions of North Vietnamduring the autumn season.

Table 2. Relationship between temperature and methane emissionTemperature increase Methane emission increase1C 20 %2C 46.5 %3C 68 %

Source: PU 2005.


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4.4 The measurement fieldsDetailed maps of the three fields can be seen in figures11, 13 and 15 respectively and pictures of the threefields are found in figures 12, 14 and 16. All pictures by

Sara Sandin.

The first field (GPS: X=0580642, Y=2332480, Z=7meters) is in a constant water filled rice field, withflowering rice. The water level was 15 cm and the cropwas 75 cm high. Approximately 350 m north of the firstfield a brick and cement factory is located. The windspeed was approximately 2 m/s northeast, when themeasurements were made. The location of the factoryand the low wind speed suggests that the emissionsfrom the factory could not have

influenced the particle counter. Aheavily trafficked road is locatednorth of measuring point one.This should not have affected themeasurements due to the filteringof CO and CO2 before measuringthe methane amount. 35 m fromthe first measuring point a 20 mwide ditch filled with water waslocated. The weather was sunny with a heat hazecovering 8 octas of the sky. Two days before the first measurement there had been heavy rain in

the area for two-three hours during the afternoon. The measurements were made at a 10 cm levelabove the crop.

100 m100 m

Figure 11. Map of field 1.

Picture 3

Figure 12. The big black arrow indicates measurement point 1. There is approximately 40

meters to the black arrow. Picture 3 taken towards the south 2004-10-17, indicated by the small

black arrow on the map.


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The second field (GPS: X=0580125, Y=2331242,Z=3 meters) was also in a rice field filled withwater, but the water level was only 10 cm, and thecrop 50 cm. The in situ measurements were doneat 10 cm height over the water level and duringthis measurement the mud was heavily trampled.

The crop was flowering. A heavily trafficked roadis located north of measuring point two. The windspeed was approximately 1 m/s northwest andincreased during the day to approximately 3 m/sat the end of the measuring day. The methanemeasurements should notbe affected by the trafficdue to the filter. Theweather was cloudy withthicker clouds covering 8octas of the sky.

100 m100 m


Picture 4

Figure 14. The big black arrow indicates measurement point 2. There is approximately 30

meters to the black arrow. Picture 4 is taken towards east 2004-10-20, indicated by the small

black arrow on the map.


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Field three (GPS: X=0581145, Y=2332656, Z=7meters) was located in a dry rice field, with 60 cmhigh flowering rice. Close to the measuring point aditch of 0.5 m width and a wildly grown cemeterywas located. This in situ measurement was also

done at 10 cm height over the mud level duringheavily trampling. The weather was clear and thewind speed was approximately 3 m/s northeast.

100 m100 m


Picture 5

Figure 16. The big black arrow indicates the measurement point 3. There is approximately 1

meter to the black arrow. Picture 5 is taken towards the north-northeast 2004-10-24, indicated

by the small black arrow on the map.


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5. Results

5.1 Soil properties

The properties of the soils of the three fields can be seen in table 3. The soil texture is shown infigure 17.

Table 3. Soil properties of the fields.

Total (%) mg/100g soil (available) meq/100g soil

pH (KCl) Org C % N P2O5 K2O P2O5 P2O5 K2O Ca2+

Mg2+

CEC

Field 1 4.2 2.1 0.23 0.22 1.1 4.9 4.7 3.6 3.6 0.97 17

Field 2 4.2 1.6 0.19 0.22 1.3 6.3 5.9 4.8 3.9 2.0 16

Field 3 4.6 2.2 0.21 0.25 0.8 9.4 5.4 3.6 5.1 2.1 10

The pH (KCl) is acid for all three points; however if the pH (H2O) had been measured it wouldprobably be 0.5-1 unit higher. But it is still an acid soil. Most plants perform best in a soil that isslightly acid to neutral, which is a pH (H2O) between 6.0 to 7.0. Acidic condition (pH< 5.5) in soils

impair microbial activity and plant growth, decrease organic matter decomposition rate andenhance toxicity of heavy metals. However, rice plants function well in an acid soil (BS 2005).According to IFA (2005) the soils of the humid tropics (as in Vietnam) are often acid and in needof liming. Indeed all the fields had low pH and may be in need of liming in order to reach highcrop productivity.

High salinity in a soil inhibits the availability of Ca2+ for the rice plants. The combination of highamounts of H+ and low Ca2+ and Mg2+ generally inhibits root growth and plant health. This isprobably the case for fields 1 and 2. Even though the health of the roots and plants are questionablein fields 1 and 2, the fields emit high levels of methane. As written in section 5.3.4 it is the fieldsthat may have improper treatment that emit most methane.

The organic content of the three fields are pretty low and the second field has the lowest valuecompared to a soil with normal organic matter content. For the total N (%) measurements a valuebetween 0.1 and 0.2 means that the soil needs moderate amount of N and a value over 0.2 meansthat there is a low requirement of N fertilizer. Fields 1 and 3 seems to have a healthy amount of Nfertilizer added. For the available P a value below 5 needs P fertilizer acutely and between 5 and 10the requirement of P fertilizer is moderate (IFA 2005). Field 2 acutely needs P fertilizer, field 1needs a contribution of P fertilizer and field 3 have low acquirement of P fertilizer. There is often ashortage of available P, N, K and Mg in the soils, hence appropriate fertilizer input is relevant forefficient crop productivity and probably also for lowering the methane emission. Section 5.2.3treats what kind of fertilizers that were added to the fields.

CEC is an estimate of soils ability to attract, retain, and exchange cation elements. A soil with lowclay content has a CEC value from 1-10 and has fewer positions to hold cations and a soil withhigh clay content has a CEC value from 11-50 and is more able to hold cations. The third field hasthe lowest CEC value and the lowest clay content, see figure 17 on soil texture below. Low CECsoils hold fewer nutrients, and will likely be subject to leaching of mobile anion nutrients.Leaching also increases with more coarse grains. None of the soils had a particular high CEC valueand a high CEC soil requires a higher soil cation level to provide adequate crop nutrition.

meq=millequivalents


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-10

10

30

50

70

90

110


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Table 4. The farming calendar for the three rice fields.Cropping systems Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov DecSpring rice1. Landpreparation2. Transplanting+Fertilizer3. Weeding 14. Weeding 2+Fertilizer5. Harvest

Summer rice1. Landpreparation2. Transplanting+Fertilizer3. Weeding 14. Weeding 2

+Fertilizer5. Harvest

5.2.2 Water management

The water management of the first two fields are the same; water all year round. During the time ofmeasurement the second rice field had a lower water level than the first rice field. At the third fieldthe water was drained at the time of measurement. At the third field the farmer irrigates the fieldwhen the rice seems to need it and when there is a supply of water. This is a kind of intermittentirrigation.

5.2.3 Fertilizer usage

As can be seen in table 4, the fertilizer was applied to the fields during transplanting and secondweeding time. How the fertilizer was applied and what kind of fertilizer e.g. pig manure, rice strawetc it was is unknown. The interviewees have answered how they have tried to improve the fields.The application of N, P and K fertilizer is shown in table 5 and 6. Unfortunately the application isonly known in kg fertilizer per owner not in kg/m2 or equivalent. The areas of the fields are notknown. Therefore it is difficult to compare the fertilizer application rate with other areas. After thesecond crop season the farmers leave 50 % the crop residue on the fields.

Table 5. Fertilizer application to spring rice fields.Nitrogen (kg) Phosphorus (kg) Potassium (kg)

Owner of field 1

(total application ofall the owners fields)High water level

9-12 10-14 2-7

Owner of field 2(total application ofall the owners fields)Medium water level

5-12 10-15 4-8

Owner of field 3(total application ofall the owners fields)Low water level

9 10 4


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Table 6. Fertilizer application to summer rice fields.Nitrogen (kg) Phosphorus (kg) Potassium (kg)

Owner of field 1(total application ofall the owners fields)High water level

8-9 10-11 2-7

Owner of field 2(total application ofall the owners fields)Medium water level

5-10 10-15 4-8

Owner of field 3(total application ofall the owners fields)Low water level

7 15 4

5.2.4 Rice variety and mitigation options

The rice variety is not known, either because the farmers do not know what they use or thequestion wasnt asked. The farmers could be unaware of what kind of rice they grow. None of the

questions about mitigation options were answered or asked.

5.3 Methane emissionThe methane emissions were measured in situ (table 7) and in a laboratory (figure 18). Themeasurements in the laboratory have higher accuracy.

5.3.1 Methane emission measured in situ

Table 7. Methane emission (ME) rates in ppm and temperature (C) in situ.

Field 1 Field 2 Field 3

Time(LT)

ME (Highwater level)

Temperature(HWL)

ME (Mediumwater level)

Temperature(MWL)

ME (Lowwater level)

Temperature(LWL)

8.30 3 29 2 28 0 32.5

10.30 3 34 2 32 1 32

12.30 3 36 3 34.5 1 33

14.30 3 34 3 34 1 34

16.30 3 30 3 29 1 29

The measurements of methane emission in situ show only small changes. But as can be seen intable 7 there is a difference between the water levels. As expected the highest water level has thehighest emission and the lowest water level has the lowest emission of methane. Themeasurements of the methane emission of the medium water level and low water level were done

with heavy trampling of the soil, the effects of this is discussed in chapter 6.2. Even though thiswas done, the emission from the paddy fields was pretty low.


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5.3.2 Methane emission measured in situ and in laboratory

0

0.5

1

1.5

2

2.5

3

3.5

4

8.30 10.30 12.30 14.30 16.30Time

Methaneemission(ppm

)

0

5

10

15

20

25

30

35

40

Temperature(oC)

Methane HWL

Methane MWL

Methane LWL

TemperatureHWLTemperature

MWLTemperatureLWL

As can be seen in figure 18 the methane emissions follow the temperature pretty well. There aresome accuracy problems with the temperature measurements. The measurements were madewithout a professional shelter from incoming solar radiation or reflection effects, this is discussedin section 6.2. These effects could not be corrected since the only measurements to correct themwith are from central Hanoi, which is an urban area characterized by an urban climate. Thethermometer was kept in shade during the measurements but reflection from the water was notobstructed.

The same pattern as the in situ measurements can be seen in these measurements. The highestmethane emission comes from the field with the highest water level.

5.3.3 Diurnal methane emission

The diurnal methane emission was calculated from a polynomial equation of the second degree(see figure 19 below). For the emissions of methane from the rice growing in a low water levelpaddy two values were excluded in order to fit into the polynomial equation. The nighttime

emission, from 8 pm to 6 am is excluded in the diurnal emission calculation. In table 8 a summaryof the results are found.

Figure 18. Methane emission rates from three rice fields. Measurements made in situ

and laboratory.


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y = -0.0007x2

+ 0.0465x + 1.8355

y = -0.0005x2

+ 0.0431x + 2.4381

y = -0.0005x2

+ 0.0481x + 0.3111

0

0.5

1

1.5

2

2.5

3

3.5

4

0 20 40 60 80 100

Time (minutes after 6 am)

Me

thaneem

iss

ion

(mgm-2

h-1

)

MWL

HWL

LWL

Values from LWLignored to fit thecurvePoly. (MWL)

Poly. (HWL)

Poly. (LWL)x10

5.3.4 Seasonal methane emission

The equation 4 described earlier in section 2.1.1.2 could not be used, since the rice strawapplication rate at the three fields could not be found out. The seasonal emission was thereforecalculated from the measurements made by Corton et al. (2000). Corton et al. have measured on a

silty clay soil with pH = 6.1, Organic content = 1.3 and total N = 0.09 %. These soil properties arenot close to the three fields, see table 8. The measurement were made on soil prepared with freshrice straw and urea (90 kg/ha) during the dry season of 1996 and the mean temperature was26.3C. Central Luzon is situated close to the latitude of North Vietnam and therefore the climateis similar. However, Central Luzon is on an island in the Pacific Ocean, which makes the climate abit different. The water management is the same as for the first two fields in this project, but asremarked in section 4.3.1 no measurements from rainfed paddies could be found in similar climatearea.

From table 8 one can see that the highest emissions of methane were from the rice fields with highwater level. The lowest methane emissions came from field 3 with low water level and the

healthiest soil of the three fields. This is as expected, since a well kept soil results in low emissionsof methane. It is probably a combination of the healthy soil and the low water level that results in alow emission of methane from field 3.

Figure 19. Polynomial equations used to calculate methane emissions during a day.


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Table 8. Summary of results of the methane emission.Field Water

managementSoiltexture

Soil pH(KCl)

OrgC(%)

Total N(%)

Total methaneemission(mg/m

2and day)

Total methaneemission (mg/m

2

and season)

1Irrigated(HWL)

Clayey,siltysand

4.2 2.14 0.23 228.022081

57947

2Irrigated(MWL)

Clayey,siltysand

4.2 1.56 0.19 167.116174

42443

3Intermittent

irrigation(LWL)

Silty,clayey

finesand

4.6 2.18 0.21 86.08329

21857

5.3.5. Future methane emission

The future methane emission was calculated from the studies by Sandin (2005) using a climatemodel called CGCM2 based on the IPCC IS92a scenario. The relationship between temperature

and methane emission found by Xu et al. (PU 2005) is used in the calculation. For more detailedinformation see section 4.3.2 and table 2. The future methane emission (from 2019 to 2095) can beseen in figure 20. This example is done with the emissions of point 1, but since the samerelationship would create the same curve only at different starting points. The highest emissionchanges is during 2060 to 2070 and this is due to the temperature patterns during these years.

0,9366 18,70833912 1,187083 270,6550132 42,65501

0,942238889 18,82256991 1,188226 270,9154594 42,91546

0,946417222 18,90729528 1,189073 271,1086332 43,10863

0,991551111 19,82694055 1,198269 273,2054244 45,20542

0,99197 19,83551396 1,198355 273,2249718 45,22497

1,0311 20,63948442 1,206395 275,0580245 47,05802

1,053469444 21,10184157 1,211018 276,1121988 48,1122

1,041818333 20,86077338 1,208608 275,5625633 47,56256

1,087273889 21,80434819 1,218043 277,7139139 49,71391

1,07054 21,45602178 1,21456 276,9197297 48,91973

1,071098333 21,46762578 1,214676 276,9461868 48,94619

1,090002222 21,86124636 1,218612 277,8436417 49,84364

1,105241667 22,17960578 1,221796 278,5695012 50,5695

1,131399444 22,72822024 1,227282 279,8203421 51,82034

1,146128889 23,03834953 1,230383 280,5274369 52,52744

1,130032778 22,69948899 1,226995 279,7548349 51,75483

1,177578889 23,70343575 1,237034 282,0438335 54,04383

1,159525556 23,32117069 1,233212 281,1722692 53,17227

1,162145 23,3765545 1,233766 281,2985443 53,29854

1,136181111 22,82880337 1,228288 280,0496717 52,04967

1,181536667 23,78741279 1,237874 282,2353012 54,2353

0

0,5

1

1,5

2

2,5

3

3,5

2019

2023

2027

2031

2035

2039

2043

2047

2051

2055

2059

2063

2067

2071

2075

2079

2083

2087

2091

Temperatureanomaly(

C)

0

20

40

60

80

100

120

140

160

180

Me

thaneem

isison

increase

(mg

CH4/m2an

dday

)

Floating mean value

ME anomaly

The crop variety used for this calculation is IR72 The crop variety used for this calculation is IR64

Figure 20. Future change in methane emission calculated from the floating mean value

2019-2095 in North Vietnam during the autumn (SON).

Methaneemissionincreas

e

(mgCH4/m2a

ndday)

Temperature(C)


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6. Discussion

6.1 Field studies

The laboratory measurements (with a sampling plastic container) have higher accuracy. The in situ

measurements were zero, but with heavy trampling of the soil (as when the crop is harvested) 1ppm (low water level) and 2-3 ppm (medium water level) was measured. Since the measurementsgot higher during trampling, there is a possibility that during the flowering and ripening stages ofthe rice the emissions comes mainly from the soil, thus from ebullition. If this is the case, then theemissions during harvest could be large. According to Schtz et al. (1989) 90 % of the methaneemitted originates from the rice plant itself. If this would be the case then the emissions from themeasurement field would be very high indeed. However, when the methane was measured in thisproject nothing but trampling gave higher methane emission. According to Wassmann et al. (2000)the aerenchyma (where it emits methane) formation of the plant is affected by nutrient availability,physical impedance and soil redox potential. The nutrient availability values were low, perhapsexplaining the low emissions from the rice plant. Field 3 had the highest Ca2+ and Mg2+ and could

leak these nutrients due to the low CEC-value. Acid conditions impair microbial activity and plantgrowth, which could be a reason for the low ppm methane emissions. If a soil properties profilehad been established it would be easier to draw a conclusion on this subject. The soil propertieswere only sampled at 5 cm depth and a soil profile with different measured parameters at differentlevels would increase the accuracy of the measured parameters.

The high measurements from the first field were received without trampling. These high methaneemissions could have been induced by the heavy rains a few days earlier. The in situ measurementsneed high emissions to be correct. Only 1 to 3 ppm has been measured and that is low values.

There are several parameters that should have been measured in Hanoi, and two of the parameters

are soil temperature, water temperature and wind speed. It would have been interesting to see howwell the methane emissions correlated with the other temperatures as well as air temperature. Acorrection of the air temperature is needed, because no professional shelter from reflection of raysfrom the water or shelter for incoming radiation was used and this could have increased thetemperature measurements. On the other hand, a correction is hard to do, because the closest airtemperature measurements are done in central Hanoi, where other parameters affect the localclimate, such as the urban heat island and different building materials. The wind speed was lowduring the measurements. If the wind speed would have been large the measurements could havebeen affected. Instead of measuring the methane emission, emissions from roads or factories couldhave been measured. However, a low wind speed increases the reflection effect of the water.

There are also several questions that were not answered in the questionnaire. It would have beenpreferable for me to accompany the interviewer and be able to read the body language of both theinterviewer and interviewee first hand. There is a risk that the answers will be improved if theyare not asked with care. However, there is little chance that this was the case, since the interviewerand the assistant on the fields were not the same person. There were some difficulties regarding theinterviews. Many of the questions were not asked (see appendix 1) and some were poorlyanswered. This could be due to many reasons; badly formulated questions, absence of author andthe fact that the interviewer and the author of this project never met. This resulted in difficulties tointerpret the results. Questions such as fertilizer values were not received in per square meter, therice sort is not known, water management is not known in detail and so forth. The rice variety may

not have been known, even by the farmer and there are many different rice species. The water


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management would be preferable to know, since Wassmann et al. (2000) have concluded that adrainage period of 7-10 days is adequate.

6.2 CalculationsThe diurnal variations of methane for the three fields are high, at least when compared to

measurements in Jakenan, Prachinburi and Los Baos, as seen in table 9. The model is based upondaytime emissions only. The educated guess is that the diurnal emissions will be a bit higher if thenighttime emissions could be calculated. The difficulty was to figure out how high the emissionswere during the nighttime. Several different scenarios could be plausible. If the methane emissionfollows temperature, it could be lower during the night and rise early morning when the sun rises.The emissions could be quite low during the nighttime hours and stay low during the earlymorning. According to Yang & Chang (1999) the emissions are highest at 2 pm and lowest at 6am, but in a Beijing rice field methane emission had two peaks, one between noon and 2 pm andone between midnight to 1 am (Wang & Shangguan 1995). It is therefore difficult to know theemissions during nighttime in Hanoi, due to the lack of measurements.

From table 8 it is clear that the highest emission is connected to the paddy with the highest waterlevel (15 cm) and the lowest emission comes from the intermittent irrigated paddy. When thesemeasurements are compared to other measurements made in Southeast Asia it is also clear thatthey are higher (table 9). The three areas which are compared with the measurements fromVietnam, namely Jakenan (in Indonesia), Los Baos (in the Philippines) and Prachinburi (inThailand) have been measured the same way, with the closed chamber method. The averageemission per day is including nighttime emissions.

Table 9. Emissions of methane in different areas in Southeast Asia.Geographical area Soil properties Water

managementAverage CH4emission

1

(mg CH4 m

-2

d

-1

)

CH4 emission(kg CH4 ha

-1season

-1)

Hanoi, VietnamField 1

pH=4.2,Org C=2.14%

Irrigated 228 220

579

Hanoi, VietnamField 2

pH=4.17,Org C=1.56%

Irrigated 167 162

424Hanoi, VietnamField 3

pH=4.56,Org C=2.18%

Rainfed/intermittentirrigation

86 83

219

Jakenan, Indonesia pH=4.2,Org C=0.33%

Rainfed 60 (52) 58

Jakenan, Indonesia pH=4.2,Org C=0.33%

Irrigated 145 (72) 137

Los Baos, Philippines pH=6.3,Org C=1.46% Rainfed 19 (8) 40

Los Baos, Philippines pH=6.3,Org C=1.46%


Prachinburi, Thailand pH=3.9,Org C=1.22%

Deepwater 50 (36) 81

Prachinburi, Thailand pH=3.9,Org C=1.22%


Source: Wassmann et al. 2000 (excluding the first three rows from Hanoi, Vietnam)The values in parenthesis are the average standard deviation.

1 The average emission from Hanoi, Vietnam was measured during a short period and during a period when the

methane emissions was especially high. The other measurements were made during 1994-1998. See text for more. The crop variety used for this calculation is IR72 The crop variety used for this calculation is IR64


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The measurements of Wassmann et al. (2000) were subdued to strong day-to-day fluctuations inmethane emission. The seasonal emissions also varied within a year during different crop seasonsand between annual cycles. This makes it more important with continuous measurements over along time period. The methane emission was measured during 1994-1998 and the figures in table 9above are a mean of the measurements. The high organic compound in Los Baos did not result in

higher emissions, compared to Jakenan. This is also confirmed by the measurements made in thisproject. Field 1 has as high organic content as field 3 but higher emissions of methane than field 3.This is instead probably due to the differences in water management. Field 3 has the lowestmethane emission, where intermittent irrigation is applied. Even though it is a low value it meanstaking a risk as a poor farmer. The farmer cannot store water so there is a risk to loose the yield ifthere is a lack in precipitation. In these tropical areas the precipitation can sometimes becomeheavy, and then the crop can be destroyed.

The seasonal calculations of methane are under strong influence of Corton et al. (2000)measurements. Those measurements were chosen due to the similarities in water management (ofthe two first points) and the climate. However, the soils are different. There were no measurementsto be found that were easily comparable to the measurements made in this project. Therefore it wasof importance to find measurements that was as comparable as possible and since many scientistshave propagated for the influence of climate and water management on methane emission, themeasurements of Corton et al. was selected. However the climate is not the same in Hanoi as inCentral Luzon. Luzon has a tropical marine climate, but with a rainy season (May to October) anda dry season (December to February) similar to Vietnam. The temperature patterns are not similar;Luzon has two peaks; one in April-May and one lower peak in September. Vietnam has one peakduring the summer months. The precipitation patterns are similar, but the maximum value is laterin Luzon. The measurements are made on irrigated paddy fields. If the measurements were madeon rainfed paddies the differences between the Hanoi values and Central Luzon would have been

greater. As described earlier the maximum value was tried to be pinpointed and this was thought tobe during the reproductive stage of the rice. The maximum could have been during thetransplanting stage. However, the values measured correspond to the measurements of Corton et al.The seasonal emissions are difficult to compare to other measurements due to different measuringmethods.

The research of seasonal emissions of methane has shown different patterns. According to Bharatiet al. (2001) the highest methane emissions were emitted during the reproduction and ripeningstages of rice. According to Liu & Wu (2004) the highest emissions are emitted during thetransplanting stage of the second crop season. It is therefore difficult to calculate the seasonalemissions with high accuracy. To calculate an annual value is even harder because the second and

first crop season differs in many

2005.present & future methane emissions from rice filed in Đông ngạc - từ liêm - việt nam

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