Factors affecting methane emission from rice fields

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Atmospheric Environment Vol. 30, Nos 10/11, pp. 1751 - 1754, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1352-2310/96 $15.00 + 0.00

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FACTORS AFFECTING METHANE EMISSION FROM RICE FIELDS H. U. N E U E , * R. WASSMANN,I" R. S. L A N T I N , * M A C. R. A L B E R T O , * J. B. A D U N A * a n d A. M. J A V E L L A N A * *International Rice Research Institute (IRRI), P.O. Box 933, Manila 1099, Philippines; and tFraunhofer Institute fiir Atmospheric Environmental Research, D-82467 Garmisch-Partenkirchen, Germany (First received 3 January 1995 and in final form 1 September 1995)

Abstract--Emission of CH4 from ricefields is the result of anoxic bacterial methane production. Global estimates of annual CH4 emission from ricefields is 100 Tg. CH4 emission data from limited sites are tentative. It is essential that uncertainty in individual sources is reduced in order to develop feasible and effective mitigation options which do not negate gains in rice production and productivity. Field studies at the InternationalRice Research Institute show that soil and added organic matter are the sources for initial methane production. Addition of rice straw enhances methane production. Roots and root exudates of wetland rice phmts appear to be the major carbon sources at ripening stage. The production and transport of CH4 to the atmosphere depend on properties of the rice plant. Under the same spacing and fertilization, the traditional variety Dular emitted more CH4 per day than did the new plant type IR65597. Upon flooding for land preparation anaerobic conditions result in significant amount of methane being formed. Drying the field at midtillering significantlyreduced total CH4 emissions.Large amounts of entrapped CH4 escape to the atmosphere when floodwater recedes upon drying at harvest. Cultural practices may account for 20% of the overall seasonal CH4 emissions. Key word index: Organic amendment, rice cultivars, agronomic practices, methane production, water management.

INTRODUCTION Methane (CH4), a very important greenhouse gas, has major natural and anthropogenic sources (Bouwman, 1989). Methane strongly absorbs and emits infrared radiation (with great warming potential) at bands of wavelengths where CO2 and water vapor do not absorb (Wang et at., 1976). Isotope measurements of atmospheric CH~ indicate that 80% is of biogenic origin (Wahlen et al., 1989). Up to 20% or 100 Tg is estimated to emit from ricefields annually (IPCC, 1992). The present atmospheric CH4 concentration of 1.8 ppmV is more than double its pre-industrial value (Dickinson and Cicerone, 1986). The increase has remarkably slowed down in the last 2-3 yr. It is not known whether the declining rate of increase is due to decreasing emission,; or to increasing oxidation of CH4. More information and mechanistic understanding of methane fluxes from individual sources and their fate in the atmosphere are needed to reduce uncertainties of current and future emissions and to provide a reliable base for mitigations. The world's annual rice production must increase from 518 million torts in 1990 to 760 million tons in 2020 (IRRI, 1989). The required expansion and intensification of rice cultivation will likely increase CH4 fluxes to the atmosphere if current technologies are

continued. Changes in water management, nutrient management, cultural practices, and cultivar selection may have potential to increase rice production and productivity and reduce CH4 fluxes at the same time. Reducing uncertainties and predicting future emission trends, as well as developing mitigation technologies which do not negate gains in rice production require information about processes and geographic distribution of factors which control CH4 fluxes in ricefields. Therefore, the effects of different factors such as organic amendments, varietal differences, and water management on methane emission were monitored.

MATERIALSAND METHODS Methane emission rates were determined by an automatic measurement system based on the "dosed chamber technique". The principles of the sampling and analytical procedure were given by Schlitz et al. (1989) and technical details of the system used in these measurements were described by Wassmann (IAEA, 1992). Methane emission was monitored in an irrigated Aquandic Epiaqualf (pH 6.4, organic carbon 1.57%, total N 0.174%, CEC 37.3 meq 100 g-t, texture: clay) at the Research Farm of the International Rice Research Institute, Los Bafios, Laguna, Philippines (14° ll'N, 121° 15'E) with an Ama climate type (K6ppen classification).

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The plots were laid out in complete randomized block with four replicates. The followingtreatments were imposed during each season: 1992 wet season 1993 dry season: 1993 wet season - 1994 dry season: 1994 wet season - 1995 dry season

Organic amendment, 5 t ha-1 rice straw Cultivar: IR72 Cultivar differences using IR69957, Dular and IR72 Water management:continuous flooding; drying the field at 2 weeks before harvest; and drying at midtillering and 2 weeks before harvest Cultivar: IR72

et al., 1989; Yagi and Minami, 1990; Sass et al., 1991; Cicerone et al., 1992) by lowering the Eh and providing more carbon sources. Addition of 5 t rice straw h a - 1 increased CH4 emission tenfold compared with mineral fertilizers (Fig. 1). The difference in CH4 emission rates between plots treated with urea and those amended with rice straw decreases over time and becomes insignificant at the end of the growing season. This indicates an increasing impact of the rice plant over the season not only in mediating CH4 emission but also in providing carbon for CH4 production. Rice cultivars

RESULTSAND DISCUSSION Organic amendments Anaerobic fermentation of organic matter produces an array of organic substances not found in wellaerated soils. Methanogens constitute the last step in the anaerobic degradation of organic matter. Readily mineralizable soil organic matter is the main source of fermentation products that finally drive the CH4 formation in wetland rice soils. It is evident that organic amendments to flooded soils increase CH4 production and emission (Schiitz

Rice plants play an important role in the flux of CH4. Up to 90% of the CH4 released from ricefields to the atmosphere may be emitted through the rice plant (Schlitz et al., 1989). Well-developed intracellular air spaces (aerenchyma) in leaf blades, leaf sheaths, culm, and roots provide an efficient gas exchange medium between the atmosphere and the anaerobic soil. Gases such as methane which are formed in the soil diffuse from the reduced layer through the aerenchyma to the atmosphere. The production and transport of CH4 to the atmosphere depend on properties of the rice plant. Root exudates and degrading roots are also important sources of CH4 production,

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Days after transplanting Fig. 1. Methane emission under two N sources. IRRI, 1992 wet season. Table 1. Tiller characteristics at optimum crop development and CH4 emission of three cultivars. Cultivar

New plant type Improved variety Traditional

Tiller number m- 2 Total

Productive

300 1000 1200

300 600 500

Cumulative CH4 emission (g)! Tiller 12 5 13

g biomass 3 4 8

Factors affecting methane emission from rice fields especially at the later growth stages. The flux of gases in the aerenchyma depends on concentration gradients and diffusion coefficients of roots and the internal structure of the aerenchyma. The n u m b e r of tillers m - z, root mass, rooting pattern, total biomass, and metabolic activity also influence gas fluxes (Neue and Sass, 1994). All these traits and related emission rates vary widely among cultivars and rice crops (Neue and Roger, 1!)93; Parashar et al., 1990).

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We found that the traditional variety Dular emitted about 30% more CH4 per day than did the new plant type IR65597. The traditional variety has more tillers and longer roots and these appear to enhance CH4 emission (Table 1).

Cultural practices Cultural and agronomic practices have been developed to suit the physical, biological, and

Methane emission (1994 wet season) 200:getative

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Days after transplanting Fig. 2. Methane emission under three water regimes. IRRI, 1994 wet season.

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Fig. 3. Methane emission after harvest. IRRI, 1992 dry season.

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socioeconomic conditions of the different rice-growing regions and environments. The effects of agronomic practices on rice growth and yield are welldocumented but their relationships to CH4 emissions are not well-established. Wet field preparation requires at least two weeks and comprises land soaking (until soil is saturated), plowing, puddling, leveling, and harrowing. O n flooding, anaerobic condition commences and a significant amount of methane is formed (Fig. 2). Total methane emissions may be underestimated if reported values include only the emissions during crop growth. Water control is one of the most important factors in rice production. In irrigated rice, short aeration period at tillering has been shown to increase yield. A single mid-season drainage may reduce seasonal emission rates by about 50% (Sass et al., 1992; Kimura, 1992). In a controlled experiment, CH4 emissions can significantly be reduced when the field is drained and dried at midtillering and before harvest (Fig. 2). Large portions of CH4 formed in an anaerobic soil may remain trapped while the soil is flooded. When the flood-water is drained entrapped CH4 is partly oxidized but large amount escapes to the atmosphere immediately after the floodwater recedes and macropores become aerated (Fig. 3). Cultural practices may account for 1 0 - 2 0 % of the overall seasonal e l l 4 emision. CONCLUSIONS Water management, organic amendments, fertilization, cultural practices, and rice cultivars are promising mitigation candidates. Current understanding of processes controlling CH4 fluxes and rice growth is sufficient to develop mitigation technologies. Information is lacking on extrapolation domains and socioeconomic feasibilities of the various technical opportunities to realistically predict mitigation potential and minimize possible trade-offs. Mitigation technologies will be adapted only if they increase rice production and productivity. Acknowledgements--Although the research described has been funded by the US-Environmental Protection Agency, under agreement EPA No. CR-817-426-01-0 to the International Rice Research Institute, it has not been subjected to the Agency's review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. REFERENCES

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