Mitigation of Methane Emissions from Irrigated Rice Agriculture
Contributed by R.L. Sass, Rice University, USA
It is recognized that to meet the rice supply of growing populations, rice cultivation will continue to increase at or beyond its current rate. It is estimated, for example, that the world's annual rough rice production must increase from a 1990 value of 518 million tons to 761 million tons by 2020 -- a 47% increase -- just to maintain current nutritional levels. Because arable land is highly limited in major rice growing areas, increased production has to be achieved mainly by intensifying cropping (i.e. two or three crops per year) rather than expanding the area of rice cultivation. Irrigated rice will continue to dominate production. Irrigated rice land now comprises about half the total harvested area but contributes more than two-thirds the total grain production. With present agronomic practices, this will lead to increased methane emissions.
Because rice agriculture is one of the few sources of methane emission where management of the system is possible, it has become a critical focus of mitigation efforts. However because rice is also the world's most important wetland crop and the primary calorie source of a large fraction of the world's population, mitigation efforts must be based on sound agricultural practices as well as good scientific judgment.
A primary mitigation "switch" of the production and emission of methane is the presence of oxygen in the rhizosphere environment. Removal of oxygen from the rhizosphere is normally through consumption by soil bacteria. The presence of the flood water impedes the diffusion of oxygen from the atmosphere into the soil and thus keeps it anoxic. It has been observed by Sass et al. (1992) that a single drain of the flood water at the end of the vegetative stage allowed the soil to be reoxidized, reducing the seasonal methane emission by nearly 50%. Repeated drains every three or four weeks throughout the rice growing season, reduced seasonal methane emission by 88% without affecting grain yield. Yage et al. (1994) observed methane emission reductions of approximately 50% in intermittently drained plots when compared with continuously flooded Japanese rice paddies.
An important contributor to variations in observed methane emissions and a strong candidate for mitigation is the use of different rice cultivars. There are currently some 80,000 different rice cultivars available through the germplasm bank at the International Rice Research Institute in the Philippines and others are being sought. Most of these were developed for specific areas of the world and many are in current use. Yet, very few methane emission studies have considered cultivar differences. Methane emissions from eight different cultivars grown under similar conditions near New Delhi, India differed by as much as an order of magnitude (Parashar, et al., 1991). A study of five rice cultivars in irrigated fields near Beijing, China indicated that methane emission during the tilling-flowering stage varied by a factor of two (Erda, 1993). A preliminary study by Sass and Fisher (private communication) using ten cultivars showed seasonal methane emissions ranging from 18.2 to 41.0 g m-2. All three studies show a significant variation in methane emission that is solely dependent on cultivar choice. Cultivar choice by individual farmers could thus greatly influence regional and global estimates of methane emission from rice fields. The wide variation of traits and related emission rates among cultivars opens the possibility for the choice of existing cultivars and the breeding of new cultivars as a method for mitigation of methane emission. However, the relationships between different cultivars characteristics and methane emission have yet to be elucidated. Some cultivars may have more or less efficient conduits for the removal of methane from the soil through the rice plant, others may deposit different amounts of organic matter in the soil during the growing season or may differ in the ability to transfer oxygen to the rhizosphere, thus altering the redox potential of the soil system or modifying the bacterial response of the rhizosphere. In other cultivars, differential allocation of translocatable carbon may even promote higher grain yield in preference to root processes and eventual methane production and emission.
The reported effects of different mineral fertilizer applications on methane emission are inconsistent. SchÄtz et al. (1989) concluded that the type and method of application strongly influenced methane emission rates. Lindau et al. (1991) observed increased methane emissions with increased urea application. Cicerone and Shetter (1981) reported large increases in emission after fertilization with ammonium sulfate while other studies (SchÄtz et al. 1989, Yagi and Minami 1990) show a decrease. Lindau et al. (1990a, 1990b, 1991, 1993) found significantly different rates of methane emission for a variety of fertilizer types and treatments (urea, ammonium sulfate, potassium nitrate). Others have found that methane emission rates are affected by the method of fertilizer application (SchÄtz et al. 1989, Kimura et al. 1992). Many other studies agree that the application of organic matter to rice paddies strongly increases methane emission rates over that from mineral fertilization. Emission rates are dependent on amount, kind, and prior treatment of the organic components (Sass et al. 1991, Chen et al. 1993, Lindau and Bollich 1993, Wassmann et al 1993, Yagi and Minami 1993, Neue et al 1994).
Current research efforts clearly indicate that realizable options are available to mitigate methane emissions from flooded rice fields. Successful implementation of these options will depend upon the collective acceptance by the rice farmers of Asia and the rest of the world. In order for that to happen, research results must be able to demonstrate that: (1) grain yield will not be decreased and may increase by a particular mitigation practice, (2) that by adopting recommended mitigation practices the farmer will benefit through better water utilization, reduction of labor, or a decrease in production costs, and (3) the rice cultivars that lead to reduced methane emission are those desired by local consumers.
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