Environ. Sci. Technol. 1991, 25, 979-981
Methane Emissions from Rice Fields in China M. A. K. Khalil" and R. A. Rasmussen
Center for Atmospheric Studies, Oregon Graduate Institute, Beaverton, Oregon 97006 Ming-Xing Wang and Lixin Ren
Institute of Atmospheric Physics, Academia Sinica, Beijing, People's Republic of China Methane emissions from rice fields in China are found to be 4-10 times higher than emission rates from rice fields in the United States and Europe. Average emission rates during the growing season were -60 mg m-2 h-l from rice fields a t TuZu in the Sczhuan Province of China. These results show that rice fields are a major source of methane to the atmosphere. 1. Introduction For more than 25 years it has been recognized that rice fields are a large source of atmospheric methane ( I ) . During the last 10 years every global budget of methane has included the world's rice paddies as a major source. The interest in emissions from rice fields intensified when it was discovered that methane is increasing in the atmosphere and could contribute to global climatic change. With increasing population, the area under rice fields has nearly tripled during the last century, making rice agriculture a potentially major cause of increasing methane (2-4). The first systematic studies of direct flux measurements from rice fields, over the entire growing season, were undertaken only recently in United States and Europe (5-8). These studies showed fairly low emission rates, leading to a downward revision of the global estimates of methane from rice fields (9-11)- The most remarkable result of our present work is that the seasonal average fluxes of methane from Chinese rice fields that we studied are 4-10 times higher than from the European and American fields. Since about a third of the world's rice is grown in China, extrapolation of our results shows that up to 50 Tg/year of methane may be emitted from China alone. Global emission rates of around 100 Tg/year, out of total of -550 Tg/year from all sources (20%) are therefore justified (see, for example, ref 12). Our studies also show that the flux of methane is greatly influenced by soil temperature (Qlo -3) and that there is a pronounced seasonal variation of methane flux in which peak emissions occur after the middle part of the growing season and then fall rapidly after the rice plants have matured.
2. Experiments a n d Methods Our experiments were conducted at TuZu, a rural village some 170 km from Chengdu in the heart of the ricegrowing area of Sczhuan province in China. In the fields we studied, organic fertilizer was used, which is usually fermented sludge from the biogas generators and is applied before the rice is planted (see also ref 13). The rice fields were irrigated to maintain standing water. Common varieties of rice (regular and hybrid) were planted, farmed, and harvested by the local farmers according to age-old practices. We believe that because our methods do not interfere with the agricultural practices, our flux measurements are representative of rice fields, a t least in the Chengdu region. We chose four rice fields and six sites in each field. A permanent aluminum base was installed in the soil around the time when the rice was first planted. During the 0013-936X/91/0925-0979$02.50/0
growing season we took flux measurement a t these 24 sites about every other day. Each flux experiment consisted of placing a rigid polyethylene chamber gently onto the aluminum base and drawing samples of air from within the chamber after 3 , 6 , 9 , and 12 min. The chamber fitted onto a groove in the aluminum base. The groove was filled with paddy water so that there was no exchange of air. Samples were drawn in glass syringes and analyzed either the same day or the next day, using a robust field gas chromatograph equipped with a flame ionization detector optimized to measure only methane (GC/FID; see ref 14). We took great pains to avoid disturbing the spongy paddy soil so as not to cause spurious releases of methane. The permanent base ensured that the chamber would not touch the soil. The plots were chosen near the edge of the field so that no one had to walk into the paddy. We took four measurements in each experiment to be able to detect cases where the soil may have been disturbed. Such disturbances are reflected in the variability of the accumulation rates. About 15% of the measurements showed some sign of disturbance and were eliminated, which slightly reduced the estimated seasonal average flux but did not change the main conclusions. These cases occurred mostly toward the end of the growing season, when the rice plants were big and filled the chambers. 3. Results The rate of increase of methane dC/dt in the chamber, here estimated by nonparametric statistical methods, is related to the flux by the following equation:
where M is the molecular weight of methane, No is Avogadro's number, po is the density of air, and A is the surface area covered by the chamber (-560 cmz). The volume V into which methane accumulated was corrected for the changing depth of the standing water in the rice field. We used chambers with volumes of 5,10,15, and 20 gal. The smaller chambers were used when the plants were small and the larger chambers were used as the rice grew. Measurements of soil temperatures were taken -5 cm below the soil surface. Over the 2 years (1988, 1989) some 13000 individual measurements of methane were taken, resulting in -3000 flux estimates. The composite results are shown in Figure 1. From these measurements the median flux is -50 mg m-2 h-l. Ninety-five percent of the fluxes are greater than 10 mg m-2 h-' and 5% are greater than 120 mg m-2 h-l. This range, however, does not reflect the precision (of the mean), which is
