Measurements and influencing factors

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. D13, PAGES 17,231-17,242, JULY 16, 2000 Methane emission from rice fields in China: Measurements and influencing factors Z. C. Cai Institute of Soil Science, Chinese Academy of Sciences, Nanjing H. Tsuruta and K. Minami National Institute of Agro-Environmental Sciences, Kannondai, Tsukuba, Japan Abstract. Methane emissions from rice fields in China were measured at eight sites in five provinces under conditions representative of local practices for rice cultivation. Methane emission rates during the rice growth period varied greatly from site to site and with treatments at the same site, ranging from 0.3 to 205 g CH4/m 2. Flooded or waterlogged rice fields in the nonrice growth season continuously emitted CH4 substantially. The average CH4 emission rate from a rice field in Chongqing was as high as 36.2 g CH4/m 2 in the nonrice growing season. Furthermore, flooding the nonrice growth season also significantly stimulated CH4 emission during the rice growth period in the next year. Increases in the rate of CH4 flux after rice transplanting were less when the number of consecutive upland crops grown before rice transplanting was greater. CH4 emissions from rice fields located on downslope was larger than from those on midslope and upslope in hilly areas due to poor drainage of the former. Application of rice straw in fall when winter wheat was sown did not increase CH4 emission significantly during the following rice growth period. CH4 emission was depressed by the application of ammonium sulfate but was, in general, not significantly affected by urea application. 1. Introduction China is an important rice production country with 22.6% of the world rice harvested area and 36.3% production of the world rice grain in 1990 [International Rice Research Institute, 1991]. Early field measurements reported by Khali! e! al. [1991] revealed that CH 4 emissions from Chinese rice fields were significantly higher than those elsewhere in the world. It was estimated that the CH 4 emission from the rice fields in China was as high as around 30 Tg CH 4 [Khali! e! al., 1991]. Sass [1994] estimated that CH 4 emission from the rice fields in China accounted for 37.6% of the total from wetland rice fields in the world. With the accumulation of field measured data, it was found that not all rice fields in China emitted CH 4 as large as those from the earlier studies. The estimates made recently showed emissions from 8 to 15 Tg CH4/yr [Yao and Zhuang, 1996; Cai, 1997; Kern e! al., 1997]. However, the factors driving the large CH4 emissions reported in earlier studies have not been broughto light. Rice production now exists in all of China except for Qinghai Province, from the tropical south to the cold-temperate north, and from the coastal plain in the east to the desert oasis in the west. As for topographical conditions, rice is cultivated not only in marshy depressions and coastal lowlands approaching sea level but also on the plateau in the southwest at an elevation above 2700 m. The rice cropping areas in China are regionalized into six regions (Figure 1) and the harvested area of each region in 1993 are as follows: region I, northeast China, early maturing and single rice cropping region (1.647 Mha); region II, northwest Copyright 2000 by the American Geophysical Union. Paper number 2000JD /00/2000JD China, single rice cropping (0.372 Mha); region III, north China, single rice cropping (0.755 Mha); region IV, southwestern plateau, single and double rice cropping (4.69 Mha); region V, central China, double and single rice cropping ( Mha); region VI, south China, double rice cropping (6.822 Mha). The climate, soil properties, crop rotation, and practices for rice cultivation are very different in different rice cropping areas. Thus all which have been demonstrated to influence CH 4 emissions from flooded rice fields could play a role to a different extent in CH 4 emission from the rice fields in China. Methane emissions from flooded rice fields are affected by many factors such as water regime [ Yagi and Minami, 1990; Sass et al., 1992; Chen et al., 1993; Cai et al., 1994; Yagi et al., 1997], fertilization [Minami, 1995; Banik et al., 1996], rice cultivar [Parashar e! al., 1991; Watanab e! al., 1995], plant density [Schutz e! al., 1989], soil properties related with soil Eh status and with biological activities [Rajagopal et al., 1988; Yagi and Minami, 1990]. The purpose of the present research was to unmask the determining factors controlling CH 4 emission from rice fields in China by measuring CH 4 emission under conditions as closely representative as possible of local practices for rice cultivation. Therefore cn 4 emission measurements were carried out at eight sites in five provinces of China (latitude from 23ø15'N to 35ø24'N and longitude from 106ø18'E to 121ø12'E) from 1993 to Materials and Methods 2.1. Observation Sites Methane emissions were measured in rice fields at eight sites: one site in the south China region with double rice cropping, five sites in the central China region with double and single rice cropping, one in the southwestern plateau region with single and double rice cropping, and one in the north China region with 17,231

2 17,232 CAI ET AL.: METHANE EMISSION FROM RICE FIELDS IN CHINA.70 75' 8i O Figure 1. Rice cultivation regions [China National Rice Research Institute, 1988] and the sites where observations were carried out in this experiment. Region I, northeast China early maturing and single rice cropping region; region II, northwest China single rice crop; region III, north China single rice cropping region; region IV, southwestern plateau region of single and double rice cropping region; region V, central China double and single rice cropping region; region VI, south China double rice cropping region cropping region in dry region; region VII, nonrice growing region. Solid circles, observation site: 1, Guangzhou; 2, Yingtan; 3, Changsha; 4, Chongqing; 5, Suzhou; 6, Jurong; 7, Nanjing; 8, Fengqiu. single rice cropping (Figure 1). The treatments at each site were week to 10 days. This was the same in other sites). G-Cont: the designed so that the measured CH 4 emissions from the treatments plot was flooded continuously from the start of the experiment were representative of local farmers' practices. The crop rotation, soil type, and properties of each site are listed in Table 1. The treatments of each site are described below briefly. in 1994 to the end of the experiment in 1995 and the crop rotation was double rice crops and fallow in winter. Other practices were the same as those adopted in G-Routine. G-Early: Guangzhou, Guangdong province (23ø15'N, single early rice crop plus two upland crops a year. G-Late: 113ø6'E). This site belongs to the south China region with single late rice crop plus two upland crops a year. G-Turn: only double rice cropping (Figure 1). However, with economic upland crops in 1994 and routine crop rotation as in G-Routine development, rice harvested area is being reduced in the region [Cai, 1997]. The rice harvested area has decreased recently because two rice crops a year have been replaced by either a single early rice crop or a single late rice crop a year and by planting rice crops every year with 1 year rice crops and 1 year in During the rice growth period, all practices in the treatments of G-Early, G-Late, and G-Turn were the same as for G-Routine. The experiment was conducted in a rice plot at the Experimental Farm of China South Agricultural University divided into five subplots. upland crops in the suburbs of Guangzhou city. Under single Yingtan, Jiangxi province (28ø12'N, 117ø6'E). rice crop rotation, upland vegetable crops are grown in the This is a hilly area and rice fields are located in sideslopes and nonrice season. Although this change was limited to the suburbs, downslopes. Topographical effects on CH 4 emission were the effect of the change on CH 4 emission during the rice growing period was also investigated to explore the trend of CH4 investigated in the region to quantify CH 4 emission from rice fields in hilly areas. The experiment was carried out in farmers' emission caused by the change of crop rotation. Five treatments rice fields located at upslope(y-up), midslope (Y-Mid), and were designed as follows (Table 2). G-Routine: local routine practices were followed, that is, the crop rotation was double rice crops and a winter upland crop with a water regime during the rice growth period of multiple midseason aeration (usually downslope (Y-Down) in 1993 and 1994 (Table 2). Double rice cropping and fallow in winter and multiple midseason aeration were practiced as by local farmers. Because the interval between early rice harvesting and late rice transplanting is short, farmers drained two to three times with the length of each drainage do not have sufficient time to collect early rice straw completely period depending upon the weather, usually varying from 1 and usually incorporate the rest of the rice straw directly into the

3 CAI ET AL.: METHANE EMISSION FROM RICE FIELDS IN CHINA 17,233,, I Z Z t--... Z Z ZZZ Z Z o6o0 m m Z Z ZZZ m Z Z Z Z Z Z Z zzz ' = ' o

4 17,234 CAI ET AL.: METHANE EMISSION FROM RICE FIELDS IN CHINA Table 2. Summary of Field Treatments and Mean CH4 Flux Over One Rice Crop Season and Total Emission Rate Throughout Rice Crop Season(s) From Flooded Rice Fields in Eight Sites of China Treatment Crop Rotation Rice Growing Fertilization* Water Management Flux œ, Total õ, Period mg CH4/m2/h g CH4/m 2 Guangzhou 1994 G-Cont Early/late rice/ Early rice, April Urea, 306 kg/ha; Annually flooding. 3.81/ fallow. 15 to July 14 superphosphate, 576kg/ha; G-Routine Early/late rice/ Late rice, KCI, 238kg/ha. No organic Multiple aeration 0.28/ vegetable. Aug. 4 to manure. during rice G-Early Early rice/ Nov. 11. growth period; 0.22/- 0.5 vegetables. drainage at G-Late Vegetables/late interval of rice -/ rice. crops. G-Turn Vegetables Drainage whole whole year. year. Guangzhou 1995 G-Cont Same as Similar to Similar to previous year. Annually flooding. G-Routine previous previous year. Multiple aeration G-Early year. during rice G-Late growth period; G-Turn Early/late rice/ drainage at vegetable. interval of rice crops. Iqngtan 1993 Y-Up Y-Mid Y-Down Early/late rice. May 10 to July 28; July 30 to Nov. 6; fallow Urea, 263 kg/ha; Superphosphate, 375 kg/ha; KCI, 150kg/ha; rice straw Multiple aeration during rice growth period; in winter before late rice transplanting, drainage at season kg/ha (fresh). interval of rice Y-Up Y-Mid Y-Down Early/late rice. May 6 to July 15; July 28-Oct. 25; fallow in winter season. N-CK Single rice/ May 30 to Sept. fallow in 19. N-S 100 N-S300 N-U100 N-U300 winter season, S-CK Wheat/rice. June 21 to Oct. S-Min 28. S-Org S-Inh S-Flood J-CK Wheat/rice. June 19 to Oct. 2. J-S100 J-S200 J-S300 J-N200 Yingtan 1994 Same as previous year. Nanting 1994 Superphosphate, 525 kg/ha; KC1, 75 kg/ha. No N. Same as N-CK except 100 kg N/ha as (NH4)2804 Same as N-CK except 300 kg N/ha as (NH4)2804 Same as N-CK except 100 kg N/ha as urea Same as N-CK except 300 kg N/ha as urea No fertilizers. Suzhou kg N/ha. S-Min+ 15t/ha pig manure. S-Min+nitrification inhibitor. Same as S-Min durong 1995 Superphospate, 525 kg/ha; KC1, 75 kg/ha; rice straw, 1.5 t/ha applied in fall; No N. Same as CK except 100 kg N/ha Same as CK except 200 kg N/ha Same as CK except 300 kg N/ha Same as CK except 200 kg N/ha and no rice straw. crops. Same as previous year. Multiple aeration during the rice growth period; drainage at interval of rice crops. Multiple aeration during the rice growth period. Continuously flooding. Multiple aeration during the rice growth period and well drainage during wheat growth season / / / / / / / / / soil when fields are prepared for late rice transplanting. In this experiment, early rice straw was also incorporated into the soil at a rate of 7125 kg/ha (fresh straw) before transplanting the late rice crop. Chemical fertilizers were applied for each rice crop at rates of 375 kg urea, kg superphosphate, and 150 kg KC1 per hectare, of which 70% was applied as a basal and 30% as a single topdressing. Before carrying out observations in 1993, the owner of the midplot land had applied pig manure at a rate of about 7000 kg/ha Changsha, Hunan province (28ø9'N, 113ø6'E). In Hunan province, rice fields are typically double rice cropped and managed similarly during the rice growing periods but are used and managed differently in the nonrice growth season. There are four main kinds of utilization of rice fields in the nonrice growth season. They are (1) fallow under drained conditions (accounting for 17.4% of total area), (2) planted with Chinese milk vetch (38.7%) that is used as green manure next year, (3) planted with oil-seed rape (35.5%) that is used as green manure as well, and

5 CAI ET AL.' METHANE EMISSION FROM RICE FIELDS IN CHINA 17,235 Table 2. (continued) Treatment Crop Rotation Rice Growing Fertilization* Water Management Period J-CK J-S100 J-S200 J-S300 J-N200 F-S F-L F-C F-S F-L F-C C-Fallow C-GM Wheat/rice. Wheat/rice. Wheat/rice. Early/late rice/ fallow. Rice/rice/green manure. lurong 1997 June 21 to Oct. Same as in Similar to previous year. 11. June 28 to Oct. 14. June 21 to Oct. 7. May 6 to July 17; July 24 to Oct. 20. Fengqiu 1993 N, 292 kg/ha; Multiple aeration during superphosphate, 750 the rice growth period kg/ha; pig manure, 4.5 and well drainage t/ha. during wheat growth œengqiu 1994 N, kg/ha; superphosphate, 750 kg/ha; pig manure, 5.0 t/ha. Changsha 1995 Weeds, 5.25 t/ha. N,-86 kg/ha; KC1, 150 kg/ha. Green manure, 3.75 t/ha. season. Similar to previous year. Multiple aeration during the rice growth period and well drainage during winter. Flux œ, Total õ, mg CH4/m2/h g CI-I4/m O / / Changsha 1996 C-Fallow Same as previo- May 4 to July 2 25 t/ha Multiple aeration during 18.12/ C-GM us year. 21; July t/ha the r ce growth period 15.89/ C-Rape R ce/rice/rape. to Oct 17. Rape, 26.9 t/ha. and well drainage 54 57/ during winter. C-Flood Rice/rice/fallow Flooding in winter Changsha 1997 C-Fallow Same as previo- May 4 to July 4.5 t/ha Same as previous year / C-GM us year 21; July t/ha 9 37/ C-Rape to Oct t/ha 12.88/ C-Flood 29 73/ Chongqing 1995 Ch-FF Rice May 15 to Urea, 273 kg/ha; Flooding fallow Ch-FFR Rice Aug. 23 superphosphate, Ch-Wheat R ce/wheat. ' kg/ha; KC1, 150 g/ha. Winter drainage Chongqing 1996 Ch-FF Same as previo- May 23 to Sept. Chemical fertilizers were Same as previous year Ch-FFR us year. 2. as same as previous Ch-Wheat year; human manure, Ch-RW R ce/wheat 20 t/ha. Winter drainage Chongqing 1997 Ch-FF Same as May 10 to Aug Same as in Same as previous year Ch-FFR prewous Ch-Wheat year Ch-RW * Application rates shown in the table were for one season of rice crop. œ 2 Mean flux over one rice crop season (rag CH4/m/h). The values before and after the slant were the mean fluxes of early rice and late n. ce crop seasons, respectively, n the regions w th double rice crops. The values w thout slant were the mean flux in the regions with a single rice crop. Total was the annual CH4 emission rate durin g the rice growth period(s) (g CH4/m2). l BeIbre the experiment, the owner of the lan had applied pig manure at a rate of about 7.0 t/ha. (4) fallow under flooded conditions (8.4%). To explore the effect of land management in the nonrice growth season on CH4 emission during the rice growth period, four treatments were designed (Table 2): winter-fallow under drained conditions (C-Fallow), winter-green manure (C-GM), winter- fallow under flooded conditions (C-Flood), and winter-oil-seed rape (C-Rape). The study was conducted at the Experimental Farm of Hunan Agricultural University. CH 4 emissions during the rice growth period were measured for two treatments (C-Fallow and C-GM) in 1995 and for all four treatments in 1996 and Green manure (Chinese milk vetch) and oil-seed rape were incorporated into the soil where produced when the fields were prepared for early rice transplanting at rates from 3.75 to 12.6 t/ha for green manure and from 10.9 to 26.9 t/ha for oil-seed rape (Table 2). Similarly, the large amount of weeds ( t/ha) that was produced in the treatment of C-Fallow was also incorporated into the soil during preparation for early rice transplanting. Few weeds were produced in the flooded plot (C- Flood). The typical water regime of the region is that rice fields are flooded from the time of land preparation for rice transplanting to the tillering stage. Afterwards and until rice harvesting, so-called wet irrigation is commonly employed, which means the fields are irrigated to water saturation without formation of a floodwater layer. This water regime was adopted for all treatments Chongqing, Chongqing municipality (29ø48'N, 106ø18'E). Chongqing is a mountainous area where most rice fields are located at the foot of the mountains. About half of the rice fields are flooded permanently even after rice harvesting. The crop system is a single rice crop and fallow in winter. An innovative approach to manage permanently flooded rice fields is

6 17,236 CAI ET AL.: METHANE EMISSION FROM RICE FIELDS IN CHINA the construction of fixed ridges of about 30 cm wide, planted with a rice crop and winter wheat on both sides of the ridge under free tillage instead of a single rice crop a year [Xie, 1988]. Our study was carried out at the Experimental Farm of China Southwest Agricultural University. Four treatments were studied (Table 2): a routinely managed rice field, that is, plain cultivation, permanently flooded with a single rice crop and fallow in winter (Ch-FF); a ridged rice field, continuously filled with water in ditches between ridges, planted with a single rice crop and fallow in winter (Ch-FFR); a ridged rice field planted with a rice crop and winter wheat (Ch-Wheat) in which ditches were filled with water during the rice growing period similar to Ch-FFR, but drained during the winter wheat growing season; and a plain-cultivated rice field planted with a rice crop and winter wheat (Ch-RW) and continuously flooded during the rice growth period but drained during the winter wheat growth period. Typically, at harvest, rice plants are cut off about 30 cm above the ground and the stems stand until incorporation into the soil in the next year when fields are ploughed. No additional organic manure was added except in 1996 when rice transplanting, each plot was sprayed with nonfermented human excrement at a rate of about 20 t/ha Nanjing, Jiangsu province (31ø58'N, 118ø48'E). This is a region where lands are intensively used and the practices of rice cultivation and land use are relatively homogenous. The crop system is dominated by a single rice crop and a winter upland crop (dominant crops are winter wheat and oil-seed rape). Unlike Changsha, in Nanjing oil-seed rape is harvested to produce food oil. Chemical fertilizers are intensively applied at normal rates of kg N/ha for each crop and organic manure is seldom applied for rice cultivation. Rice fields are multiply aerated during the rice growing period and are well drained during the winter crop season. In this experiment the effect of nitrogen form and application rate on CH4 emission from rice fields was investigated (Table 2). Five treatments were studied in Nitrogen application rates were 0, 100, and 300 kg N/ha in the form of urea or ammonium sulphate, referred to as N-CK, N-U 100, N-U300, N-S 100, and N-S300, respectively. The detailed description of the treatments is given by Cai et al. [1997] Jurong, Jiangsu province (31ø56'N, 119ø9'E). This is a hilly area in the same region as Nanjing and the agricultural practices and water regimes are similar to those of Nanjing. The experiment was conducted at the Experimental Farm of Jurong Agricultural College which is located at the top of a moderate hillslope. The effects of rice straw amendment and N application rate on CH 4 emission were investigated (Table 2). The treatments included no chemical N application (as control, J-CK), 100 (J-S100), 200 (J-S200), and 300 kg N/ha (J-S300) with rice straw applied to the soil surface at a rate of 1500 kg/ha after winter wheat was sown in fall and 200 kg N/ha without rice straw (J-N200). The measurements were carried out in 3 years (1995 through 1997) Suzhou, Jiangsu province (31ø18'N, 121ø12'N). This belongs to the same rice cultivation region as Jurong and Nanjing, and the crop system, water regime, and fertilization are substantially identical. But it is a plain landscape and soil fertility is usually higher than that in Jurong and Nanjing (Table 1). Five treatments were designed in Suzhou (Table 2); no fertilizer application (S-CK), chemical fertilizers (S-Min), chemical fertilizers at the same rate as S-Min plus pig manure (S-Org), chemical fertilizer plus nitrification inhibitor (thiourea, S-Inh), and chemical fertilizers as with S-Min but continuously flooded (S-Flood) during the rice growing period. The fertilizer application rates and water management details are given by Cai et al. [ 1994] Fengqiu, Henan province (35ø24'N, 114ø24'E). Our experiment was conducted at the Experimental Farm of Fengqiu Ecological Experimental Station, Chinese Academy of Sciences, which is located in the Huang-Huai-Hai Plain. Soils in the area are derived from the alluvium of the Yellow River. Due to multiple depositions in a long history, there are large spatial variations of soil texture in the horizontal and with depth. To investigate the effect of soil texture on CH4 emission, the measurements were conducted in rice fields with soils of sandy (F-S), loam (F-L), and clay texture (F-C). The measurements were carried out in 1993 and 1994 (Table 2). More details of the treatments and the management of the rice fields are given by Cai et al. [ 1999] Sampling Program and Analytical Methods Methane fluxes were usually measured twice a week (every 3-4 days) throughouthe rice growth period using a closed chamber method as described by Yagi and Minami [1991]. Methane fluxes were also measured at intervals of about 2 weeks in the nonrice growing season when the fields were flooded in Chongqing. Sampling chambers were made of Plexiglas with a size of 5 l x5 l x 100 (height) cm. In Chongqing, chambers with a height of 50 cm were used when rice plants were small and in the nonrice growing season. For each site, except Nanjing and Jurong, one gas sample was collected from each type of treatment. At Nanjing and Jurong, three gas samples were collected from three replicate plots of each type of treatment. Gas samples were usually collected in the morning ( ) or the afternoon ( ) or both, mostly finished in 1 hour except in Nanjing and Jurong where it took 3 to 4 hours to finish the sample collection. To minimize the introduction of biases from diurnal variation of CH 4 flux during collection, the gas samples in Nanjing and Jurong were collected in three passes where each pass involved collection of one gas sample from each type of treatment. More details of the sampling schedule are given by Cai et al. [ 1994, 1997, 1999]. Chambers were placed on a fixed frame in each plot, and gas samples were collected with a syringe and transferred immediately to evacuated vial bottles stopped with butyl rubber septa. Methane concentration in the gas samples was determined with a gas chromatograph/flame ionization detector in the Laboratory of Material Cycling in Pedosphere, Institute of Soil Science, Chinese Academy of Science within 6 months. Pretest showed that the samples in the bottles were stable for at least 1 year. The standard gases were cross checked with the laboratory in the Japan National Institute of Agro-Environmental Sciences. Methane fluxes were calculated from the concentration changes in the chamber with time at 0, 10, 20, and 30 min after the chamber was placed on the fixed frame, using the equation F = 0.714Sh(273/(273+T)) where F is CH4 flux in mg CH4/m2/h, S is the linear increase of CH 4 concentration increased with time, h is the available height of the chamber, and T is the absolute temperature. The mean flux over one season of rice crop was the average of the measured fluxes weighted by the intervals between two measurements. The total CH 4 emission over one season of rice crop was the product of the mean flux and the duration of the rice growing period. The annual CH 4 emission rate during the rice growing period(s) was the sum of CH 4 emissions from either two seasons of double rice

7 CAI ET AL.' METHANE EMISSION FROM RICE FIELDS IN CHINA 17,237 crops (double rice cropping areas) or one season of single rice crop (single rice cropping areas) in a unit area (m2). Soil properties were determined following standard procedures. Among them, amorphous Fe (Feo) was extracted with ammonium acetate (ph3.2), available P (Olson-P) was extracted with 0.5 mol/l NaHCO3, available K was extracted with NH4OAc, and soil ph was measured at a soil to water ratio of 1: Results and Discussion 0.0 Mean CH4 fluxes during the rice growth period are shown in Table 2. Since this experiment included single rice crop and double rice crops a year, the duration of the rice growth season varied from less than 80 days to 160 days. For comparison of CH 4 emission rate [g CH4/m 2] CH 4 emission during the rice growing season(s), both mean CH 4 Figure 2. Frequency distribution of rice season CH4 emission fluxes (in mg CH4/m2/h) throughout one rice growing period and rate from the rice fields in China. rice season (single or double rice) emission rates (in g CH4/m 2) are listed in Table 2. The mean CH4 flux over one period of rice growth varied greatly from site to site and among treatments in the same site, ranging from <1 to CH4/m2/h. There was also a very large variation of rice season CH4 emission rates in irrigation, compared to continuously flooded fields [Yagi and Minami, 1990; Sass et al., 1992; Chen et al., 1993; Cai et al., 1994; Bronson e! al., 1997; Yagi e! al., 1997]. The suppressive China, ranging from 0.3 to 205 g CH4/m 2. The lowest emission effect of multiple midseason aeration on CH 4 emission is also rate was observed for single late rice (G-Late) in Guangzhou and demonstrated by the results of this experiment (Table 2). In the highest was observed for downslope field (Y-Down) in Suzhou the mean CH 4 flux from the treatment of S-Min, which Yingtan. However, the emission rates were concentrated in the was imposed by intermittent irrigation, was 3.2 mg CH4/m2/h, low end of the range. A rate less than 10 g CH4/m 2 accounted for only 48.5% of that from S-Flood (6.22 mg CH4/m2/h), which 44% of the total measurements, between g CH4/m 2 for 10% and more than 100 g CH4/m 2 accounted for less than 10%. The frequency distribution of CH 4 emission followed a regression equation of y= In(x)-0.079, R 2= (p<0.01) where y was the frequency and x was the CH 4 emission rate (Figure 2). The arithmetic mean CH4 emission rate of all measurements was 35.9 g CH4/m 2 and the median value was was continuously flooded during the rice growth period. The effect of multiple midseason aeration on CH4 flux was even more significant in Guangzhou. Before the start of the experiment in 1994, the rice field was homogenous, which was demonstrated by the similar mean CH4 fluxes from G-Routine and G-Early during the early rice growing period in the first year (1994). Compared to the mean CH4 flux from the continuously 16.3 g CH4/m 2. The CH4 emission rates from Japanese rice fields flooded treatment of C-Cont, the fluxes from G-Routine and G- were reported to range from 13.4 to 20.8 g CH4/m 2 with an average of 18.1 g CH4/m 2 [Soil Science Society of Japan, 1996]. Parashar e! al. [1994] reported that CH4 emission rates from Early were reduced to 7.3% and 5.8% under multiple midseason aeration conditions during the early rice growth period in 1994 (Table 2). The results indicate that the mean CH4 flux over one Indian rice fields were from 8 to 44 g CH4/m 2 for waterlogged period of rice growth could be reduced by multiple midseason (flooded) fields and from 0.1 to 2.1 g CH4/m 2 for intermittently aeration, but the extent of the effect varied from site to site. flooded irrigated fields. Compared to these emission rates, the CH4 emission rates from the rice fields in China were characterized by larger variation and a higher mean rate Effect of Water Regime in the Nonrice Growing Season on CH4 Emissions in the Next Rice Crop The CH4 emission from the same rice field varied greatly Our study shows that the water regime during not only the among treatments as well. For instance, the rice season CH4 rice but also the nonrice growing period plays an important role emission rate varied from 0.3 to 101 g CH4/m 2 in Guangzhou, in CH4 emission during the rice growing period. Before the from 49.8 to 205 g CH4/m 2 in Yingtan and 27.4 to 128 g CH4/m 2 transplanting of early rice in 1995, the number of consecutive in Changsha (Table 2). No regression relationships were found upland crops was one in G-Routine, two in G-Early, and three in between soil organic matter content, total N, soil ph, amorphous G-Turn at the Guangzhou site. The results in Figure 3 show that Fe, Mn oxides, and the rice season CH4 emission. The rice yields the increasing rate of CH4 fluxes after the early rice transplanting ranged widely from 3.5 to 8.0 t/ha in this experiment and were was suppressed more when more consecutive upland crops were not related to the mean CH 4 fluxes either using all data at the grown under the same water regime and fertilization conditions eight measurement sites or using data from the individual sites. during the rice growing period. The mean fluxes during the early Management practice such as water regime both during the rice rice growing period decreased from 2.63 mg CH4/m2/h in the and non-rice growing periods and organic material amendment treatment of G-Routine to 0.72 mg CH4/m2/h in G-Early and to seem to play more important roles in controlling CH 4 emission 0.21 mg CH4/m2/h in G-Turn in 1995 (Table 2). rate from the rice fields in China during the rice growth period. Similar results were observed in the late rice crop season in 1994 and Late rice crops in G-Routine and G-Turn were 3.1. Effect of Water Regime in the Rice Growing Season transplanted following an early rice crop but in G-Late following on CH 4 Emission upland crops. The mean CH 4 fluxes during the late rice growth The effect of the water regime of rice fields on CH4 emission has been studied intensively and has mainly focused on the period of rice growth. Generally speaking, mean CH4 fluxes over one rice growing period are reduced by intermittent period were much lower from G-Late than those from G-Routine and G-Turn under the same treatments in the late rice growing season (Table 2). The results suggest that before rice transplanting, the longer a soil is under drainage conditions, the I.079 R2 =

8 17,238 CAI ET AL.: METHANE EMISSION FROM RICE FIELDS IN CHINA smaller the mean CH 4 flux is during the following rice growing period. At the site of Changsha, the water regime was designed to be the same during the rice growth period, but different in the nonrice growing season. On average, the rice season CH4 emission rate was the highest from C-Flood ( g CH4/m2), which was flooded in the nonrice season but not amended with organic materials, followed by C-Fallow period but also produces and emits CH4 in the nonrice growing season (Figure 4). The mean CH4 flux during the nonrice growth period measured in Chongqing was 5.50 mg CH4/m2/h, which was equivalent to 36.2 g CH4/m 2 for the season. This rate was much higher than those measured during the rice growth period in Nanjing, Suzhou, Jurong, and Fengqiu (Table 2). In China the area of rice fields flooded in the nonrice growth season varied from 2.7 to 4.0 Mha, distributed mainly in south and southwest ( g CH4/m2), which was well drained the nonrice China [Lee, 1992] where soil temperature is high enough for growing season but amended with weeds, and the lowest from the treatments amended with green manure (C-GM and C-Rape, g CH4/m2). Under annually flooded conditions (G- Cont) in Guangzhou, the CH 4 emission significantly increased with flooding time. The mean flux was high up to 32.8 mg CH4/m2/h during the late rice growth period in 1995 (Table 2) and 235 times higher than the corresponding mean fluxes from G-Late, in which two consecutive upland crops had grown before methanogenesis. For example, the mean soil temperature at 5 cm in Chongqing was 17.3øC in the nonrice growth season in 1995/1996 and 15.9øC in 1996/1997. Correlation analysis shows a significant relationship between CH 4 flux and soil temperature in Chongqing during the nonrice growth season in 1995/1996 (r = , p< 0.01 for Ch-FF and r = , p<0.01 for Ch-FFR, respectively), but not significant in 1996/1997 (r = for Ch-FF and r = , for Ch-FFR, respectively). the transplanting of late rice in The effect of water regime between two crop seasons on methanogenesis and CH 4 emission was also demonstrated by pot 3.4. Effect of Water Regime as Influenced by Topography on CH 4 Emission experiments reported by Trolldenier [1995] and Cai and Xu The results obtained from Yingtan showed that the rice season [1998]. After flooding, the Eh of the soils which were under well CH4 emission rates were higher from the downslope plot than drained conditions during the nonrice growth period, decreased from the midslope and upslope plots (Table 2). Because gradually and was a key factor in controlling CH4 fluxes during depressive topography leads to invasion by side leaching water, the following rice growth period. The soil that was flooded the rice field at the downslope was waterlogged. Midseason during the nonrice growing period was already reduced when aeration was also practiced at the downslope plot during the rice rice was transplanted and thus Eh was low and not a controlling growth period as was done at Y-Mid and Y-Up plots, but it was factor for CH 4 production [Cai and Xu, 1998]. The pot difficult for the soil to be really aerated. Ishibashi et al. [1997] experiment conducted by Cai and Xu [1998] showed that the reported that in waterlogged paddy fields a short-term aeration mean CH 4 flux during the rice growth period was mg was less effective in depressing CH 4 emission. During the CH4/m2/h from the well drained treatment, while it was nonrice growth period, the field was drained freely, but it was, in mg CH4/m2/h from the treatment floodeduring the fact, still poorly drained and water saturated or even flooded nonrice growing period. Flooding in the nonrice growth season most of the time due to invasion by side leaching water. As was also favorable for methanogenetic activity during the rice discussed above, this was the reason that the CH 4 emission was growth period [Trolldenier, 1995]. Analysis of data available on higher from the downslope plot than from the midslope and CH 4 fluxes supports the hypothesis that permanent flooding was upslope plots. In fact, the rice season CH 4 emission rate from Y- an important factor leading to high CH 4 fluxes from some rice Down in 1993 was the largest among all measurements (Table 2). fields in China [Cai, 1997]. The location of rice fields at the foot of the mountains in 3.3. Effect of Water Regime in the Nonrice Growing Season: Chongqing is an important reason for the large CH 4 emission Direct Emissions during the rice growth period. Following draining and winter wheat planting, CH 4 emission Flooding of rice fields in the non-rice growing seasonot only from the rice fields in Chongqing was greatly decreaseduring stimulates CH 4 emission during the following rice growing the next rice growth season in 1995 [Cai et al., 1998]. However, the effect of drainage was not observed in the following 2 years 18 in the same plots (Table 2). Due to the depressive topography, the 16 soil was not easily aerated even under drainage conditions and soil moisture depended mainly on the precipitation during the 14 season. It might be that CH4 emissions could be significantly 12 reduce during the following rice growth period if the soil was lo G-Routine dried to a certain extent in the nonrice growth season. Unfortunately, we did not measure soil moisture in this G Early 6 experiment. Ridge cultivation in the rice fields in Chongqing did G-Turn not change CH4 emission (Table 2), contrary to the results 4 obtained by Liet al. [1993]. They found that ridge cultivation of 2 a perennially waterlogged rice paddy significantly reduced CH 4 emission compared to plain cultivation Days after transplanting Figure 3. Effect of the number of consecutive upland crops grown before rice transplanting on CH4 flux during the following early rice growth period in Guangzhou in The number of consecutive upland crops grown was one in G-Routine, two in G-Early, and three in G-Turn Effect of Organic Material Amendment on CH 4 Emission The stimulating effect of organic material amendment on CH4 emission from flooded rice fields has been well documented [Yagi and Minami, 1990; Chen et al., 1993; Lauren et al., 1994; Oyediran et al., 1996]. In this experiment, no treatments were specially designed to investigate the effect of organic material

9 _ CAI ET AL.: METHANE EMISSION FROM RICE FIELDS IN CHINA 17, Ch-FF "t 60 = Ch-FFR :',... ß ' :r: 50 - '*ø i 30,:,, 20 0, Figure 4. Methane fluxes from the treatments of Ch-FF and Ch-FFR in Chongqing during the rice growth period and nonrice growth period, Solid line, rice growing season; dashed line, nonrice growing season. amendment on CH4 emission because the main purpose of the experiment was to investigate CH4 emissions from irrigated rice fields under prevailing conditions. However, the stimulating effect is apparent in this experiment. The CH4 emissions from all treatments in Chongqing were significantly higher (on the average, 99.6% higher) in 1996 than those in 1995 and 1997 submerged soil with straw was suppressed to 25% by aerobic incubation fbr 20 days prior to submergence compared with no preincubation treatment. These results are consistent with the conclusion that promoting aerobic degradation of organic matter in the fields can decrease CH4 emission from rice paddy fields [ Yagi et al., Therefore application of rice straw during the (Table 2). This is attributed to the application of nonfermented upland crop season, instead of during the rice crop season, seems human excrement as basal fertilizer in 1996 because it was the to be a practical option for reducing CH4 emission and only factor that differed in 1995 and In addition, we noted that different CH 4 emissions from the next rice crop season were measured for each kind of manure that was applied before early rice transplanting. The results obtained in C-GM and C-Rape in maintaining soil organic matter content in cultivated regions with rice and upland crop rotation Other Factors Influencing CH4 Emission Changsha showed that CH4 emission rates were much lower N-fertilizers. The N-fertilizer application studies during the late rice growing period than during the early rice growing period, while the differences were not substantial in C- Flood and C-Fallow (Table 2). These results are consistent with the finding that the effect of green manure on increasing CH 4 emissions did not last as long as other organic materials. Denier van der Gon and Neue [ 1995] found that toward the end of the season, the difference in CH 4 production between green manure treated and nontreated fields was less pronounced. The results indicated that the stimulating effect of Chinese milk vetch and oil-seed rape lasted for a shorter term than that from weeds, which might be attributed to the different components of the organic materials [¾agi and Minami, 1990; l/vatanab et al., 1993]. The studies in Jurong suggesthat CH4 emissions from crop showed that CH 4 emissions decreased with an increase in N application rate for both urea and ammonium sulphate. The results obtained in Nanjing showed a trend that ammonium sulphat effectively depressed CH 4 emission more than urea did, which is consistent with previous reports [Minami, 1995; Ban& et al., 1996]. Compared with the control (no application of N fertilizer), on average, urea at rates of 100 kg N/ha and 300 kg N/ha reduced CH4 emission by 7 and 14%, respectively, while ammonium sulphate reduced CH 4 emission by 42 and 60%, respectively, although the differences were not statistically significant at p<0.05 due to large variations among the repeats [Cai et al., 1997]. During the rice growth period, the individual CH4 fluxes were always lower from the plots treated with ammonium sulphate than from the control plot. CH 4 emission residue incorporation can be reduced by aerobic decomposition decreased with an increase in the application rate of ammonium of the residue before rice transplanting. The rice straw was placed on the soil surface at a rate of 1500 kg/ha after winter wheat was sown and then was incorporated into the soil when the field was ploughed and prepared for rice transplanting. The mean sulphate (Figure 5a). This is probably related to the input of sulphate, which may cause competition between CH4-producing bacteri and sulfate-reducing bacteria [Ponnamperuma, 1972]. In contrast, the CH 4 fluxes from urea-treated plots were always CH 4 flux was 0.67 mg CH4/m2/h from the treatments with straw higher than those from the control in the early part of the and 0.46 mg CH4/m2/h from the treatment without straw in 1995 growing season and afterwards lower than those from the control and they were 2.23 mg CH4/m2/h and 1.01 mg CH4/m2/h, (Figure 5b). This is probably explained by the competitive respectively, in The amendment of rice straw increased inhibition of NH4 + on CH4 oxidation in the early stage and the CH 4 emission slightly but not significantly. A pot experiment using the stone soil showed that compared with rice straw amendrnent in the fall when winter wheat was sown, amendment contribution of nitriflers to the CH4 oxidation [Cai and Mosier, 2000]. The effect of urea application rate on CH 4 emission in Jurong was not significant (Table 2). at the same rate in spring before rice transplanting increased the Soil texture. The soil texture in the Huang-Huai- CH4 emission 2.75 times during the rice growth period [Xu, 1997]. An incubation and pot experiment carried out by Inubushi et al. [1994] also demonstrated that formation of CH4 in Hai plain varies greatly, while agricultural practices for rice cultivation are substantially constant. Therefore the effect of soil texture on CH 4 emission was investigated in Fengqiu. The results

10 17,240 CAI ET AL.' METHANE EMISSION FROM RICE FIELDS IN CHINA 16, 14 ff 12 N-CK N-S N-S300 o 8 6- C b N-CK N-U N-U300 8 = 6 : 4 Date [Month/Day] Figure 5. Effect of N-fbrtilizers on CH 4 flux during the rice growth period in Nanjing in 1994: (a) ammonium sulphate and (b) urea. showed that the mean CH4 fluxes during the rice growing period CH 4 emission was observed previously in rice fields [Bronson were low, ranging from 0.16 to 1.86 mg CH4/m2/h and close to those from one rice crop and two upland crops in Guangzhou and those in Jurong (Table 2). In the experimental plots, the soil had and Moiser, 1991; Keerthisinghet al., 1993]. However, the CH 4 emission from the treatment with nitrification inhibitor (thiourea, S-Inh) was larger than the treatment without inhibitor (S-Min) in low organic matter contents (Table 1) and high percolation rates. the rice field in Suzhou. This suggests that different nitrification Usually, a floodwater layer of 3-5 cm in thickness disappeared in inhibitors can have different effects on CH4 emission. a few days. Possibly, the high water percolation rate [Yagi et al., 1990] and low organic matter content (Table 1) led to low CH4 emissions from the rice fields in Fengqiu. Statistical analysis by a pair-t test showed that the CH 4 emissions from clay-textural soil 4. Conclusions The rice season CH4 emission rates from rice fields in China were significantly less than those from loam and sand plots in both years. The mean soil Eh in the clay plot was 150 mv, which was higher than those in loam and sandy plots. The mean CH 4 flux during the rice growing period was negatively related to the ranged from 0.3 to 205 g CH4/m 2, but more than half of the measured rates were less than 20 g CH4/m 2 and less than 10% were higher than 100 g CH4/m 2. The emissions from the rice fields in Yingtan, Changsha, and Chongqing were much higher mean soil Eh [Cai et al., 1999]. Because of the high water than from the rice fields in Guangzhou, Nanjing, Suzhou, Jurong, percolation rate, the soils were not strongly reduceduring the and Fengqiu. The large emissions could be attributed to organic rice growth period, especially, the clay soil, which had a larger material amendments, poor drainage due to depressive buffering capacity for oxidation/reduction than loam and sandy topography, and flooding in the nonrice growth season. The soils did. An incubation experiment carried out by Wassmann et water regime in the nonrice growth season played a key role in al. [1998] indicated that CH 4 production was related negatively CH 4 emission during the following rice growth period. Flooding to soil clay content. They assumed that the protection of organic during the nonrice growth period not only stimulated the CH4 matter from microbial breakdown by clay particles was the reason for lower CH4 production Nitrification inhibitors. The depressiveffect of the addition of encapsulated and wax-coated calcium carbide on emission during the following rice growth period but also led to CH4 emission from the fields in the nonrice growth season. Furtheresearch on the effect of flooding in the nonrice growth season on CH4 emission during the following rice growth period

11 CAI ET AL.: METHANE EMISSION FROM RICE FIELDS IN CH1NA 17,241 is necessary for mitigating total CH4 emission from the rice fields in China. A period of aerobic decomposition of rice straw in fields before submergence for rice transplanting did not significantly increase CH 4 emission during the rice growth period. The CH 4 emission decreased with an increase in the application rate of ammonium sulphate, but there were mixed effects from urea application on CH4 emission. Acknowledgments. This project was supported by the National Natural Science Foundation of China (grant ), Chinese Academy of Sciences (KZ952-J1-203), the National Key Basic Research Support Foundation (G ), and by the Global Environmental Research Program of the Environmental Agency, Japan. We thank K. Yagi at Japan International Center for Agricultural Sciences for his valuable suggestions running the project. We also wish to thank X. Y. Yan, H. Xu, and X. P. 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12 17,242 CAI ET AL.: METHANE EMISSION FROM RICE FIELDS IN CHINA Agro-environmental Sciences Res. Rep. Div. Environ. Plan., 6, Z. C. Cai, Institute of Soil Science, Chinese Academy of Sciences, , P.O.Box 821, Nanjing , China. ( zccai ns.issas.ac.cn ) Yagi, K., H. Tsuruta, and K. Minami, Possible options for mitigating K. Minami and H. Tsuruta, National Institute of Agro-Environmental methanemission from rice cultivation, Nutr. Cycles. Agroecosysterns, Sciences, Kannondai, Tsukuba 305, Japan. 49, , Yao, H., and Y. H. Zhuang, Estimation of methane emission from rice paddies in mainland China, Global Biogeochern. Cycles, 10, , (Received May 19, 1999; revised December 30, 1999; accepted January 7, 2000.)