Emission rates of CH4 from four Japanese paddy fields were measured throughout the cultivation period in 1988 by using the closed chamber method. Large seasonal variations of the CH4 flux were observed. The emission was closely related to the decrease of the redox potential (Eh) in paddy soils. Drainage and supplementary application of mineral fertilizer substantially reduced the CH4 emission. Emission rates of CH4 differed markedly with the soil types. The highest rate was observed in a paddy field consisting of Peat soil (44.8 g-CH4/ m 2 during a cultivation period), followed by Gley soil (8.0-27.0). The emission rates in the Andosols were significantly lower (0.6-12.6). Application of rice straw at a rate of 6-9 t/ha to the paddy fields increased the CH4 emission rates 1.8-to 3.5-fold. Application of compost slightly increased the CH4 emission. Annual emission rates of CH4 from individual plots were positively correlated with the contents of readily mineralizable carbon (RMC) in paddy soils collected before flooding, suggesting that RMC is one of the main factors affecting CH4 emission from flooded soils.

The effect of differing water management schemes on the emission of methane (CH4) from rice paddies to the atmosphere was studied in a Japanese paddy field. Using an automated sampling and analyzing system, the test site was divided into two plots: a continuously flooded plot which was maintained flooded by constant irrigation from May to August, and an intermittently drained plot in which short‐term draining practices were performed several times during the flooding period . The draining practice had a strong effect on CH4 emission. A large flush of CH4 emission was observed in the intermittently drained plot immediately after each drainage. It was followed by a rapid decrease in CH4 flux in most of the cases. A large flush of CH4 was observed after the final drainage at the end of August in the continuously flooded plot, accounting for about 7% of the total CH4 emitted in the plot. Total emission rates of CH4 during the cultivation period were 14.8 and 8.63 g m−2 for 1991 and 9.49 and 5.18 g m−2 for 1993 in the continuously flooded and intermittently drained plots, respectively. Companion N2O flux measurements showed that almost no N2O was emitted from either plot until the final drainage. These results indicate that short‐term draining practices strongly reduce CH4 emission from rice paddy fields, and that improvement in water management can be one of the most important mitigation strategies for CH4 emission from rice paddy fields.

A.R. M o s i e r I , J.M. D u x b u r y 2, J.R. F r e n e y 3, O. H e i n e m e y e r 4 and K. M i n a m i 5 I USDA/ARS, AbstractIn this paper we discuss three topics concerning N20 emissions from agricultural systems. First, we present an appraisal of N20 emissions from agricultural soils (Assessment). Secondly, we discuss some recent efforts to improve N20 flux estimates in agricultural fields (Measurement), and finally, we relate recent studies which use nitrification inhibitors to decrease N20 emissions from N-fertilized fields (Mitigation).To assess the global emission of N20 from agricultural soils, the total flux should represent NzO from all possible sources; native soil N, N from recent atmospheric deposition, past years fertilization, N from crop residues, N20 from subsurface aquifers below the study area, and current N fertilization. Of these N sources only synthetic fertilizer and animal manures and the area of fields cropped with legumes have sufficient global data to estimate their input for N20 production. The assessment of direct and indirect N20 emissions we present was made by multiplying the amount of fertilizer N applied to agricultural lands by 2% and the area of land cropped to legumes by 4 kg N20-N ha -1 . No regard to method of N application, type of N, crop, climate or soil was given in these calculations, because the data are not available to include these variables in large scale assessments. Improved assessments should include these variables and should be used to drive process models for field, area, region and global scales.Several N20 flux measurement techniques have been used in recent field studies which utilize small and ultralarge chambers and micrometeorological along with new analytical techniques to measure N20 fluxes. These studies reveal that it is not the measurement technique that is providing much of the uncertainty in N20 flux values found in the literature but rather the diverse combinations of physical and biological factors which control gas fluxes. A careful comparison of published literature narrows the range of observed fluxes as noted in the section on assessment. An array of careful field studies which compare a series of crops, fertilizer sources, and management techniques in controlled parallel experiments throughout the calendar year are needed to improve flux estimates and decrease uncertainty in prediction capability.There are a variety of management techniques which should conserve N and decrease the amount of N application needed to grow crops and to limit N20 emissions. Using nitrification inhibitors is an option for decreasing fertilizer N use and additionally directly mitigating N20 emissions. Case studies are presented which demonstrate the potential for using nitrification inhibitors to limit N20 emissions from agricultural soils. Inhibitors may be selected for climatic conditions and type of cropping system as well as the type of nitrogen (solid mineral N, mineral N in solution, or organic waste materials) and applied with the fertilizers.

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 in 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.

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