A four-year record of methane emissions from irrigated rice fields in the Beijing region of China
Methane (CH sub 4) emissions from irrigated rice fields were measured using an automatic sampling-measuring system with closed chamber method in 1995-98. Average emission rates ranged from 11 to 364 mg m high -2 d high -1 depending on season, water regime, and fertilizer application. Crop management typical for this region (i. e. midseason drainage and organic/mineral fertilizer application) resulted in emission of 279 and 139 mg CH sub 4 m high -2 d high -1 in 1995 and 1997, respectively. This roughly corresponds to emissions observed in other rice-growing areas of China. Emissions were very intense during the tillering stage, which accounted for 85 % of total annual emission, but these were suppressed by low temperature in the late stage of the season. The local irrigation practice of drying at midseason reduced emission rates by 23 %, as compared with continuous flooding. Further reduction of CH sub 4 emissions could be attained by (1) alternate flooding/drying, (2) shifting the drainage period to an earlier stage, or (3) splitting drainage into two phases (of which one is in an earlier stage). Emission rates were extremely sensitive to organic amendments: seasonal emissions from fields treated with pig manure were 15-35 times higher than those treated with ammonium sulfate in the corresponding season. On the basis of identical carbon inputs, CH sub 4 emission potential varied among organic amendments. Rice straw had higher emissions than cattle manure but lower emissions than pig manure. Use of cultivar Zhongzhuo (modern japonica) reduced CH sub 4 emission by 45 % and 50 %, in 1995 and 1997, respectively, as compared with Jingyou (japonica hybrid) and Zhonghua (tall japonica). The results give evidence That CH sub 4 emissions from rice fields in northern China can be reduced by a package of crop management options without affecting yields.