Changes in CH4 emission from rice fields from 1960 to 1990s 2. The declining use of organic inputs in rice farming

Previous studies have shown that organic amendments used in paddy rice cultivation strongly enhance methane emission from rice fields. Because of the increased availability of mineral fertilizer and the labor-intensive nature of many organic amendments, it is likely that the use of organic amendments in rice farming has decreased in recent years. If so, a significant decrease in CH4 emissions from rice fields could be the result, but quantification is difficult because data on organic amendments are scarce. In this paper data on green manure application in China are used to quantify the impact of changes in an organic amendment on CH4 emission. The trend in CH4 emissions over time, taking green manure application explicitly into account, suggests that CH4 emissions from Chinese rice fields from the 1970s to the early 1980s may have been considerably higher than at present. The results presented here and in a previous study [Denier van der Gon, 1999] indicate that changes in agricultural management and technology need to be considered in assessments of the CH4 source strength of wetland rice fields and their role in the global CH4 budget of the past, present, and future.