RELEASE OF ENTRAPPED METHANE FROM WETLAND RICE FIELDS UPON SOIL DRYING

Methane emissions from Philippine rice paddies, fertilized with either urea or green manure, were monitored for several weeks after harvesti ng the dry and the wet season crops of 1992. The fields were still flo oded during harvest but irrigation was stopped after harvest and the f ields were allowed to evaporatively dry while CH4 emissions were monit ored with a closed chamber technique. In all plots we observed a sudde n, strong increase of CH4 emissions to the atmosphere for 2 to 4 days just after the soil fell dry. As soil drying continued, the soils bega n to crack and CH4 emissions decreased to nil. The release of CH4 duri ng soil drying was observed for fields on three different soil types a nd both for urea or organically manured rice fields in both seasons. T he absolute amounts of CH4 emitted during soil drying differed greatly depending on fertilizer treatment. However, the ratio between the amo unt of CH4 released upon soil drying and CH4 emitted during the growin g season was quite constant (0.10 +/- 0.04). This suggests that about 10% of the CH4 emitted during a full rice crop cycle is released durin g drying of the fields and thus needs to be included in estimates of t he total CH4 emission from rice agriculture.