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 harvesting the dry and the wet season crops of 1992. The fields were still flooded during harvest but irrigation was stopped after harvest and the fields were allowed to evaporatively dry while CH 4 emissions were monitored with a closed chamber technique. In all plots we observed a sudden, strong increase of CH 4 emissions to the atmosphere for 2 to 4 days just after the soil fell dry. As soil drying continued, the soils began to crack and CH 4 emissions decreased to nil. The release of CH 4 during soil drying was observed for fields on three different soil types and both for urea or organically manured rice fields in both seasons. The absolute amounts of CH 4 emitted during soil drying differed greatly depending on fertilizer treatment. However, the ratio between the amount of CH 4 released upon soil drying and CH 4 emitted during the growing season was quite constant (0.10 ±0.04). This suggests that about 10% of the CH 4 emitted during a full rice crop cycle is released during drying of the fields and thus needs to be included in estimates of the total CH 4 emission from rice agriculture.