TEMPORAL PATTERNS OF METHANE EMISSIONS FROM WETLAND RICE FIELDS TREATED BY DIFFERENT MODES OF N-APPLICATION

Methane emission rates from wetland rice fields were determined in Los Banos (Philippines) using an automatic system that allows continuous measurements over time. Methane emission was monitored in an irrigated Aquandic Epiaqualf planted to rice cultivar IR72. Urea fertilizer was applied using four modes: (1) broadcast 10 days after transplanting, (2) broadcast at transplanting, (3) broadcast and incorporated at fina l harrowing, and (4) deep placement as sulfur-coated granules. The tre atments were laid out in a randomized complete block design with four replicates. Measurements were done in the 1991 wet season, 1992 dry se ason (four treatments), and the 1992 wet season (only treatment 3). Me thane emission rates from the experimental plots showed pronounced sea sonal and diel variations. The diel pattern of methane emission rates followed a consistent pattern, with highest rates observed in the earl y afternoon and lowest rates in the early morning. Methane emission ra te was generally highest at the ripening stage. The average methane em ission rate during the 1992 dry season (190 mg CH4 m-2 d-1) exceeded t he average flux rates of the 1992 wet season (79 mg CH4 m-2 d-1) by a factor of 2.4. The total methane emitted from these flooded rice field s amounted to 19 g CH4 m-2 in the dry season with rice yields of 5.2-6 .3 t ha-1 and 7 g CH4 M-2 in the wet season with rice yields of 2.4-3. 3 t ha-1 regardless of the mode of N application. Significant amounts corresponding to 20% of the methane released under waterlogged conditi ons were released when the soil was drained after harvest. Emission ra tes increased sharply when the floodwater receded and macropores start ed to drain. Emission of methane stopped only when the soil became ful ly aerated.