Mechanisms of crop management impact on methane emissions from rice fieldsin Los Banos, Philippines

This article comprises 4 yr of field experiments on methane (CH4) emissions from rice fields conducted at Los Banos, Philippines. The experimental lay out allowed automated measurements of CH4 emissions as affected by water re gime, soil amendments (mineral and organic), and cultivars. In addition to emission records over 24 h, ebullition and dissolved CH4 in soil solution w ere recorded in weekly intervals. Emission rates varied in a very wide rang e from 5 to 634 kg CH4 ha(-1), depending on season and crop management. In the 1994 and 1996 experiments, field drying at midtillering reduced CH4 emi ssions by 15-80% as compared with continuous flooding, without a significan t effect on grain yield. The net impact of midtillering drainage was dimini shed when (i) rainfall was strong during the drainage period and (ii) emiss ions were suppressed by very low levels of organic substrate in the soil. F ive cultivars were tested in the 1995 dry and wet season. The cultivar IR72 gave higher CH4 emissions than the other cultivars including the new plant type (IR65597) with an enhanced yield potential. Incorporation of rice str aw into the soil resulted in an early peak of CH4 emission rates. About 66% of the total seasonal emission from rice straw-treated plots was emitted d uring the vegetative stage. Methane fluxes generated from the application o f straw were 34 times higher than those generated with the use of urea. App lication of green manure (Sesbania rostrata) gave only threefold increase i n emission as compared with urea-treated plots. Application of ammonium sul fate significantly reduced seasonal emission as compared with urea applicat ion. Correlation between emissions and combined dissolved CH4 concentration s (from 0 to 20 cm) gave a significant R-2 of 0.95 (urea + rice straw), and 0.93 (urea + Sesbania), whereas correlation with dissolved CH4 in the inor ganically fertilized soils was inconsistent. A highly significant correlati on (R-2 =0.93) existed between emission and ebullition from plots treated w ith rice straw. These findings may stimulate further development of diagnos tic tools for easy and reliable determination of CH4 emission potentials un der different crop management practices.