Automated Chamber Measurements of Methane and Nitrous Oxide Flux in a Flooded Rice Soil: II. Fallow Period Emissions

Methane and N2O are radiatively important gases that are emitted from waterlogged soils. Automated chamber measurements of CH4 and N2O fluxes were carried out at the International Rice Research Institute in the Philippines in flooded rice (Oryza sativa L.) and fallow rice fields 24 h a day between December 1992 and April 1994. This period included three 5‐ to 11‐wk rainfed fallow periods. During the first two fallows, the soil was generally aerobic, and moderate amounts of NO3 accumulated (7–20 kg NO3‐N ha‐1). Moderately high, continuous N2O fluxes were evident during these two fallow periods. This N2O was apparently emitted during nitrification of mineralized organic N in the topsoil and possibly from denitrification in the wet subsoil. Nitrous oxide fluxes were highest (up to 80 mg N2O‐N m‐2 d‐1) immediately after rainfalls >20 mm, and following the establishment of flooding for rice at the end of the fallows. Acetylene inhibition in intact cores at these times showed that more N2 was produced than N2O. Denitrification of accumulated NO3 was therefore occurring after the wetting events. Methane emissions were generally absent during the fallow periods. Two exceptions were immediately after rice harvest and 1 to 2 wk after the establishment of the permanent flood. Following flooding and green manure (GM; Sesbania rostrata L.) incorporation, CH4 fluxes appeared within 7 d. During the third fallow period, which was unusually wet, no NO3 accumulated in the soil. Nitrous oxide emissions were not significant, and low levels of CH4 fluxes persisted throughout this fallow period. This study demonstrates that rice soils in the fallow periods can be significant sources of N2O, and during wet fallow seasons, important sources of CH4 as well.