FACTORS AND PROCESSES CONTROLLING METHANE EMISSIONS FROM RICE FIELDS

Understanding the major controlling factors of methane emissions from ricefields is critical for estimates of source strengths. This paper r eports results on the relationship of different plant characteristics and methane fluxes in ricefields. Methane fluxes in ricefields show di stinct diel and seasonal variations. Diel variations are mainly contro lled by soil solution temperature and the partial pressure of methane. One or two distinct seasonal maxima are observed in irrigated ricefie lds. The first is governed by methane production from soil and added o rganic matter and a second at heading is plant derived. During ripenin g and maturity, root exudation, root porosity and root oxidation power may control methane emission rates. Rice plants play an important rol e in methane flux. The aerenchyma conduct methane from the bulk soil i nto the atmosphere. The amount of carbon utilized in methane formation varied among cultivars. A strong positive effect of rice root exudate s on methane production imply that cultivar selections for lower metha ne emissions should not only be based on the gas transport capabilitie s but also on the quality and quantity of root exudates. Soils show a wide range of methane production potential but no simple correlation b etween any stable soil property and methane production is evident. Var ious cultural practices affect methane emissions. Defined aeration per iods reduce methane emissions. Soil entrapped methane is released to t he atmosphere as a result of soil disturbances. Mineral fertilizers in fluence methane production and sulfate containing fertilizer decrease methane production. The methane release per m(2) from different rice e cosystems follow the order: deepwater rice>irrigated rice>rainfed rice . Abatement strategies may only be accepted if the methane source stre ngth of ricefields is reliably discriminated and if mitigation technol ogies are in accordance with increased rice production and productivit y.