Methane emission from wetland rice fields

Abstract

Methane (CH 4 ) is an important greenhouse gas and plays a key role in tropospheric and stratospheric chemistry. Wetland rice fields are an important source of methane, accounting for approximately 20% of the global anthropogenic methane emission. Methane fluxes from wetland rice fields in the Philippines were monitored with a closed chamber technique in close cooperation with the International Rice Research Institute (IRRI). The field studies were complemented by laboratory and greenhouse experiments. Up to 90 % of the methane emitted from a rice field may be transported from soil to atmosphere through the rice plant. It was shown that this plant-mediated transport is diffusion controlled. Methane emitted from a rice field is the net effect of methane production and methane oxidation. Methane oxidation in the rice rhizosphere depended on the growth stage of the rice plant and becomes of much less importance when the rice plant reaches the ripening stage. Maximum rhizospheric methane oxidation efficiency observed was about 30%, which is much lower than the 70-90% estimated from indirect measurements in previous studies. A higher percentage of oxygen in the air resulted in lower methane emission indicating that breeding rice cultivars that transport more oxygen to their rhizosphere may be a promising mitigation option. Field studies with several soil related factors that influence methane emission were conducted; salinity, sulfate availability, organic amendments and soil types. Organic amendments strongly stimulated methane emission. The impact of organic amendments on methane emission can be described by a dose-response curve. This approach proofed successful for data from various locations of the world. Salinity partly inhibited methane production but methane oxidation in the salt-amended plot was even more inhibited, indicating that a reduction in CH 4 production does not necessarily cause a proportional reduction in CH 4 emission. This illustrates the importance of both production and oxidation of methane when designing mitigation strategies to reduce methane emission. Different soil types had different methane emission levels. Wetland rice fields on saline, low-sulfate soils emit less methane than comparable non-saline rice fields. On soils high in sulfate or amended with large amounts of sulfate containing substances, methane emissions are reduced even more. Continuous monitoring of methane fluxes showed that upon soil drying considerable amounts of soil-entrapped methane may be released. In previous monitoring studies these periods were not included, which may cause an underestimation of, total seasonal emission by 10-15%