Trace Gas Emissions from Rice Fields

Abstract

Wetland rice cultivation is considered to be one of the larger sources of atmospheric methane, a gas which is an important potential driver of global warming. The atmospheric methane concentration is increasing at about 1% per year and it is an unanswered question as to how much of this increase is due to increased emissions from wetland ricefields. Objectives in current research are to reduce uncertainties concerning how much methane and other climatically active trace gases are annually emitted from irrigated, rainfed, and flood prone rice ecosystems at present, to predict future emissions for given management scenarios, and to develop feasible rice technologies that will reduce emissions and yet will meet the required increase in rice production.

Recent global estimates of methane emission from ricefields range from 20 to 100 Tg/yr corresponding to 6 to 30% of total anthropogenic methane emission. A part of the methane emitted from naturally flooded ricefields may not be considered anthropogenic. Because of the limited number and locations of comprehensive seasonal flux measurements, global extrapolations of emission rates from ricefields are still highly uncertain and tentative. They do not account for varying floodwater regimes, soil properties, organic amendments, cultural practices, and rice cultivars. Irrigated ricefields seem to be the major potential source for increased methane emission. Methane emissions are lower and highly variable in rainfed rice because of periodic droughts during the growing season. Flood prone rice may also emit less methane because of deep flooding or tidal influence. Upland rice is not a source of methane because it is grown like wheat on aerobic soils.

Flooding a ricefield cuts off the oxygen supply from the atmosphere causing an anaerobic fermentation of organic matter in the soil. Methane is a major end product of this process. Zero to over 90% of the methane produced may be oxidized in the soil depending on flood condition and time of growing season. Methane is released to the atmosphere by diffusion, ebullition, and through rice plants. A well developed vascular system, common to wetland plants, provides an effective vent to supply atmospheric oxygen to the rice roots for respiration and to release methane from the soil. Methane fluxes are influenced by: temperature; water regime; low molecular carbon supply from decomposing soil organic residues and root exudates; soil physical, chemical and biological properties; plant physiology; rice cultivars; and cultural practices. Methane emissions from ricefields show distinct diurnal and seasonal variations. Diurnal variation strongly correlates with soil temperature while seasonal variation seems to be more influenced by plant development.

The world’s annual rice production must increase by 65% in the next 30 years to feed the expected population. With present agronomic practices, such increased production will lead to further increases in methane emission. Promising mitigation candidates that are in accord with increased production are: shortening of flooding periods through direct seeding and multiple-drainage aeration, minimizing application of easily decomposable organic matter, use of sulfate-containing fertilizer, application of chemicals that inhibit nitrification and methane formation at the same time, breeding of rice cultivars with a lower methane emission potential, and cultural practices that cause less soil disturbance.