Abstract
This paper examines primary controlling factors that affect methyl halide emissions from rice paddy ecosystems. Observations of four cultivars under multiple growth conditions during studies in commercial fields and the University of California, Irvine, greenhouse lead to the conclusion that daily emissions of methyl halides are primarily determined by the growth stage of the rice plant, with the exception that methyl chloride emissions show no clear seasonal pattern. Methyl chloride emissions appear to be more from the paddy water and/or soil as opposed to the plants; however, in soils with high chloride content, these emissions appear to peak during the reproductive phase. Strong secondary influences include air temperature, soil halide concentration, and soil pore water saturation. The cultivars studied had statistically separate seasonally integrated emissions. Irradiant light and aboveground biomass appear to have little effect on emissions. Emissions of methyl chloride, methyl bromide, and methyl iodide are estimated to be 3.5, 2.3, and 48 mg/m2/yr, or 5.3, 3.5, and 72 Gg/yr, from rice paddies globally.
Highlights
The effect of simulated ‘‘rain-fed’’ field water-management conditions on methyl halide emissions from rice is qualitatively described, as well as the emission patterns of methyl halides over the growing season
[18] Results from all studies for methyl chloride, methyl bromide and methyl iodide are shown in Figures 2, 3, and 4
[20] For a complete description of rice growth stages, the reader is referred to International Rice Research Institute (IRRI) journals or our previous papers [Redeker et al, 2000, 2002]
Summary
[2] A significant portion of halogen radicals found in the atmosphere is derived from methyl halide gases, which are produced at the surface of the Earth through both natural and anthropogenic processes [Davis et al, 1996; Khalil, 1999]. In the troposphere, elevated methyl iodide concentrations (MeI or CH3I) may increase aerosol formation, affecting regional radiative balance and rates of heterogeneous reactions [O’Dowd et al, 2002]. [4] There are several terrestrial ecosystems that emit methyl halides in globally significant quantities. All ecosystems and most plants measured to date have shown some capacity to influence atmospheric methyl chloride, methyl bromide, and methyl iodide concentrations. [5] Accurate global estimates of ecosystem emissions of methyl halides are hampered by lack of knowledge of mechanisms and of extent of variability. Previous studies of terrestrial ecosystems have focused on single parameters, such as aboveground biomass, soil halide content, or diurnal response (the combined effects of light and temperature), and have rarely examined emissions from multiple sites over entire growing seasons/annual cycles. The effect of simulated ‘‘rain-fed’’ field water-management conditions (experimental soils were primarily unsaturated with short periods of complete saturation) on methyl halide emissions from rice is qualitatively described, as well as the emission patterns of methyl halides over the growing season
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