Maggie L. Walser

Coastal salt marshes as global methyl halide sources from determinations of intrinsic production by marsh plants

Emissions of CH3Cl, CH3Br and CH3I were measured biweekly for 12- to 24-month periods between March 2002 and March 2005 from monospecific stands of four dominant southern California coastal salt marsh plants. These measurements revealed large inherent differences between species and more detailed patterns of seasonal production than previously reported. Marsh plants displayed intrinsic abilities to produce methyl halides. Salt marsh plants produced 92% of CH3Cl and 90% of CH3Br emitted and only 41% of the emitted CH3I. Unvegetated areas emitted 7.9% of CH3Cl, 9.9% CH3Br, and 59% of the emitted CH3I. The accuracy of the estimated methyl halide emissions from a coastal marsh and probably other ecosystems can be dramatically improved with increasing the number of species being measured and including emission from barren (mudflats and soil) areas. Estimates of global salt marsh emissions based on vegetated and barren area are 130, 21, 5.5 (mg m-2 yr-1) for CH 3Cl, CH3Br, and CH3I, respectively, or 1.2, 3.9, and 0.8% of total global fluxes of these gases. Copyright 2006 by the American Geophysical Union.

Physiological and biochemical controls over methyl halide emissions from rice plants

This paper investigates physiological and biochemical aspects of methyl halide production in rice plants over two growing seasons. Multiple separate mechanisms appear to be responsible for production of methyl halides in rice plant tissues. Evidence for multiple mechanisms is found in timing of peak emissions of methyl halides from rice, inconsistent effects of competitive inhibitors on methyl halide emissions, and large differences in methyl halide emission rates when compared to plant tissue halide concentrations. Other results show that chloride, bromide, and iodide ion concentrations in plant tissue appear to be regulated throughout the season, and observed changes in leaf tissue concentration cannot explain observed methyl halide emissions. The Km for methyl iodide formation in leaf tissue cell-free extract is 0.018 mM, suggesting a very efficient mechanism. Of the seven competitive inhibitors used, only thiol had a consistently strong effect on both methyl iodide and methyl bromide. Copyright 2004 by the American Geophysical Union.

Seasonal mass balance of halogens in simulated rice paddies

Halogens released from soil reservoirs to the atmosphere play important roles in atmospheric chemistry, including ozone loss and aerosol formation. Closed system experiments to determine controlling factors in halogen movement between the pedosphere, hydrosphere, terrestrial biosphere, and atmosphere are needed. This paper presents results from a closed system experiment on simulated rice paddies. It was observed that most water-extractable (bioavailable), halogens were swept downward from the surface during the initial watering pulse (∼50, 70, and 75% of chloride, bromide, and iodide in unadulterated soils). Soil halogens were sequestered by rice plants with 28, 4, and 24% of the remaining bioavailable chlorine, bromine, and iodine processed by the plant tissue by the end of the season. Of the bioavailable halogens taken into the rice plant, less than 1% of chlorine or bromine is volatilized as a methyl halide while over 90% of iodide is emitted as gaseous CH3I. Copyright 2004 by the American Geophysical Union.