Water, temperature, and vegetation controls on halomethane fluxes from semiarid temperate grasslands

Methyl bromide (CH3Br) and methyl chloride (CH3Cl) are the dominant carriers of organic bromine and chlorine to the upper atmosphere, where they catalyze stratospheric ozone loss. Prior experiments indicate that temperate grasslands may be a significant sink for these gases, although large uncertainties exist regarding the range and magnitude of fluxes. Here we report the results of a combined field and laboratory experiment that investigated the role of water, temperature, and vegetation composition in regulating fluxes of CH3Br, CH3Cl, methyl iodide (CH3I), and chloroform between the land surface and atmosphere. A novel stable isotope tracer technique was employed to deconvolute simultaneous production and oxidation of CH3Br and CH3Cl. Fluxes were measured over a range of cover types, topographic positions, and soil water contents. Gross CH3Br and CH3Cl production was observed in all our grassland sites. Gross CH3Br production rates averaged 4.17 ± 1.09 nmol m-2 day-1 and gross CH3Cl production averaged 310.14 ± 152 nmol m-2 day-1. However, gross CH3Br and CH3Cl oxidation greatly exceed production in most of our field sites, such that net fluxes tended to be consumptive. Net fluxes of CH3Br and CH3Cl averaged -27 ± 0.87 and -8.45 ± 102.93 nmol m-2 day-1, respectively. The exception was in floodplain sites dominated by the common shrub Atriplex canescens, where net CH3Cl emissions were observed (458.90 ± 85.59 nmol m-2 day-1. This suggests that Atriplex canescens is a significant emitter of CH3Cl. All field sites showed modest emissions of CH3I and chloroform. Gross oxidation of CH3Br and CH3Cl was positively correlated with soil water content and appeared insensitive to soil temperature variations in the range of 16-22 °C. This suggests that oxidation of CH3Br and CH3Cl is more strongly limited by water availability than by temperature in semiarid grasslands.