Measuring Simultaneous Production and Consumption Fluxes of Methyl Chloride and Methyl Bromide in Annual Temperate Grasslands

Abstract

Methyl chloride (CH3Cl) and methyl bromide (CH3Br) are both produced and consumed by terrestrial ecosystems, but large uncertainties remain about the magnitude of their emission and uptake rates. Most field-based studies report net fluxes, but knowledge of gross fluxes is required to assess partial atmospheric lifetimes and potential mechanisms. Here, we present the first field results using a stable isotope tracer technique to determine gross emission and uptake fluxes of CH3Cl and CH3Br at two temperate annual grasslands in California. These grasslands generally showed modest emission and uptake rates of CH3Cl and CH3Br, although large net emissions were observed at riparian and dry playa sites within these grasslands. While gross production rates of the methyl halides are not correlated to each other, gross consumption rates of CH3Cl and CH3Br show a molar uptake ratio of approximately 40: 1, consistent with results from other biomes. Gross consumption rates appear to be strongly affected by soil moisture. Temperate grassland soils have been previously identified as a globally significant sink for CH3Br, accounting for approximately 25% of the total soil sink, but our results suggest that the uptake rate could be much smaller.