Shrubland fluxes of methyl bromide and methyl chloride

Flux measurements in coastal sage scrub, chamise chaparral, and creosote bu sh scrub environments show that methyl bromide (CH3Br) and methyl chloride (CH3Cl), compounds that are involved in stratospheric ozone depletion, are both produced and consumed by southern California shrubland ecosystems. CH3 Br and CH3Cl are produced in association with a variety of plants and are c onsumed by the soils, although there is a large variability in the fluxes, depending on predominant vegetation and environmental conditions. At sites with a net uptake of both compounds the fluxes of CH3Cl and CH3Br show a st rong correlation, with a molar ratio of roughly 40:1, pointing to a similar mechanism of consumption. In contrast, the net production rates of these c ompounds show no apparent correlation with each other. The average observed net CH3Br uptake rates are an order of magnitude smaller than the previous ly reported average soil consumption rates assigned to shrublands. Extrapol ations from our field measurements suggest that shrublands globally have a maximum net consumption of <1 Gg yr(-1) for CH3Br and < 20 Gg yr(-1) for CH 3Cl and may, in fact, be net sources for these compounds. Consequently, the measured net fluxes from shrubland ecosystems can account for part of the present imbalance in the CH3Br budget by adding a new source term and poten tially reducing the soil sink term. These results also suggest that while s hrubland soil consumption of CH3Cl may be small, soils in general may be a globally significant sink for CH3Cl.