Measurements of Methyl Halide Fluxes in California Salt Marshes and Grasslands Using a Stable Isotope Tracer Technique

Methyl bromide (CH3Br) and methyl chloride (CH3Cl) transport halogens to the stratosphere where they can catalyze the destruction of stratospheric ozone. Large uncertainties remain in their global budgets, especially in regard to the role of the terrestrial biosphere. A complication in measuring terrestrial fluxes of methyl halides is that sources (e.g. biosynthesis by certain plants and fungi or abiotic production in soils) and sinks (e.g. oxidation by soil bacteria) are typically co-located, such that field measurements typically represent net fluxes rather than gross production and consumption rates. Previously, a laboratory method was developed to separate production and consumption of these methyl halides using a stable isotope tracer technique. We now present results from field measurements using stable isotope tracers in conjunction with flux chambers. Field measurements were conducted in 2004-2005 at a coastal salt marsh (Bolinas Lagoon) and at two annual grasslands (Jepson Prairie and Sibley Volcanic Reserve) in northern California. Initial outings established the presence of substantial net production and consumption at these sites. Subsequent outings employed the use of stable isotope tracers to demonstrate that gross production and gross consumption simultaneously occur within these ecosystems. In coastal salt marsh sites, where large rates of net production were observed, relatively low rates of gross uptake were determined. In temperate grasslands where net consumption predominates, gross production rates were also observed. These results demonstrate the need for measuring gross rates in terrestrial ecosystems in order to establish better estimates of natural terrestrial sources and sinks.