Atmospheric methyl bromide (CH3Br) and methyl chloride (CH3Cl), compounds that are involved in stratospheric ozone depletion, originate from both natural and anthropogenic sources. Current estimates of CH3Br and CH3Cl emissions from oceanic sources, terrestrial plants and fungi, biomass burning and anthropogenic inputs do not balance their losses owing to oxidation by hydroxyl radicals, oceanic degradation, and consumption in soils, suggesting that additional natural terrestrial sources may be important. Here we show that CH3Br and CH3Cl are released to the atmosphere from all vegetation zones of two coastal salt marshes. We see very large fluxes of CH3Br and CH3Cl per unit area: up to 42 and 570 micromol m(-2) d(-1), respectively. The fluxes show large diurnal, seasonal and spatial variabilities, but there is a strong correlation between the fluxes of CH3Br and those of CH3Cl, with an average molar flux ratio of roughly 1:20. If our measurements are typical of salt marshes globally, they suggest that such ecosystems, even though they constitute less than 0.1% of the global surface area, may produce roughly 10% of the total fluxes of atmospheric CH3Br and CH3Cl.

Carbonyl sulfide (COS) is a reduced sulfur gas that is taken up irreversibly in plant leaves proportionally with CO 2 , allowing its potential use as a tracer for gross primary production. Recently, wheat field soil at the Southern Great Plains Atmospheric Radiation Measurement site in Lamont, Oklahoma, was found to be a measureable source of COS to the atmosphere. To understand the mechanism of COS production, soil and root samples were collected from the site and incubated in the laboratory over a range of temperatures (15-34°C) and light conditions (light and dark). Samples exhibited mostly COS net uptake from the atmosphere in dark and cool (<22-25°C) trials. COS emission was observed during dark incubations at high temperatures (>25°C), consistent with field observations, and at a lower temperature (19°C) when a full spectrum lamp (max wavelength 600 nm) was applied. Sterilized soil and root samples yielded only COS production that increased with temperature, supporting the hypothesis that (a) COS production in these samples is abiotic, (b) production is directly influenced by temperature and light, and (c) some COS consumption in soil and root samples is biotic.

Methyl chloride (CH(3)Cl) and methyl bromide (CH(3)Br) are the primary carriers of natural chlorine and bromine, respectively, to the stratosphere, where they catalyze the destruction of ozone, whereas methyl iodide (CH(3)I) influences aerosol formation and ozone loss in the boundary layer. CH(3)Br is also an agricultural pesticide whose use is regulated by international agreement. Despite the economic and environmental importance of these methyl halides, their natural sources and biological production mechanisms are poorly understood. Besides CH(3)Br fumigation, important sources include oceans, biomass burning, tropical plants, salt marshes, and certain crops and fungi. Here, we demonstrate that the model plant Arabidopsis thaliana produces and emits methyl halides and that the enzyme primarily responsible for the production is encoded by the HARMLESS TO OZONE LAYER (HOL) gene. The encoded protein belongs to a group of methyltransferases capable of catalyzing the S-adenosyl-L-methionine (SAM)-dependent methylation of chloride (Cl(-)), bromide (Br(-)), and iodide (I(-)) to produce methyl halides. In mutant plants with the HOL gene disrupted, methyl halide production is largely eliminated. A phylogenetic analysis with the HOL gene suggests that the ability to produce methyl halides is widespread among vascular plants. This approach provides a genetic basis for understanding and predicting patterns of methyl halide production by plants.

[1] We conducted measurements of methane (CH 4 ) emission and ecosystem respiration on >200 points across the Arctic coastal tundra near Barrow, Alaska, United States, in July 2007 and August 2008. This site contains broad diversity in tundra microtopography, including polygonal tundra, thaw lakes, and drained lake basins. In 2007, we surveyed CH 4 emissions across this landscape, and found that soil water content was the strongest control of methane emission rate, such that emission rates rose exponentially with water content. However, there was considerable residual variation in CH 4 emission in the wettest soils (>80% volumetric water content) where CH 4 emissions were highest. A statistical analysis of possible soil and plant controls on CH 4 emission rates from these wet soils revealed that vegetation height (especially of Carex aquatilis) was the best predictor, with ecosystem respiration and permafrost depth as significant copredictors. To evaluate whether plant height served as a proxy for aboveground plant biomass, or gross primary production, we conducted a survey of CH 4 emission rates from wet, Carex-dominated sites in 2008, coincidently measuring these candidate predictors. Surprisingly, vegetation height remained the best predictor of CH 4 emission rates, with CH 4 emissions rising exponentially with vegetation height. We hypothesize that taller plants have more extensive root systems that both stimulate more methanogenesis and conduct more pore water CH 4 to the atmosphere. We anticipate that the magnitude of the climate change-CH 4 feedback in the Arctic Coastal Plain will strongly depend on how permafrost thaw alters the ecology of Carex aquatilis.

[1] The Arctic tundra is a major source and sink of carbon-containing gases, but the biogeochemical cycling of halocarbons in this ecosystem has been largely unexplored. In this study, coastal tundra fluxes of methyl halides (CH 3 Cl, CH 3 Br, and CH 3 I) and methane (CH 4 ) were measured near Barrow, Alaska (71°N, 157°W) during the 2005 growing season. Sites covered a range of microtopographic features including drained lake basins, channels, and high-and low-centered ice-wedge polygons. CH 3 Cl and CH 3 Br fluxes varied significantly with hydrologic conditions, with progressively higher net uptake rates observed with decreasing soil saturation. Drained tundra sites averaged À620 nmol

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Atmospheric methyl chloride (CH3Cl) and methyl bromide (CH3Br), compounds involved in the destruction of stratospheric ozone, are simultaneously produced and consumed by the terrestrial biosphere. Here we present a stable isotope incubation technique using 13CH3Cl and 13CH3Br as tracers to simultaneously determine production and consumption fluxes in boreal forest soils from Alaska, USA. Measured uptake rates are consistent with previously reported boreal soil results for CH3Br and show a CH3Cl:CH3Br molar consumption ratio of 40:1. Boreal forest soils appear to produce small amounts of these methyl halides as well, but at rates that are negligible in their global budgets. This isotope tracer technique can be applied to laboratory studies of plants and other soils and to field measurements where disturbances to the system can be minimized.

show a strong correlation, with a molar ratio of roughly 40:1, pointing to a similar mechanism of consumption. In contrast, the net production rates of these compounds show no apparent correlation with each other. The average observed net CH3Br uptake rates are an order of magnitude smaller than the previously reported average soil consumption rates assigned to shrublands. Extrapolations from our field measurements suggest that shrublands globally have a maximum net consumption of <1 Gg yr -• for CH3Br and <20 Gg yr -• for CH3C1 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 potentially reducing the soil sink term. These results also suggest that while shrubland soil consumption of CH3C1 may be small, soils in general may be a globally significant sink for CH3C1.