RESUMEN

Carbon tetrachloride (CCl4) is an anthropogenic gas with a long atmospheric lifetime and can catalyze the destruction of stratospheric ozone. Natural soils are believed to be important and widespread sinks of atmospheric CCl4, although poorly characterized due to a limited number of measurements. In this study, for the first time in situ static-chamber measurements and laboratory-based incubations for CCl4 fluxes were conducted at coastal Antarctic tundra. Results showed that soil in remote Antarctica is also acting as a CCl4 sink, with an average uptake rate of -2.2 ± 0.6 nmol m-2 d-1, which is comparable to the reported soil sinks in other regions of the world. No significant difference (p > 0.05) was found across different types of tundra, such normal upland tundra, coastal marsh tundra, and tundra in the sea animal colonies. Soil CCl4 fluxes did not show significant correlations (p > 0.05) with soil moisture, pH, TOC, TN, TP and Cl contents. Laboratory-based anoxic incubations showed that the uptake rates of CCl4 in tundra soil were suppressed; post-thermal sterilization incubations showed that soil CCl4 sink was enhanced; these results suggested that CCl4 degradation in tundra soil was likely an abiotic process preferring oxic environments. A rough extrapolation suggested that Antarctic tundra may degrade about 2.4 metric tons of atmospheric CCl4 each year. Combining soil CCl4 fluxes from this study and other literature reports, CCl4 partial lifetime with respect to the soil sink was evaluated to be 354 (235-474) years, which supported the recent viewpoint that the soil sink of CCl4 is smaller than previously thought.

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RESUMEN

Methyl bromide (CH3Br) and methyl chloride (CH3Cl) are major carriers of atmospheric bromine and chlorine, respectively, which can catalyze stratospheric ozone depletion. However, in our current understanding, there are missing sources associated with these two species. Here we investigate the effect of copper(II) on CH3Br and CH3Cl production from soil, seawater and model organic compounds: catechol (benzene-1,2-diol) and guaiacol (2-methoxyphenol). We show that copper sulfate (CuSO4) enhances CH3Br and CH3Cl production from soil and seawater, and it may be further amplified in conjunction with hydrogen peroxide (H2O2) or solar radiation. This represents an abiotic production pathway of CH3Br and CH3Cl perturbed by anthropogenic application of copper(II)-based chemicals. Hence, we suggest that the widespread application of copper(II) pesticides in agriculture and the discharge of anthropogenic copper(II) to the oceans may account for part of the missing sources of CH3Br and CH3Cl, and thereby contribute to stratospheric halogen load.

RESUMEN

Methyl chloride (CH3Cl) and methyl bromide (CH3Br) are the predominant carriers of natural chlorine and bromine from the troposphere to the stratosphere, which can catalyze the destruction of stratospheric ozone. Here, penguin colony soils (PCS) and the adjacent tundra soils (i.e., penguin-lacking colony soils, PLS), seal colony soils (SCS), tundra marsh soils (TMS), and normal upland tundra soils (UTS) in coastal Antarctica were collected and incubated for the first time to confirm that these soils were CH3Cl and CH3Br sources or sinks. Overall, tundra soil acted as a net sink for CH3Cl and CH3Br with potential flux ranges from -18.1 to -2.8 pmol g-1 d-1 and -1.32 to -0.24 pmol g-1 d-1, respectively. The deposition of penguin guano or seal excrement into tundra soils facilitated the simultaneous production of CH3Cl and CH3Br and resulted in a smaller sink in PCS, SCS, and PLS. Laboratory-based thermal treatments and anaerobic incubation experiments suggested that the consumption of CH3Cl and CH3Br was predominantly mediated by microbes while the production was abiotic and O2 independent. Temperature gradient incubations revealed that increasing soil temperature promoted the consumption of CH3Cl and CH3Br in UTS, suggesting that the regional sink may increase with Antarctic warming, depending on changes in soil moisture and abiotic production rates.

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RESUMEN

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.

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RESUMEN

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.

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