Abstract
Abstract. Vegetation, soil and ecosystem level carbonyl sulfide (COS) exchange was observed at Duke Forest, a temperate loblolly pine forest, grown under ambient (Ring 1, R1) and elevated (Ring 2, R2) CO2. During calm meteorological conditions, ambient COS mixing ratios at the top of the forest canopy followed a distinct diurnal pattern in both CO2 growth regimes, with maximum COS mixing ratios during the day (R1=380±4 pptv and R2=373±3 pptv, daytime mean ± standard error) and minimums at night (R1=340±6 pptv and R2=346±5 pptv, nighttime mean ± standard error) reflecting a significant nighttime sink. Nocturnal vegetative uptake (−11 to −21 pmol m−2s−1, negative values indicate uptake from the atmosphere) dominated nighttime net ecosystem COS flux estimates (−10 to −30 pmol m−2s−1) in both CO2 regimes. In comparison, soil uptake (−0.8 to −1.7 pmol m−2 s−1) was a minor component of net ecosystem COS flux. In both CO2 regimes, loblolly pine trees exhibited substantial COS consumption overnight (50% of daytime rates) that was independent of CO2 assimilation. This suggests current estimates of the global vegetative COS sink, which assume that COS and CO2 are consumed simultaneously, may need to be reevaluated. Ambient COS mixing ratios, species specific diurnal patterns of stomatal conductance, temperature and canopy position were the major factors influencing the vegetative COS flux at the branch level. While variability in branch level vegetative COS consumption measurements in ambient and enhanced CO2 environments could not be attributed to CO2 enrichment effects, estimates of net ecosystem COS flux based on ambient canopy mixing ratio measurements suggest less nighttime uptake of COS in R2, the CO2 enriched environment.
Highlights
With an atmospheric lifetime of approximately 5 years and an average global mixing ratio of 500 pptv, carbonyl sulfide (COS) is the most abundant sulfur-containing gas in the troposphere (Chin and Davis, 1995; Notholt et al, 2003) and the most important sulfur gas available for transport to the stratosphere (Chin and Davis, 1995)
We present measurements made at the Duke Forest (DF) Free Atmospheric Carbon Enrichment (FACE) site, which offered a unique opportunity to examine the magnitude and controls of vegetative and soil COS uptake in a temperate forest grown under present day and elevated CO2 (200 μL L−1 higher than ambient) conditions
Vegetation, soil, and canopy COS measurements in 2004 and 2005 all indicate that the Duke Forest FACE site was generally a net sink for COS
Summary
With an atmospheric lifetime of approximately 5 years and an average global mixing ratio of 500 pptv, carbonyl sulfide (COS) is the most abundant sulfur-containing gas in the troposphere (Chin and Davis, 1995; Notholt et al, 2003) and the most important sulfur gas available for transport to the stratosphere (Chin and Davis, 1995). Terrestrial ecosystems play a major role in the global COS budget as vegetative uptake is the largest global sink for the gas (Brown and Bell, 1986; Goldan et al, 1988; Kjellstrom, 1998; Kettle et al, 2002) while microbial consumption in soils is the second largest (Kesselmeier et al, 1999; Kettle et al, 2002) Oceanic emissions, including both indirect production from the oxidation of marine emissions of carbon disulfide (CS2) and dimethyl sulfide (DMS) (Barnes et al, 1994; Chin and Davis, 1993) and direct COS photochemical production from dissolved organic matter (Weiss et al, 1995), are the major COS sources to the troposphere. Since the same enzymes that utilize CO2 during photosynthesis consume COS (Protoschill-Krebs and Kesselmeier, 1992), the global COS
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