Carbonyl sulfide exchange in a temperate loblolly pine forest grown under ambient and elevated CO 2

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) CO 2. During calm meteorological conditions, ambient COS mixing ratios at the top of the forest canopy followed a distinct diurnal pattern in both CO 2 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−2 s−1, negative values indicate uptake from the atmosphere) dominated nighttime net ecosystem COS flux estimates ( −10 to−30 pmol m−2 s−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 CO 2 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. Correspondence to: M. L. White (mwhite@gust.sr.unh.edu)

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, 10 including both indirect production from the oxidation of marine emissions of carbon disulfide (CS 2 ) 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. Additionally, biomass burning and anthropogenic activities such as aluminum production and coal combustion are 15 significant terrestrial sources of COS (Chin and Davis, 1993;Kettle, 2000).
Currently, the magnitude of the natural vegetative and soil sinks is poorly constrained. For example, observations of seasonal variations in COS mixing ratios at a variety of Northern Hemisphere surface sites appear to be much larger than can be explained by current estimates of the COS vegetative sink (Montzka et al., 2007).
Since the same enzymes that utilize carbon dioxide (CO 2 ) during photosynthesis also consume COS (Protoschill-Krebs and Kesselmeier, 1992), the global COS vegetation sink has been calculated by scaling estimates of net primary production (NPP) by the ratio of the mean mixing ratios of COS and CO 2 (Kjellstrom, 1998;Kettle et al., 2002;Goldan et al., 1988 (Sandoval-Soto et al., 2005). In support of this, a recent modeling study using gross primary production (GPP) to estimate COS flux was able to accurately represent continental scale COS variability over the United States compared to models based on NPP estimates (Campbell et al., 2008). Initially, soils were considered sources of COS to the atmosphere (e.g. Aneja et al., 5 1979). However, this conclusion was based upon soil fluxes measured using dynamic chambers swept with S-free air. Under these conditions, an artificial gradient of COS within the chamber often forced soil emission of COS (Castro and Galloway, 1991). More recent studies using enclosures flushed with ambient air have revealed that most soils actually consume COS (Kuhn et al., 1999;Simmons et al., 1999;Yi et al., 2007;10 Steinbacher et al., 2004;deMello and Hines, 1994). However, the environmental controls over soil uptake processes were highly variable between sites and soil types. As a result, estimates of the global soil sink strength have a wide range of uncertainty (Watts, 2000;Kettle et al., 2002). The potential effect of rising CO 2 levels on COS consumption in terrestrial ecosys- 15 tems is also currently unclear. The warmer temperatures that will accompany higher CO 2 levels would likely increase plant NPP and growing seasons suggesting an enhanced vegetation sink (IPCC, 2007). However, laboratory studies indicate that the rate of COS consumption drops with the decreased expression of the photosynthetic enzyme carbonic anhydrase in algae grown under elevated CO 2 (Protoschill-Krebs et 20 al., 1995). For many plant species, COS uptake is also largely controlled by stomatal conductance (Geng and Mu, 2006;Sandoval-Soto et al., 2005) and there is evidence that enhanced CO 2 conditions will result in reduced stomatal conductance and frequency for many plant species, potentially further limiting the COS plant sink (Herrick et al., 2004;Kouwenberg et al., 2003;Kurschner et al., 1997;Greenwood et al., 2003; and vegetation and soil fluxes within control (ambient CO 2 ) and CO 2 enriched plots at DF in 2004 and. This study presents an analysis of the diel cycles and major factors influencing COS consumption processes within the two CO 2 regimes. Estimates of net ecosystem COS flux were also made and compared to measured vegetation and soil fluxes to better understand the impacts of individual sinks on COS uptake at the 15 site.

Methods
The measurements presented here are from three field studies conducted at the Duke Forest FACE site in September 2004 and June and September 2005. Descriptions of the significant measurements and calculations from each study are presented below. 20 All data is presented in local time (LT), which is UTC−04:00. All means are presented as mean ± standard error unless otherwise indicated. Statistical analyses were conducted using SPSS v. 15 sweetgum (Liquidambar styraciflua), elm (Ulmus alata), red maple (Acer rubrum), dogwood (Cornus florida) and a variety of oak-hickory species. The soils are classified as Enon series (fine, mixed, weathered alfisols) (Oh and Richter, 2005). The FACE experiment consists of 6 circular experimental plots, or rings, each 30 m in diameter and surrounded by vertical vent pipes that extend to the top of the canopy. 10 Three experimental rings receive additional CO 2 through the vent pipes to supplement atmospheric concentrations by 200 µL L −1 . The remaining three control rings are Interactive Discussion on a three gas chromatograph (GC) system equipped with two flame ionization detectors (FIDs), two electron capture detectors (ECDs), and a mass spectrometer (MS) for C 2 -C 10 nonmethane hydrocarbons (NMHCs), C 1 -C 2 halocarbons, C 1 -C 5 alkyl nitrates, and select oxygenated volatile organic compounds (OVOCs) and organic sulfur compounds including COS, carbon disulfide (CS 2 ) and dimethyl sulfide (DMS) (Sive et 5 al., 2005(Sive et 5 al., , 2007Zhou et al., , 2008. Carbon dioxide was sampled from 11-28 September simultaneously at the 16 m sampling height in R1 and R2 using LiCor 7000 infrared gas analyzers. Mixing ratios were measured every 5 s and averaged over one-minute intervals. Wind speed and direction, air temperature and photosynthetically active radiation (PAR) were collected at one minute intervals by the FACTS-I meteorological instruments in R2. The Climatronics F460 anemometer and wind vane and LiCor quantum sensor were mounted above the tree canopy, while air temperature was collected using thermocouples mounted within the mid-canopy. Meteorological data were averaged hourly to facilitate comparison to volatile organic compound (VOC) measurements. Precipitation data for the 15 FACE site was collected on a daily basis from the FACTS-I rain gauge.

Vertical profile measurements, September 2004 and June 2005
Vertical profiles of VOC mixing ratios from ground level to above the canopy were also measured on four occasions during the September 2004 and June 2005 field campaigns. During each profile measurement, evacuated canisters were filled to ambient 20 pressure at 5 height intervals from 1 m to 20 m above ground in each ring.

Vegetation flux measurements, June 2005
During the second field campaign (1-12 June 2005), direct vegetation VOC fluxes were measured within R1 and R2 (Sive et al., 2007). Vegetation flux measurements were collected approximately every two hours for loblolly pine and sweetgum, the two dominant trees at the site, over two 48-h sampling periods using dynamic branch enclo-5 sures. The enclosures were made of large clear Teflon bags supported by an external frame. A single tree branch was placed within the enclosure and exposed to a continuous flow of air from the canopy to maintain ambient pressure. A mass flow meter continuously monitored the rate of air flow into the bag (3-6 L min −1 ) while a cold palladium catalyst was used for ozone (O 3 ) removal. In addition to O 3 , the palladium catalyst 10 also removed a fraction of COS (0-400 pptv) and CO 2 (0-170 ppmv) from the ambient canopy air pumped into the bag. A more detailed description of the effect of catalyst COS depletion on flux measurements is given in Sect. 3.3.1. Air from the enclosure was vented through the outlet located deep within the bag to ensure that samples were well-mixed. Branches were enclosed and exposed to O 3 scrubbed canopy air for 24 h 15 prior to sampling. Temperature, relative humidity, and photosynthetically active radiation (PAR) within the bag were monitored continuously using thermocouples, relative humidity sensors, and photodiodes mounted on the branch sampled. During each flux measurement, three air samples were collected: ambient (A) air from the canopy drawn from the initial air intake, post-catalyst (PC) air sampled from the 20 air flow entering the bag, and bag (B) air sampled from the outlet vent. The sampling flow rate was controlled using a needle valve to make sure that positive outlet vent pressure was maintained continuously. Air samples were collected for VOC analysis as described above in 2-L electropolished stainless steel canisters and pressurized to 35 psig. A LiCor 7000 infrared gas analyzer and a miniature O 3 sensor (Mao et al.,25 2006) attached to the sampling manifold monitored CO 2 and O 3 levels in the sampled air flow. Flux measurements were made every 2 h from 4-6 June 2005 for the loblolly pines and 8-10 June 2005 for the sweetgum trees. After sampling, the branch enclosed Introduction

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Printer-friendly Version Interactive Discussion was removed from the tree and returned to UNH for direct measurement of leaf dry weight and leaf area. Vegetation fluxes (pmol m −2 LA s −1 ) were calculated for COS and CO 2 as follows: where C is the concentration of COS and CO 2 in pmol m −3 and µmol m −3 , respectively, 5 in both the bag (B) and post-catalyst (PC) air samples, flow is the rate of air flow into the bag in m 3 s −1 , and LA represents the single sided leaf area (m 2 ) of the enclosed branch. Flux errors were propagated as described by Taylor (1982) from individual measurement uncertainties for COS (5%), CO 2 (5%), leaf area (10%), and the standard error of the mean inlet flow rate during measurement (2%).

Soil flux measurements, June and September 2005
Soil fluxes were measured within Ring 1 and 2 on 9 June and 20 September 2005 using static enclosures as described in Varner et al. (1999). Measurements were made using four 30 cm×30 cm Teflon coated aluminum collars placed in the soil a month prior to field sampling. Two collars (labeled R1 collar A, R1 collar B, R2 collar A, and R2 col-15 lar B) were placed in both rings. During sampling, a Teflon coated aluminum chamber (30 L volume) was placed over each collar for approximately 30 min. An ambient pressure canister sample was collected from the chamber headspace every 10 to 12 min for a total of 4 samples. On both 9 June and 20 September, all soil measurements were collected between 08:00 and 19:00 local time with each collar being sampled 3 20 to 4 times throughout the day. Canister air samples were returned to UNH for VOC analysis as described above. Chamber and soil temperatures at 0, 5 and 10 cm depths were measured each time a collar was sampled. Additionally, soil moisture data for 9 June and 20 September 2005 was obtained from the FACTS-1 time-domain reflectometry (TDR) instruments Introduction

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Printer-friendly Version Interactive Discussion ring were calculated as the average of half-hourly measurements recorded by the four TDR instruments located within each plot. Soil fluxes (pmol m −2 s −1 ) were calculated as the initial change in chamber COS concentration vs. time, d C d t t=0 , in pmol m −3 s −1 multiplied by the chamber volume, V c (m 3 ), and divided by collar area, A c (m 2 ): In 11 of the 30 soil flux measurements made in June and September 2005, the change in chamber concentration was linear over the time of chamber deployment. In these cases, d C d t t=0 was calculated as the linear regression slope of chamber concentration versus time. In the remaining 19 soil flux measurements, the rate of change in chamber 10 concentration decreased with time and d C d t t=0 was calculated by fitting the following exponential equation to the data (deMello and Hines, 1994): where, "a" was C eq , or the concentration reached when COS within the chamber reached equilibrium with the concentration in soil micropores, "b" was C eq minus C t=0 , 15 the COS concentration at t=0, and k is a rate constant. The values of a and k were calculated iteratively for each flux measurement and used to determine d C d t t=0 as follows: This value for d C d t t=0 was then used in Eq.
(2) to calculate net COS flux. 20 Soil flux errors were propagated as described by Taylor (1982) from the standard errors for the linear regression slope or non linear coefficients, k and a, used during flux calculation as well as the individual measurement uncertainties for the chamber volume (2%), collar area (2%) and COS mixing ratios (5% Net ecosystem COS uptake rates were calculated from changes in ambient canopy mixing ratios under stable nocturnal boundary layer conditions. This approach for calculating ecosystem COS flux was employed by Kuhn et al. (1999) in a temperate oak 5 forest and has been used in a variety of studies on other trace gases (e.g. White et al., 2008;Sive et al., 2007;Varner et al., 2008). Under stagnant wind conditions associated with the stable boundary layer, changes in ambient mixing ratios should reflect local sources and sinks. As a result, net nighttime fluxes, NF, can be estimated as: Where dC d t is the change in concentration over time (pmol m −3 s −1 ) and ML is the height of the mixed layer (m).
Only measurements taken when wind speeds were ≤0.8 m s −1 were used to calculate NF to limit the influence of large-scale horizontal advection on our flux esti- 15 mates. Unfortunately, only one night during each campaign met these conditions (15-16 September 2004 and4-5 June 2005). As nocturnal mixing layer height was not measured during either campaign, ML height was assumed to be 125 m, which represents the median value of the nocturnal inversion layer height range (50-200 m) measured at a variety of similar midlatitudinal rural locations (Galbally, 1968;Güsten et al., 20 1998;Hastie et al., 1993;Shepson et al., 1992). Concentration changes were not linear over time and individual fluxes were calculated for each hour time step. Flux errors were propagated from the individual measurement uncertainties in the canopy COS measurements (5%) and the estimated uncertainty in the ML height (60%) according to Taylor (1982 For comparison, total vegetation and soil fluxes scaled to ground area were also estimated for both nights. The vegetation fluxes were calculated as follows: errors of the means used in calculation as described by Taylor (1982

Canopy measurements, September 2004
Comparison of simultaneous R1 and R2 measurements of COS and ethane at 16 m height is presented in Fig. 1 for the entire data collection period. Ethane mixing ratios demonstrate variability between the two rings independent of any natural VOC sources 5 or sinks. This anthropogenic trace gas exhibited a very strong 1:1 relationship indicating minimal variability associated with either ring. In contrast, the R1 versus R2 relationships for COS showed much greater scatter that reflects its natural sinks within temperate forests. The time series of meteorological data, CO 2 , and COS measurements conducted Exceptions to this pattern occurred during passage of Hurricanes Ivan and Jeanne on 16-19 and 27-29 September, respectively, which resulted in revolving wind directions and a maximum wind speed of 11 m s −1 (Fig. 2a). Winds from the southeast associated 15 with these hurricanes also coincided with elevated levels of COS indicative of marine influence ( Fig. 2a and c). In contrast, moderate wind speeds from the north and east, warm daytime temperatures, and sunny conditions generally prevailed at the site during 21-27 September ( Fig. 2a and b). During this time period, ambient CO 2 mixing ratios in R1 followed a pronounced diel pattern with minimum levels in the late afternoon 20 corresponding to photosynthetic uptake and maximum levels overnight. COS mixing ratios in R1 and R2 also exhibited a diel pattern during this time period, although less pronounced than CO 2 and with minimum mixing ratios at night ( Fig. 2c and 3, non-hurricane night means: R1=340±6 and R2=346±5). Similar diurnal patterns have been observed in a temperate oak forest in California (Kuhn et al., 1999), a 25 tropical rainforest in Cameroon (Kesselmeier et al., 1993), and a spruce forest in Germany (Steinbacher et al., 2004). The diurnal variability is attributed to a combination of turbulent transport of COS during the day (Kesselmeier et al., 1993) (Steinbacher et al., 2004;Kuhn et al., 1999). Daytime mixing ratios during non-hurricane periods at DF averaged 380±4 pptv in R1 and 373±3 pptv in R2, well below the global mean mixing ratio of 500 pptv. These lower mixing ratios of COS are consistent with observations at low altitude, Northern Hemisphere continental sites during summer and reflect the influences of a significant 5 vegetation sink (Kuhn et al., 1999;Montzka et al., 2007;Steinbacher et al., 2004). The lowest COS mixing ratios (R1: 227 pptv, R2: 257 pptv), observed at DF on the night of 15 September, coincided with the lowest wind speeds observed during the study period and further indicate a strong nighttime deposition for this gas to the forest (Fig. 2). From 18:00 to 01:00 LT that night, COS mixing ratios in R1 were also con-10 sistently lower than in R2 by 10-60 pptv. Close examination of the hourly mean COS mixing ratios in each ring during non-hurricane periods (15)(16) revealed that COS mixing ratios were generally lower overnight (23:00 to 04:00 LT) in R1 than in R2 (Fig. 3). While these differences between the two rings were small on average (0-20 pptv), they do imply that the nighttime COS sink was slightly stronger in 15 R1 than in R2 during September 2004.

Vertical profile measurements, September 2004 and June 2005
Further support for a forest sink for COS is evident in the vertical profile measurements from September 2004 and June 2005 (Fig. 4). Taken during day and night periods and in autumn and summer, these vertical profiles all indicated decreasing COS mixing 20 ratios from above the forest canopy to ground level. However, the pattern of COS decreases over the vertical gradient varied considerably reflecting different sampling times and wind conditions as well as spatial variability in the two rings. For example, on 26 September 2004 at 18:00 LT (Fig. 4a), R1 showed a relatively consistent reduction in COS mixing ratios from 450 pptv above the canopy (20 m) to 362 pptv at 1 m above 25 the ground suggesting a strong canopy and ground level sink. In contrast, COS mixing ratios in R2 decreased from 413 pptv above the canopy to 370 pptv at 10 m above ground before increasing to 394 pptv again at ground level (1 m surface source at that location. Decreased COS mixing ratios occurred at ground level in all profiles in June 2005, including those in R2, indicating that conditions favored a stronger surface sink during the second field study (Fig. 4b, c, and d). Furthermore, the profile obtained at 00:30 LT on 6 June 2005 (Fig. 4c) indicated strong nighttime canopy sinks in both R1 and R2 5 with COS mixing ratios from 1 to 15 m above ground (R1=309-359 and R2=316-412 pptv) well below levels observed above the canopy (406 and 417 pptv at 20 m above ground in R1 and R2, respectively). Evidence of a canopy sink was more limited in the daytime vertical profiles taken on 5 and 10 June (Fig. 4c and d). A decrease in average COS mixing ratios in the three elevated CO 2 rings was observed on 5 June 10 from 490±20 pptv at 15 m to 435±9 pptv at 10 m above ground. However, profiles collected simultaneously in the three control rings indicated a slight increase in COS mixing ratios from 460±20 pptv to 490±10 pptv over the same elevation difference (Fig. 4c). Furthermore, profiles obtained on 10 June showed little change in COS mixing ratios throughout the canopy in R2 and an increase in R1 from 460±20 pptv 15 at 20 m above ground to 520±20 pptv between 10 and 15 m height within the canopy (Fig. 4d).
Previous studies of ambient COS mixing ratios in loblolly pine forests indicated that COS uptake is highly variable. For example, vertical profiles obtained at the University of Georgia B.F. Grant research forest, a loblolly pine stand located southeast of Atlanta, 20 GA, suggested significant release of COS within the forest canopy (Berresheim and Vulcan, 1992). Ambient mixing ratios in the Georgia pine stand (500-900 pptv) were also significantly higher than those observed at DF indicating COS sources dominated over sinks in that region.

Vegetation flux measurements, June 2005
Branch enclosure measurements performed in June 2005 indicate both loblolly pine and sweetgum trees at DF were sinks for COS (Figs. 5 and 6). Negative fluxes, indicating uptake of COS from the atmosphere, dominated COS exchange for both species over 48 h sampling periods. The distinct diurnal patterns of COS fluxes observed for the 5 two tree species reflect the multiple factors that can impact vegetative COS exchange such as ambient COS levels, environmental controls over stomatal conductance, and leaf photosynthetic capacity. The relationships between the COS vegetation fluxes measured at DF and these factors will be discussed in more depth in the following two sections.

Post-catalyst COS levels
For all trees sampled, net COS flux exhibited the strongest correlation with the postcatalyst (PC) COS levels flowing into the bag (Fig. 7). Similar strong correlations have been observed for crop plants (Kesselmeier and Merk, 1993) and deciduous trees in northern China (Geng and Mu, 2006) indicating that the major factor controlling the rate 15 of COS uptake for many plants is the amount of ambient COS. The linear regressions for these relationships (Fig. 7) can be used to calculate compensation points, or the COS mixing ratios at which production and consumption processes balance each other and net flux equals zero. Interactive Discussion depletion in PC COS mixing ratios responsible for the minimum uptake rates (least negative fluxes) observed for both R1 and R2 loblolly pines did not reflect ambient (A) COS conditions. As Fig. 5c indicates, ambient COS levels during loblolly pine measurements actually reached maximum values during the day following a diurnal pattern similar to that observed during non-hurricane periods in September 2004. The 5 levels of PC COS were more consistent throughout the two day sampling period for the sweetgum trees although PC COS was depleted compared to ambient COS levels (Fig. 6c). The removal of COS from the inlet airflow suggests that the magnitude of COS uptake rates measured in both loblolly pine and sweetgum enclosures was lower than expected for the open canopy. As the strong negative correlations between PC 10 COS levels and net COS uptake rates indicate (Fig. 7), higher ambient COS mixing ratios would result in higher uptake rates.
To provide a representative picture of loblolly pine and sweetgum COS uptake, independent of catalyst COS depletion, deposition velocities, V d (cm s −1 ), are provided in  Kuhn et al., 1999;Xu et al., 2002;Sandoval-Soto et al., 2005;Geng and Mu, 2006).

Stomatal conductance and photosynthetic capacity
The day and night relationships between COS net flux and PC COS levels displayed 25 in Fig. 7  Interactive Discussion uptake rates and PC COS mixing ratios for R1 and R2 loblolly pines ( Fig. 7a and b) resulted in relatively consistent day and night COS V d ( indicates that a wide variety of plants, including several pine species, exhibit significant nighttime stomatal conductance (Caird et al., 2007). The COS uptake patterns observed in this study indicate that loblolly pine may fall into this category. The absence of a strong relationship between PAR and COS V d for the loblolly pine trees is particularly significant when considering the nighttime depletion observed in 10 the ambient canopy measurements from September 2004. Previous observations of nighttime depletion in ambient forest COS mixing ratios have been attributed primarily to soil sinks as vegetative uptake was largely limited to daylight hours (Kuhn et al., 1999;Steinbacher et al., 2004). However, comparable day and night V d for the dominant tree species in the FACE rings at DF suggests vegetative uptake was a major 15 nocturnal sink at this site, a possibility that will be explored in more depth in Sect. 3.5.
It is also interesting to note that COS uptake by loblolly pine trees did not have a significant correlation with CO 2 uptake (Fig. 8). Several previous studies of vegetative COS consumption have indicated a strong relationship between photosynthetic CO 2 assimilation and COS exchange (Kesselmeier and Merk, 1993;Kuhn et al., 1999;20 Geng and Mu, 2006;Xu et al., 2002). However, Geng and Mu (2006) noted that strong correlations between CO 2 and COS uptake in two Chinese deciduous trees were only observed during diurnal measurements made over one day. No significant linear correlations were observed when a larger data set of measurements was considered, which was attributed to the large variation in ambient COS mixing ratios (Geng and Mu, 2006). 25 Montzka et al. (2007) also noted the important influence of respiration and CO 2 loss during photosynthesis on vegetative CO 2 exchange, processes which are not mirrored in vegetative COS exchange. It is possible that any daytime relationship between COS uptake and CO 2 assimilation for the loblolly pines was obscured by the significant influ-

Conclusions
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Interactive Discussion ence of the wide range of PC COS mixing ratios on net COS flux in combination with loblolly pine respiration and CO 2 loss. However, the large nighttime COS V d measured in this study indicates significant COS consumption occurred independently of CO 2 assimilation in the loblolly pines. Interestingly, the mean nighttime R2 loblolly pine COS V d was significantly lower than 5 the day and night means for the R1 loblolly pine (Table 1). Long-term photosynthetic acclimation to elevated CO 2 in loblolly pines at DF has been linked to reduced activity of the carboxylation enzyme Rubisco, involved in the light-independent reactions of the Calvin cycle, in one-year old needles (Rogers and Ellsworth, 2002). As Rubisco can enhance COS uptake by consuming the CO 2 or HCO − 3 produced by COS hydrolysis 10 (Protoschill-Krebs and Kesselmeier, 1992), any reduction in its activity is likely to affect COS uptake rates. As a result, reduced COS V d for the R2 loblolly pine could reflect photosynthetic acclimation to long-term elevated CO 2 exposure (Rogers and Ellsworth, 2002). In contrast to the loblolly pines, the clear separation between day and nighttime flux 15 relationships with PC COS for the deciduous sweetgum trees does reflect substantial stomatal closure at night ( Fig. 7c and d) For the R1 sweetgum, net nighttime flux and PC COS mixing ratios did exhibit a significant correlation (Fig. 7c) fluxes for the R2 sweetgum were consistently near zero and nighttime V d values had no significant relationship to PC COS levels (Fig. 7d) suggesting that the R2 sweetgum stomates were generally closed overnight. Sweetgum trees exposed to elevated CO 2 , like those in R2, have shown significant reductions in stomatal conductance (Herrick et al., 2004). If this CO 2 enrichment effect was responsible for the minimal nighttime COS 5 V d observed in R2, a similar reduction in the R2 sweetgum day COS V d should also have occurred. Instead, mean daytime sweetgum COS V d were significantly higher in R2 compared to R1 (Table 1; R1=1.1±0.1 cm s −1 , R2=1.5±0.1 cm s −1 ). Significantly higher daytime CO 2 V d in R2 (Table 1; R1=0.6±0.1 cm s −1 , R2=0.9±0.2 cm s −1 ) actually suggests that the sweetgum branch sampled in R2 had a higher photosynthetic 10 capacity than that in R1. These differences in COS and CO 2 V d most likely reflect variations in the canopy position of the two sweetgum branches sampled as the branch in R1 was more sheltered (less open sunlight) than the R2 branch. Observations of sweetgum sun and shade leaves at DF indicates that the photosynthetic capacity of sun leaves at the top of the canopy was 80% more than that of shade leaves in the 15 understory (Herrick and Thomas, 2001). Similarly, sun leaf stomatal conductance was more than twice that of shade leaves (Herrick et al., 2004). Similar effects of canopy position on COS consumption rates have also been noted for a tropical rainforest tree species in Cameroon where COS deposition at the top of the canopy was nearly 4 times that measured for the same species growing at ground level (Kesselmeier et al., . It should be noted that the limited sample size in this study may have prevented direct observations of the effect of CO 2 enrichment on COS uptake. As noted previously, long-term CO 2 enrichment does have significant effects on both photosynthetic capacity and stomatal conductance in loblolly pines and sweetgums (Rogers and Ellsworth,25 2002; Herrick et al., 2004). Because stomatal conductance and photosynthetic capacity variability resulting from environmental and spatial heterogeneity were major factors impacting COS uptake rates in this study, it is probable that CO 2 effects on these characteristics will also have an impact on vegetative COS uptake within the two environments. This potential influence on the net ecosystem flux estimates for the two rings will be considered more in Sect. 3.5.

Soil flux measurements, June and September 2005
Soil was also a net sink for COS at DF (Table 2, Fig. 9). The range of net fluxes observed in the soil static chamber measurements (R1: −3 to −0.53 pmol m −2 s −1 ; R2:  (Yi et al., 2007)). There were no significant differences between R1 and R2 mean net fluxes indicating that CO 2 enrichment had 10 little effect on soil COS consumption. However, one significant difference was observed in the soil fluxes at DF between R2 collar A and B on 9 June 2005 (Fig. 9a). The mean R2 collar B flux for that sampling day was −0.4±0.2 pmol m −2 s −1 compared to −2.0±0.2 pmol m −2 s −1 for R2 collar A. This was not a consistent difference as the mean R2 collar fluxes were . Laboratory studies indicate that soil temperature, moisture content, porosity, and ambient levels of COS all can affect COS consumption in soils (Kesselmeier et al., 1999;Van Diest and Kesselmeier, 2008). A recent study of soil uptake indicates that water-filled pore space, which reflects soil moisture content, struc-20 ture, and porosity, may be the primary factor influencing COS uptake and suggests that soil gas diffusivity regulates COS uptake (Van Diest and Kesselmeier, 2008). While there were no significant differences in soil temperature or ambient COS levels between the two collars on 9 June, the variability in measured fluxes could reflect a spatial difference in soil moisture content, or porosity in R2 that was not captured by our measurements. For example, spatial variability from differences in topography and/or vegetation was not captured in the measurements of soil moisture content, which were averages of measurements made from four TDR instruments located in each ring. Fur-17239 Introduction

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Printer-friendly Version Interactive Discussion thermore, the effect of soil structure and porosity on gas diffusivity was not measured. Localized differences in soil moisture or gas transport conditions might have favored COS consumption at R2 collar A and the two R1 collars compared to R2 collar B. The only significant correlation observed for the DF soil fluxes was between deposition velocities, V d , and soil surface temperature (Fig. 10, r 2 =0.32, p=0.001). The 5 proportion of V d variability attributable to the temperature relationship, indicated by the coefficient of determination (r 2 ), was small and suggests that there were other major unidentified factors affecting the COS uptake flux at DF. Other field studies of forest soils have shown widely variable responses to soil moisture and temperature. For example, optimum net fluxes were observed between 8 and 9 • C in a German spruce 10 forest soil while significant uptake was observed at soil moisture contents well above those observed in laboratory studies (Steinbacher et al., 2004). In three subtropical forest types in China, net COS soil uptake was not correlated with soil moisture or temperature except when considered together in multiple regression second order polynomial fits (Yi et al., 2007). The strongest individual correlations observed in these 15 Chinese forests were actually with soil respiration and ambient levels of COS. Such highly variable relationships with COS net flux reflect both the complexity of interactions in natural environments as well as the potential effect of different soil types and microbial communities on COS gas exchange. For example, differences in moisture dependent maximum COS deposition velocities observed in boreal soil incuba-20 tions from several different European and Asian locations were primarily accounted for by differences in soil porosity (Van Diest and Kesselmeier, 2008). In this same study, the temperature range of maximum COS deposition velocities was consistently higher for the boreal soil samples than for the one temperate soil sample, suggesting variable adaptations of the microbial communities within different climate zones. Further stud- 25 ies of COS uptake in a variety of soils in both laboratory and field settings are clearly needed to more fully characterize this COS sink.

Estimates of net ecosystem COS flux, September 2004 and June 2005
The net ecosystem flux results, NF, calculated using Eq. (5) for 15-16 September 2004 and4-5 June 2005 represent the best estimate of ecosystem level nighttime exchange processes at DF (Fig. 11). It should be noted that the limited number of low wind speed nights and the lack of ML height measurements contributed to the large uncertainty for 5 these calculations. However as a first approximation, the NF estimates allow a valuable comparison between the individual vegetation and soil sinks measured and the net COS uptake rates observed at DF. The NF estimates were generally consistent with the combined vegetation and soil flux estimates for both nights (Fig. 11). It should be noted that the vegetation and 10 soil fluxes for 15 September 2004 (−19±6 and −13±4 pmol m −2 s −1 combined for R1 and R2, respectively) were calculated using deposition velocities measured at other times during the year and the slightly higher mean NF estimates for that night (−30±20 pmol m −2 s −1 for both R1 and R2) could reflect the effect of seasonal differences in temperature and/or soil moisture on stomatal conductance and COS uptake. combined in R1 and R2, respectively) were all calculated from measurements made within the same week. This agreement between the COS flux calculations indicates that the NF estimates and enclosure measurements reasonably represent ecosystem 20 exchange processes. It is worth noting that NF estimates of R2 ecosystem COS uptake in June 2005 (−10±10 pmol m −2 s −1 ) were approximately half the NF estimates in R1 (−20±10 pmol m −2 s −1 ; Fig. 11). Because of the large uncertainties associated with the NF calculations, these differences cannot be considered significant. However, 25 the observations of reduced stomatal conductance in sweetgum trees and photosynthetic downregulation in loblolly pines associated with long-term CO 2 enrichment at DF (Rogers and Ellsworth, 2002;Herrick et al., 2004) suggests that lower ecosystem ACPD Introduction

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Interactive Discussion COS uptake rates in R2 could reflect the effects of CO 2 enrichment. A reduction in the vegetative COS uptake capacity due to CO 2 enrichment effects would also explain why higher overnight canopy COS levels in R2 on 4-5 June 2005 (R1=290±20 pptv, R2=350±20 pptv) did not result in greater R2 net ecosystem COS uptake rates. However, similar NF estimates in R1 and R2 in September 2004 suggest that the magnitude 5 of this effect was not consistent throughout the year and may depend upon the variability of other environmental controls over stomatal conductance and photosynthetic capacity such as temperature, soil moisture, or dominant leaf age. Compared to other forest studies, the estimates of nighttime COS uptake at DF were large. Similar NF estimates of net ecosystem flux in a California oak forest 10 (−2.4 pmol m −2 s −1 (Kuhn et al., 1999)) were an order of magnitude smaller than those calculated for DF, while nighttime COS fluxes measured using relaxed eddy accumulation (REA) methods in a German spruce forest were generally positive indicating emission (Xu et al., 2002). Soil was the only observed COS nighttime sink in both of those studies. In contrast, the significant overnight deposition rates observed for 15 loblolly pines at DF strongly influenced nighttime COS fluxes. Overall, COS uptake by vegetation comprised 37 to 100% of observed NF net ecosystem flux estimates (Fig. 11).
This suggests that as much as 33% of the daily COS vegetative consumption at DF occurred independently of CO 2 assimilation. These results have important implications 25 for global estimates of the vegetative COS sink strength. Sandoval-Soto et al. (2005) and Montzka et al. (2007) have pointed out that estimates of the global COS vegetative sink made by scaling net primary productivity (NPP=CO 2 photosynthetic consumption -respiration) by the ratio of the mean atmospheric mixing ratios for COS and

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Interactive Discussion CO 2 should be reconsidered to account for the larger vegetative V d of COS compared to CO 2 . In support of this, recent model results of COS atmospheric concentrations based on gross primary productivity (GPP=CO 2 photosynthetic consumption only) and leaf scale relative uptake rates measured in chamber experiments were in much closer agreement with observations of vertical COS depletion in the atmospheric boundary 5 layer of the continental United States (Campbell et al., 2008). Observations of significant nighttime stomatal conductance in loblolly pine and a wide variety of other plant species (Caird et al., 2007) suggests that COS vegetative uptake that occurs independently of CO 2 photosynthetic consumption could also be substantial and must be considered in global vegetative sink strength estimates.

Conclusions
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. Vegetative uptake patterns for the loblolly pine and sweetgum trees sampled were significantly different in R1 and R2, but these differences most likely reflected environmental and canopy spa-15 tial effects on the stomatal conductance and photosynthetic capacity of the branches measured rather than CO 2 enrichment effects.  Chin, M. and Davis, D. D.: A reanalysis of carbonyl sulfide as a source of stratospheric background sulfur aerosol, J. Geophys. Res., 100, 8993-9005, 1995. deMello, W. Z. and Hines, M. E.: Application of static and dynamic enclosures for determining dimethyl sulfide and carbonyl sulfide exchange in Sphagnum peatlands: Implications for the magnitude and direction of flux, J. Geophys. Res., 99, 14601-14607, 1994. 5 Elliot, S., Lu, E., and Rowland, F. S.: Rates and mechanisms for the hydrolysis of carbonyl sulfide in natural waters, Environ. Sci. Technol., 23, 458-461, 1989.   Interactive Discussion 2.6. Error bars for the vegetation uptake rate estimates were propagated from the standard error of the mean night vegetation V 5 6 7 8 9 d values from June 2005 (n ≥ 23) and the mean ambient COS values for the nights represented (n ≥ 5). Error bars for the soil uptake rate estimates were propagated from the standard error of the mean nighttime soil V d values from September 2005 (n = 8) and June 2005 (n = 7) and the mean ambient COS values for the nights represented (n ≥ 5).