Measuring Simultaneous Production and Consumption Fluxes of

Oct 16, 2007 - 507 McCone Hall #4740, Department of Geography and Berkeley Atmospheric Sciences Center, University of California at Berkeley, Berkeley...
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Environ. Sci. Technol. 2007, 41, 7837-7843

Measuring Simultaneous Production and Consumption Fluxes of Methyl Chloride and Methyl Bromide in Annual Temperate Grasslands ROBERT C. RHEW* AND TRIFFID ABEL 507 McCone Hall #4740, Department of Geography and Berkeley Atmospheric Sciences Center, University of California at Berkeley, Berkeley, California 94720

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 ∼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.

Introduction Methyl chloride (CH3Cl) and methyl bromide (CH3Br) transport halogens to the stratosphere where they catalyze the destruction of ozone (1, 2). CH3Cl and CH3Br have both natural and anthropogenic sources, including oceanic production, terrestrial plants and fungi, biomass burning, industrial and agricultural use, and leaded gasoline combustion (3). Because of its ozone depleting potential, the use of CH3Br as a fumigant is being phased out internationally in accordance with the Copenhagen Amendments to the Montreal Protocol. Known loss mechanisms of atmospheric CH3Cl and CH3Br include oxidation by hydroxyl radicals (OH), oceanic degradation, and consumption by soil (3). Current estimates of methyl halide losses greatly exceed quantified known emissions, suggesting that new sources need to be identified and/or that the sink terms need to be reduced. Reconciling the budget is essential to determine the anthropogenic contribution to overall source strengths, which are currently estimated at ∼10-15% for CH3Cl and ∼2040% for CH3Br (3). Clarifying the role of the terrestrial biosphere in the atmospheric budgets of these compounds has been complicated by the fact that terrestrial ecosystems act as both sources and sinks of these compounds. In general, methyl * Corresponding author e-mail: [email protected]. 10.1021/es0711011 CCC: $37.00 Published on Web 10/16/2007

 2007 American Chemical Society

halide production has been associated with plants and fungi, while consumption has been associated with soil bacteria (3). However, some recent laboratory studies indicate that plants and soils can play opposite roles as well: methyl halides may be produced in soils or litter through the decomposition of organic matter (4, 5) and consumed by plants exposed to high concentrations (6). Terrestrial field studies typically do not separate the activities of plants, soils, and fungi at individual sites, and the results tend to reveal a mosaic of seasonal sources and sinks. Flux measurements in shrublands, temperate forests, peatlands, tundra, and rice paddies all show net emissions and net uptake, depending on the plant species present, soil moisture levels, and season of measurement (7-11). While net ecosystem flux measurements are useful in balancing the budgets of the methyl halides, it is the gross production and consumption rates that are used to estimate atmospheric lifetimes and provide greater insight into the factors that regulate surface fluxes. Therefore, an essential next step is to separate the net flux into its gross production and consumption components. Grasslands cover roughly 20% of the total global land surface (12), occupying 1.4 million square miles in North America alone. Previous studies report that temperate grasslands account for a quarter of the natural soil sink for CH3Br (13, 14). Given the close correlation between CH3Br and CH3Cl uptake rates in other biomes (8, 15), temperate grasslands could be an important sink for CH3Cl as well. On the other hand, recent field measurements in southern California (8), and Tasmania, Australia (16), show that grasscovered sites (Avena fatua, Bromus diandrus, Nassella pulchra, Poa poiformis, and Lolium perenne) are frequently net sources for CH3Br and CH3Cl, with large net emissions from sites containing Brassica plants. Incubation techniques using small additions of stable carbon isotope tracers can separate production and consumption fluxes without the disturbances caused by physically separating components or introducing high concentrations of the compounds of interest (17). A laboratorybased isotope tracer method was recently developed to separate gross fluxes of CH3Br and CH3Cl in soils (15), modified from a technique to separate biological and chemical degradation rates of CH3Br in seawater (18). Here, we present field results using this isotope tracer technique in conjunction with flux chambers to quantify gross emission and uptake rates of these methyl halides.

Experimental Procedures Field Sites. Field studies were conducted between July 2004 and April 2005 at two annual grasslands in California (Table 1): the Jepson Prairie Preserve (38°,17′ N, 121°,49′ W) in Solano County and Sibley Volcanic Regional Preserve (37°,51.1′ N, 122°,11.4′ W) in Alameda County. The Mediterranean climate (cool wet winter and warm dry summer) favors winter annual grasses, such Avena, Bromus, Hordeum, Festuca, and Lolium (19). The growing season typically runs from October to May, and both grasslands are managed primarily through seasonal grazing. At Jepson Prairie, three field outings were conducted (July 9, 2004; October 30, 2004; and March 12, 2005; Table 1) with measurements in both grassland and riparian habitats. Annual grasses such as the wild oat (Avena sp.), Italian ryegrass (Lolium multiflorum), and medusahead (Taeniatherum caput-medusae) are widespread, but a variety of dicots, including yellow star-thistle (Centaurea solstitialis), perennial pepperweed (Lepidium latifolium), and filaree VOL. 41, NO. 22, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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3.0 ( 2.0 -4.9 to 9.9 -617 to -399 14.8 ( 1.4 22.1 ( 2.0 5 5

10.3 ( 1.0 3 3

14.0 ( 3.6

27.0 ( 2.9

1.2 ( 4.9 -14 to -7.2 -540 to -440

14 to 74 4.0 ( 1.7 26.6 ( 1.8 22.8 ( 1.3 3

0 2

3

August 24, 2004: dry, non-growing season Grassland annual grasses (dead) December 20, 2004: wet, early growing season Grassland annual grasses (young, unidentified) April 10, 2005: wet, mid-growing season Grassland Bromus sp., Hordeum sp., P. aquilinum, C. maculatum

3 3

26.9 ( 0.3

-2.9 ( 7.7 -0.2 to 19.6

0.2 ( 1.1 5.9 to 90.5 78 to 2460 46.4 ( 6.5 17.9 ( 2.8 20.1 ( 5.4

2 2 2 2

Sibley Volcanic Regional Park (37°,51.1′ N, 122°,11.4′ W)

0.2 ( 1.2 -1.4 to 2.2 16.3 ( 6.1 17.5 ( 1.1 22.7 ( 2.1

-121 to -59

1.8 ( 5.6 -1.3 ( 2.8 -14 to -4.4 22 to 44 -466 to -247 159 16.9 ( 1.7 15.5 ( 0.8 21.7 ( 1.2 20.1 ( 0.6

0 0 2 2

21.4 ( 0.8 48.5 ( 3.4

n.d. n.d. 0.7 to 4.5 2040 to 2650 -33 to -20 33700 to 42100 22.3 ( 3.0 27.8 ( 2.1 25.3 ( 0.7 26.5 ( 0.3

17 to 271 1.3 ( 0.3 27.5 ( 3.8 24.8 ( 2.8 0

Jepson Prairie Preserve (38°,17′ N, 121°,49′ W)

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July 9, 2004: dry, non-growing season Grassland Avena sp. (dead), T. caput-medusae (dead), L. latifolium Riparian Leymus triticoides, D. spicata Dry playa C. truxillensis, F. salina October 30, 2004: wet, end of non-growing season Grassland Avena sp. (dead), Erodium sp. Riparian L. triticoides, D. spicata March 12, 2005: wet, mid-growing season Grassland Avena sp., L. multiflorum, Medicago polymorpha, T. caput-medusae, flowering Asteraceae Riparian D. spicata, L. latifolium

spiked unspiked predominant species field outing

9

32.8 ( 0.8 4.3 ( 1.1

-0.6 to 3.5

n.d.

F-113 CH3Br CH3Cl

unspiked net fluxes (nmol/m2/day)

soil H2O (% wet wt) soil temp (5 cm) (°C) air temp (°C) no. of expts

TABLE 1. Predominant Vegetation, Experimental Conditions, and Net Fluxes at Two California Annual Grasslands 7838

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(Erodium sp.) are also common. The northern portion of the Prairie is crossed by a small creek, Barker Slough, and grasses along its banks include saltgrass (Distichlis spicata) and creeping wildrye (Leymus triticoides). The first outing (July 2004) consisted of net flux measurements only, with two additional sites on a seasonally dry playa that was sparsely vegetated with the salt tolerant species alkali heath (Frankenia salina) and alkali weed (Cressa truxillensis). At the Sibley Volcanic Regional Preserve, three field outings were conducted (August 24, 2004; December 20, 2004; and April 10, 2005; Table 1), with measurements focusing on the annual grasses. The predominant grasses were bromes (Bromus sp.) and hare barley (Hordeum murinum), although a set of measurements covered bracken fern (Pteridium aquilinum) and poison hemlock (Conium maculatum). All flux chamber sites at the Sibley Preserve were located within 70 m of each other. Field Sampling. Gas fluxes were measured using a twocomponent, temperature-controlled, transparent static flux chamber (20), covering a basal area of 0.233 m2 and a maximum headspace of 152 L (Figure S1). Air samples were collected in previously evacuated 1 L electropolished (LabCommerce, San Jose, CA) or 3 L fused silica lined (Restek, Bellefonte, PA) stainless steel canisters. When sampling, a vent tube on the chamber was opened for pressure equilibration. Internal temperatures were maintained close to ambient temperatures by pumping chilled water through an internal aluminum coil. The number of moles of air in the chamber was estimated based on the volume of enclosed headspace, chamber temperature, and ambient pressure. Net fluxes (from unspiked chamber experiments) were measured at every study site (Table 1), which entailed taking chamber air samples at 1, 15, and 30 min following enclosure. Ambient air samples were also taken during these chamber experiments. Fluxes were determined by the concentration changes in the chamber headspace over time, with small corrections for the input of ambient air through the pressure vent. Starting with the second outing, 18 of 20 unspiked chamber experiments were followed by a spiked chamber experiment at the same plot to determine gross fluxes. In these experiments, 33-40 mL of a gas mixture was injected into the headspace through a septum in the chamber lid immediately after closure. This spiking gas consisted of 464 parts per billion (ppb) 13CH3Br, 4.62 parts per million (ppm) 13CH Cl, and 4.61 ppm CFC-113 (F-113, CCl FCClF ) in 3 2 2 nitrogen, yielding initial concentrations inside the chamber of ∼100 parts per trillion (ppt) 13CH3Br, ∼1000 ppt 13CH3Cl, and ∼1000 ppt CFC-113. The CFC-113 served as a tracer for physical loss of the added compounds and is expected to be inert to soil microbial activity (21). After allowing the spiking gas to mix, air samples were collected at 2, 12, 22, and 32 min following enclosure. Above-ground vegetation was harvested for identification as well as for determination of moisture content and dry biomass. Small soil samples were also taken at each chamber site for gravimetric moisture determination in the laboratory. Soil moisture is reported as a percentage wet weight. Control experiments were conducted under sunlit conditions, using an aluminum sheet with a Viton gasket to seal off the chamber base bottom. A small but consistent emission of CH3Cl and CH3Br was observed (38 ( 22 nmol m-2 day-1 and 1.3 ( 1.1 nmol m-2 day-1, respectively; n ) 4), which we believe is due to outgassing from the silicone sealant used in the Lexan chamber lid. The production of silicone (polydialkylsiloxane) involves CH3Cl as an intermediate feedstock, and laboratory incubations confirmed that the silicone sealant emits relatively large amounts of CH3Cl and CH3Br within the first few days of application and much smaller amounts thereafter (Rhew and Atwood, unpublished

chamber headspace can be modeled with the following equations:

FIGURE 1. Simplified schematic of production and consumption fluxes of methyl halides within a flux chamber experiment. (A) Physical loss of tracer ) kL[CFC-113]. (B) Microbial consumption in soils ) k[CH3X]. (C) Emission from soils and/or fungi. (D) Emission from above-ground plants and/or litter. data). Field flux measurements reported here do not correct for this bias. Analytical Methods. Two sub-samples (40 and 80 mL) were drawn from the air canisters and analyzed using an Agilent 6890N/5973 quadrupole gas chromatograph/mass spectrometer. Further description of the sample trapping, injection, and chromatographic separation methods can be found in Rhew et al. (7). A pair of common stable isotopes exists for bromine (79Br and 81Br) and chlorine (35Cl and 37Cl); thus, the addition of 13C labeled methyl halides to flux chambers yields eight isotopologues to measure. The four stable isotopologues of CH3Br (12CH379Br, 12CH381Br, 13CH379Br, and 13CH381Br) and the four stable isotopologues of CH3Cl (12CH335Cl, 12CH337Cl, 13CH335Cl, and 13CH337Cl) were detected using selective ion monitoring of the parent ions (m/z ) 94-97 for CH3Br and m/z ) 50-53 for CH3Cl). F-113 was also monitored (m/z ) 153). To measure all four isotopologues simultaneously, peak area corrections need to be applied because of overlap caused by the ion fragmentation of the parent ions (22). For example, 13CH 81Br (m/z ) 97) has no peak interference, but its 3 fragment 13CH281Br (m/z ) 96) has the same mass-to-charge ratio as the 12C parent ion: 12CH381Br (m/z ) 96). Separate injections of high concentration 13CH3X standards (X ) Br or Cl) are used to determine these ion fragmentation ratios (Table S1, right), which are then used to correct for peak overlap in the unknown samples. To correct the peak areas of the lighter halogen 13CH3X isotopologues (13CH379Br and 13CH 35Cl), a similar correction is applied using ratios 3 determined from injections of high concentration standards of 12CH3X (Table S1, left). Calibration curves were constructed by trapping and injecting differently sized aliquots of a synthetic standard calibrated on the SIO-1998 scale (23). Separate calibration curves were constructed using m/z ) 94 and 96 for total 12CH Br and m/z ) 50 and 52 for total 12CH Cl. Concentration 3 3 measurements based on all eight isotopologues confirm that the standard and unknown samples do not have consistent measurable offsets in halogen isotopic compositions and that the method of correcting for peak overlap yields valid concentrations. Calculations. By adding a stable isotope 13CH3X spike to the chamber headspace and monitoring 12CH3X and 13CH3X concentrations, we can solve for gross production rates (P) and uptake rate constants (k), assuming that production rates are linear and that uptake rates are first-order (24) over the time scale of the experiment. The gas pathways are illustrated in Figure 1, and the concentration changes in the flux

d[12CH3X] ) (F12)P - (k12)[12CH3X] dt

(1)

d[13CH3X] ) (F13)P - (k13 + kL)[13CH3X] dt

(2)

where bracketed terms represent concentrations (pmol/mol or ppt). F12 and F13 represent the fraction of CH3X production (P) that is in the form of 12CH3X (98.9%) and 13CH3X (1.1%), respectively. While known sources have a range of isotopic values (25, 26), the differences are negligible in our model, as the typical δ13C value of -40 ( 20 ‰ translates to an F12 value of 0.9893 ( 0.0002. The first-order uptake rate constants (min-1), k12 and k13, are related to each other by stable carbon isotopic fractionation factors (R ) k12/k13) of 1.046 ( 0.004 for CH3Cl and 1.069 ( 0.009 for CH3Br (27, 28). The uncertainty in R yields uncertainties that are typically minor as compared to experimental error (15). kL represents the nonbiological, physical loss of the added compounds, either into soil pore space or due to leakage from the chamber. We approximate this combined advective and diffusive loss rate (20) as a firstorder function and solve for this by plotting the CFC-113 concentrations (minus 75 ppt ambient concentration) versus time. For the 18 spiked chambers, the first-order fit describes the CFC-113 concentration change very well (r2 > 0.98) in all but two cases (r2 ) 0.88 and 0.97) (Table S2). We then use the slope of the least-squares fit of ln[13CH3X] versus time to yield an initial estimate for (k13 + kL). Using this initial estimate for k13 and the observed concentration changes for each carbon isotope pair of methyl halides, we simultaneously solve for P, k12, and initial concentrations through a recursive error minimization procedure (15). To calculate gross uptake fluxes, k12 is then multiplied by Northern Hemisphere background concentrations (535.7 ppt CH3Cl and 10.4 ppt CH3Br) (23). For each spiked chamber, observed and modeled concentrations versus time were plotted for 12CH3Br, 13CH3Br, 12CH Cl, and 13CH Cl (Figure S2 shows a flux chamber 3 3 experiment with simultaneous production and consumption). Concentrations and uptake rates were calculated using both major parent ions for each gas: m/z ) 94 and 96 for 12 CH3Br, m/z ) 95 and 97 for 13CH3Br, m/z ) 50 and 52 for 12CH Cl, and m/z ) 51 and 53 for 13CH Cl. Because con3 3 centrations and flux calculations based on both parent ions agreed very well (Figures S2 and S3), we conclude that any halogen isotope exchange or kinetic isotope effect associated with halogen isotopes is not significant for this study. Applying the peak corrections and isotope tracer model, our spiked control experiments (n ) 4, Table S2) yield small gross production fluxes of 42 ( 33 nmol CH3Cl m-2 day-1 and 0.7 ( 0.7 nmol CH3Br m-2 day-1, consistent with the unspiked control experiments. The gross consumption fluxes in the control experiment (-21 ( 30 nmol CH3Cl m-2 day-1 and -0.4 ( 0.5 nmol CH3Br m-2 day-1) are considered to be negligible.

Results and Discussion Net Fluxes Are Influenced by Plant Community, Soil Moisture, and Season of Study. Net fluxes of CH3Cl and CH3Br from the unspiked chambers vary widely in direction and magnitude (Table 1 and Figures S3 and S4), but some important factors affecting the fluxes can be deduced. Net emissions, which range over several orders of magnitude, dominate during the dry season and are typically associated with certain species of plants. Net uptake, on the other hand, dominates during the wet season and shows a much more VOL. 41, NO. 22, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Comparison of net fluxes observed from spiked chamber vs unspiked chambers for (A) 12CH3Cl and (B) 12CH3Br. 1:1 line shown for comparison. Error bars represent experimentally derived errors on individual flux measurements. limited range of values, up to 617 nmol m-2 day-1 for CH3Cl and 14 nmol m-2 day-1 for CH3Br. Calculating the net fluxes from either parent ion of CH3Cl and CH3Br yields the same flux within errors (Figure S3), and all fluxes hereafter represent the averaged fluxes using both parent ions. At Jepson Prairie, the habitat type proves to be important, as net fluxes strongly vary between dry playa, riparian, and grassland sites (Table 1). The largest emissions, by far, come from the two sites located on the dry playa in July (in the wet months, the playa became a vernal pool). Net emissions were 34 000-42 000 nmol CH3Cl m-2 day-1 and 2000-2600 nmol CH3Br m-2 day-1, at least an order of magnitude greater than the next largest net fluxes. These sparsely vegetated plots contained salt tolerant species, F. salina and C. truxillensis. Emissions on a biomass (dry weight) basis were 650-710 nmol CH3Cl g-1 day-1 and 41-43 nmol CH3Br g-1 day-1, twice the emission rates as those reported from coastal saltmarshes for Frankenia (29), perhaps because the playa soils were hypersaline. The next largest set of emissions was observed at a riparian site in March in association with L. latifolium with 2450 nmol m-2 day-1 CH3Cl and 91 nmol m-2 day-1 CH3Br. Net emissions on a live biomass (dry wt) basis were 11 nmol g-1 day-1 CH3Cl and 0.4 nmol g-1 day-1 CH3Br. At grassland habitat sites, net emissions dominate in July, while net uptake dominates during the wetter months. At Sibley Volcanic Regional Preserve, only grassland habitats were sampled. Like Jepson Prairie, however, net emissions of CH3Cl dominated in the dry summer season (14-74 nmol m-2 day-1), even at sites where no live vegetation was found, only decaying grasses. Net emissions of CH3Br are associated with sites containing Brassica sp., live Bromus grasses, and one site with Pteridium (bracken fern) and Conium (poison hemlock). In the wet season, CH3Cl and CH3Br fluxes at Sibley Preserve are generally dominated by net uptake rather than net emissions. In the unspiked chamber experiments, CFC-113 shows no significant fluxes (Table 1), demonstrating that it is inert over the time scale of incubation and hence an appropriate tracer for physical loss within the system. Consecutively Run Unspiked and Spiked Chambers Yield Similar Net Fluxes. In testing the isotope tracer technique, it is essential to confirm that the addition of a small spike of stable isotope gas does not significantly alter the behavior of the system. The net 12CH3X fluxes observed in unspiked chamber experiments were compared with consecutively run spiked chamber experiments. Net fluxes agreed very well (Figure 2), considering that the measurements were not conducted at exactly the same time. The average difference of unspiked versus spiked chamber fluxes 7840

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(n ) 18) was -15 ( 55 nmol m-2 day-1 CH3Cl and 0.4 ( 3.2 nmol m-2 day-1 CH3Br. Gross Flux Measurements Show Simultaneous Production and Consumption. While net fluxes present a highly variable picture of local net sources and sinks, the gross fluxes clarify the contributions of production and consumption to the overall net flux. For the following discussion, gross emissions are considered to be gross production rates/fluxes by the enclosed plants, soils, and/or microbes, while gross uptake rates are considered to be gross consumption rates/ fluxes. Figure 3 shows the gross production, gross consumption, and net fluxes calculated for all the spiked chamber experiments between August 2004 and April 2005. Gross production of CH3Cl and CH3Br is nearly ubiquitous, ranging from 0 to 535 nmol m-2 day-1 for CH3Cl and 1.5 to 40.7 nmol m-2 day-1 for CH3Br. The largest gross production fluxes of CH3Br were observed at sites that similarly showed large net emissions: the riparian sites of Jepson Prairie (for which spiked samples were measured only in October 2004). The largest gross production rates of CH3Cl, on the other hand, were observed at sites that show overall net uptake, demonstrating that small net fluxes may be the result of two large opposing gross fluxes. Consumption fluxes vary by habitat (grassland vs riparian) and season of measurement, with the largest uptake rates in the grassland during wet season and the lowest uptake rates in the summer and also at the two riparian sites. The overall picture supports the general result of the net flux experiments: that production rates are highly variable and closely associated with the type of vegetation coverage and season of measurement, while the consumption rates are constrained to a more limited range of values. A similar result was found in southern California coastal sagebrush, chaparral, and desert shrublands (8). Gross Fluxes versus Environmental Conditions. Separately determining the production and consumption rates can help clarify which environmental or biological factors drive the fluxes. Gross production rates of CH3Cl and CH3Br do not correlate well with each other, to soil moisture, or to air temperatures, correlations that might exist if abiotic degradation of soil organic matter was the dominant source of these methyl halides (4). Instead, the variability of observed gross emissions appears to be most influenced by the species of plants enclosed, consistent with the observation that the capacity to produce methyl halides varies widely among plant species (30). For example, the largest gross emission of CH3Br is observed at a riparian site with D. spicata (salt grass), a species that emits methyl halides in saltmarsh ecosystems (31).

FIGURE 3. Net (gray bars), gross production (black bars), and gross consumption (white bars) fluxes for (A) CH3Cl and (B) CH3Br. August, December, and April measurements are from Sibley Volcanic Regional Preserve (SIB), and October and March measurements are from Jepson Prairie (JEP). The first column represents spiked control (C) chamber experiments. R represents the two riparian sites at Jepson Prairie. Gross flux errors include uncertainties in the number of moles of chamber air.

FIGURE 4. Gross uptake rate correlations. (A) CH3Cl (black diamonds) and CH3Br (gray circles) vs gravimetric soil moisture (by wet wt %) at grassland sites. Riparian sites are excluded from the linear regressions. (B) Gross uptake rates of CH3Cl vs CH3Br. [CH3Cl flux] ) 38 × [CH3Br flux] - 21.5. Gross uptake rates of CH3Cl and CH3Br are strongly and positively correlated to soil moisture at the grassland sites (Figure 4A). When the riparian sites are included (45-50% soil moisture), however, the uptake rates show a significant decline, perhaps owing to the limited diffusive transport of air in saturated soils. The decrease in uptake rates at high soil saturation levels has also been found in Alaskan tundra (7). An overall parabolic relationship of CH3Br uptake with soil moisture was also observed in experiments with forest and agricultural soils (24). CH3Cl and CH3Br uptake rates are also correlated with each other (r2 ) 0.865), with a molar ratio of 38:1 (Figure 4B). This ratio of uptake is similar to the net uptake rate ratios found in southern California shrublands (40:1), Alaskan tundra (49:1), and gross uptake rates in incubations of boreal forest soils (39:1) and in the shortgrass steppe of Colorado (50:1) (7, 8, 15, 32). Because the uptake rates reported here have been normalized to average Northern Hemisphere concentrations of 536 ppt CH3Cl and 10.4 ppt CH3Br (23), which yields a CH3Cl/CH3Br ratio of 52:1, the first-order uptake rate constants are on average slightly greater for CH3Br than CH3Cl, which might be expected given that the C-Cl bond strength is ∼20% greater than the C-Br strength and that CH3Br is destroyed both chemically and biologically while CH3Cl is removed only biologically (28). However, the nearly consistent ratio of uptake rates among diverse biomes adds further evidence that CH3Cl uptake in soils occurs by

the same primary mechanism and perhaps due to the same microbes as CH3Br uptake. Implications for Temperate Grassland Budgets. The average gross production rates at grassland sites (riparian and playa sites excluded) during the growing season were 156 ( 161 nmol m-2 day-1 CH3Cl and 7 ( 6 nmol m-2 day-1 CH3Br (October to April outings weighted equally). Nongrowing season gross production rates (August measurements only) were negligibly small at 23 ( 9 nmol m-2 day-1 CH3Cl and 4 ( 5 nmol m-2 day-1 CH3Br, similar to rates from the control experiments. However, production rates are closely tied to actual species measured as well as their growth stage, so averaged rates are misleading until additional sampling is conducted and a better handle on the variability is achieved. The average gross uptake rates during the growing season (October to April outings weighted equally) were 504 ( 38 nmol m-2 day-1 CH3Cl and 12 ( 3 nmol m-2 day-1 CH3Br, while soil moisture ranges from 12 to 30% (riparian sites excluded). Non-growing season uptake rates were much smaller at 10 ( 9 nmol m-2 day-1 CH3Cl and 0.6 ( 0.8 nmol m-2 day-1 CH3Br, while soil moisture was less than 6%. All of the CH3Br uptake rates reported here are much lower than the average soil uptake rates previously reported for temperate grasslands (47 ( 16 nmol m-2 day-1 for a site in New Hampshire (13) and 43 ( 14 nmol m-2 day-1 for a site in Colorado (14)). The reasons for this discrepancy are VOL. 41, NO. 22, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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likely related to the disparate grassland types and different experimental conditions in which each of these flux chamber experiments was conducted. The New Hampshire study measured a grassy lawn with moderate initial CH3Br concentrations (∼600 ppt), whereas the Colorado study measured dry bunchgrass prairie with very high initial concentrations of CH3Br (∼10 000 ppt). A grassy lawn would likely have a much higher soil moisture and organic carbon content than the natural grasslands of Colorado or California, which would then be expected to yield higher uptake rates (24). It is somewhat surprising, then, that the New Hampshire and Colorado studies yield similar fluxes. It may be that the large Colorado soil uptake rate is related to the large spike of CH3Br added, which raised initial concentrations to 3 orders of magnitude higher than ambient. This may have been large enough such that other biological, chemical, or physical processes may have occurred. A more recent study of CH3Br fluxes in the Colorado shortgrass steppe (32) using the isotope tracer method described in this paper reported average gross uptake rates of 9.1 nmol m-2 day-1 during the growing season, which are comparable to those reported here but much lower than those reported in previous studies. Soil diffusion-controlled uptake rates using an inert tracer are sometimes assumed as a minimum estimate for CH3Br uptake (13). However, laboratory studies of steam sterilized soils show that tracer loss rates correspond with the physical redistribution of 13CH3X tracers (15). In addition, chamber air leakage can occur. Thus, we explicitly subtract the CFC113 tracer uptake rate constant from the 13CH3X uptake rate constants. This correction is typically small under moist soil conditions (Table S2), when leakage rates are low and biological degradation rates are expected to be greatest. Without the correction, CH3Br uptake rates during wet (>20% soil H2O) conditions would be larger but much more variable (20 ( 6 nmol m-2 day-1), while drier soil uptake rates would be dubiously large (32 ( 18 nmol m-2 day-1), suggesting that the tracer-derived uptake rate correction is necessary. Given the apparent sensitivity of gross methyl halide uptake rates to soil moisture levels, we use the relationships shown in Figure 4A to model uptake rates at a different annual grassland of California (Sierra Foothill Research and Extension Center in Browns Valley, Yuba County, 39°,15′ N, 121°,17′ W) where soil moisture (0-10 cm) was measured at a much higher frequency (33). For 2004, we calculate an uptake rate of 98.8 µmol m-2 year-1 for CH3Cl and 2.21 µmol m-2 year-1 for CH3Br (Figure S5). In 2005, when conditions were much wetter overall, we calculated a larger uptake rate of 133 µmol m-2 year-1 for CH3Cl and 3.17 µmol m-2 year-1 for CH3Br (Figure S5). On average, our annual modeled uptake rates for CH3Br are one-fourth the value of previous studies (10.4-11.4 µmol m-2 year-1) (13, 14). Assuming that these annual grasslands are representative of “tall/medium/short grasslands with shrub cover” as classified by Matthews (12), we multiply our modeled annual uptake rates with a global surface area of 9.34 × 1012 m2 to yield 0.05-0.06 Tg CH3Cl per year (1 Tg ) 1012 g) and 2.0-2.8 Gg CH3Br per year (1 Gg ) 109 g). This category represents one-third of the total grassland surface area. Previous estimates for global CH3Br uptake by temperate grasslands range from 9.7 Gg year-1 using a modest surface area of 9.0 × 1012 m2 (13) to 31.4 Gg year-1 using a larger surface area of 31.9 × 1012 m2 (14). Further measurements are clearly necessary over a range of grassland ecosystems to assess the overall role grasslands play in global methyl halide budgets. The results presented here, however, suggest that the estimated range of CH3Br uptake by temperate grasslands needs to be revised downward. 7842

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Acknowledgments We thank J. Garcia, C. Krumme, L. Olier, Y.A. Teh, M. Abel, and S. Yee for field and laboratory assistance; E. Saltzman for the isotopically labeled gases; the UC Natural Reserve System and the East Bay Regional Parks for field site access; O. Maze´as for helpful reviews; and the NSF Atmospheric Chemistry Program.

Supporting Information Available Diagram of flux chamber. Graphs of chamber concentration changes, net methyl halide fluxes based on both parent ions, grassland net fluxes by site, and modeled annual methyl halide uptake rates. Tables showing GC/MS ion fragmentation ratios, first-order uptake rate constants, and gross flux results. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review May 10, 2007. Revised manuscript received August 1, 2007. Accepted September 4, 2007. ES0711011

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