Global climate change can potentially increase the frequency of climate anomalies. Anomalously warm years may cause an increase in soil nitrogen (N) availability by stimulating N mineralization. To date, most studies addressing the effects of ecosystem warming have been conducted in relatively cold ecosystems and few studies have addressed impacts of interannual as opposed to continuous, multiyear warming. In this study, 12 intact soil monoliths were excavated from a tallgrass prairie site near Purcell, Oklahoma, USA and divided among four large flux chambers (EcoCELLs). During the first year, all four EcoCELLs were subjected to Oklahoma air temperatures and precipitation. During the second year, air temperature in two EcoCELLs was increased by 4 degrees C throughout the year resulting in an increase in soil temperature of 2.3 degrees C at 7.5 cm depth. During the third and fourth years, temperatures in the warmed EcoCELL returned back to 'normal' conditions. During the warming year, vegetation N content was not significantly affected by the warming treatment suggesting no change in N availability. Other N availability indicators (soil solution chemistry, leaching, and N adsorption by ion exchange resins) did not show any effect of warming. Soil solution, leaching, and ion exchange resins showed a large pulse of NH(4)(+) at the start of the study most likely due to disturbance caused by monolith excavation and transport but these effects were short-lived and had disappeared before the treatment started. The lack of a clear warming effect may be explained by a reduction in soil moisture in the warming treatments compared with the controls offsetting a potential stimulation of N mineralization in response to increased temperatures. As a result, effects of an anomalously warm year on N availability in warmer ecosystems may be small compared with colder ecosystems but are likely to depend on soil moisture status.

Ecosystem CO 2 uptake: Prolonged after-effects of an extremely warm year Earth's terrestrial ecosystems strongly modulate levels of CO2 in the atmosphere through seasonal changes in net plant productivity (CO2 absorbance) and soil microbial respiration (CO 2 release). It has been known for decades that these processes respond to seasonal shifts in climate, especially temperature, resulting in the zig-zag form of the global CO 2 curve, but the data necessary to quantify impacts of a single climate variable at interannual timescales have been lacking. A four-year study using intact tallgrass prairie ecosystems in controlled environment chambers (like the one on the cover, showing plant communities a few weeks after summer mowing) now provides some of the missing data. The results show that one anomalously warm year reduces net ecosystem CO 2 exchange for that year and the year after. Carbon sequestration in ecosystems exposed to high temperatures for a year is a third of that in controls. These findings suggest that more frequent anomalously warm years, a possible consequence of rising anthropogenic CO 2 levels, could lead to a sustained decrease in CO 2 uptake by terrestrial ecosystems.

The atmosphere is an important pathway by which mercury is transported and distributed to pristine ecosystems. The significance of anthropogenic versus natural mercury contributions to the atmosphere is controversial, and the importance of re-emission of deposited mercury from ecosystems is not known. Here we present a continuous year-long data set of gaseous elemental mercury exchange between intact soil–plant monoliths of tallgrass prairie and the atmosphere. Mercury fluxes were measured using large open-flow gas exchange chambers (7.3×5.5×4.5 m3, L×W×D). Approximately 60 μg m−2 of elemental gaseous mercury was lost from four replicate grassland ecosystems (9 m2 surface area each) to the atmosphere over the course of 1 yr. Deposition was an important flux in the winter and emissions were dominant in spring, summer, and fall. Solar radiation and air temperature were most strongly correlated with mercury emissions. Gaseous elemental mercury losses to the atmosphere exceeded other measured fluxes of mercury in and out of the grassland ecosystems. These results indicate that mercury emissions from uncontaminated terrestrial ecosystems to the atmosphere may be a significant source of atmospheric mercury. We hypothesize that most of the mercury being emitted is previously deposited mercury and that re-emissions of mercury from terrestrial ecosystems is an important process whereby mercury is continually cycled between the air and terrestrial ecosystems.

Modeling analyses suggest that an increase in growth rate of atmospheric CO2 concentrations during an anomalously warm year may be caused by a decrease in net ecosystem production (NEP) in response to increased heterotrophic respiration (Rh). To test this hypothesis, 12 intact soil monoliths were excavated from a tallgrass prairie site near Purcell, Oklahoma, USA and divided among four large dynamic flux chambers (Ecologically Controlled Enclosed Lysimeter Laboratories (EcoCELLs)). During the first year, all four EcoCELLs were subjected to Oklahoma air temperatures. During the second year, air temperature in two EcoCELLs was increased by 4°C throughout the year to simulate anomalously warm conditions. This paper reports on the effect of warming on soil CO2 efflux, representing the sum of autotrophic respiration (Ra) and Rh.

Whole system elemental mercury (Hg0) flux was measured for approximately 1.5 years using two large gas exchange mesocosms containing approximately 100 two-year old aspen trees (Populus tremuloides) planted in soil with elevated mercury concentrations (12.3 microg/g). We hypothesized that during leafout, whole mesocosm Hg0 flux would increase due to movement of Hg0 in the transpiration stream from the soil to the air. This hypothesis was not supported; plants were found to assimilate Hg0 from the contaminated air, and whole system Hg0 emissions were reduced as plants leafed-out due to shading of the soil. Surface disturbance, watering, and increases in soil moisture, light, and temperature were all found to increase whole system Hg0 flux, with light being a more significant factor. Although surface soils were maintained at 15-20% moisture, daily watering caused pulses of Hg0 to be released from the soil throughout the experiment. Data developed in this experiment suggested that those processes acting on the soil surface are the primary influence on Hg emissions and that the presence of vegetation, which shields soil surfaces from incident light, reduces Hg emissions from enriched soils.

Increased belowground labile C inputs under elevated [CO2] could stimulate nonsymbiotic N2 fixation, thereby enhancing growth responses of vegetation to elevated [CO2] on nutrient-poor sites. To test this hypothesis, nonsymbiotic N2 fixation rates in soils planted with 3-year-old Jeffrey pine (Pinus jeffreyi Grev. & Balf.) trees grown under 365 and 700 µL·L–1 atmospheric [CO2] were measured by exposing the soil to 15N2-enriched air for 78 d. Nitrogen fixation rates were estimated by measuring 15N content of trees and soil. Compared with the ambient CO2 treatment, the elevated CO2 treatment did not affect biomass, N content, or δ15N of individual plant parts and soils, indicating that elevated [CO2] did not stimulate nonsymbiotic N2 fixation. Because belowground C inputs did not increase under elevated [CO2], the initial hypothesis could not be accepted or rejected. The results from the 15N2 labeling study agree with other studies showing that nonsymbiotic N2 fixation is not likely to provide a large input of N in forest ecosystems. The 15N2 labeling technique was promising for studying N2 fixation in plant–soil systems, but the preliminary nature of this study did not allow for firm conclusions with regard to the effects of elevated [CO2].

Increases in net primary production (NPP) may not necessarily result in increased C sequestration since an increase in uptake can be negated by concurrent increases in ecosystem C losses via respiratory processes. Continuous measurements of net ecosystem C exchange between the atmosphere and two experimental cheatgrass (Bromus tectorum L.) ecosystems in large dynamic flux chambers (EcoCELLs) showed net ecosystem C losses to the atmosphere in excess of 300 g C m−2 over two growing cycles. Even a doubling of net ecosystem production (NEP) after N fertilization in the second growing season did not compensate for soil C losses incurred during the fallow period. Fertilization not only increased C uptake in biomass but also enhanced C losses through soil respiration from 287 to 469 g C m−2, mainly through an increase in rhizosphere respiration. Fertilization decreased dissolved inorganic C losses through leaching of from 45 to 10 g C m−2.

Foliar accumulation of mercury has been demonstrated to occur as plants leaf out, yet the primary source of this mercury is not known. Using closed‐system growth chambers, uptake of mercury by quaking aspen ( Populus tremuloides ) foliage was measured over time as a function of soil mercury concentrations (0.01, 6.2, and 25.6 μg/g) and atmospheric mercury exposure concentrations (1.4, 14.9, and 68.5 ng/m 3 ). Foliar mercury concentrations increased as a function of time for all exposures. Twice during the experiment, leaf washes were analyzed for mercury to assess surface deposition, and little mercury was removed (0.02‐0.04 ng/m 2 ), suggesting that direct deposition to the leaf surface was not significant during this experiment. At the end of the four‐month experiment, whole‐plant mercury concentrations were determined. It was found that whereas mercury in the atmosphere primarily influenced foliar uptake, root concentrations were related to the soil mercury concentration. The implication of this study is that litterfall may serve as a pathway for new, atmospherically derived mercury to be deposited to forest soils. This has significant implications for watershed management of ecosystems where mercury is of concern.

We used unique mesocosms to examine the role that plants play in accumulating and transforming atmospheric Hg. Several stands of quaking aspen were grown in large gas-exchange chambers in Hg-enriched soil (12.3±1.3μgg−1), and the Hg content in the vegetation was determined over time. Foliar Hg concentrations increased as a function of leaf age and leveled off after 2–3 months in the oldest tissue with a mean tissue concentration of 150ngg−1. Approximately 80% of the total Hg accumulated in the aboveground biomass was found in the leaves, and roughly 1% of that Hg was methylated. Leaves of additional aspen grown within the mesocosms in containers of low Hg soil (0.03±0.01μgg−1) exhibited foliar Hg concentrations similar to those of trees grown in the Hg-enriched soil. Leaf rinses and surrogate Teflon surfaces were analyzed to characterize surface deposition processes. Small gas-exchange systems were used to measure stomatal uptake of Hg vapor, and the mean Hg flux was −3.3ngm−2h−1. These experiments showed that almost all of the Hg in foliar tissue originated from the atmosphere. Thus, in the fall when deciduous trees enter dormancy and leaves senesce, litterfall would represent a new Hg input to terrestrial ecosystems.

Use of plant phenological variables in models predicting evapotranspiration (ET) has largely relied on relatively simple (e.g., linear) relationships which may not be sufficiently accurate to predict small—yet ecologically significant—changes in plant phenology that are expected to occur in response to global climate change. A dearth of experimental data reflects the difficulties in quantifying these relationships against the background of large environmental variability that occurs in the field. Our main objective was to quantify how plant phenology (leaf area index [LAI] and root length density [RLD]) affect ET and its components during an entire vegetation cycle in large-scale model grassland (Bromus tectorum) ecosystems using the Ecologically Controlled Enclosed Lysimeter Laboratory (EcoCELL)—a unique open flow and mass balance laboratory. We also aimed to compare the three methods employed by the EcoCELL laboratory to measure ecosystem ET (whole-ecosystem gas exchange, weighing lysimetry, and weighing lysimetry combined with time domain reflectometry [TDR]) in order to independently confirm the performance of the unique gas exchange technology. Cumulative ET during the 190 days of the experiment measured with the three different methods compared very well with each other (mean errors <1%). We found that ET reached maximum levels at relatively low LAI (2–3), but as LAI increased beyond this value, small increase in transpiration were more than offset by decreases in soil evaporation, thereby causing declines in ET. A combined rectangular hyperbola (effects on transpiration) and linear (effects on soil evaporation) function between LAI and ET accounted for almost 90% of all variability in measured daily ET. RLD showed relationships to ET similar to those observed for LAI due to high covariance between RLD and LAI, but root length densities did not explain any additional variability in daily ET beyond that explained by LAI under the well-watered conditions of the experiment. Taken together, our results show that: (i) the EcoCELL mesocosm laboratory can precisely and accurately quantify hydrologic processes of large soil–plant monoliths under controlled environmental conditions; (ii) plant canopy phenological changes affect ecosystem ET, and the contribution of transpiration, in non-linear ways; (iii) these non-linear responses must be accounted for when assessing the consequences of changes in plant phenology—e.g., due to global environmental change—on ecosystem hydrology.

This report evaluates tailpipe and nontailpipe hydrocarbon (HC) emissions from light-duty spark-ignition (SI) vehicles. The sources of information were unpublished data sets, generated mainly from 1990 through 1994, on emissions from volunteer fleets of in-use vehicles in chassis dynamometer and sealed housing for evaporative determination tests, and published chemical mass balance (CMB) source apportionments of HC in roadway tunnels and in urban air. The nontailpipe emissions evaluated comprise running-loss, hot soak, diurnal emissions, and resting-loss emissions. Relations between pressure and purge test failures and actual nontailpipe emissions were also examined.

Warm cloud processes in which dissolved peroxides and ozone oxidize SO2 to sulfate are important contributors to acidic sulfate deposition, and may also contribute to visibility impairment if the clouds evaporate prior to rainfall, e.g. under summer monsoonal conditions in the Southwest U.S.A. However, data from summer season gaseous peroxide measurements are sparse for this region, specifically in the area of the Grand Canyon National Park (GCNP). Results are reported herein from a gaseous peroxide measurement campaign at Meadview, AZ (between GCNP and various sources of SO2 to the west and south) during July-August 1992. Total peroxide and H2O2 concentrations were measured by the concurrent coil collection/enzyme catalyzed fluorescent technique (Lazrus et al., 1986, Anal. Chem. 58, 594-597). Total gaseous peroxide levels observed were generally in the range of 1.0-5 ppbv.Results from measurements of peroxides are used with other preliminary data for ozone, SO2, temperature and humidity, and from other observations to estimate the potential contribution of in-cloud oxidation processes to visibility degradation due to increased sulfate levels. The data show that peroxide concentrations and estimated H2O2 concentrations nearly always exceeded SO2 levels at Meadview, indicating the absence of ''oxidant limited'' conditions in air in which clouds might be formed. Only modest quantities of aerosol sulfate (< 1 mug m-3) can be formed in clouds that evaporate, for calculated rates of conversion at the usually observed ambient SO2 concentrations of 100-500 pptv, when these rates are averaged over mean boundary layer thicknesses. Estimated initial rates of in-cloud reaction of ozone with S(IV) are comparable to rates for peroxides when cloudwater pH values exceed about 6. The self-quenching of the ozone-S(IV) reaction makes this process unimportant for typical summer conditions at Meadview with cloud liquid water contents (LWC) of about 0.1, but the process may become important at higher LWC and pH.

A technique is described for low parts-per-trillion (ppt) detection of SO 2 with high temporal resolution compared to current filter sampling methods. A sulfur chemiluminescence detector, equipped with a quartz burner chamber and a sample probe with a critical orifice, was used as a single detector to analyze SO 2 in ambient air by alternately cycling the sample airstream through a SO 2 denuder. The method provides one SO 2 concentration reading every 10 min. The limits of detection and quantitation were found to be 20 and 70 pptv, respectively. Results from field measurements at a remote site on the south rim of the Grand Canyon during November 1991 are presented, and they indicate a highly variable distribution of SO 2 from the limit of detection to near 2 ppbv

Measurements are reported from integrating nephelometers modified to sample alternately from a fine-particle (D(aero) < 2.5 mum) cyclone and an unrestricted inlet. These nephelometers were successfully operated at two sites in the southwest U.S.A. as part of the SCENES program in the spring and summer of 1989. One of the nephelometers was collocated with a transmissometer monitoring total extinction. Fine- and total-particle samples were collected at each site for determination of mass and light absorption.Intercomparisons of the collocated daytime scattering, absorption, and total extinction measurements indicate that the nephelometer reported somewhat less than half the actual scattering by coarse particles. When this under-response is corrected for, the nephelometer and transmissometer show good agreement. The corrected data indicate that coarse particles were responsible from one-quarter to one-third of the total particle scattering. Predominantly coarse-particle dusts are estimated to have contributed one-third to one-half of the total-particle scattering.

The halocarbons CFCl3, CH3CCl3, CCl4, and C2Cl4 have been monitored at four locations in southern California, southern Nevada, and northwestern Arizona. Summertime concentrations of CH3CCl3 and C2Cl4 at the Nevada and Arizona sites exhibit strong weekly cycles that lag similar cycles observed in the Los Angeles Basin by 1–2 days. The observed patterns imply a nearly complete weekend shutdown of emissions, and remarkably consistent long‐range (300–400 km) transport through complex terrain. The average amplitude of the summertime CH3CCl3 cycle is about 1200 ppt in air leaving the Los Angeles Basin, 80 ppt at the mountain‐top Nevada observatory, and 40 ppt at the level‐terrain Arizona observatory.

A two-year record of hourly concentrations of halocarbon tracers (methylchloroform and perchloroethylene) and hourly averages of particle light scattering (Bsp) has been analyzed In an effort to understand the sources of haze In the U.S. southwestern deserts and mountains. Measurements were taken on top of Spirit Mountain in southern Nevada. In conjunction with photographs used to interpret visual quality, haze episodes at Spirit Mountain were usually coincident with elevated concentrations of tracers originating from urban sources. Haze obscured an 88-km-distant mountain 17 percent of the total observation time. Of those Incidents, 69 percent were associated with long-range transport of haze from the Los Angeles Basin.