Abstract  The policy-relevant background (PRB) ozone is defined by the US Environmental Protection Agency (EPA) as the surface ozone concentration that would be present over the US in the absence of North American anthropogenic emissions. It is intended to provide a baseline for risk and exposure assessments used in setting the National Ambient Air Quality Standard (NAAQS). We present here three-year statistics (2006–2008) of PRB ozone over the US calculated using the GEOS-Chem global 3-D model of atmospheric composition with 1/2° × 2/3° horizontal resolution over North America and adjacent oceans (2° × 2.5° for the rest of the world). We also provide estimates of the US background (no anthropogenic US emissions) and natural background (no anthropogenic emissions worldwide and pre-industrial methane). Our work improves on previous GEOS-Chem PRB estimates through the use of higher model resolution, 3-year statistics, better representation of stratospheric influence, and updated emissions. PRB is particularly high in the intermountain West due to high elevation, arid terrain, and large-scale subsidence. We present for this region a detailed model evaluation showing that the model is successful in reproducing ozone exceedances up to 70 ppbv. However, the model cannot reproduce PRB-relevant exceptional events associated with wildfires or stratospheric intrusions. The mean PRB estimates for spring–summer are 27 ± 8 ppbv at low-altitude sites and 40 ± 7 ppbv at high-altitude sites. Differences between the PRB simulation and the natural simulation indicate a mean enhancement from intercontinental pollution and anthropogenic methane of 9 ppbv at low-altitude sites and 13 ppbv at high-altitude sites. The PRB is higher than average when ozone exceeds 60 ppbv, particularly in the intermountain West. Our PRB estimates are on average 4 ppbv higher than previous GEOS-Chem studies and we attribute this to higher lighting, increasing Asian emissions, and improved model resolution. Whereas previous studies found no occurrences of PRB exceeding 60 ppbv, we find here some occurrences in the intermountain West. The annual 4th-highest PRB values in the intermountain West are typically 50–60 ppbv, as compared to 35–45 ppbv in the East or on the West Coast. Such high PRB values in the intermountain West suggest that special consideration of this region may be needed if the ozone NAAQS is decreased to a value in the 60–70 ppbv range.

Abstract  Rates of microbial reduction of O2, Fe3+, Mn4+, NO3-, and SO42-, and total generation of CO2 and CH4 were measured in the hypolimnia of three Canadian Shield lakes. Methanogenesis accounted for 72-80 carbon generation, while sulfate reduction contributed 16-20 of anoxic carbon generation (2-8 combined (nitrate, iron, and manganese reduction). In lakes affected by acid deposition,inputs of sulfate and nitrate will increase, and it is expected that reducing power normally going to methane production will be diverted to nitrate and sulfate reduction. The last two reduction reactions can result in alkalinity production, whereas methane production does not. A model was developed to predict the significance of hypolimnetic alkalinity production which could result from these reactions in lakes with known hypolimnetic reducing power (methane production). The model showed that the hypolimnia of two ELA lakes which have been made eutrophic artificially could potentially produce enough persistent alkalinity to neutralize "typical" acid deposition, while a lake that was not eutrophic could not. Besides trophic state, other factors important in determining a lake's capability for hypolimnetic alkalinity production were watershed area: surface area ratio, the watershed retentions of H+, SO42-, NO3-, and NH4+, and the degree of precipitation of FeS in the sediment.

Abstract  Upper San Gabriel Valley Water Association. #Atmospheric N deposition, associated with chronic urban air pollution, has produced stream water nitrate concentrations as high as 7.0 mg of N/l in chaparral watersheds in the San Gabriel Mountains of Los Angeles County, California. Stream water [NO3-] and discharge were greatest at high flow and may have contributed significantly to existing groundwater NO3- pollution. Annual NO3- discharge ranged from 0.04-10.0 kg of N/ha over 4 yr. Canopy throughfall and precipitation inputs of 23.3 and 8.2 kg N ha/yr were high relative to other undisturbed ecosystems nationwide. Dry deposition was apparently a major source of the throughfall N. NO3- concentrations from nearby, relatively unpolluted watersheds were lower by 1-3 orders of magnitude. NO3-yield was elevated on watersheds where chaparral was converted to grassland in 1960 and may be have been accelerated after wildfire because of high postfire NH4+ concentrations and rapid nitrification in terrestrial and aquatic ecosystems.

Abstract  NO2 uptake of eight pot plants (Ficus elastica 'Robusta', Ficus benjamina, Hedera helix 'Anne Marie', Hedera canariensis 'Montgomery', Hibiscus rosa-sinensis 'Red'. Hibiscus rosa-sinensis 'Moesiana', Dieffenbachia maculata 'Compacta' and Nephrolepis exaltata 'Bostoniensis') was shown to correlate with transpiration, while NO uptake had no significant dependence on stomatal opening. NO was taken up at a constant rate throughout the light period, while the uptake of NO2 decreased towards the end of the day in the same manner as transpiration. Uptake of NO in the dark was as high as in the light, while uptake of NO2 in the dark was reduced by as much as transpiration. Relative to pollutant uptake, the toxicity of NO towards photosynthesis is 22 times higher than that of NO2. NO must therefore have a mechanism of toxicity which is differnt from that of NO2. A sufficiently high dose of NO (4 ml l-1 for 5 hours) to reduce transpiration (close stomata) decreased photosynthesis than a lower dose (1 .mu.l l-1 for 12 h). Since total NO uptake was not dependent on stomatal aperture, but stomatal closure reduced the effect of NO on photosynthesis, it is suggested that stomatas regulate the effective NO uptake, i.e., the small fraction of NO uptake that affects photosynthesis. The larger ineffective portion of the uptake is thought to be absorbed on the outer leaf surfce. The sensitivity of cultivars or species to NO or NO2 (i.e. effects on photosynthesis and transpiration) did not correlate with the uptake of either pollutant gas.

Abstract  By reviewing the physics of the processes leading to dry deposition in the atmospheric surface layer, a number of corrections and possible errors in the determination of the dry deposition fluxes are identified. The concept of deposition velocity is examined, its usefulness discussed, and some pitfalls are pointed out. An overview is given of the various micrometeorological methods with which dry deposition can be determined and some of their limitations are indicated. With this background a number of recent publications on dry deposition have been analyzed. It is apparent that in most cases a more complete documentation is needed of the structure of the surface layer and the adjacent planetary boundary layer. Errors related to advection and entrainment are especially difficult or impossible to assess because of inadequate information about the surroundings and the height of the boundary layer.

Abstract  #A generalized, lumped-parameter model of carbon (C), water, and nitrogen (N) interactions in forest ecosystems (PnET-CN) is presented. The model operates at a monthly time step and at the stand-to-watershed scale, and is validated against data on annual net primary productivity, monthly carbon and water balances, annual net N mineralization, nitrification, foliar N concentration and annual and monthly N leaching losses for two sites, Hubbard Brook (West Thornton, NH) and Harvard Forest (Petersham, MA). It is then used to predict transient responses in function resulting from changes in land use and N deposition, as well as the maximum rate of N cycling which can be sustained for any given combination of site, climate and species. Model predictions suggest a very long legacy effect of land use history on N cycling. Even with only one `active' soil organic matter pool, complete recovery from three modest harvests at Hubbard Brook is predicted to require more than two centuries at current N deposition rates. Complete recovery is predicted to take even longer at the Harvard Forest where biomass removals have been more intense. PnET-CN is used to predict maximum sustainable rates of N cycling for 14 sites throughout the northeastern USA. Predicted maximum values were higher, as expected, than measured N mineralization rates for all but one site. The measured fraction of N mineralization nitrified at these 14 sites showed a general relationship with the ratio of measured to maximum net N mineralization. This latter ratio is discussed as a potentially useful indicator of the degree of nitrogen saturation in forest ecosystems. A regional map of predicted maximum N cycling rates is presented based on regressions between model predictions and summary climatic variables.

Abstract  We revisited 68 plots of forest vegetation in the San Bernardino Mountains that had been quantitatively described In 1929-1935 from the California Vegetation Type Map (VTM) Survey. By using the same sampling methods, we documented changes-over approximately 60 years and during a period of fire suppression management-in tree density by both species and size class. In general we found increasing stand densities, a transformation from old growth age structure to young growth, and a compositional shift from Pinus ponderosa and P. jeffreyi to Abies concolor and Calocedrus decurrens. Density of trees of more than 12 cm diameter at breast height (dbh) increased by 79%, including three to ten-fold increases in the youngest cohorts 12-66cm dbb. The magnitude of change depended upon initial forest composition and local annual precipitation. Monotypic stands of P. jeffreyi or those initially dominated by Abies concolor showed the least change in species composition; the most xeric stands of P. jeffreyi showed the least gain in density; and mesic mixed P. ponderosa stands showed the most dramatic change in composition and density. We compared these data to records of past and present forests in the Sierra Nevada and found parallel trends, but magnified by the Increased precipitation of the Sierra Nevada We also compared V7M data from the San Bernardino Mountains to mixed conifer forests in the Sierra San Pedro Martir of Baja California. These Mexican sites and forests are ecologically similar to those in California, but they still experience unmanaged fire regime& Californian forests of 60 years ago are remarkably similar to modern forests in the Sierra San Pedro Martir. Thus, we conclude that forest changes in the San Bernardino Mountains are primarily due to lengthening fire intervals. Forest changes as a result of fire suppression have important conservation consequences for bird species diversity In general and for Spotted Owl and Neotropicat migrants in particular.

Abstract  1st PAR.Forest ecosystems throughout the world are exposed to acid rain, a complex solution consisting largely of H+, SO42-, NH4+, and NO3- pollutant ions derived from sulfur and nitrogen oxides. Although the public in the United States may view acid deposition as a localized pollution issue specific to certain regions of North America and northern Europe, recent evidence of acidic deposition throughout much of Asia, including tropical forest regions, has demonstrated that acid deposition is actually a global phenomenon (Abate 1995).

Abstract  This study examined the growth responses of exotic annuals and native shrubs to elevated N levels to test the hypothesis that increased N availability favors nitrophilous annuals over the slower-growing shrubs. The vegetation structure of the coastal sage scrub ecosystems in southern California is shifting from shrubland to annual grasslands. Over the last 30 years large tracts of wildlands, particularly those adjacent to urban centers, have lost significant native shrub cover, which has been replaced by exotic annuals native to the Mediterranean Basin. During this same time, air pollution has led to increased terrestrial eutrophication by atmospheric deposition. Changes in vegetation are often the result of changes in resource availability. The results of our experiments showed the three native shrubs tested to be more nitrophilous than the three annuals tested, which contrasts with most models of perennial species' adaptation to stressful environments. Under greenhouse conditions the annual grasses exhibited yield depression at the highest N treatments of 80 Ág g-1 in soil. The three shrub species evaluated continued. to increase shoot biomass at 80 Ág g-1 N in soil. The grasses also exhibited increased tissue N concentrations with increased soil N in contrast with the shrubs where there was little difference in tissue N concentrations with increasing availability. Although the differential yield responses to elevated N do not explain the success of the annual vegetation in replacing shrubs, the inability of the shrubs to regulate growth under elevated N levels may explain the poor survival of mature individuals.

Abstract  The effect of atmospheric aerosols and regional haze from air pollution on the yields of rice and winter wheat grown in China is assessed. The assessment is based on estimates of aerosol optical depths over China, the effect of these optical depths on the solar irradiance reaching the earth's surface, and the response of rice and winter wheat grown in Nanjing to the change in solar irradiance. Two sets of aerosol optical depths are presented: one based on a coupled, regional climate/air quality model simulation and the other inferred from solar radiation measurements made over a 12-year period at meteorological stations in China. The model-estimated optical depths are significantly smaller than those derived from observations, perhaps because of errors in one or both sets of optical depths or because the data from the meteorological stations has been affected by local pollution. Radiative transfer calculations using the smaller, model-estimated aerosol optical depths indicate that the so-called "direct effect" of regional haze results in an approximately 5-30% reduction in the solar irradiance reaching some of China's most productive agricultural regions. Crop-response model simulations suggest an approximately 1:1 relationship between a percentage increase (decrease) in total surface solar irradiance and a percentage increase (decrease) in the yields of rice and wheat. Collectively, these calculations suggest that regional haze in China is currently depressing optimal yields of approximately 70% of the crops grown in China by at least 5-30%. Reducing the severity of regional haze in China through air pollution control could potentially result in a significant increase in crop yields and help the nation meet its growing food demands in the coming decades.

Abstract  The herbaceous layer (vascular plants | 1 m in height) is an important component of forest ecosystems and a potentially sensitive vegetation stratum in response to acid deposition. This study tested several hypotheses concerning soil and herbaceous layer response to experimental acidification at the Fernow Experimental Forest in north-central West Virginia. Fifteen circular sample plots (0.04 ha) were established in each of three watersheds: WS3 (and receiving acidification treatment with (NH4)2SO4), WS4 (>80-yr-old control), and WS7 ( 20-yr-old control). The herb layer was sampled intensively in 10 1-m2 subplots within each sample plot, including determination of species composition, cover, and random biomass harvests. Harvested plant material was separated by species and analyzed for macronutrients, micronutrients, and Al. Soil was sampled from harvest subplots and analyzed for texture, pH, organic matter, and macro- and micronutrients. Few differences among wate

Abstract  This paper compares spatial and seasonal variations and temporal trends in modeled and measured concentrations of sulfur and nitrogen compounds in wet and dry deposition over an 18-year period (1988-2005) over a portion of the northeastern United States. Substantial emissions reduction programs occurred over this time period, including Title IV of the Clean Air Act Amendments of 1990 which primarily resulted in large decreases in sulfur dioxide (SO(2)) emissions by 1995, and nitrogen oxide (NO(x)) trading programs which resulted in large decreases in warm season NO(x) emissions by 2004. Additionally, NO(x) emissions from mobile sources declined more gradually over this period. The results presented here illustrate the use of both operational and dynamic model evaluation and suggest that the modeling system largely captures the seasonal and long-term changes in sulfur compounds. The modeling system generally captures the long-term trends in nitrogen compounds, but does not reproduce the average seasonal variation or spatial patterns in nitrate. (C) 2010 Elsevier Ltd. All rights reserved.

Abstract  Technological advances in industrial processes have resulted in reduced atmospheric emissions from metal industries all over the globe, but the response of the environment is not well known. The authors studied metal (As, Cd, Cu, Ni, Pb, and Se) accumulation in passerine birds (pied flycatcher, Ficedula hypoleuca, and great tit, Parus major) following almost 20 years of reduced metal emissions from the largest nonferrous smelter in Finland. Close to the industry, emission reductions resulted in reduced exposure to several of the elements and, more importantly, reduced accumulation by 58 to 95% in liver tissue. Albeit showing significant tissue reductions, nestlings had elevated concentrations of arsenic, cadmium, lead, and selenium close to the industry. Single-element concentrations were below critical levels associated with subclinical effects, but the mixture of toxic elements indirectly affected health and reproduction. Our study indicates that in environments with moderate duration of industrial activity, impact, and soil pollution, metal accumulation in organisms can decrease relatively rapidly when atmospheric emissions are reduced.

Abstract  We characterized the seasonal and interannual variation in macrophytes, epiphytes, invertebrate herbivores, small demersal predators, and physicochemical characteristics of an eelgrass (Zostera marina) bed in Chesapeake Bay, Virginia, over 10 yr, to explore the relative importance of abiotic and biotic forcing on community composition and abundance. Our hypotheses were (1) physicochemical drivers affect community structure directly, (2) bottom-up trophic control is evidenced by positive covariance among trophic levels, (3) top-down control generates inverse patterns of abundance at adjacent trophic levels, and (4) species diversity among herbivores contributes to temporal stability. Composition and abundance of eelgrass-associated species varied strongly among seasons and years. Much of this variation correlated with temperature and salinity anomalies, and multivariate analysis grouped communities roughly by season, supporting our first hypothesis. Severe seagrass loss during the hot summer of 2005 shifted the community toward a novel composition, but community structure rebounded within a year. Evidence for trophic control was mixed: selected taxa showed patterns consistent with top-down or bottom-up control, but these patterns generally disappeared at the level of whole years and entire trophic levels. Our ability to detect trophic effects may have been limited, however, by consumer movement or changing behavioral responses to resource availability and predation. There was also little evidence that diversity stabilized total herbivore abundance. Although consumer effects on lower levels were inconsistent, the strong physicochemical forcing of community structure supports suggestions that eelgrass communities are highly vulnerable to natural and anthropogenic changes in climate and hydrography.

Abstract  A previously observed shift in the relationship between Chesapeake Bay hypoxia and nitrogen loading has pressing implications on the efficacy of nutrient management. Detailed temporal analyses of long-term hypoxia, nitrogen loads, and stratification were conducted to reveal different within-summer trends and understand more clearly the relative role of physical conditions. Evaluation of a 60-year record of hypoxic volumes demonstrated significant increases in early summer hypoxia, but a slight decrease in late summer hypoxia. The early summer hypoxia trend is related to an increase in Bay stratification strength during June from 1985 to 2009, while the late summer hypoxia trend matches the recently decreasing nitrogen loads. Additional results show how the duration of summertime hypoxia is significantly related to nitrogen loading, and how large-scale climatic forces may be responsible for the early summer increases. Thus, despite intra-summer differences in primary controls on hypoxia, continuing nutrient reduction remains critically important for achieving improvements in Bay water quality.

Abstract  * Plasticity of leaf nutrient content and morphology, and macronutrient limitation were examined in the northern pitcher plant, Sarracenia purpurea subsp. purpurea, in relation to soil nutrient availability in an open, neutral pH fen and a shady, acidic ombrotrophic bog, over 2 yr following reciprocal transplantation of S. purpurea between the wetlands. * In both wetlands, plants were limited by nitrogen (N) but not phosphorus (P) (N content < 2% DW(-1), N : P < 14) but photosynthetic quantum yields were high (F(V)/F(M) > 0.79). Despite carnivory, leaf N content correlated with dissolved N availability to plant roots (leaf N vs , r(2) = 0.344, P < 0.0001); carnivorous N acquisition did not apparently overcome N limitation. * Following transplantation, N content and leaf morphological traits changed in new leaves to become more similar to plants in the new environment, reflecting wetland nutrient availability. Changes in leaf morphology were faster when plants were transplanted from fen to bog than from bog to fen, possibly reflecting a more stressful environment in the bog. * Morphological plasticity observed in response to changes in nutrient supply to the roots in natural habitats complements previous observations of morphological changes with experimental nutrient addition to pitchers.

Abstract  The WHAM-F-TOX model uses chemical speciation to describe the bioavailability and toxicity of proton and metal mixtures (including Al) to aquatic organisms. Here, we apply the previously parameterised model to 45 UK and Norwegian upland surface waters recovering from acidification, to compare its predictions of the maximum species richness of the macroinvertebrate Orders Ephemeroptera, Plecoptera and Trichoptera (SR-EPT) with time-series observations. This work uses data from two national scale survey programmes, the Acid Waters Monitoring Network in the UK and a lakes survey in Norway. We also investigate data from a long-studied catchment, Llyn Brianne in Wales. For the national surveys, model results relate well with actual trends, with Regional Kendall analysis indicating biological recovery rates for both actual and predicted species richness that are generally consistent (1.2-2.0 species per decade). However, actual recovery rates in AWMN lakes were less than in the rivers (0.6 vs. 2.0 species per decade), whilst predicted rates were similar (1.7 vs. 2.0). Several sites give a very good fit between model predictions and observations; at these sites chemistry is apparently the principal factor controlling limits of species richness. At other sites where there is poorer agreement between model predictions and observations, chemistry can still explain some of the reduction in species richness. However, for these sites, additional (un-modelled) factors further suppress species richness. The model gives a good indication of the extent of these un-modelled factors and the degree to which chemistry may suppress species richness at a given site. (C) 2011 Elsevier Ltd. All rights reserved.

Abstract  The Tropospheric Emission Spectrometer (TES) on the EOS Aura satellite makes global measurements of infrared radiances which are used to derive profiles of species such as O3, CO, H2O, HDO and CH4 as routine standard products. In addition, TES has a variety of special modes that provide denser spatial mapping over a limited geographical area. A continuous-coverage mode (called "transect", about 460 km long) has now been used to detect additional molecules indicative of regional air pollution. On 10 July 2007 at about 05:37 UTC (13:24 LMST) TES conducted such a transect observation over the Beijing area in northeast China. Examination of the residual spectral radiances following the retrieval of the TES standard products revealed surprisingly strong features attributable to enhanced concentrations of ammonia (NH3) and methanol (CH3OH), well above the normal background levels. This is the first time that these molecules have been detected in space-based nadir viewing measurements that penetrate into the lower atmosphere.

Abstract  Nearshore waters of the California Current System (California CS) already have a low carbonate saturation state, making them particularly susceptible to ocean acidification. We used eddy-resolving model simulations to study the potential development of ocean acidification in this system up to the year 2050 under the Special Report on Emissions Scenarios A2 and B1 scenarios. In both scenarios, the saturation state of aragonite Ω(arag) is projected to drop rapidly, with much of the nearshore region developing summer-long undersaturation in the top 60 meters within the next 30 years. By 2050, waters with Ω(arag) above 1.5 will have largely disappeared, and more than half of the waters will be undersaturated year-round. Habitats along the sea floor will become exposed to year-round undersaturation within the next 20 to 30 years. These projected events have potentially major implications for the rich and diverse ecosystem that characterizes the California CS.

Abstract  In this review, we present a conceptual model which links plant communities and saprotrophic microbial communities through the reciprocal exchange of growth-limiting resources. We discuss the numerous ways human-induced environmental change has directly and indirectly impacted this relationship, and review microbial responses that have occurred to date. We argue that compositional shifts in saprotrophic microbial communities underlie functional responses to environmental change that have ecosystem-level implications. Drawing on a long-term, large-scale, field experiment, we illustrate how and why chronic atmospheric N deposition can alter saprotrophic communities in the soil of a wide-spread sugar maple (Acer saccharum) ecosystem in northeastern North America, resulting in the slowing of plant litter decay, the rapid accumulation of soil organic matter, and the accelerated production and loss of dissolved organic carbon (DOC). Compositional shifts in soil microbial communities, mediated by ecological interactions among soil saprotrophs, appear to lie at the biogeochemical heart of ecosystem response to environmental change.

Abstract  Ozonolysis is a major tropospheric removal mechanism for unsaturated hydrocarbons and proceeds via "Criegee intermediates"--carbonyl oxides--that play a key role in tropospheric oxidation models. However, until recently no gas-phase Criegee intermediate had been observed, and indirect determinations of their reaction kinetics gave derived rate coefficients spanning orders of magnitude. Here, we report direct photoionization mass spectrometric detection of formaldehyde oxide (CH(2)OO) as a product of the reaction of CH(2)I with O(2). This reaction enabled direct laboratory determinations of CH(2)OO kinetics. Upper limits were extracted for reaction rate coefficients with NO and H(2)O. The CH(2)OO reactions with SO(2) and NO(2) proved unexpectedly rapid and imply a substantially greater role of carbonyl oxides in models of tropospheric sulfate and nitrate chemistry than previously assumed.

Abstract  This paper aims to advance regional worldwide source receptor relationships, providing fate factors for acidifying and eutrophying air emissions (NOx, HNO3, SO2, SO4 and NH3) to be used within life cycle impact assessment. A simulation for the reference year 2005 of the three-dimensional global scale tropospheric GEOS-Chem model was used as the basis of a novel methodological approach to derive source receptor matrices (SRMs) whose elements are fate factors at a global 2 degrees x 2.5 degrees grid. This new approach makes it possible to assess the impact of transboundary emissions while maintaining regional scale emission differentiation. These 2 degrees x 2.5 degrees grid resolution fate factors were later aggregated at continental and country resolutions using emission weighting. Continental fate factor results showed that 50-70% of nitrogen oxides (NOx, HNO3) and sulfur oxides (SO2, SO4) and approximately 80% of ammonia (NH3) emissions will deposit on the same continent. Results showed that the developed fate factor derivation approach was within a +/-10% agreement with GEOS-Chem simulations in which fate factors were determined by withdrawing the regional emission inventory over Canada and in +/-50% agreement with current state-of-the-art LCIA fate factors (calculated with the European Monitoring and Evaluation Programme (EMEP) model). The SRMs outlined in this paper facilitate further modeling developments without having to run the underlying tropospheric model, thus opening the door to the assessment of the regional life cycle inventories of a global economy. (C) 2012 Elsevier Ltd. All rights reserved.

Abstract  Sphagnum mosses are assumed to be effective at acquiring low amounts of nitrogen (N) in precipitation to support annual growth. However, N concentrations in precipitation have increased from anthropogenic sources over the last 150 years. I hypothesized that N retention from wet atmospheric deposition decreases with increased N availability, by comparing Sphagnum mosses in a high N deposition region in the Adirondack Park, New York, to a low-deposition region in eastern Maine. A 15NH4 15NO3 tracer was applied to mosses in both regions, and retention after 24 h was estimated. Nitrogen retention ranged from 50 to 90% of N applied. Most 15N was recovered from the apical capitula and upper stems. Nitrogen retention was greater in the Maine sites in 1998. However, in 1999, a drought year, particularly in Maine, N retention was less in Maine than in New York. The drier climate appeared to lower N retention, possibly through its physiological effects on the mosses. Although atmospheric deposition might be the only exogenous source of N, it satisfied only a small fraction of N required for annual growth. These data suggest that internal cycling processes, such as mineralization, may be much more important N sources to support Sphagnum growth.