Abstract  Epiphytic lichen communities are highly sensitive to excess nitrogen (N), which causes the replacement of native floras by N-tolerant, "weedy" eutrophic species. This shift is commonly used as the indicator of ecosystem "harm" in studies developing empirical critical levels (CLE) for ammonia (NH3) and critical loads (CLO) for N. To be most effective, empirical CLE and/or CLO must firmly link lichen response to causal pollutant(s), which is difficult to accomplish in field studies in part because the high cost of N measurements limits their use. For this case study we synthesized an unprecedented array of atmospheric N measurements across 22 long-term monitoring sites in the Los Angeles Basin, California, USA: gas concentrations of NH3, nitric acid (HNO3), nitrogen dioxide, and ozone (n = 10 sites); N deposition in throughfall (n = 8 sites); modeled estimates of eight different forms of N (n = 22 sites); and nitrate deposition accumulated on oak twigs (n = 22 sites). We sampled lichens on black oak (Quercus kelloggii Newb.), and scored plots using two indices of eutroph (N tolerant species) abundance to characterize the community-level response to N. Our results contradict two common assertions about the lichen-N response: (1) that eutrophs respond specifically to NH3 and (2) that the response necessarily depends upon the increased pH of lichen substrates. Eutroph abundance related significantly but weakly to NH3 (r2 = 0.48). Total N deposition as measured in canopy throughfall was by far the best predictor of eutroph abundance (r2 = 0.94), indicating that eutrophs respond to multiple forms of N. Most N variables had significant correlations to eutroph abundance (r2 = 0.36-0.62) as well as to each other (r2 = 0.61-0.98), demonstrating the risk of mistaken causality in CLE/CLO field studies that lack sufficient calibration data. Our data furthermore suggest that eutroph abundance is primarily driven by N inputs, not substrate pH, at least at the high-pH values found in the basin (4.8-6.1). Eutroph abundance correlated negatively with trunk bark pH (r2 = 0.43), exactly the opposite of virtually all previous studies of eutroph behavior. This correlation probably results because HNO3 dominates N deposition in our study region.

Abstract  Nitrification is a microbially mediated process that plays a central role in the global cycling of nitrogen and is also of economic importance in agriculture and wastewater treatment. The first step in nitrification is performed by ammonia-oxidising microorganisms, which convert ammonia into nitrite ions. Ammonia-oxidising bacteria (AOB) have been known for more than 100 years. However, metagenomic studies and subsequent cultivation efforts have recently demonstrated that microorganisms of the domain archaea are also capable of performing this process. Astonishingly, members of this group of ammonia-oxidising archaea (AOA), which was overlooked for so long, are present in almost every environment on Earth and typically outnumber the known bacterial ammonia oxidisers by orders of magnitudes in common environments such as the marine plankton, soils, sediments and estuaries. Molecular studies indicate that AOA are amongst the most abundant organisms on this planet, adapted to the most common environments, but are also present in those considered extreme, such as hot springs. The ecological distribution and community dynamics of these archaea are currently the subject of intensive study by many research groups who are attempting to understand the physiological diversity and the ecosystem function of these organisms. The cultivation of a single marine isolate and two enrichments from hot terrestrial environments has demonstrated a chemolithoautotrophic mode of growth. Both pure culture-based and environmental studies indicate that at least some AOA have a high substrate affinity for ammonia and are able to grow under extremely oligotrophic conditions. Information from the first available genomes of AOA indicate that their metabolism is fundamentally different from that of their bacterial counterparts, involving a highly copper-dependent system for ammonia oxidation and electron transport, as well as a novel carbon fixation pathway that has recently been discovered in hyperthermophilic archaea. A distinct set of informational processing genes of AOA indicates that they are members of a distinct and novel phylum within the archaea, the 'Thaumarchaeota', which may even be a more ancient lineage than the established Cren- and Euryarchaeota lineages, raising questions about the evolutionary origins of archaea and the origins of ammonia-oxidising metabolism.

Abstract  Anthropogenic nitrogen (N) addition may substantially alter the terrestrial N cycle. However, a comprehensive understanding of how the ecosystem N cycle responds to external N input remains elusive. • Here, we evaluated the central tendencies of the responses of 15 variables associated with the ecosystem N cycle to N addition, using data extracted from 206 peer-reviewed papers. • Our results showed that the largest changes in the ecosystem N cycle caused by N addition were increases in soil inorganic N leaching (461%), soil NO₃⁻ concentration (429%), nitrification (154%), nitrous oxide emission (134%), and denitrification (84%). N addition also substantially increased soil NH₄+ concentration (47%), and the N content in belowground (53%) and aboveground (44%) plant pools, leaves (24%), litter (24%) and dissolved organic N (21%). Total N content in the organic horizon (6.1%) and mineral soil (6.2%) slightly increased in response to N addition. However, N addition induced a decrease in microbial biomass N by 5.8%. • The increases in N effluxes caused by N addition were much greater than those in plant and soil pools except soil NO₃⁻, suggesting a leaky terrestrial N system.

Abstract  Mass balance studies conducted in the 1970s in Great Sippewissett Salt Marsh, New England, showed that fertilized plots intercepted 60 to 80% of the nitrogen (N) applied at several treatment levels every year from April to October, where interception mechanisms include plant uptake, denitrification and burial. These results pointed out that salt marshes are able to intercept land-derived N that could otherwise cause eutrophication in coastal waters. To determine the long-term N interception capacity of salt marshes and to assess the effect of different levels of N input, we measured nitrogenous materials in tidal water entering and leaving Great Sippewissett experimental plots in the 2007 growing season. Our results, from sampling over both full tidal cycles and more intensively sampled ebb tides, indicate high interception of externally added N. Tidal export of dissolved inorganic N (DIN) was small, although it increased with tide height and at high N input rates. NH4+ export was generally 2 to 3 times NO3– export, except at the highest N addition, where DIN export was evenly partitioned between NO3– and NH4+. Exports of dissolved organic N were not enhanced by N addition. Overall, export of added N was very small, <7% for all treatments, which is less than earlier estimates. Apparent enhanced tidal export of N from N-amended plots ceased when N additions ended in the fall. Nitrogen cycling within the vegetated marsh appears to limit N export, such that interception of added N remains high even after over 3 decades of external N inputs.

Abstract  Anthropogenic N deposition can slow the decay of plant detritus, leading to an accumulation of soil organic matter and the production of phenolic dissolved organic C (DOC), which can leach from soil to ground and surface waters. Actinobacteria are one of the few groups of saprotrophic microorganisms that oxidatively depolymerize lignin, producing substantial soluble polyphenolics in the process. In combination, these observations present the possibility that lignolytic Actinobacteria may become more important agents of lignin decay as atmospheric N deposition continues to increase during the next decade. To test this idea, we quantified actinobacterial abundance and community composition in a well-replicated field study in which atmospheric N deposition has been experimentally increased since 1994. Actinobacterial abundance was assessed using quantitative polymerase chain reaction of 16S rRNA and community composition was evaluated using clone libraries and phylogenetic community analyses (i.e., LIBSHUFF and UniFrac). Contrary to our expectation, experimental atmospheric N deposition had no effect on actinobacterial abundance in the forest floor (∼1010 gene copies kg−1); however, it significantly decreased actinobacterial abundance by 47% and total DNA by 31% in surface soil. Our analyses revealed that experimental N deposition further elicited a significant membership change in forest floor and surface soil communities, as well as significant differences in the phylogenetic diversity of forest floor Actinobacteria This shift in community composition occurred in concert with a slowing of plant litter decay, accumulation of soil organic matter, and a greater production of phenolic DOC. These observations are consistent with the idea that changes in actinobacterial community composition may underlie biogeochemical responses to experimental N deposition.

Abstract  Watershed recovery from acidic deposition, such as the Noland Divide Watershed in the Great Smoky Mountains National Park, is difficult to predict because of complex biogeochemical processes exhibited in soils. Laboratory soil columns and in situ pan lysimeters were used to investigate soil solution response to simulated reductions in acid deposition. Controlling for influent SO2−4, NO−3, and NH+4 concentrations in the column experiments, effluent pH declined similarly to 4.4 among five experimental scenarios from an initial pH of approximately 4.7 and 6.1. Influent-effluent chemical comparisons suggest nitrification and/or SO2−4 desorption controls effluent pH. Sulfate adsorption occurred when SO2−4 influent was greater than 25  μmol L−1 and desorption occurred below 15  μmol L−1, which would equate to approximately a 61% reduction in current SO2−4 deposition levels. Base cation depletion occurred in column experiments, in which 64–60  μmol L−1 Ca2+ and 24–27  μmol L−1 Mg2+ reductions were measured. Cation depletion rates were pH dependent, primarily caused by soil cation exchange and not weathering. In these soils with base saturation below 7%, complete Ca2+ and Mg2+ depletion was estimated as 90 to 140 years. Protons released by SO2−4 desorption via ligand exchange are expected to cause further base cation depletion, thereby delaying watershed recovery. Field experiments found SO2−4 sorption dynamics to be limited by kinetics and hydrologic interflow rates, illustrating how precipitation intensity can influence ion transport from soil to stream. Results from this study provide important information for predicting watershed recovery in the future and suggest needs for further research.

Abstract  Isoprene is an important atmospheric volatile organic compound involved in ozone production and NO(x) (NO+NO(2)) sequestration and transport. Isoprene reaction with OH in the presence of NO can form either isoprene hydroxy nitrates ("isoprene nitrates") or convert NO to NO(2) which can photolyze to form ozone. While it has been shown that isoprene nitrate production can represent an important sink for NO(x) in forest impacted environments, there is little experimental knowledge of the relative importance of the individual isoprene nitrate isomers, each of which has a different fate and reactivity. In this work, we have identified the 8 individual isomers and determined their total and individual production yields. The overall yield of isoprene nitrates at atmospheric pressure and 295K was found to be 0.070(+0.025/-0.015). Three isomers, representing nitrates resulting from OH addition to a terminal carbon, represent 90% of the total IN yield. We also determined the ozone rate constants for three of the isomers, and have calculated their atmospheric lifetimes, which range from similar to 1-2 h, making their oxidation products likely more important as atmospheric organic nitrates and sinks for nitrogen.

Abstract  Studies of the long-term impacts of acidic deposition in Europe and North America have prompted growing interest in understanding the dynamics linking the nitrogen (N) and calcium (Ca) cycles in forested watersheds. While it has been shown that increasing concentrations of nitrate (NO3-) through atmospheric deposition or through nitrification can increase Ca loss, the reciprocal effects of Ca on N transformation processes have received less attention. We studied the influence of soil Ca availability on extractable inorganic N (NO3- + NH4+) across a Ca gradient in the Adirondack Mountains, New York, USA. Our results did not show the direct Ca-N interaction that we had expected, but instead showed that exchangeable Ca coupled with soil moisture, soil organic matter, and ambient temperature accounted for 61% of the variability in extractable inorganic N across 11 sites over two growing seasons. Soil Ca concentrations were, however, positively related to sugar maple (Acer saccharum) and American basswood (Tilia americana) basal areas and negatively related to American beech (Fagus grandifolia) basal area. Based on litter chemistry differences among these tree species and reported potential N mineralization values, we suggest that the influence of Ca on soil inorganic N is through a multistep pathway: reciprocal interactions between soil Ca concentrations and species composition, which in turn affect the quality of litter available for N mineralization. If chronic soil Ca depletion continues, as reported in some forested ecosystems, potential shifts in biotic communities could result in considerable alterations of N cycling processes.

Abstract  High levels of atmospheric nitrogen (N) deposition may result in greater terrestrial carbon (C) storage. In a northern hardwood ecosystem, exposure to over a decade of simulated N deposition increased C storage in soil by slowing litter decay rates, rather than increasing detrital inputs. To understand the mechanisms underlying this response, we focused on the saprotrophic fungal community residing in the forest floor and employed molecular genetic approaches to determine if the slower decomposition rates resulted from down-regulation of the transcription of key lignocellulolytic genes, by a change in fungal community composition, or by a combination of the two mechanisms. Our results indicate that across four Acer-dominated forest stands spanning a 500-km transect, community-scale expression of the cellulolytic gene cbhI under elevated N deposition did not differ significantly from that under ambient levels of N deposition. In contrast, expression of the ligninolytic gene lcc was significantly down-regulated by a factor of 2-4 fold relative to its expression under ambient N deposition. Fungal community composition was examined at the most southerly of the four sites, in which consistently lower levels of cbhI and lcc gene expression were observed over a two-year period. We recovered 19 basidiomycete and 28 ascomycete rDNA 28S operational taxonomic units; Athelia, Sistotrema, Ceratobasidium and Ceratosebacina taxa dominated the basidiomycete assemblage, and Leotiomycetes dominated the ascomycetes. Simulated N deposition increased the proportion of basidiomycete sequences recovered from forest floor, whereas the proportion of ascomycetes in the community was significantly lower under elevated N deposition. Our results suggest that chronic atmospheric N deposition may lower decomposition rates through a combination of reduced expression of ligninolytic genes such as lcc, and compositional changes in the fungal community.

Abstract  Microbial communities play a pivotal role in soil nutrient cycling, which is affected by nitrogen loading on soil fungi and particularly mycorrhizal fungi. In this experiment, we evaluated the effects of allochthonous nitrogen addition on soil bacteria and fungi in two geographically distinct but structurally similar scrub oak forests, one in Florida (FL) and one in New Jersey (NJ). We applied allochthonous nitrogen as aqueous NH4NO3 in three concentrations (0 kg ha−1 yr−1 (deionized water control), 35 kg ha−1 yr−1 and 70 kg ha−1 yr−1) via monthly treatments over the course of 1 yr. We applied treatments to replicated 1 m2 plots, each at the base of a reference scrub oak tree (Quercus myrtifolia in FL and Q. ilicifolia in NJ). We measured microbial community response by monitoring: bacterial and fungal biomass using substrate induced respiration, and several indicators of community composition, including colony and ectomycorrhizal morphotyping and molecular profiling using terminal restriction fragment length polymorphism (TRFLP). Bacterial colony type richness responded differently to nitrogen treatment in the different sites, but ectomycorrhizal morphotype richness was not affected by nitrogen or location. Both experimental sites were dominated by fungi, and FL consistently supported more bacterial and fungal biomass than NJ. Bacterial biomass responded to nitrogen addition, but only in FL. Fungal biomass did not respond significantly to nitrogen addition at either experimental site. The composition of the bacterial community differed between nitrogen treatments and experimental sites, while the composition of the fungal community did not. Our results imply that bacterial communities may be more sensitive than fungi to intense pulses of nitrogen in sandy soils.

Abstract  Influences of specific sources of inorganic PM(2.5) on peak and ambient aerosol concentrations in the US are evaluated using a combination of inverse modeling and sensitivity analysis. First, sulfate and nitrate aerosol measurements from the IMPROVE network are assimilated using the four-dimensional variational (4D-Var) method into the GEOS-Chem chemical transport model in order to constrain emissions estimates in four separate month-long inversions (one per season). Of the precursor emissions, these observations primarily constrain ammonia (NH(3)). While the net result is a decrease in estimated US NH(3) emissions relative to the original inventory, there is considerable variability in adjustments made to NH(3) emissions in different locations, seasons and source sectors, such as focused decreases in the midwest during July, broad decreases throughout the US in January, increases in eastern coastal areas in April, and an effective redistribution of emissions from natural to anthropogenic sources. Implementing these constrained emissions, the adjoint model is applied to quantify the influences of emissions on representative PM(2.5) air quality metrics within the US. The resulting sensitivity maps display a wide range of spatial, sectoral and seasonal variability in the susceptibility of the air quality metrics to absolute emissions changes and the effectiveness of incremental emissions controls of specific source sectors. NH(3) emissions near sources of sulfur oxides (SO(x)) are estimated to most influence peak inorganic PM(2.5) levels in the East; thus, the most effective controls of NH(3) emissions are often disjoint from locations of peak NH(3) emissions. Controls of emissions from industrial sectors of SO(x) and NO(x) are estimated to be more effective than surface emissions, and changes to NH(3) emissions in regions dominated by natural sources are disproportionately more effective than regions dominated by anthropogenic sources. NO(x) controls are most effective in northern states in October; in January, SO(x) controls may be counterproductive. When considering ambient inorganic PM(2.5) concentrations, intercontinental influences are small, though transboundary influences within North America are significant, with SO(x) emissions from surface sources in Mexico contributing almost a fourth of the total influence from this sector.

Abstract  Forest nitrogen (N) retention and soil carbon (C) storage are influenced by tree species and their associated soil microbial communities. As global change factors alter forest composition, predicting long-term C and N dynamics will require understanding microbial community structure and function at the tree species level. Because atmospheric N deposition is increasing N inputs to forested ecosystems across the globe, including the northeastern US, it is also important to understand how microbial communities respond to added N. While prior studies have examined these topics in mixed-species stands, we focused on the responses of different tree species and their associated microbial communities within a single forest type – a northern hardwood forest in the Catskills Mountains, NY. Based on prior studies, we hypothesized that N additions would stimulate extracellular enzyme activities in relatively labile litters, but suppress oxidative enzyme activities in recalcitrant litters, and tested for independent tree species effects within this context. During the 2007 growing season (May–June), we measured enzyme activities and microbial community composition (using phospholipid fatty acid analysis - PLFA) of the forest floor in single-species plots dominated by sugar maple (Acer saccharum), yellow birch (Betula alleghaniensis), red oak (Quercus rubra), American beech (Fagus grandifolia) and eastern hemlock (Tsuga canadensis), species whose litters range from relatively labile to recalcitrant. Half the plots were fertilized with N by adding NH4NO3 (50 kg ha−1 y−1) from 1997 to 2009. Non-metric multidimensional scaling (NMS) and multi-response permutation procedures (MRPP) were used to examine microbial community structure and relationship to enzyme activities. We found that in response to N additions, both microbial community composition and enzyme activities changed; however the strength of the changes were tree species-specific and the direction of these changes was and not readily predictable from prior studies conducted in mixed-species stands. For example, in contrast to other studies, we found that N additions caused a significant overall increase in fungal biomass that was strongest for yellow birch (24% increase) and weakest for sugar maple (1% increase). Contrary to our initial hypotheses and current conceptual models, N additions reduced hydrolytic enzyme activities in hemlock plots and reduced oxidative enzyme activity in birch plots, a species with relatively labile litter. These responses suggest that our understanding of the interactions between microbial community composition, enzyme activity, substrate chemistry, and nutrient availability as influenced by tree species composition is incomplete. NMS ordination showed that patterns in microbial community structure (PLFA) and function (enzyme activity) were more strongly influenced by tree species than by fertilization, and only partially agreed with the structure–function relationships found in other studies. This finding suggests that tree species-specific responses are likely to be important in determining the structure and function of northeastern hardwood forests in the future. Enhanced understanding of microbial responses to added N in single and mixed-species substrates with varying amounts of lignin and phenols may be needed for accurate predictions of future soil C and N dynamics.

Abstract  The effects of forest management on soil carbon (C) and nitrogen (N) are important to understand not only because these are often master variables determining soil fertility but also because of the role of soils as a source or sink for C on a global scale. This paper reviews the literature on forest management effects on soil C and N and reports the results of a meta analysis of these data. The meta analysis showed that forest harvesting, on average, had little or no effect on soil C and N. Significant effects of harvest type and species were noted, with sawlog harvesting causing increases (+18%) in soil C and N and whole-tree harvesting causing decreases (-6%). The positive effect of sawlog harvesting appeared to be restricted to coniferous species. Fire resulted in no significant overall effects of fire on either C or N (when categories were combined); but there was a significant effect of time since fire, with an increase in both soil C and N after 10 years (compared to controls). Significant differences among fire treatments were found, with the counterintuitive result of lower soil C following prescribed fire and higher soil C following wildfire. The latter is attributed to the sequestration of charcoal and recalcitrant, hydrophobic organic matter and to the effects of naturally invading, post-fire, N-fixing vegetation. Both fertilization and N-fixing vegetation caused marked overall increases in soil C and N. (C) 2001 Elsevier Science B.V. All rights reserved.

Abstract  Pollutant nitrogen deposition effects on soil and foliar element concentrations were investigated in acidic and limestone grasslands, located in one of the most nitrogen and acid rain polluted regions of the UK, using plots treated for 8-10 years with 35-140 kg N ha(-2)y(-1) as NH(4)NO(3). Historic data suggests both grasslands have acidified over the past 50 years. Nitrogen deposition treatments caused the grassland soils to lose 23-35% of their total available bases (Ca, Mg, K, and Na) and they became acidified by 0.2-0.4 pH units. Aluminium, iron and manganese were mobilised and taken up by limestone grassland forbs and were translocated down the acid grassland soil. Mineral nitrogen availability increased in both grasslands and many species showed foliar N enrichment. This study provides the first definitive evidence that nitrogen deposition depletes base cations from grassland soils. The resulting acidification, metal mobilisation and eutrophication are implicated in driving floristic changes.

Abstract  Fluxes of N2O, NO and NO2 between grassland and the atmosphere were measured over 1 y using three plots which have been maintained al a constant pH of 3.9, 5.9 and 7.6 over many years. Net fluxes of N2O and NO were always from the soil to the atmosphere, whilst those of NO2 were invariably from atmosphere to soil. Mean fluxes of N2O decreased appreciably with increasing acidity, whilst NO fluxes showed little dependence on pn, with the highest mean flux from the plot at pH 5.9. Sterilization of soil cores by autoclaving reduced N2O emissions almost to zero at all pH values, but residual production of NO was found, even at low pH. Increasing the pH of unsterilized soil cores from pH 3.9-6.5 +/- 0.5 led to a reduction in NO and especially N2O fluxes. It was concluded that the microbial community of the soil had adjusted to the low pH and was responsible for the entire production of N2O and much of the NO release. Chemodenitrification is also responsible for some NO production, especially at low pH. (C) 1997 Elsevier Science Ltd.

Abstract  Quarterly base flow water quality data collected from October, 1993 to November, 2002 at 90 stream sites in the Great Smoky Mountains National Park were used in step-wise multiple linear regression models to analyze pH, acid neutralizing capacity (ANC), and sulfate and nitrate long-term time trends. The potential predictor variables included cumulative Julian day, seasonality, elevation, basin slope, stream order, precipitation, surrogate streamflows, geology, and acid depositional fluxes. Modeling revealed statistically significant decreasing trends in pH and sulfate with time at lower elevations, but generally no long-term time trends in stream nitrate or ANC. The best forecasting models were chosen based on maximizing the r(2) of a holdout data set. If conditions remain the same and past trends continue, the forecasting models suggest that 30.0% of the sampling sites will reach pH values less than 6.0 in less than 10 years, 63.3% in less than 25 years, and 96.7% in less than 50 years. The pH forecasting models explain 65% of the variability in the holdout data.

Abstract  An important tool in the evaluation of acidification damage to aquatic and terrestrial ecosystems is the critical load (CL), which represents the steady-state level of acidic deposition below which ecological damage would not be expected to occur, according to current scientific understanding. A deposition load intended to be protective of a specified resource condition at a particular point in time is generally called a target load (TL). The CL or TL for protection of aquatic biota is generally based on maintaining surface water acid neutralizing capacity (ANC) at an acceptable level. This study included calibration and application of the watershed model MAGIC (Model of Acidification of Groundwater in Catchments) to estimate the target sulfur (S) deposition load for the protection of aquatic resources at several future points in time in 66 generally acid-sensitive watersheds in the southern Blue Ridge province of North Carolina and two adjoining states. Potential future change in nitrogen leaching is not considered. Estimated TLs for S deposition ranged from zero (ecological objective not attainable by the specified point in time) to values many times greater than current S deposition depending on the selected site, ANC endpoint, and evaluation year. For some sites, one or more of the selected target ANC critical levels (0, 20, 50, 100 mu eq/L) could not be achieved by the year 2100 even if S deposition was reduced to zero and maintained at that level throughout the simulation. Many of these highly sensitive streams were simulated by the model to have had preindustrial ANC below some of these target values. For other sites, the watershed soils contained sufficiently large buffering capacity that even very high sustained levels of atmospheric S deposition would not reduce stream ANC below common damage thresholds. (C) 2011 Elsevier Ltd. All rights reserved.

Abstract  The perturbation of the global nitrogen (N) cycle due to the increase in N deposition over the last 150 years will likely have important effects on carbon (C) cycling, particularly via impacts on forest C sequestration. To investigate this effect, and the relative importance of different mechanisms involved, we used the Generic Decomposition And Yield (G’DAY) forest C–N cycling model, introducing some new assumptions which focus on N deposition. Specifically, we (i) considered the effect of forest management, (ii) assumed that belowground C allocation was a function of net primary production, (iii) assumed that foliar litterfall and specific leaf area were functions of leaf N concentration, (iv) assumed that forest canopies can directly take up N, and (v) modified the model such that leaching occurred only for nitrate N. We applied the model with and without each of these modifications to estimate forest C sequestration for different N deposition levels. Our analysis showed that N deposition can have a large effect on forest C storage at ecosystem level. Assumptions (i), (ii) and (iv) were the most important, each giving rise to a markedly higher level of forest C sequestration than in their absence. On the contrary assumptions (iii) and (v) had a negligible effect on simulated net ecosystem production (NEP). With all five model modifications in place, we estimated that the C storage capacity of a generic European forest ecosystem was at most 121 kgCkg 1N deposited. This estimate is four times higher than that obtained with the original version of G’DAY (27.8 kgCkg 1 N). Thus, depending on model assumptions, the G’DAY ecosystem model can reproduce the range of dC : dNdep values found in the literature. We conclude that effects of historic N deposition must be taken into account when estimating the C storage capacity of a forest ecosystem.

Abstract  Cost-benefit analysis can be used to provide guidance for emerging policy priorities in reducing nitrogen (N) pollution. This paper provides a critical and comprehensive assessment of costs and benefits of the various flows of N on human health, ecosystems and climate stability in order to identify major options for mitigation. The social cost of impacts of N in the EU27 in 2008 was estimated between €75-485 billion per year. A cost share of around 60% is related to emissions to air. The share of total impacts on human health is about 45% and may reflect the higher willingness to pay for human health than for ecosystems or climate stability. Air pollution by nitrogen also generates social benefits for climate by present cooling effects of N containing aerosol and C-sequestration driven by N deposition, amounting to an estimated net benefit of about €5 billion/yr. The economic benefit of N in primary agricultural production ranges between €20-80 billion/yr and is lower than the annual cost of pollution by agricultural N which is in the range of €35-230 billion/yr. Internalizing these environmental costs would lower the optimum annual N-fertilization rate in Northwestern Europe by about 50 kg/ha. Acknowledging the large uncertainties and conceptual issues of our cost-benefit estimates, the results support the priority for further reduction of NH3 and NOx emissions from transport and agriculture beyond commitments recently agreed in revision of the Gothenburg Protocol.

Abstract  Increasing reactive nitrogen (N) input has been recognized as one of the important factors influencing climate system through affecting the uptake and emission of greenhouse gases (GHG). However, the magnitude and spatiotemporal variations of N-induced GHG fluxes at regional and global scales remain far from certain. Here we selected China as an example, and used a coupled biogeochemical model in conjunction with spatially explicit data sets (including climate, atmospheric CO2, O-3, N deposition, land use, and land cover changes, and N fertilizer application) to simulate the concurrent impacts of increasing atmospheric and fertilized N inputs on balance of three major GHGs (CO2, CH4, and N2O). Our simulations showed that these two N enrichment sources in China decreased global warming potential (GWP) through stimulating CO2 sink and suppressing CH4 emission. However, direct N2O emission was estimated to offset 39% of N-induced carbon (C) benefit, with a net GWP of three GHGs averaging -376.3 +/- 146.4 Tg CO2 eq yr(-1) (the standard deviation is interannual variability of GWP) during 2000-2008. The chemical N fertilizer uses were estimated to increase GWP by 45.6 +/- 34.3 Tg CO2 eq yr(-1) in the same period, and C sink was offset by 136%. The largest C sink offset ratio due to increasing N input was found in Southeast and Central mainland of China, where rapid industrial development and intensively managed crop system are located. Although exposed to the rapidly increasing N deposition, most of the natural vegetation covers were still showing decreasing GWP. However, due to extensive overuse of N fertilizer, China's cropland was found to show the least negative GWP, or even positive GWP in recent decade. From both scientific and policy perspectives, it is essential to incorporate multiple GHGs into a coupled biogeochemical framework for fully assessing N impacts on climate changes.

Abstract  The effect of low environmental pH on amphibians was studied in central Pennsylvania and the New Jersey Pine Barrens. Jefferson salamander (Ambystoma jeffersonianum) and Fowler's toad (Bufo woodhousei) embryos were intolerant of low pH and were absent from the most acidic ponds. Wood frog (Rana sylvatica) and pine barrens treefrog (Hyla andersoni) embryos were tolerant and found in ponds with the lowest pH's. Laboratory tests demonstrated that A. jeffersonianum could not hatch below pH 4.50, whereas R. sylvatica could hatch even at pH 4.25. Similarly, B. woodhousei could not hatch below pH 4.10, but H. andersoni could hatch at pH 3.70. Embryos of all four species were transplanted into several ponds ranging in pH from 3.90-7.06 to test whether embryonic mortality caused by low pH could be responsible for the absence of the two least tolerant species from acidic ponds. Hatching of embryos of R. sylvatica was not related to pond pH while mortality of embryos of A. jeffersonianum increased significantly as pond pH declined. H. andersoni also hatched in all experimental New Jersey ponds, but embryos of B. woodhousei suffered significantly higher mortality in ponds of lower pH's. Hatching success was variable at specific pond pH's, indicating significant interaction of pH with other chemical variables. In laboratory trials, tadpoles of B. woodhousei and H. andersoni grew significantly slower when exposed to low pH. This sublethal effect on body size has important implications for dynamics of amphibian populations in acidic ponds.

Abstract  Nitrogen oxides (NOx) are important components of ambient and indoor air pollution and are emitted from a range of combustion sources, including on-road mobile sources, electric power generators, and non-road mobile sources. While anthropogenic sources dominate, NOx is also formed by lightning strikes and wildland fires and is also emitted by soil. Reduced nitrogen (e.g., ammonia, NH3) is also emitted by various sources, including fertilizer application and animal waste decomposition. Nitrogen oxides, ozone (O-3) and fine particulate matter (PM2.5) pollution related to atmospheric emissions of nitrogen (N) and other pollutants can cause premature death and a variety of serious health effects. Climate change is expected to impact how N-related pollutants affect human health. For example, changes in temperature and precipitation patterns are projected to both lengthen the O-3 season and intensify high O-3 episodes in some areas. Other climate-related changes may increase the atmospheric release of N compounds through impacts on wildfire regimes, soil emissions, and biogenic emissions from terrestrial ecosystems. This paper examines the potential human health implications of climate change and N cycle interactions related to ambient air pollution.

Abstract  The absorption of atmospheric carbon dioxide (CO2) into the ocean lowers the pH of the waters. This so-called ocean acidification could have important consequences for marine ecosystems. To better understand the extent of this ocean acidification in coastal waters, we conducted hydrographic surveys along the continental shelf of western North America from central Canada to northern Mexico. We observed seawater that is undersaturated with respect to aragonite upwelling onto large portions of the continental shelf, reaching depths of approximately 40 to 120 meters along most transect lines and all the way to the surface on one transect off northern California. Although seasonal upwelling of the undersaturated waters onto the shelf is a natural phenomenon in this region, the ocean uptake of anthropogenic CO2 has increased the areal extent of the affected area.

Abstract  We examine the formation of nitrate and ammonium on five types of externally mixed pre-existing aerosols using the hybrid dynamic method in a global chemistry transport model. The model developed here predicts a similar spatial pattern of total aerosol nitrate and ammonium to that of several pioneering studies, but separates the effects of nitrate and ammonium on pure sulfate, biomass burning, fossil fuel, dust and sea salt aerosols. Nitrate and ammonium boost the scattering efficiency of sulfate and organic matter but lower the extinction of sea salt particles since the hygroscopicity of a mixed nitrate-ammonium-sea salt particle is less than that of pure sea salt. The direct anthropogenic forcing of particulate nitrate and ammonium at the top of the atmosphere (TOA) is estimated to be -0.12 W m(-2). Nitrate, ammonium and nitric acid gas also affect aerosol activation and the reflectivity of clouds. The first aerosol indirect forcing by anthropogenic nitrate (gas plus aerosol) and ammonium is estimated to be -0.09 W m(-2) at the TOA, almost all of which is due to condensation of nitric acid gas onto growing droplets (-0.08 W m(-2)).

Abstract  We examined long-term changes in soil solution chemistry associated with experimental, whole watershed-acidification at the Bear Brook Watershed in Maine (BBWM). At BBWM, the West Bear (WB) watershed has been treated with bimonthly additions of ((NH4)(2) SO4) since 1989. The adjacent East Bear (EB) watershed serves as a biogeochemical reference. Soil solution chemistry in the EB watershed was relatively stable from 1989-2007, with the exception of declining SO4-S concentrations associated with a progressive decline in SO4-S deposition during this period. Soil solution chemistry in WB reflected a progressive change in acid-neutralization mechanisms from base cation buffering to Al buffering associated with treatment during this period. Total dissolved Al concentrations progressively increased over time and were similar to 4x higher in 2007 than in 1989. Treatment of WB was also associated with long-term increases in soil solution H+, SO4-S, and NO3-N, whereas soil solution dissolved organic carbon (DOC) was unresponsive to treatment. For solutes such as Ca, H+, and SO4-S, changes in stream chemistry were generally parallel to changes in soil solution chemistry, indicating a close coupling of terrestrial and aquatic processes that regulate the chemistry of solutions in this first-order stream watershed. For other solutes such as Al and DOC, solute concentrations were higher in soil solutions compared with streams, suggesting that sorption and transformation processes along hydrologic flow-paths were important in regulating the chemistry of solutions and the transport of these solutes.