Air pollution can lead to environmental impacts. Over the past decades there have been some success stories reducing pollutant emission, namely sulphur dioxide (SO2). However, impacts on ecosystems from atmospheric nitrogen (N) pollution are still seen as a major threat for European biodiversity. Across Europe over 70% of Natura 2000 sites are at risk for eutrophication with over 70% of the Natura 2000 area in Europe (EU28( exceeding critical loads for nutrient nitrogen deposition. Agricultural ammonia is a key contributor to the threat to these sites due to the close proximity of agricultural activities and protected sites. Source attribution modelling using an atmospheric transport model showed that agricultural livestock production in the UK is the dominant nitrogen source for N disposition across the UK Natura 2000 network. Nearly 90% of all sites had livestock as their dominant source, contributing 32% of the total nitrogen deposition across the whole network. 76% of all Special Areas of Conservation (SAC) sites exceeded their critical load for nutrient nitrogen, representing 74% of the entire SAC area. The extent of exceedance is also notable with many sites experiencing depositions of >50 kg N/ha/yr over the critical load. the situation for acidity critical load exceedance is less sever, by 51% of sites are still exeeded. Legislation to regulate pollutant emissions to air and protect biodiversity are often not integrated, and there has been no common European approach for determining the impacts of nitrogen deposition on individual Natura sites, or on conservation status. Off-site sources of air pollution present difficulties in assessing and attributing impacts, because deposition can result from local sources (1-2 km), or very far away sources (>1000 kms). Managing nitrogen losses on the farm and improving the efficient use of nitrogen are key components for overall reduction in NH3 emissions. Many nitrogen management options are available to abate ammonia from agricultural activities. On the one hand, technical and management measures include controlling emissions from manure storage and spreading, livestock feeding strategies, and improving housing systems. Trees, on the other hand, are effective scavengers of both gaseous and particulate pollutants from the atmosphere, making tree belts potentially effective landscape features to support ammonia abatement strategies. Using a coupled deposition and turbulence model the recapture efficiency of tree planting around ammonia sources was estimated. Using different canopy structure scenarios, tree depths and differing leaf area density (LAD) and leaf are index (LAI) were adjusted for a main canopy and a backstop canopy. Recapture efficiency for ammonia ranged from 27% (trees planted around housing systems), up to 60% (under-story livestock silvopastoral systems). Practical recapture potential was set at 20% and 40% for housing and silvopastoral systems respectively. Model results from scaling up to national level suggest that tree planting in hot spot areas of ammonia emissions would lead to reduced N deposition on nearby sensitive habitats. Scenarios for on-farm emission control through tree planting showed national reductions in nitrogen deposition to semi-natural areas of 0.14% (0.2 kt N-NHx) to 2.2% (3.15 kt N-NHx). Scenarios mitigating emissions from cattle and pig housing yielded the highest reductions. The afforestation strategy showed national-scale emission reductions of 6% (8.4 kt N-NHx) to 11% (15.7 kt N-NHx) for 25% and 50% afforestation scenarios respectively. Increased capture by the planted trees also generated an added benefit of reducing long-range transport effects, including a decrease in wet deposition of up to 3.7 kt N-NHx (4.6%) and a decrease in export from the UK of up to 8.3 kt N-NHx (6.8%). Agroforestry measures for ammonia abatement were shown to be cost-effective for both planting downwind of housing and in silvopastoral systems, when costs to society were taken into account. Planting trees was also cost-effective from a climate change perspective. Comparing the cost per kg of NH3 abated showed that planting trees is a method of ammonia emission mitigation comparable with other (technical) measures. The costs for planting trees downwind of housing were calculated at €2.6-7.3/kg NH3. Agroforestry for ammonia abatement offers multiple benefits for the farmer and synergistic effects for society as a whole including i) carbon sequestration. ii) visibility screening around housing units, iii) imporved animal welfare for silvopastoral systems, iv) reducing critical load exceedance on protected sites, v) price advantage of 'woodland chick' productions, vi) supporting the Industrial Emission Directive (IED) requirements for emission reduction, vii) supporting national afforestation policies. The results of this work support the notion that in the emerging discussion about the values of ecosystem services and the role of nature-based solution to tackle persistent environmental challenges, tree planting has a large potential in rural and urban environments.

The Clean Air Act of 1970 was promulgated after thousands of lives were lost in four catastrophic air pollution events. It authorized the establishment of National Ambient Air Quality Standards or (NAAQS) for six pollutants that are harmful to human health and welfare: carbon monoxide, lead, nitrogen dioxide, particulate matter, ozone and sulfur dioxide. The Clean Air Act also led to the establishment of the United Stated Environmental Protection Agency (US EPA) to set and enforce regulations. The first paper in this dissertation studies ozone in the Lake Michigan region (Foley, T., Betterton, E.A., Jacko, R., Hillery, J., 2011. Lake Michigan air quality: The 1994-2003 LADCO Aircraft Project (LAP). Atmospheric Environment 45, 3192-3202.) The Chicago-Milwaukee-Gary metropolitan area has been unable to meet the ozone NAAQS since the Clean Air Act was implemented. The Lake Michigan Air Directors' Consortium (LADCO) hypothesized that land breezes transport ozone precursor compounds over the lake, where a large air/water temperature difference creates a shallow conduction layer, which is an efficient reaction chamber for ozone formation. In the afternoon, lake breezes and prevailing synoptic winds then transport ozone back over the land. To further evaluate this hypothesis, LADCO sponsored the 1994-2003 LADCO Aircraft Project (LAP) to measure the air quality over Lake Michigan and the surrounding areas. This study has found that the LAP data supports this hypothesis of ozone formation, which has strong implications for ozone control strategies in the Lake Michigan region. The second paper is this dissertation (Foley, T., Betterton, E.A., Wolf, A.M.A., 2012. Ambient PM10 and metal concentrations measured in the Sunnyside Unified School District, Tucson, Arizona. Journal of the Arizona-Nevada Academy of Science, 43, 67-76) evaluated the airborne concentrations of PM10 (particulate matter with an aerodynamic diameter of 10 microns or less) and eight metalloids and metals (arsenic, beryllium, cadmium, chromium, cobalt, lead, manganese, and nickel) in the southern Tucson metropolitan area. A Tucson company that uses beryllium oxide to manufacture thermally conductive ceramics has prompted strong citizen concern. This study found that the study area has good air quality with respect to PM₁₀ and metals, with ambient concentrations meeting US Environmental Protection Agency and World Health Organization standards. Beryllium was detected only once (during a dust storm) and was ascribed to naturally-occurring beryllium in the suspended soil. The third paper (to be submitted to the Journal of Great Lakes Research) studies nitrogen dry deposition over Lake Michigan and Lake Superior. Numerous studies have shown that wet and dry deposition of nitrogen has contributed to the eutrophication of coastal waters and declining productivity of marine fisheries. Nitrogen dry deposition over the Great Lakes themselves, as opposed to the shorelines, has not been documented in the peer-reviewed literature. This paper calculates nitrogen dry deposition over Lake Michigan and Lake Superior, using aircraft measurements from the LADCO Aircraft Study, and finds that over-water, nitrogen dry deposition is a significant source of nitrogen to Lake Michigan and Lake Superior.

Nitrogen (N) deposition in Europe has recently increased and is expected to continue to increase in the future. There is a well-documented decline in lichen diversity with higher N availability, although the mechanisms behind this are poorly known. In this study, I tested whether attacks by fungal parasites increase with higher N deposition. This pattern has been found in a number of studies on vascular plants, but it has never been investigated for lichens. I surveyed dark lesions and discolourings caused by fungi on the pollution-sensitive lichen Alectoria sarmentosa, after 4 years of increased N deposition in a whole tree fertilisation experiment in a boreal spruce forest. I found two species of fungi growing on the investigated lichen thalli. One of these species responded positively to increased N deposition. The results show that lichens can suffer from increased parasite attacks under a higher N load. Further studies using multiple lichen species and many years of recording are needed to understand the importance of parasites for the response of whole lichen communities to an increased N load.

The main aim of this study was to analyse the long-term effects on epiphytic lichen dry mass development, diversity and community structure after exposure to a simulated nitrogen deposition gradient. A whole tree experiment was set up in a boreal forest in the County of Västerbotten in Sweden, whereby 15 trees were artificially irrigated and nitrogen fertilized during seven consecutive years (2006-2012). The treatments were equal to an additional deposition of 0.6, 6, 12.5, 25 and 50 kg N ha-1 yr-1. Branches from the trees were collected in October 2012 and the lichen material was harvested and further analysed during the spring 2013. The results from this study showed that the lichens were directly affected by the long-term increased nitrogen deposition. Generally, lichen dry mass and species richness declined at high nitrogen loads and the initial positive effects of low nitrogen loads reported in a preceding study had thus depressed with time. The results from this study also showed that the composition of the lichen community changed benefitting nitrogen-tolerant species, probably not because of competition but more likely because of nitrogen-sensitive species disappearance. This study strongly indicates that it takes more than a few years to detect changes in lichen communities exposed to enhanced nitrogen loads. In addition low concentrations seem to have a cumulative impact. Consequently, this study stresses the importance of also considering the cumulative effect of low nitrogen loads when determining critical values. The critical load for lichen communities in boreal forests might be below 6 kg N ha-1 yr-1.

Global temperatures are expected to increase by 1.1 to 6.4°C over the next century and, over the same period, nitrogen inputs to terrestrial ecosystems are expected to increase as a result of increased crop fertilisation and atmospheric nitrogen deposition. Both of these global change drivers are expected to affect net carbon balance by increasing both gross primary production and ecosystem respiration, yet the balance between these processes, and the potential interactive effects of the drivers, require quantification. The ability to accurately predict the effects of warming and nitrogen addition on all components of terrestrial carbon balance will be critical in determining the likely positive feedback to rising atmospheric CO₂ from terrestrial ecosystems. Tussock grasslands are a widespread and important carbon store within New Zealand and are representative of temperate grasslands worldwide. This thesis addresses the question: Will tussock grasslands act as a positive feedback to rising atmospheric CO₂ concentration in response to soil warming and nitrogen addition? Using a combination of controlled-environment and field-scale studies of tussock grassland, net ecosystem carbon exchange was partitioned into gross primary production, ecosystem respiration and the autotrophic and heterotrophic components of soil respiration. Soil respiration in the field increased by 41% in response to a 3°C soil warming treatment and by 12% in response to a 50 kg N ha⁻¹ y⁻¹ nitrogen addition treatment. Only warming resulted in enhanced heterotrophic decomposition of soil organic matter (37% increase). However, a controlled-environment study indicated that caution must be used when interpreting temperature responses of heterotrophic respiration from root-free soils, as priming effects were shown to decrease the sensitivity of heterotrophic respiration to temperature. Measurements of net ecosystem exchange in the field showed that warming-enhanced heterotrophic respiration lead to a significant 49 g m⁻² reduction in net ecosystem carbon uptake. Neither nitrogen addition nor combined warming and nitrogen addition treatment resulted in significant changes in net ecosystem carbon balance. These results suggest that tussock grasslands will act as a positive feedback to rising atmospheric CO₂ concentration. However, increased nitrogen deposition will serve as a potential mitigating factor for climate driven feedbacks.

Indiana University-Purdue University Indianapolis (IUPUI) / Invasive exotic species have caused severe ecological and economic damages to many communities in the United States and elsewhere. It is therefore important to improve our understanding of how global environmental changes will affect the invasiveness and severity of these invasive species. Over the last century, anthropogenic activities have caused multiple environmental changes. Previous studies have generally focused on the impact of the increasing atmospheric CO2 level on the physiology and growth of invasive species. With atmospheric nitrogen (N) deposition on the rise over the past decades, it is essential to recognize how an increase in soil N will affect the invasiveness of some exotic species. To determine the impact of increased atmospheric N deposition and drought stress on invasive species, I studied the impact of different levels of N on Alliaria petiolata (garlic mustard), an exotic invasive species. In addition, I examined the interactive effects of N deposition and drought stress on garlic mustard. Multiple morphological measurements were used to analyze the growth rate at varying levels of N and soil moisture. The study on N deposition on plant growth will improve our understanding of the invasiveness of garlic mustard. The changes in precipitation patterns must also be examined to foresee if plants in increased atmospheric N conditions can overcome drought stress conditions. I found an increase in plant growth and photosynthetic rate at higher levels of N. Plants with adequate water displayed a continued increase from the lowest level to the highest level of N. Increases in drought stressed plants plateaued at an intermediate N level of 20 kg ha-1. My results demonstrated that during drought stress garlic mustard does not benefit from an increase in N above a certain level. These results are important to take into consideration when we analyze the spreading of invasive weeds due to global environmental changes, including increased atmospheric N deposition and regional drought, in order to apply the optimal management strategies for controlling invasive species.

Two types of nitrogen have been studied in this report, nitrate and ammonium. Nitrate mainly originates from incomplete combustion like fossil fule combustion from sea traffic, land traffic and energy use. Ammonium mainly origins from farming like fertilizers and animal waste. Nitrate and ammonium are water soluble and can be transported long distances in the atmosphere before depositing with precipitation. In order to record the nitrogen deposition, precipitation was collected and the amount of nitrogen was measured. This was done in 6 places in Eastern Svealand over a two month period, middle of March to middle of May 2014. The source regions of the precipitation was studied and it was found out that if the precipitation originated from areas north of Sweden the amount of deposited nitrogen was significantly lower then if the air masses originated from areas south, south east and south west of Sweden. The results from the measurements shows a decrease in nitrogen deposition with distance from the pollution source and that there is a higher amount of nitrogen in the precipitation in urban areas. / I denna rapport har två typer av kväve studerats, nitrat och ammonium. Nitrat har främst sitt ursprung från förbränningar, till exempel förbränning av fossila bränslen från sjöfart, fordonstrafik och energiförbrukning. Ammonium har framför allt källor från jordbruk, till exempel gödsling och djuravföring. Nitrat och ammonium är vattenlösliga och kan transporteras långa sträckor för att sedan deponera med nederbörden. Genom att samla in nederbörd och mäta halterna av nitrat och ammonium i proverna kunde depositionen av kväve beräknas. Detta gjordes över en tvåmånaders period, mitten av mars till mitten av maj 2014, på 6 platser i östra Svealand. Luftmassornas ursprung undersöktes och det visade sig att när luftmassorna och nederbörden hade sitt ursprung norr om Sverige innehöll den betydligt lägre halter kväve än när de kom från söder, sydöst och sydväst. Resultaten från mätningarna visade att det är en tydlig minskning av kvävehalten i nederbörden med avståndet från källan och att det är en förhöjd kvävehalt i nederbörden i städerna.

Nitrogen (N) fertilization due to atmospheric deposition (NDEP ) may explain some of the net carbon (C) sink (0.6-0.7 Pg y-1) in temperate forests, but estimates of the additional C uptake due to atmospheric N additions (∆C/∆N) can vary by over an order of magnitude (5 to 200 ∆C/∆N). High estimates from several recent studies [e.g. Magnani (2007), Nature 447 848-850], deriving ∆C/∆N from regional correlations between NDEP and measures of C uptake (such as eddy covariance -derived net ecosystem production, or forest inventory data) contradict estimates from other studies, particularly those involving 15N tracer applications added as fertilizer to the forest floor. A strong ∆C/∆N effect requires nitrogen to be efficiently acquired by trees and allocated to high C:N, long-lived woody tissues, but these isotope experiments typically report relatively little (~ 20 %) of 15N added is found above-ground, with less than 5 % of the total 15N applied found in wood. Consequently the high correlation-derived ∆C/∆N estimates are often attributed to co-variation with other factors across the range of sites investigated. However, 15N-fertilization treatments often impose considerably higher total N loads than ambient NDEP , while almost all exclusively only apply mineral 15N treatments to the soil, often in a limited number of treatment events over relatively short periods of time. Excessive N deposition loads can induce negative physiological effects and limit the resulting ∆C/∆N observed, and applying treatments to the soil ignores canopy nitrogen uptake, which has been demonstrated in numerous studies. As canopies can directly take up nitrogen, the chronic, (relatively) low levels of ambient NDEP inputs from pollution may be acquired without some of the effects of heavy N loads, with trees obtaining this N before it reaches the soil, allowing canopies to substitute for, or supplement, edaphic N nutrition. The strength of this effect depends on how much N uptake can occur across the canopy under field conditions, and if this extra N supplies growth in woody tissues such as the stem, as well as the canopy. Similarly, such mineral fertilizer isotope trace experiments are also unable to trace N in the decomposing litter and humus layers of the soil, which even under heavy NDEP loading contribute most of the N utilised for forest growth. Recent literature suggests that some organic (early decomposition) forms of N may be taken up by roots. If this litter N is not retained or distributed in the same way as mineral fertilizers, its contribution to plant nutrition and ∆C/∆N may need to be reassessed under nitrogen deposition. We tested some of these assumptions in the nursery and the field. In order to facilitate litter 15N tracing, we conducted an experiment injecting large trees with 15N-NH4NO3 to create 15N-labelled litter, tracing the applied isotope into a full harvest of the canopy. Such labelled litter substitute was used to replace the litter layer in a Sitka Spruce plantation (Picea sitchensis L. (Bong.)), where the fate of this 15N from litter decomposition in the soil system was compared against the fate of 15N in deposition. Similarly, in potted Sitka Spruce saplings, we used combination treatments of 15N-labelled litter, soil-targeted 15N-deposition, and canopy targeted 15N-deposition, investigating 15N return in different age classes of above and below ground biomass. We found that i) 15N recovery in canopies (needles and branches) in our injected trees was almost all of the injected 15N five months after injection, ii) canopy application of NDEP led to 60 % 15N return in above-ground parts of saplings compared to 21 % in soil applications and iii) a litter-derived 15N source was retained 55 % more in topsoil, and 36 % more in roots, than a similar deposition 15N source applied as mineral fertilizer. We discuss the implications of such findings in the context of 15N return in different plant organs and ecosystem pools, seasonal variation in N content, and overall inferences of a forest ∆C/∆N effect. Our results suggest that the total ∆C/∆N effect driven by a high N sequestration from canopy uptake in wood is ~ 114:1, more than double that of 15N tracer experiments but not as high as upper estimates from correlative studies, and that litter-derived organic N is better retained in trees and soils in excess of similar amounts of mineral 15N from deposition. Existing forest 15N-fertilization experiments could under-estimate the overall ∆C/∆N effect of atmospheric N deposition.

The effect of moderate rates of nitrogen deposition on ground floor vegetation is poorly predicted by uncontrolled surveys or fertilization experiments using high rates of nitrogen (N) addition. We compared the temporal trends of ground floor vegetation in permanent plots with moderate (7–13 kg ha−1 year−1) and lower bulk N deposition (4–6 kg ha−1 year−1) in southern Sweden during 1982–1998. We examined whether trends differed between growth forms (vascular plants and bryophytes) and vegetation types (three types of coniferous forest, deciduous forest, and bog). Trends of site-standardized cover and richness varied among growth forms, vegetation types, and deposition regions. Cover in spruce forests decreased at the same rate with both moderate and low deposition. In pine forests cover decreased faster with moderate deposition and in bogs cover decreased faster with low deposition. Cover of bryophytes in spruce forests increased at the same rate with both moderate and low deposition. In pine forests cover decreased faster with moderate deposition and in bogs and deciduous forests there was a strong non-linear increase with moderate deposition. The trend of number of vascular plants was constant with moderate and decreased with low deposition. We found no trend in the number of bryophyte species. We propose that the decrease of cover and number with low deposition was related to normal ecosystem development (increased shading), suggesting that N deposition maintained or increased the competitiveness of some species in the moderate-deposition region. Deposition had no consistent negative effect on vegetation suggesting that it is less important than normal successional processes.

Peatlands are peat forming ecosystems in which not fully decomposed plant material builds up the soil. The sequestration of carbon into peat is mainly associated with the bryophyte genus Sphagnum (peat mosses), which dominate and literally form most peatlands. The responses of Sphagnum to environmental change help us to understand peatland development and function and to predict future changes in a rapidly changing world. In this thesis, the overarching aim was to use ecophysiological traits to investigate mechanisms behind the response of Sphagnum to elevated N deposition, and, processes connected to ecosystem shift and ecosystem function of peatlands. Regarding elevated N deposition, three experiments were performed at different scales (country-wide to greenhouse). Independent of scale and species, apical tissue N concentration increased with increasing N input until N saturation was reached. Maximum photosynthetic rate, a trait evaluating photosynthetic capacity, increased with N input and could be well predicted by tissue N concentration. Thus, the physiological responses of Sphagnum to N deposition are often positive and I found no evidence of toxic effects. Production did, however, not increase with N input, and results of the N:P ratio suggested that P limitation, and possibly other elements, might hamper growth under high N input. The effect of P limitation was, in contrast to current view, most pronounced in fast growing species indicating species specific responses to nutrient imbalance. I explored the puzzling, but historically frequently occurring, rich fen to bog ecosystem shift; a shift from a species-rich ecosystem dominated by brown mosses, to a species-poor one with greater carbon storage that is Sphagnum-dominated. The bog-dwelling species of Sphagnum grew well, to our surprise, when in contact with rich fen water but was not a strong competitor compared to rich fen Sphagnum species. If submerged under rich fen water (high pH), the bog Sphagnum species died while rich fen species of Sphagnum were unaffected. These results show that differences in two physiological traits (growth rate and tolerance to flooding) among species, can explain when a peatland ecosystem shift might occur. In the last study, the function of peatlands was related to trade-offs between traits and allometric scaling in Sphagnum. Results suggested that growth strategies are determined by the distribution of Sphagnum relative to the water table in order to minimize periods with suboptimal hydration. Allometric analyses stressed the importance of resource allocation among and within shoots (apical part vs. stem), although the allocation patterns in Sphagnum were not always consistent with those of vascular plants. Interestingly, data indicated a trade-off between photosynthetic rate and decomposition rate among Sphagnum species.