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Peatlands can buffer the impact of external perturbations, but can also rapidly shift to a new ecosystem type, with large gains or losses of stored carbon. Meter for meter, peatlands store more carbon than any other terrestrial ecosystem. Covering only about 3% of Earth's land area, they hold the equivalent of half of the carbon that is in the atmosphere as CO 2 ( 1 , 2 ). Waterlogged conditions slow decomposition, and slow rates of subsurface flow allow the partly decayed organic matter to accumulate in place. But the same processes of anaerobic decomposition that allow carbon to accumulate also produce the strong greenhouse gas methane (CH 4 ). Over the time span of centuries, peatlands exert a net cooling effect on the global radiation balance, because the effect of removing long-lived atmospheric CO 2 ultimately surpasses that of releasing short-lived CH 4 ( 3 ). However, should peatlands begin to degrade on a large scale, this stored carbon could be released, reducing—or even reversing—their climate cooling effect. How will the carbon balance of peatlands change over coming centuries?

The demand for more food is increasing fertilizer and land use, and the demand for more energy is increasing fossil fuel combustion, leading to enhanced losses of reactive nitrogen (Nr) to the environment. Many thresholds for human and ecosystem health have been exceeded owing to Nr pollution, including those for drinking water (nitrates), air quality (smog, particulate matter, ground-level ozone), freshwater eutrophication, biodiversity loss, stratospheric ozone depletion, climate change and coastal ecosystems (dead zones). Each of these environmental effects can be magnified by the ‘nitrogen cascade’: a single atom of Nr can trigger a cascade of negative environmental impacts in sequence. Here, we provide an overview of the impact of Nr on the environment and human health, including an assessment of the magnitude of different environmental problems, and the relative importance of Nr as a contributor to each problem. In some cases, Nr loss to the environment is the key driver of effects (e.g. terrestrial and coastal eutrophication, nitrous oxide emissions), whereas in some other situations nitrogen represents a key contributor exacerbating a wider problem (e.g. freshwater pollution, biodiversity loss). In this way, the central role of nitrogen can remain hidden, even though it actually underpins many trans-boundary pollution problems.

Reactive nitrogen (N) deposition from intensive agricultural and industrial activity has been identified as the third greatest threat to global terrestrial biodiversity, after land-use and climate change. While the impacts of N deposition are widely acknowledged, their magnitude is poorly quantified. We combine N deposition models, empirical response functions, and vegetation mapping to simulate the effects of N deposition on plant species richness from 1900 to 2030, using the island of Great Britain as a case study. We find that current species richness values - when averaged across five widespread habitat types - are approximately one-third less than without N deposition. Our results suggest that currently expected reductions in emissions will achieve no more than modest increases in species richness by 2030, and that emissions cuts based on habitat-specific \"critical loads\" may be an inefficient approach to managing N deposition for the protection of plant biodiversity. The effects of N deposition on biodiversity are severe and are unlikely to be quickly reversed.

In peatland ecosystems, plant communities mediate a globally significant carbon store. The effects of global environmental change on plant assemblages are expected to be a factor in determining how ecosystem functions such as carbon uptake will respond. Using vegetation data from 56 Sphagnum -dominated peat bogs across Europe, we show that in these ecosystems plant species aggregate into two major clusters that are each defined by shared response to environmental conditions. Across environmental gradients, we find significant taxonomic turnover in both clusters. However, functional identity and functional redundancy of the community as a whole remain unchanged. This strongly suggests that in peat bogs, species turnover across environmental gradients is restricted to functionally similar species. Our results demonstrate that plant taxonomic and functional turnover are decoupled, which may allow these peat bogs to maintain ecosystem functioning when subject to future environmental change. Peatland plant communities are expected to be affected by environmental change, though how assemblages respond is not fully understood. Here, Robroek et al. show that peatland species occur in two distinct clusters, and functional identity and redundancy was maintained under taxonomic turnover.

A transect of 68 acid grasslands across Great Britain, covering the lower range of ambient annual nitrogen deposition in the industrialized world (5 to$35 kg N ha^{-1} year^{-1}$), indicates that long-term, chronic nitrogen deposition has significantly reduced plant species richness. Species richness declines as a linear function of the rate of inorganic nitrogen deposition, with a reduction of one species per 4-m2quadrat for every$2.5 kg N ha^{-1} year^{-1}$of chronic nitrogen deposition. Species adapted to infertile conditions are systematically reduced at high nitrogen deposition. At the mean chronic nitrogen deposition rate of central Europe ($17 kg N ha^{-1} year^{-1}$), there is a 23% species reduction compared with grasslands receiving the lowest levels of nitrogen deposition.

In this paper, we assess and map the risk that atmospheric nitrogen (atN) pollution poses to biodiversity in Natura 2000 sites in mainland Portugal. We first review the ecological impacts of atN pollution on terrestrial ecosystems, focusing on the biodiversity of Natura 2000 sites. These nature protection sites, especially those located within the Mediterranean Basin, are under-characterized regarding the risk posed by atN pollution. We focus on ammonia (NH3) because this N form is mostly associated with agriculture, which co-occurs at or in the immediate vicinity of most areas of conservation interest in Portugal. We produce a risk map integrating NH3 emissions and the susceptibility of Natura 2000 sites to atN pollution, ranking habitat sensitivity to atN pollution using expert knowledge from a panel of Portuguese ecological and habitat experts. Peats, mires, bogs, and similar acidic and oligotrophic habitats within Natura 2000 sites (most located in the northern mountains) were assessed to have the highest relative risk of biodiversity change due to atN pollution, whereas Natura 2000 sites in the Atlantic and Mediterranean climate zone (coastal, tidal, and scrubland habitats) were deemed the least sensitive. Overall, results allowed us to rank all Natura 2000 sites in mainland Portugal in order of evaluated risk posed by atN pollution. The approach is of great relevance for stakeholders in different countries to help prioritize site protection and to define research priorities. This is especially relevant in countries with a lack of expertise to assess the impacts of nitrogen on biodiversity and can represent an important step up from current knowledge in such countries.

Nitrogen (N) deposition significantly affects forest dynamics, carbon stocks and biodiversity, and numerous assessments of N fluxes and impacts exist in temperate latitudes. In tropical latitudes, however, there are few such assessments. In this study, we measured the inorganic N concentration (wet deposition) deposited in rainfall and rainfall pH throughout one year at the boundary of a forest reserve in Malaysian Borneo. We considered that the N deposition may be either from forest and agricultural fires or derived from agricultural fertiliser. Therefore, we determined the wind trajectories using the HYSPLIT model provided by NOAA, the location of fires throughout the landscape throughout one year using NASA’s FIRM system, and obtained the land use cover map of Malaysia and Indonesia. We then correlated our monthly cumulative wet N deposition with the cumulative number of fires and the cumulative area of oil palm plantation that wind trajectories arriving at our study site passed over before reaching the rainfall sampling site. At 7.45 kg N ha−1 year−1, our study site had the highest annual wet inorganic N deposition recorded for a Malaysian forest environment. The fire season and the cumulative agricultural area crossed by the winds had no significant effect on N deposition, rainfall N concentration, or rainfall pH. We suggest that future research should use 15N isotopes in rainfall to provide further information on the sources of N deposition in tropical forests such as this.

Experimental studies have shown that deposition of reactive nitrogen is an important driver of plant community change, however, most of these experiments are of short duration with unrealistic treatments, and conducted in regions with elevated ambient deposition. Studies of spatial gradients of pollution can complement experimental data and indicate whether the potential impacts demonstrated by experiments are actually occurring in the ‘real world’. However, targeted surveys exist for only a very few habitats and are not readily comparable. In a coordinated campaign, we determined the species richness and plant community composition of five widespread, semi-natural habitats across Great Britain in sites stratified along gradients of climate and pollution, and related these ecological parameters to major drivers of biodiversity, including climate, pollution deposition, and local edaphic factors. In every habitat, we found reduced species richness and changed species composition associated with higher nitrogen deposition, with remarkable consistency in relative species loss across ecosystem types. Whereas the diversity of mosses, lichens, forbs, and graminoids declines with N deposition in different habitats, the cover of graminoids generally increases. Considered alongside previous experimental studies and survey work, our results provide a compelling argument that nitrogen deposition is a widespread and pervasive threat to terrestrial ecosystems.

This field manipulation study tested the effect of weekly pulses of solutions of NH4NO3 and (NH4)2SO4 salts on the evolution of CH4 and N2O from peatland soils. Methane and nitrous oxide emission from a nutrient-poor fen in northern Minnesota USA was measured over a full growing season from plots receiving weekly additions of NH4NO3 or (NH4)2SO4. At this relatively pristine site, natural additions of N and S in precipitation occur at 8 and 5 kg ha-1 y-1, respectively. Nine weekly additions of the dissolved salts were made to increase this to a total deposition of 31 kg N ha-1 y-1 on the NH4NO3-amended plots and 30 and 29 kg ha-1 y-1 of N and S, respectively, in the (NH4)2SO4-amended plots. Methane flux was measured weekly from treatment and control plots and all data comparisons are made on plots measured on the same day. After the onset of the treatments, and over the course of the growing season, CH4 emission from the (NH4)2SO4-amended plots averaged 163 mg CH4 m-2 d-1, significantly lower than the same-day control plot mean of 259 mg CH4 m-2 d-1 (repeated measures ANOVA). Total CH4 flux from (NH4)2SO4 treatment plots was one third lower than from control plots, at 11.7 and 17.1 g CH4 m-2, respectively. Methane emission from the NH4NO3-amended plots (mean of 256 mg CH4 m-2 d-1) was not significantly different from that of controls measured on the same day (mean of 225 mg CH4 m-2 d-1). Total CH4 flux from NH4NO3 treatment plots and same-day controls was 16.9 and 15.1 g CH4 m-2, respectively. In general, stable, relatively warm and wet periods followed by environmental 'triggers' such as rainfall or changes in water table or atmospheric pressure, which produced a CH4 'pulse' in the other plots, produced no observable peak in CH4 emission from the (NH4)2SO4-amended plots. Nitrous oxide emission from all of the plots was below the detection limit over the course of the experiment.

Dissolved organic carbon (DOC) concentrations have risen in upland waters across large areas of Europe and North America. Two proposed drivers of these increases are (1) deposition of atmospheric pollutant nitrogen (N) with consequent effects on plant and decomposer carbon dynamics, and (2) soil recovery from acidification associated with decreasing sulphur deposition. Examination of 12 European and North American field N addition experiments showed inconsistent (positive, neutral, and negative) responses of DOC to N addition. However, responses were linked to the form of N added and to resulting changes in soil acidity. Sodium nitrate additions consistently increased DOC, whereas ammonium salts additions usually decreased DOC. Leachate chemistry was used to calculate an index of “ANC forcing” of the effect of fertilization on the acid-base balance, which showed that DOC increased in response to all de-acidifying N additions, and decreased in response to all but three acidifying N additions. Exceptions occurred at two sites where N additions caused tree mortality, and one experiment located on an older, unglaciated soil with high anion adsorption capacity. We conclude that collectively these experiments do not provide clear support for the role of N deposition as the sole driver of rising DOC, but are largely consistent with an acidity-change mechanism. It is however possible that the unintended effect of acidity change on DOC mobility masks genuine effects of experimental N enrichment on DOC production and degradation. We suggest that there is a need, more generally, for interpretation of N manipulation experiments to take account of the effects that experimentally-induced changes in acidity, rather than elevated N per se, may have on ecosystem biogeochemistry.