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QUESTIONS: Sod cutting has been used extensively as an effective measure in removing excess N and restoring dwarf shrub dominance in heathlands affected by increased nitrogen deposition. However, recovery of other plant species is often very limited. One barrier is high soil acidity following sod cutting, which results in soil aluminium (Al³⁺) and ammonium (NH₄⁺) reaching toxic concentrations. Sod‐cutting management also removes most of the major nutrients from the system, so intensified nutrient limitation could be an additional barrier to the recovery of species‐rich communities. Soil phosphorus (P) is of special interest as research indicates sod‐cutting management can shift the system to P limitation. LOCATION: Hoge Veluwe National Park, The Netherlands. METHODS: We set up a full‐factorial experiment in sod‐cut heathland formerly encroached by Molinia caerulea, adding phosphate (P+) and lime (Ca+) and over the next three growing seasons, we recorded soil chemistry and plant responses. RESULTS: Soil inorganic N, especially soil NH₄⁺, strongly declined after liming compared to the control situation, confirming that liming alleviates NH₄⁺ toxicity. Addition of P resulted in a similar decline, also suggesting a role for soil phosphate availability in this process. Acid‐sensitive plant species richness increased significantly in both Ca+ and P+ treatment, whereas acid‐insensitive plant species richness only increased significantly as a result of P+ treatment. Mean vegetation relative growth rate increased significantly in both Ca+ and P+ treatments. CONCLUSIONS: Excessive P removal due to sod cutting constitutes a second barrier to restoration in addition to soil acidity. We discourage the large‐scale use of sod cutting to reduce soil N availability in heathlands and propose to use interventions that leave the soil nutrient balance intact, such as burning and grazing, mowing or litter removal, in combination with measures that restore the soil buffer capacity.

We tested the long-term effects of liming on soil micro-arthropods in a stand of Scotch pine on former drift sand in the Netherlands. To counteract the effects of acidification, liming was applied in increasing quantities from 0 (control), 3, 6, 9 and 18 ton ha−1 on experimental plots over the course of 1985 and 1986. Soil samples for chemical analyses and those for extraction of soil micro-arthropods were taken in October 2017, 32 years after application. Liming did restore the buffer capacity of the soil and did increase pH and increased plant biomass of the understory. Liming, however, also created P limitation, due to Al precipitation and an excess of free Ca to bind on. The consequence of the observed P limitation was a significant decrease in herbivorous and herbofungivorous micro-arthropods, whereas fungivorous and opportunistic herbofungivores were unaffected. P availability in acidified soils had become limited, due to higher N input that also caused acidification and due to remediation with added buffer material. Decomposition of organic matter was accelerated and increased N release to the system. The forest ecosystem had become P limited, where it originally was N limited or N and P co-limited.

Nitrogen deposition is influencing the quality of Habitat types. To assess at what deposition level the quality is influenced significantly critical loads have been estimated. To estimate the extent of the quality loss we estimated doses-response curves for nitrogen deposition per habitat type. This was done by collecting field data from empirical gradient studies and by estimating response curves for habitat types based on response curves of qualifying species per habitat type. The latter also includes a study to the response of so-called displacement species. The number of empirical studies is very limited but the results from the view that are available are very promising. Statistical relations could be estimated for 60 out of 61 nitrogen deposition sensitive habitat types occurring in the Netherlands. Comparison between the empirical and statistical relations showed that they showed very similar responses for part of the habitat types and no relation for a minor part of the types. Stikstofdepositie beïnvloedt in hoge mate de kwaliteit van habitattypen. Om te bepalen vanaf welke stikstofdepositie de kwaliteit van de habitattypen significant afneemt, zijn kritische depositiewaarden opgesteld. Om te bepalen in welke mate de kwaliteit afneemt bij stikstofdeposities hoger dan de kritische depositiewaarde, zijn er in dit onderzoek dosis-effectrelaties opgesteld. Dat is enerzijds gedaan op basis van empirische gradiëntstudies en anderzijds is verkend in hoeverre dat ook kan op basis van statistische relaties tussen het voorkomen van kwalificerende soorten van habitattypen en stikstofdepositie, waarbij ook is gekeken naar de respons van verdringingssoorten. Het aantal dosis-effectrelaties op basis van empirische studies is beperkt door gebrek aan data, maar wel zeer veelbelovend. Op basis van statistische relaties kon voor 60 van de 61 habitat(sub)typen een responscurve worden geschat. In vergelijking met de empirische relaties komen de statistische relaties voor een deel goed overeen en voor een klein deel slecht.

Until the large-scale application of artificial fertilizer in the 20th century, heathlands were a characteristic and essential element of the agricultural system on sandy soils in NW Europe. Attempts to increase the total area of the small remaining heathlands by converting agricultural areas are only partially successful. Reasons for such low success rates are (1) excessive soil nutrient concentrations, (2) elevated pH, (3) a low local availability of diaspores of characteristic biota, and (4) an ill-adapted soil community. The present review discusses these topics in more detail and evaluates potential solutions. Based on this review in combination with results of a long-lasting large-scale field experiment, we evaluate the prospects of alternative techniques to restore heathlands from agricultural fields.

Summary The authors examine how papyrological sources from Roman Egypt written in Greek on antichresis relate to classical Roman law. Antichresis attested in papyrological antichretic contracts had a lot in common with antichresis emerging from Roman dispute resolutions. There was only one substantive difference: in classical Roman law, protection of the debtor was emphasized, whereas in the Greek papyrological antichretic contracts the position of the creditor was favoured. Given the similarities found, the authors conclude that antichretic loan both as an independent legal institution and as a pactum antichreticum was a pan-Mediterranean legal concept.

Dispersal limitation, biotic interactions and environmental filters interact to drive plant and fungal community assembly, but their combined effects are rarely investigated. •This study examines how different heathland plant and fungal colonization scenarios realized via three biotic treatments ‐ addition of mature heathland derived sod, addition of hay and no additions ‐ affect soil fungal community development over six years along a manipulated pH gradient in a large‐scale experiment starting from an agricultural, topsoil removed state. •Our results show that both biotic and abiotic (pH) treatments had a persistent influence on the development of fungal communities, but that sod additions diminished the effect of abiotic treatments through time. Analysis of correlation networks between soil fungi and plants suggests that the reduced effect of pH in the sod treatment, where both soil and plant propagules were added, might be due to plant‐fungal interactions since the sod additions caused stronger, more specific, and more consistent connections compared to no addition treatment. •Based on these results, we suggest that the initial availability of heathland fungal and plant taxa, that reinforce each other, can significantly steer further fungal community development to an alternative configuration, overriding otherwise prominent effect of abiotic (pH) conditions.

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Although plant-soil interactions are increasingly recognized as an important factor in ecosystem restoration, their effects on community assembly during de novo ecosystem establishment are largely unknown. In a heathland restoration trial after topsoil removal we introduced either only aboveground heathland species with fresh herbage or both above- and belowground heathland species with sods to facilitate community assembly. Sod inoculation increased resemblance of the microbial community to the reference system, with a higher fungal and lower bacterial proportion to the community structure. Also densities of bacteriophagous and phytophagous nematodes, Acari and Collembola increased after sod inoculation. The cover of heathland plant species increased by 49% after sod inoculation. The introduction of solely aboveground heathland species increased the cover of these species by only 13%, and did not affect soil community assembly. Additionally, the increase in cover of heathland species over time was inversely correlated to the cover of mesotrophic grassland species. Inverse correlations were also observed between changes in fungal and bacterial abundances. Simultaneous introduction of key species of both above- and below-ground communities had a critical effect on the establishment of both communities, providing a potential shortcut for successful restoration of target ecosystems on disturbed soils.

Phosphorus content of mineral-poor sandy soils is steadily decreasing due to leaching caused by continuous and cumulative acidification and N deposition. Sod-cutting as a traditional restoration measure for heathland vegetation appears to increase P limitation, as most of the P present is in the organic matter being removed by sod-cutting. Mineral weathering, the natural inorganic source of P, becomes limiting or has even ceased as a result of the depletion of minerals. Previous investigations indicate a P limitation of the macrofauna under these circumstances. If this also holds for the soil fauna, hampering of decomposition may occur. To test experimentally whether soil fauna is indeed limited by the amount of P in the system, we set up an experiment in sod-cut heathland in which we added P or Ca (as Dolokal), resulting in: P + Ca+, P + Ca-, P-Ca+ and P-Ca- (control) treatments and an extra reference block in the original grass encroached heathland vegetation. The Ca treatment was added because liming is used to recover from acidification effects, but as a side effect Ca may also bind P. Three growing seasons after the addition of P and Ca, we found a significant increase in herbivores and predators among the soil fauna, with herbivore numbers higher in the P+/Ca-plots than in the P+/Ca + plots, indicating a lower availability of P in the presence of added Ca. Predators increased in all P+ plots. Fungivorous browsers responded negatively to the treatment after three growing seasons, both to P and to Ca addition. Phoretic species responded rapidly either to fewer numbers (when these are fungivorous browsers) or to greater numbers (when these are herbivorous browsers) to P addition. P addition induced also an allometric effect, via the medium-sized species increasing in greater numbers than both the larger and smaller species.

This study estimates the potential losses of vascular plant species richness due to terrestrial acidification for different world's biomes. We used empirical occurrence data of 2409 species from 140 studies and estimated the relative species richness – pH response curves using logistic regressions. The regressions were then used to quantify the fraction of species that are potentially lost due to soil pH changes. Although we found considerable variability within biomes, out results show that the pH at which species richness was maximized was found to be the lowest in (sub)tropical forests (pH = 4.1) and the highest in deserts (pH = 7.4). We also found that (sub)tropical moist forests are highly sensitive to decreases of in soil pH below 4.1. This study can be coupled with existing atmospheric deposition models to quantify the risk of species richness loss following soil acidification.

Ajuts: This manuscript resulted from the 1st "Committee on Air Pollution Effects Research on Mediterranean Ecosystems" (CAPERmed) Conference celebrated in Lisbon, 2014 (http://capermed.weebly.com/). All attendants to this conference are, therefore, greatly acknowledged. Fundação para a Ciência e Tecnologia (FCT) is acknowledged through the Post Doc grant SFRH/BPD/85419/2012 to Teresa Dias and the Investigador grant to Silvana Munzi. ADM, EP and PS thank the LIFE10 ENV/FR/208 project MOTTLES. EP thanks also the Fondazione Cassa di Risparmio di Firenze (2013/7956). ROH is supported by a Juan de la Cierva Fellowship (IJCI-2014-21252) of the Spanish Ministry of Economy and Competitiveness. The authors declare no conflicts of interest. Mediterranean Basin ecosystems, their unique biodiversity, and the key services they provide are currently at risk due to air pollution and climate change, yet only a limited number of isolated and geographically-restricted studies have addressed this topic, often with contrasting results. Particularities of air pollution in this region include high O3 levels due to high air temperatures and solar radiation, the stability of air masses, and dominance of dry over wet nitrogen deposition. Moreover, the unique abiotic and biotic factors (e.g., climate, vegetation type, relevance of Saharan dust inputs) modulating the response of Mediterranean ecosystems at various spatiotemporal scales make it difficult to understand, and thus predict, the consequences of human activities that cause air pollution in the Mediterranean Basin. Therefore, there is an urgent need to implement coordinated research and experimental platforms along with wider environmental monitoring networks in the region. In particular, a robust deposition monitoring network in conjunction with modelling estimates is crucial, possibly including a set of common biomonitors (ideally cryptogams, an important component of the Mediterranean vegetation), to help refine pollutant deposition maps. Additionally, increased attention must be paid to functional diversity measures in future air pollution and climate change studies to establish the necessary link between biodiversity and the provision of ecosystem services in Mediterranean ecosystems. Through a coordinated effort, the Mediterranean scientific community can fill the above-mentioned gaps and reach a greater understanding of the mechanisms underlying the combined effects of air pollution and climate change in the Mediterranean Basin.

Heischraal grasland is een van oorsprong soortenrijk ecosysteem in het zandlandschap, het heuvellandschap en de duinen. De staat van instandhouding van de habitattypen H6230 en H2130C (waar dit ecosysteem in Nederland wordt verdeeld) is slecht, met name de droge varianten. Dat is extra zorgelijk, omdat het om prioritaire habitattypen gaat, dat wil zeggen dat er extra aandacht moet zijn voor het zo spoedig mogelijk bereiken van een gunstige staat van instandhouding. Er is landelijk gezien nog maar 30-40 ha redelijk ontwikkeld heischraal grasland over. Veel heischrale graslanden, ook die er qua soortensamenstelling nog relatief goed uitzien, zijn sterk verzuurd. Door menselijke aanvoer van eerst zwavel en nu stikstof is de zuurbuffering in de bodem ernstig aangetast, en monitoring van de stikstofbelasting laat zien dat deze nog nauwelijks is verminderd.

Question: Which environmental variables affect floristic species composition of acid grasslands in the Atlantic biogeographic region of Europe along a gradient of atmospheric N deposition? Location: Transect across the Atlantic biogeographic region of Europe including Ireland, Great Britain, Isle of Man, France, Belgium, the Netherlands, Germany, Norway, Denmark, and Sweden. Materials and Methods: In 153 acid grasslands we assessed plant and bryophyte species composition, soil chemistry (pH, base cations, metal, nitrate and ammonium concentrations, total carbon and nitrogen, and Olsen plant-available phosphorus), climatic variables, N deposition and S deposition. Ordination and variation partitioning were used to determine the relative importance of different drivers on the species composition of the studied grasslands. Results: Climate, soil and deposition variables explained 24% of the total variation in the species composition. Variance partitioning showed that soil variables explained the most variation in the data set and that climate and geographic variables accounted for slightly less variation. Deposition variables (N and S deposition) explained 9.8% of the variation in the ordination. Species positively associated with N deposition included Holcus mollis, and Leontodon hispidus. Species negatively associated with N deposition included Agrostis curtisii, Leontodon autumnalis, Campanula rotundifolia, and Hylocomium splendens. Conclusions: Although secondary to climate gradients and soil biogeochemistry, and not as strong as for species richness, the impact of N and S deposition on species composition can be detected in acid grasslands influencing community composition both directly and indirectly, presumably by soil mediated effects.

Question: High atmospheric nitrogen (N) deposition has been shown to affect productivity and species composition of terrestrial ecosystems. This study focused on the differential effects of the two inorganic N forms in atmospheric deposition (i.e. ammonium and nitrate). Methods and location: Nutrient addition experiments were carried out during 4 years in a mesotrophic fen in a low-deposition area in Ireland. In a factorial design, plots were fertilized with ammonium and/or nitrate, in two doses comparable with 35 and 70kgNha � 1 y � 1 and compared with an unfertilized control. Results: Vascular plant biomass as well as bryophyte biomass were not affected by N dose but showed significantly different responses to the N form. In the ammonium-fertilized plots, vascular plant biomass was higher and moss biomass was lower than the control, while nitrate additions had no effect. Vascular plant species density was high (16 species per 0.49m 2 ) and was not affected by any of the treatments; bryophyte species density was also high (seven species per 0.04m 2 ) but showed a significant decrease upon ammonium fertilization. Conclusion: The vulnerability of the mesotrophic vegetation to enhanced atmospheric N deposition depends strongly on the N form. If N would be mainly deposited as NOx, no detrimental effects on the vegetation will occur. If, however, the deposition is mainly in the form of NHy, the bryophyte vegetation will be seriously damaged, while the vascular plant vegetation will show an increased biomass production with possible shifts in dominance from Carex and herb species to grasses and shrubs.

Field observations and experimental data of effects of nitrogen (N) deposition on plant species diversity have been used to derive empirical critical N loads for various ecosystems. The great advantage of such an approach is the inclusion of field evidence, but there are also restrictions, such as the absence of explicit criteria regarding significant effects on the vegetation, and the impossibility to predict future impacts when N deposition changes. Model approaches can account for this. In this paper, we review the possibilities of static and dynamic multispecies models in combination with dynamic soil–vegetation models to (1) predict plant species composition as a function of atmospheric N deposition and (2) calculate critical N loads in relation to a prescribed protection level of the species composition. The similarities between the models are presented, but also several important differences, including the use of different indicators for N and acidity and the prediction of individual plant species vs. plant communities. A summary of the strengths and weaknesses of the various models, including their validation status, is given. Furthermore, examples are given of critical load calculations with the model chains and their comparison with empirical critical N loads. We show that linked biogeochemistry–biodiversity models for N have potential for applications to support European policy to reduce N input, but the definition of damage thresholds for terrestrial biodiversity represents a major challenge. There is also a clear need for further testing and validation of the models against long-term monitoring or long-term experimental data sets and against large-scale survey data. This requires a focused data collection in Europe, combing vegetation descriptions with variables affecting the species diversity, such as soil acidity, nutrient status and water availability. Finally, there is a need for adaptation and upscaling of the models beyond the regions for which dose–response relationships have been parameterized, to make them generally applicable.

Our study investigates the negative impact of nitrogen (N) deposition on species richness in acidic grasslands, based on a temporal comparison of vegetation data spanning a period of almost 70 years. We compiled a large data base of plots assigned to the Violion caninae grassland type, composed of managed, but unfertilized semi-natural grasslands on nutrient-poor, acidic soils. In total 1114 plots, mainly from Great Britain, the Netherlands and Germany, were compiled, dating back to 1939. Environmental site information included geographical and soil (mean Ellenberg values) variables as well as estimates of cumulative N and sulphur (S) deposition since 1939. Statistical analyses were carried out separately for the data subsets from the three regions. In all regions, the vegetation differentiation was mainly related to soil acidity and nutrient availability, as well as to the year of sampling and the cumulative amounts of N and S deposition. Plot-species richness of vascular plants and bryophytes (analysed for Great Britain only) decreased with time and analyses suggest these are affected by various factors, notably soil pH, but also latitude and cumulative N deposition. The latter explained more of the variation in species number than the year of sampling and cumulative S deposition, which supports the interpretation that the decline in species richness is mainly caused by increasing N availability and less by altered management and soil acidification. For Great Britain and Germany, cumulative N deposition showed a strong negative relationship with several biodiversity measures, especially the proportion of dicots, whereas it was positively related to the proportion of grass species. In general, our results give temporal evidence for the negative effect of N deposition on species richness in semi-natural vegetation.

Increased phosphorus availability may provoke serious eutrophication problems in wetlands. Strong evidence indicates that sulphate induced mobilization of phosphate (internal eutrophication) has been responsible for a strong decline of the biodiversity in wetlands during the last decades. It is currently underestimated, however, that the wide spread leaching of nitrate from agricultural lands can indirectly provoke strong internal phosphate eutrophication in wetlands, via its interference with sulphur and iron biogeochemistry in the subsoil. Nitrate can mobilize sulphate from geological pyrite deposits by the oxidation of FeSx in the aquifer, leading to a decrease of nitrate and an increase of groundwater sulphate concentrations. Furthermore nitrate immobilizes iron in the subsoil by oxidizing reduced (dissolved) iron. Increased sulphate concentrations may provoke strong phosphate eutrophication in wetlands fed directly or indirectly (via surface water) with groundwater as sulphate strongly interferes with iron phosphorus chemistry and stimulates anaerobic decomposition of organic matter. Management of wetlands should therefore be approached at a broader scale which includes the landscape-scale management of groundwater systems. Leaching of nitrate to the groundwater, for instance, should not only receive attention for its potential effects on drinking water quality but above all because of the resulting large scale mobilization of sulphate from geological pyrite deposits and the immobilization of ferrous iron.

The northwest European heathland landscape with its characteristic communities of nutrient-poor and acidic soils has a high nature value, because of its locally high biodiversity and the distinct site conditions. in order to conserve and restore the heathlands, numerous rehabilitation projects have been performed, although with varying success. This is partly due to the fact that the key biogeochemical processes distinguishing the various vegetation types within the heathlands are not known in detail.Therefore, we performed a statistical survey on the main communities and their soil characteristics. In addition, we analyzed the data for key factors determining biodiversity in the heathland landscape.Data from previous studies and surveys was used to compile a dataset of 267 vegetation releves (classified as EUNIS habitat types) with extensive soil measurements (22 parameters). A canonical discriminant analysis revealed that soil acidity explained most of the differences between the habitat types, while soil moisture content and soil fertility were less important. Acidity-related factors as Al(3+), Al/Ca-ratio and pH were also strongly correlated to plant diversity in the majority of the habitat types, respectively, the species-rich Nardus grasslands, the Rhynchosporion communities and the species-rich Molinia meadows. In the dry heaths and over the total heathland landscape, plant diversity was negatively correlated with soil NH(4)(+)-concentrations. Only in wet heath, nutrient availability, in this case P, was the primary factor in explaining plant diversity.This study presents ranges for all major soil parameters for the studied well-developed heathland habitat types, thereby providing clear guidelines for conservation and restoration. (C) 2009 Elsevier Ltd. All rights reserved.

Question: What are the long-term implications of former fertilisation for the ecological restoration of calcareous grasslands? Location: Gerendal, Limburg, The Netherlands. Methods: In 1970, ten permanent plots were established in just abandoned agricultural calcareous grassland under a regime of annual mowing in August. From 1971 to 1979, two different fertiliser treatments were applied twice a year to a subset of the plots (artificial fertiliser with different proportions of nitrogen and phosphorus). The vegetation of the plots was recorded yearly and vegetation biomass samples were taken for peak standing crop and total amounts of nitrogen, phosphorus and potassium. Species composition and floristic diversity were analysed over the research period (1970-2006) and between the treatments, including the use of multivariate techniques (Detrended Correspondence Analysis). Results: In terms of species number, there is a clear optimum 10 to 20 years after fertilisation has been terminated. Afterwards, there is a slow decrease; no new species appear and species of more nutrient-rich conditions gradually disappear. For the fertilised plots that received a relatively high proportion of N, effects are found only in the first years, whereas, for the plots that received a relatively high proportion of P, long-term after-effects are found in species composition, peak standing crop, total amounts of phosphorus in biomass, and in soil phosphorus data. Conclusions: The effect of artificial fertiliser with a large amount of nitrogen disappears in less than ten years when mown in August, including removal of the hay. This is a promising result for restoration of N-enriched calcareous grasslands, as the applied dose of nitrogen in this experiment largely exceeds the extra input of nitrogen via atmospheric deposition. Application of fertiliser with a large amount of phosphorus, however, has effects even more than 25 years after the last addition. There are no prospects that this effect will become reduced in the near future under the current mowing management.

Dutch fens, subjected to high nitrogen (N) deposition levels with reduced N (NHy) highly dominating over oxidised N (NOx), have since the second half of the past century seen a significant decline of Scorpidium and other characteristic brown moss species, while several Sphagnum species have increased rapidly. This promotes acidification and the transition from rich to poor fens. In line with the outcomes of previous short-term water culture experiments, we hypothesised that Scorpidium growth is negatively affected by NHy due to ammonium toxicity, but not by NOx deposition, and that Sphagnum grows equally well on both N forms. To test this hypothesis under field conditions, we carried out a 4-year N addition experiment (5.0 g N m−2 year−1, applied either as NO3 −-N or as NH4 +-N) on natural mixed Scorpidium revolvens–Sphagnum contortum stands in a rich fen with relatively low background N deposition. After 4 years, ammonium addition had significantly reduced Scorpidium growth, while Sphagnum had not significantly been affected by N additions. Increased ammonium levels were directly toxic to Scorpidium, while Sphagnum was not affected. Furthermore, N addition (in particular nitrate) also indirectly influenced moss growth through promoting vascular plants. Our study confirms that it is ecologically relevant to consider the specific form in which N enrichment occurs, i.e. the ratio of NHy vs. NOx. We conclude that in rich fens, the risk of rapid transition of the moss layer to dominance of poor-fen species is strongly promoted by increased deposition of reduced N.

Nitrogen (N) deposition impacts natural and semi-natural ecosystems globally. The responses of vegetation to N deposition may, however, differ strongly between habitats and may be mediated by the form of N. Although much attention has been focused on the impact of total N deposition, the effects of reduced and oxidised N, independent of the total N deposition, have received less attention. In this paper, we present new analyses of national monitoring data in the UK to provide an extensive evaluation of whether there are differences in the effects of reduced and oxidised N deposition across eight habitat types (acid, calcareous and mesotrophic grasslands, upland and lowland heaths, bogs and mires, base-rich mires, woodlands). We analysed data from 6860 plots in the British Countryside Survey 2007 for effects of total N deposition and N form on species richness, Ellenberg N values and grass:forb ratio. Our results provide clear evidence that that N deposition affects species richness in all habitats except base-rich mires, after factoring out correlated explanatory variables (climate and sulphur deposition). In addition, the form of N in deposition appears important for the biodiversity of grasslands and woodlands but not mires and heaths. Ellenberg N increased more in relation to NHx deposition than NOy deposition in all but one habitat type. Relationships between species richness and N form were habitat-specific: acid and mesotrophic grasslands appear more sensitive to NHx deposition while calcareous grasslands and woodlands appeared more responsive to NOy deposition. These relationships are likely driven by the preferences of the component plant species for oxidised or reduced forms of N, rather than by soil acidification.

Spatial variability in hydrological flowpaths and nitrate-removal processes complicates the overall assessment of riparian buffer zone functioning in terms of water quality improvement as well as enhancement of the greenhouse effect by N2O emissions. In this study, we evaluated denitrification and nitrous oxide emission in winter and summer along two groundwater flowpaths in a nitrate-loaded forested riparian buffer zone and related the variability in these processes to controlling soil factors. Denitrification and emissions of N2O were measured using flux chambers and incubation experiments. In winter, N2O emissions were significantly higher (12.4 mg N m−2 d−1) along the flowpath with high nitrate removal compared with the flowpath with low nitrate removal (2.58 mg N m−2 d−1). In summer a reverse pattern was observed, with higher N2O emissions (13.6 mg N m−2 d−1) from the flowpath with low nitrate-removal efficiencies. Distinct spatial patterns of denitrification and N2O emission were observed along the high nitrate-removal transect compared to no clear pattern along the low nitrate-removal transect, where denitrification activity was very low. Results from this study indicate that spots with high nitrate-removal efficiency also contribute significantly to an increased N2O emission from riparian zones. Furthermore, we conclude that high variability in N2O:N2 ratio and weak relationships with environmental conditions limit the value of this ratio as a proxy to evaluate the environmental consequences of riparian buffer zones.

Mesocosms filled with dune sand were planted with graminoid (Calamagrostis epigejos, Carex arenaria) and herbaceous species (Carlina vulgaris, Galium verum). Strong effects of nitrogen addition on the vegetation were found within two to three years. The above-ground biomass of C. epigejos and C. arenaria increased at deposition rates between 10 and 80 kg N ha−1 yr−1. Both grasses were limited by N. In latter stages P limitation was suggested for C. arenaria. At high N-levels, C. epigejos dominated the vegetation within two years. C. vulgaris and G. verum declined drastically as a result of increased competition for light by the highly competitive grass C. epigejos. It is concluded that increased (ambient) N inputs are of major importance for the increased dominance of tall grasses in stable dune grasslands.

In the second half of the 20th century, frequency and diversity of brown mosses strongly declined in Dutch fens while species of Sphagnum and Polytrichum increased markedly. We hypothesized that high ammonium (NH4 +) concentrations in Dutch precipitation have promoted these species changes. To test this hypothesis, we examined the effects of varying concentrations (1–1000 μM) of NH4 + on a brown moss (Calliergonella cuspidata), two species of Sphagnum (S. contortum and S. squarrosum) and Polytrichum commune in a hydroponic experiment. A second goal was to identify possible interspecific differences in the mechanisms responsible for NH4 + detoxification. Dry matter production and tissue cation concentrations indicated decreasing sensitivity to NH4 + in the order C. cuspidata>S. contortum>S. squarrosum and P. commune. In S. squarrosum and P. commune, the highest NH4 + treatments induced strong increases in tissue nitrogen (N) and amino acid concentrations. Sphagnum contortum showed a similar, but l...

The effects of increasing ammonium concentrations in combination with different pH levels were studied on five heathland plant species to determine whether their occurrence and decline could be attributed to ammonium toxicity and/or pH levels. Plants were grown in growth media amended with four different ammonium concentrations (10, 100, 500 and 1000 micromol l(-1)) and two pH levels resembling acidified (pH 3.5 or 4) and weakly buffered (pH 5 or 5.5) situations. Survival of Antennaria dioica and Succisa pratensis was reduced by low pH in combination with high ammonium concentrations. Biomass decreased with increased ammonium concentrations and decreasing pH levels. Internal pH of the plants decreased with increasing ammonium concentrations. Survival of Calluna vulgaris, Deschampsia flexuosa and Gentiana pneumonanthe was not affected by ammonium. Moreover, biomass increased with increasing ammonium concentrations. Biomass production of G. pneumonanthe reduced at low pH levels. A decline of acid-sensitive species in heathlands was attributed to ammonium toxicity effects in combination with a low pH.

Summary • In Dutch fens, subjected to high nitrogen (N) deposition, Scorpidium and other brown mosses have declined markedly. A concurrent strong increase of Sphagnum and Polytrichum has promoted acidification. We measured nitrate (NO3−) and ammonium (NH4+) availability in Dutch fens. We also tested preference for either N form of Scorpidium scorpioides, Sphagnum squarrosum and Polytrichum commune. • Ion exchange membranes were installed in the field. In a hydroponic experiment, plants were grown on 100 µm N (reflecting concentrations in Dutch precipitation since 1980), provided as NO3−, NH4NO3, or NH4+. • NH4+ availability in Sphagnum and Polytrichum stands and NH4+ : NO3− ratio in Sphagnum stands were higher than in brown moss stands. In the experiment, Scorpidium performed best on NO3−. NH4NO3 tended to decrease its growth, whereas NH4+ was very toxic. N treatment did not significantly affect growth of Sphagnum and Polytrichum. Tissue pH and nutrient concentrations confirmed the growth patterns and indicated that Scorpidium was most sensitive to NH4+ stress. • We conclude that high NH4+ inputs pose a serious threat to the brown moss flora of rich fens.

Summary 1. Restoration of formerly species-rich wet heaths and matgrass swards has not always been successful. The constraints on this restoration process are not yet fully understood and need further investigation, particularly the accumulation of ammonium in the soil after sod cutting, i.e. the removal of the vegetation and topsoil layer. This accumulation is known from sod cutting experiments in dry heaths, but had not previously been studied in wet heaths and matgrass ecosystems. 2. In 2000, sods were cut from two degraded Dutch wet heaths. Soil chemistry and germination in the sod-cut plots were measured at irregular intervals between April 2000 and August 2001. To test the influence of ammonium on germination and survival, a glasshouse dose‐response experiment was conducted with two endangered wet heath plant species. 3. In both wet heaths, an accumulation of KCl-extractable ammonium up to 600 µ mol kg − 1 dry soil was found in the upper 10 cm of the soil within the first year after sod cutting. These high ammonium concentrations lasted for about 10 months. Germination was very low in the sod-cut plots in 2000 and 2001, and few target species were found, although they were present in the vicinity. 4. The dose‐response experiment indicated a significant, negative correlation of both germination and survival with increasing ammonium addition for both plant species. Mean soil ammonium concentrations of the control, 100 and 250 µ  ammonium treatments were significantly lower than those of the 500 and 1000 µ  ammonium treatments (47, 45, 70, 144 and 252 µ mol kg − 1 dry soil, respectively). 5. Maximum concentrations of KCl-extractable ammonium in the field corresponded to water-extractable concentrations that were higher than those found to be limiting germination and growth in the glasshouse experiments. The low germination in the field is likely to have been adversely affected by high concentrations of ammonium as a result of sod cutting. 6. Synthesis and applications . High ammonium concentrations occur in wet heaths following sod cutting. Low rates of germination of restoration target plant species occur under such conditions. To increase the success of wet heath restoration, the accumulation of ammonium after sod cutting should be prevented by additional measures, such as liming. Because sod cutting is also applied as a restoration measure in the restoration of other ecosystems, such as fens, the effects on increased soil ammonium concentrations need further attention.

The decline of Cirsium dissectum in dessicatingwetlands is attributed to acidification and eutrophication. Experimentalevidence was obtained for the first time on ammonium toxicity under low pH. Inahydroculture experiment, interactive effects of nitrogen forms (250μmol NH4 + or 250μmolNO3 −) and pH (4, 5 or 6) were studied with regardtothe vitality of C. dissectum seedlings. The results showthat 250 μmol l−1 ammonium as sole nitrogensource only had negative effects on C. dissectum incombination with a low pH. Ammonium uptake at a rhizosphere pH of 4, resultedinlower nitrogen contents of both roots and shoots, lower internal pH of rootsandshoots and increased contents of basic amino-acids, resulting indecreased survival rate and biomass development. At higher pH, or when nitratewas the nitrogen source, these processes do not take place. This phenomenonstresses the importance of periodic influence of base rich groundwater duringthe winter in wet species-rich heathlands and grasslands, necessary torestore the acid neutralising capacity of the soil. Anthropogenic lowering ofthe groundwater table will lead to acidification enabling ammonium to becometoxic to herbaceous plant species such as C. dissectum.

This chapter provides an overview of geochemical indicators for nitrogen (N), acidity, and metals in soil and water (soil solution, ground water and surface water) in view of their impacts on different endpoints (tree growth/health, human health, soil biodiversity etc.). Relevant indicators for N are the soil C/N ratio, nitrate (NO3) concentration in ground water and total N concentration in soil and surface water. For acidity the most relevant endpoint indicators are the exchangeable base cation pool or base saturation in the soil, the ratio of aluminium (Al) to base cation (Bc) in soil solution, the total Al concentration in ground water and the acid neutralizing capacity (ANC) in surface water. Relevant indicators for metals are the total or reactive metal concentration in the soil and the free or total metal ion concentration in water. Using critical limits for those endpoint indicators, it is possible to assess critical loads for both terrestrial and aquatic ecosystems based on geochemical modelling. An overview is given of the derivation of those limits, mostly under laboratory circumstances, and a critical evaluation of their relevance in the field situation.

Dose-Response (D-R) relationships derived from nitrogen (N) addition experiments and N deposition gradient studies are extrapolated over natural and (semi-)natural grasslands in Europe, using a European land cover map. Based on emissions of oxidized and reduced N in 2000 and 2020, ecosystem-specific depositions of total N are computed over Europe. For 2020 two scenarios are applied, i.e. one according agreed emission reductions under national and European legislation and the other based on the (hypothetical) application of best available emission control techniques. Results show that the impact of N deposition on plant species diversity computed over European (semi-)natural grasslands is less when N addition based dose-response relationships are used than when a N deposition gradient based alternative is applied. Using the latter approach, the species richness is computed to be reduced by more than 40 % in about 5 % of European grasslands in 2000. This reduction in species richness becomes less than 25 %, when N emissions are cut back using maximum feasible abatement technologies.

Human activity in the last century has increased nitrogen (N) deposition to a level that has caused or is likely to cause alterations to the structure and function of many ecosystems across the United States. We synthesized current research relating atmospheric N deposition to effects on terrestrial and freshwater ecosystems in the United States, and estimated associated empirical critical loads of N for several receptors: freshwater diatoms, mycorrhizal fungi, lichens, bryophytes, herbaceous plants, shrubs, and trees. Biogeochemical responses included increased N mineralization and nitrification, increased gaseous N losses, and increased N leaching. Individual species, population, and community responses included increased tissue N, physiological and nutrient imbalances, increased growth, altered root-shoot ratios, increased susceptibility to secondary stresses, altered fire regime, shifts in competitive interactions and community composition, changes in species richness and other measures of biodiversity, and increases in invasive species. The range of critical loads of nutrient N reported for U.S. ecoregions, inland surface waters, and freshwater wetlands is 1–39 kg N ha−1 yr−1, spanning the range of N deposition observed over most of the country. The empirical critical loads of N tend to increase in the following sequence: diatoms, lichens and bryophytes, mycorrhizal fungi, herbaceous plants and shrubs, trees.

Empirical critical loads of nitrogen (N) were first presented in a background document for a workshop in 1992 in Sweden. Since their first presentation, the critical loads of N have been updated at regular intervals and for a large number of habitats. This chapter presents a brief history of the empirical critical loads and explains the process of determination of empirical critical loads for nitrogen and their reliability. For European habitats (defined as EUNIS and Natura 2000 habitat classes), current empirical critical loads for nitrogen are presented. For each of these habitats, the main effects of enhanced nitrogen inputs are discussed that have formed the basis for the determination of the empirical critical loads. Factors other than nitrogen, that may affect ecosystem processes or ecosystem functioning, are discussed as these may modify the nitrogen critical load under specific conditions.

Softwater lakes possess a highly characteristic vegetation adapted to limited carbon availability. Based on hydrology, vegetation and geographic distribution, a boreal and an atlantic lake type can be distinguished. In general, boreal softwater lakes occur in remote areas where eutrophication is a local phenomenon and acidifying input is low. A number of these lakes are, however, very susceptible to eutrophication and acidification. Reducing the input of nutrients and/or liming the stream or catchment is generally sufficient to restore typical softwater vegetation. The vegetation of atlantic softwater lakes are subject to many anthropogenic degradation processes. Removal of nutrient-rich, anoxic, organic sediments is a prerequisite for restoration of these lakes. In acidified or acid-sensitive lakes, subsequent controlled inlet of calcareous, nutrient-poor water is more adequate than direct liming. The effects of these restoration measures strongly depend on interaction with processes, such as atmospheric deposition, drainage, catchment acidification, eutrophication and reduced colonisation rates.

International audience; Nutrient pollution presents a serious threat to biodiversity conservation. In terrestrial ecosystems, the deleterious effects of nitrogen pollution are increasingly understood and several mitigating environmental policies have been developed. Compared to nitrogen, the effects of increased phosphorus have received far less attention, although some studies have indicated that phosphorus pollution may be detrimental for biodiversity as well. On the basis of a dataset covering 501 grassland plots throughout Europe, we demonstrate that, independent of the level of atmospheric nitrogen deposition and soil acidity, plant species richness was consistently negatively related to soil phosphorus. We also identified thresholds in soil phosphorus above which biodiversity appears to remain at a constant low level. Our results indicate that nutrient management policies biased toward reducing nitrogen pollution will fail to preserve biodiversity. As soil phosphorus is known to be extremely persistent and we found no evidence for a critical threshold below which no environmental harm is expected, we suggest that agro-environmental schemes should include grasslands that are permanently free from phosphorus fertilization.

Summary Raised bogs are among the ecosystems most susceptible to atmospheric nitrogen pollution. Based on global data ranging from pristine to heavily polluted areas, a conceptual model is presented to explain the logistic response of these terrestrial carbon reservoirs to increased airborne nitrogen fluxes.

The growth of J. bulbosus was investigated in a cultivation experiment with a two-way factorial design for liming and reacidification. The growth of J. bulbosus on limed sediment in a reacidified water layer clearly differed from the growth in the other treatments. Dry weight was approximately five times higher after a growth period of five months which was the result of more rapid shoot growth and a higher production of rosette-bearing segments. Enhanced growth of J. bulbosus corresponded with increased concentrations of ammonium in the sediment pore water and increased carbon dioxide concentrations in the water layer. It is concluded that J. bulbosus benefits from higher nutrient concentrations in limed sediment when the carbon dioxide concentration in the water layer is relatively high. This occurs after reacidification of the water layer when the bicarbonate–carbon dioxide equilibrium is shifted towards carbon dioxide. As regular liming leads to accumulation of calcium carbonate in the sediment, it is to be expected that enhanced growth of J. bulbosus will continue in reacidifying limed lakes in the future.

1. In many Dutch lakes, eutrophied, alkaline and sulphate-rich river water has been used to compensate for water losses. As a result, these waters have become eutrophied, and typical macrophyte species have disappeared. 2. The contribution of increased alkalinity to the eutrophication of softwater lakes was studied. In a mesocosm experiment, two types of sediments were flooded with demineralized water containing 2 mmol L−1 bicarbonate ions. The upper centimetres changed from a brown soil with coarse organic particles to a fine black mud. The formation of degradation intermediates and some phosphate release were observed. 3. This degradation was more evident in sediments flooded with demineralized water containing 4 mmol L−1 sulphate ions. In addition, sulphate consumption, sulphide production, bicarbonate production and enhanced phosphate release were observed in the sediment. 4. The eutrophied, softwater Lake Banen has been isolated from river water inputs, and mud layers were removed to restore the formerly oligotrophic, softwater conditions. 5. Removal of the degraded sediment layer and isolation of the lake from river water prevented sediment degradation and led to a return of the endangered macrophyte communities typical of softwater lakes. Copyright © 1999 John Wiley & Sons, Ltd.

Summary The effects of increased atmospheric nitrogen inputs, from both NOy and NHx, on diversity in various semi-natural and natural ecosystems are reviewed. The severity of these impacts depends on abiotic conditions (e.g. buffering capacity, soil nutrient status and soil factors that influence the nitrification potential and nitrogen immobilization rate) in the particular system. The sensitivity of fresh water ecosystems, wetlands and bogs, species-rich grasslands, heathlands and field layer of forests, all of which have conservational value, are discussed in detail. The most important effects of nitrogen deposition are: (i) accumulation of nitrogenous compounds resulting in enhanced availability of nitrate or ammonium; (ii) soil-mediated effects of acidification; and (iii) increased susceptibility to secondary stress factors. Long-term nitrogen enrichment has gradually increased the availability of nitrogen in several vegetation types, leading to competitive exclusion of characteristic species by more nitrophilic plants, especially under oligo- to mesotrophic soil conditions. Soil acidification (with losses of buffering capacity and increased concentrations of toxic metals) is especially important after nitrification of ammonium in weakly buffered environments: acid-resistant plant species then become dominant at the expense of the often rare plants typical of intermediate pH. The related change in the balance between ammonium and nitrate may also affect the performance of several species. The susceptibility of plant species to secondary stress factors (pathogens; frost and drought) may be affected by air-borne nitrogen but data are only available for a few communities (e.g. dry heathlands). Most global biodiversity is contained within natural and semi-natural vegetation. It is thus crucial to control emissions of nitrogenous compounds to the atmosphere, in order to reduce or prevent effects on diversity in these systems. Most research has focused on acidification in forestry stands and lakes and on the effects on trees. We highlight serious gaps in knowledge of other ecosystems.

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Arnica montana and Cirsium dissectum, typical of species-rich heathlands and acidic grasslands, have declined rapidly in The Netherlands in recent years. Field surveys suggest that the decline is caused by soil acidification as a result of enhanced atmospheric N and S deposition. Therefore, the survival, growth and development of these species were studied in a water culture experiment, using nutrient solutions which differed both in mineral nitrogen form and in ammonium concentration. For comparison, the performance of a third, acid tolerant species, Calluna vulgaris, was studied. The results showed that both Arnica and Cirsium performed better using nitrate than when using ammonium as a sole nitrogen source, whereas ammonium toxicity became apparent when ammonium concentrations were raised above 100 µM. Ammonium toxicity was expressed by an increase in mortality of Arnica plants with increasing ammonium concentrations and by a reduction of biomass in Arnica and Cirsium. Furthermore, cation concentrations in both roots and shoots decreased when ammonium was supplied as a nitrogen source. In contrast, Calluna showed optimal development when using ammonium as a sole nitrogen source. In this species, only root biomass was negatively affected by high ammonium concentrations. The ecological implications of these preferences are discussed in relation to soil acidification.

International audience; Reduction in the species richness of acid grasslands along a gradient of atmospheric nitrogen (N) deposition has previously been demonstrated in the UK (Stevens, Dise, Mountford, Gowing, Science 303:1876–1879, 2004). Further surveys of acid grasslands in the UK confirm this relationship. This chapter reports an examination of the relationship across the Atlantic region of Europe. Examining the cover of functional groups across this gradient reveals that forb cover is strongly reduced along the gradient of N deposition.

The main mechanisms of nitrogen (N) deposition impacts on terrestrial biodiversity, mainly from studies in Europe, are identified as: direct foliar impacts; eutrophication; acidification; negative effects of reduced N; and increased susceptibility to secondary stress and disturbance factors such as drought, frost, pathogens or herbivores. The relation of several of these mechanisms to aquatic ecosystems is also described, as is the relative lack of N impact studies on faunal species/communities compared to floral ones. The factors that moderate N impacts on ecosystems are also considered and are categorized as: (1) the duration and total amount of the N inputs; (2) the chemical and physical form of the airborne N input; (3) the intrinsic sensitivity to the changes in N availability of the plant and animal species present; (4) the abiotic conditions (such as the ability of soils and waters to neutralize acidification effects); and (5) the past and present land use or management. The increased susceptibility of plants (or animal) species to stresses and disturbances, induced by enhanced atmospheric N loads, is highly dependent of the large differences in the physiological functioning of individual species. Therefore, the generalization of the effects of N deposition over a range of ecosystems is hardly, if at all, possible, although these impacts have been demonstrated to be of major importance in some ecosystems.

Enhanced nitrogen (N) availability is one of the main drivers of biodiversity loss and degradation of ecosystem functions. However, in very nutrient-poor ecosystems, enhanced N input can, in the short-term, promote diversity. Mediterranean Basin ecosystems are nutrient-limited biodiversity hotspots, but no information is available on their medium- or long-term responses to enhanced N input. Since 2007, we have been manipulating the form and dose of available N in a Mediterranean Basin maquis in south-western Europe that has low ambient N deposition (

This chapter reports the findings of a Working Group on how atmospheric nitrogen (N) deposition affects both terrestrial and freshwater biodiversity. Regional and global scale impacts on biodiversity are addressed, together with potential indicators. Key conclusions are that: the rates of loss in biodiversity are greatest at the lowest and initial stages of N deposition increase; changes in species compositions are related to the relative amounts of N, carbon (C) and phosphorus (P) in the plant soil system; enhanced N inputs have implications for C cycling; N deposition is known to be having adverse effects on European and North American vegetation composition; very little is known about tropical ecosystem responses, while tropical ecosystems are major biodiversity hotspots and are increasingly recipients of very high N deposition rates; N deposition alters forest fungi and mycorrhyzal relations with plants; the rapid response of forest fungi and arthropods makes them good indicators of change; predictive tools (models) that address ecosystem scale processes are necessary to address complex drivers and responses, including the integration of N deposition, climate change and land use effects; criteria can be identified for projecting sensitivity of terrestrial and aquatic ecosystems to N deposition. Future research and policy-relevant recommendations are identified.