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.

45Evidence from an international survey in the Atlantic biogeographic region of Europe 46indicates that chronic nitrogen deposition is reducing plant species richness in acid 47 grasslands. Across the deposition gradient in this region (2 to 44 kg N ha -1 yr -1 ) species 48 richness showed a curvilinear response, with greatest reductions in species richness when 49 deposition increased from low levels. This has important implications for conservation 50 policies, suggesting that to protect the most sensitive grasslands resources should be 51 focussed where deposition is currently low. Soil pH is also an important driver of species 52 richness indicating that the acidifying effect of nitrogen deposition may be contributing to 53 species richness reductions. The results of this survey suggest that the impacts of nitrogen 54 deposition can be observed over a large geographical range.

Summary• 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 µmol 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.

While it is well established that ecosystems display strong responses to elevated nitrogen deposition, the importance of the ratio between the dominant forms of deposited nitrogen (NH(x) and NO(y)) in determining ecosystem response is poorly understood. As large changes in the ratio of oxidised and reduced nitrogen inputs are occurring, this oversight requires attention. One reason for this knowledge gap is that plants experience a different NH(x):NO(y) ratio in soil to that seen in atmospheric deposits because atmospheric inputs are modified by soil transformations, mediated by soil pH. Consequently species of neutral and alkaline habitats are less likely to encounter high NH(4)(+) concentrations than species from acid soils. We suggest that the response of vascular plant species to changing ratios of NH(x):NO(y) deposits will be driven primarily by a combination of soil pH and nitrification rates. Testing this hypothesis requires a combination of experimental and survey work in a range of systems.

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.

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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 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.

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.