Summary• Peatlands in the northern hemisphere have accumulated more atmospheric carbon (C) during the Holocene than any other terrestrial ecosystem, making peatlands long-term C sinks of global importance. Projected increases in nitrogen (N) deposition and temperature make future accumulation rates uncertain.• Here, we assessed the impact of N deposition on peatland C sequestration potential by investigating the effects of experimental N addition on Sphagnum moss. We employed meta-regressions to the results of 107 field experiments, accounting for sampling dependence in the data.• We found that high N loading (comprising N application rate, experiment duration, background N deposition) depressed Sphagnum production relative to untreated controls. The interactive effects of presence of competitive vascular plants and high tissue N concentrations indicated intensified biotic interactions and altered nutrient stochiometry as mechanisms underlying the detrimental N effects. Importantly, a higher summer temperature (mean for July) and increased *These authors contributed equally to this work.

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.

Questions:Has plant species richness in semi-natural grasslands changed over recent decades? Do the temporal trends of habitat specialists differ from those of habitat generalists? Has there been a homogenization of the grassland vegetation?Location: Different regions in Germany and the UK. Methods:We conducted a formal meta-analysis of re-survey vegetation studies of semi-natural grasslands. In total, 23 data sets were compiled, spanning up to 75 years 188 |