Summary1. The atmospheric deposition of sulphur and nitrogen compounds in the Netherlands has been responsible for decreasing plant species diversity in heathland. To unravel the relative importance of nitrogen compounds on soil acidification and eutrophication, and hence on the vegetation, we carried out a factorial addition experiment and a germination experiment in heathland on nutrient-poor sandy soil. 2. We changed nutrient availability and acidity independently in eight different treatments that, respectively, added nutrients or carbon in various combinations (N, P, glucose) or added acidifying or neutralizing compounds. One treatment also involved adding Al. Additions occurred five times per year during 5 years, in an area from which sods had been removed before the experiment began. The same design was used for the germination experiment, but the treatments were applied for 2 years. 3. Our results showed that acidification was the most important factor in reducing species diversity. In addition, the germination of several heathland species was significantly reduced in plots with a pH below 5, and germination was very poor in plots where Al had been added. 4. The number of plant species declined particularly with increasing Al in the upper soil horizons. We conclude that this relationship is responsible for the influence of acidification on plant species richness in heathland. 5. The influence of nutrient availability on species composition in heathland was subsidiary to acidity, but nutrient availability influenced species composition in an independent way. The growth of the three dominant species ( Molinia caerulea , Calluna vulgaris and Erica tetralix ) was limited by different nutrients. Erica tetralix was limited by N, Calluna vulgaris by P and Molinia caerulea by both N and P. We argue that increased N availability will change the relative availability of N and P, which can decrease species diversity. 6. Together these results show how factorial experiments can elucidate the complex ecological effects arising from sulphur and nitrogen deposition, revealing different mechanisms that change species richness and community composition.

Summary 1We studied the effects of elevated atmospheric CO 2 and increased N deposition on the plant species composition of a Sphagnum -dominated bog ecosystem in the Netherlands. Large peat monoliths (surface area 1 m 2 , depth 0.6 m) with intact bog vegetation were kept outdoors in large containers and were exposed to elevated CO 2 or increased N deposition for three growing seasons. Elevated CO 2 conditions (target concentration 560 µ mol CO 2 mol -1 ) were created using MiniFACE technology. In a separate experiment, N deposition was increased by 5 g N m -2 year -1 by adding dissolved NH 4 NO 3 at 3 week intervals during the growing season. 2 Elevated atmospheric CO 2 increased height growth of Sphagnum magellanicum , the dominant Sphagnum species, in the second and third growing seasons. Vascular plant biomass was not significantly affected by elevated CO 2 , but growth of species growing close to the moss surface was influenced negatively by the increased Sphagnum height growth. Elevated CO 2 did not change allocation to below-ground plant parts. 3 Adding N increased above-ground vascular plant biomass. The shallow-rooted species Vaccinium oxycoccus responded most to the increased N deposition. Sphagnum growth was significantly reduced in the third growing season. This reduction was likely the result of the increased vascular plant cover, given the observed negative relation between vascular plant cover and Sphagnum growth. 4 The observed shifts in species composition as a result of species-specific responses to treatments, and interactions between peat mosses and vascular plants will have important consequences for the sequestration of carbon in the bog ecosystem.

Summary• We studied the effects of N deposition on shrub-moss competition and the establishment and growth of invasive Betula pubescens and Molinia caerulea in intact bog vegetation removed from a site subject to 40 kg N ha − 1 yr − 1 .• Mesocosms with and without introduced Betula seedlings and Molinia sprouts were kept under a roof and received an equivalent of 0, 40 and 80 kg N ha − 1 yr − 1 for two growing seasons.• N concentration in both interstitial water and Sphagnum decreased when N input ceased and increased when N input was doubled. Molinia biomass was positively related to the inorganic N concentration in the interstitial water. Adding N increased production of Molinia and prolonged survival of Betula seedlings in the first year. Sphagnum height increment showed a hump-shaped relationship with light interception by vascular plants.• N deposition encouraged vascular plants to grow by enhancing N availability in the rhizosphere. Water table level and the availability of P were found to be important in explaining species-specific responses to N deposition. The underlying mechanisms and the reversibility of N effects are discussed.

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The competition between peat mosses (Sphagnum) and vascular plants as affected by raised CO2 and increased N deposition was studied in a glasshouse experiment by exposing peat monoliths with monocultures and mixtures of Sphagnummagellanicum and Eriophorumangustifolium to ambient (350 ppmv) or raised (560 ppmv) atmospheric CO2 concentrations, combined with low (no N addition) or high (5 g m−2 yr−1 added) N deposition. Growth of the two species was monitored for three growing seasons. 
The presence of Eriophorum did not affect Sphagnum biomass, because Eriophorum density did not become high enough to severely shade the moss surface. In contrast, Sphagnum had a negative effect on Eriophorum biomass, particularly on the number of flowering stems. Possibly, the presence of a living Sphagnum layer decreased nutrient availability to Eriophorum by immobilising nutrients mineralised from the peat. 
Raised CO2 and/or increased N deposition did not change these competitive relationships between Sphagnum and Eriophorum, but had independent effects. Raised CO2 had a positive effect both on Sphagnum and Eriophorum biomass, though on Eriophorum the effect was transient, probably because of P limitation. Nitrogen addition had a direct negative effect on Sphagnum height growth in the first growing season, but by the third year an increased shoot density had cancelled this out, so no N effect on Sphagnum biomass was present at the end of the experiment. The response of Eriophorum to N addition was small; N availability appeared not to limit its growth.