This paper assesses the effects of atmospheric nitrogen (N) deposition on terrestrial plant diversity across various ecosystems, from Arctic and boreal systems to tropical forests. The authors review field experiment evidence and current understanding of N deposition mechanisms, global ecoregion distribution, and future N deposition rates to identify risks to plant diversity. Key findings include: 1. **Global Impact of N Deposition**: N accumulation is the primary driver of changes in species composition, leading to shifts in plant diversity. Other effects, such as direct toxicity of nitrogen gases and aerosols, long-term negative impacts of increased ammonium availability, and soil-mediated effects like acidification, are ecosystem-specific. 2. **Ecosystem-Specific Effects**: - **Arctic and Alpine Ecosystems**: Lichens and bryophytes are highly sensitive to increased N inputs, with a threshold effect of 5-15 kg N ha⁻¹ yr⁻¹. - **Boreal Forests**: N addition affects understorey vegetation, with a threshold of 5-10 kg N ha⁻¹ yr⁻¹. Bryophytes and dwarf shrubs are particularly sensitive. - **Temperate Forests**: Excess N can reduce herb layer richness, with a threshold of <20 kg N ha⁻¹ yr⁻¹ for significant species changes. Epiphytic species are also affected, with a threshold of >20 kg N ha⁻¹ yr⁻¹. - **Temperate Non-forest Ecosystems**: Heathlands and grasslands are sensitive to N enrichment, with thresholds of 15-20 kg N ha⁻¹ yr⁻¹ for heathlands and 15-20 kg N ha⁻¹ yr⁻¹ for grasslands. 3. **Mediterranean Vegetation**: N accumulation enhances the spread of nitrophilous and invasive species, leading to biodiversity loss. A threshold of >10 kg N ha⁻¹ yr⁻¹ is identified for significant impacts. 4. **Future Projections**: The study highlights vulnerable regions outside Europe and North America, including parts of Asia, the Mediterranean, and subtropical and tropical areas in Latin America and Africa. 5. **Policy Implications**: The critical load approach has helped European governments reduce N loads on sensitive ecosystems. More efforts are needed in Europe and North America, especially for ecosystems of high conservation importance. The paper emphasizes the widespread and potentially severe impacts of increased atmospheric N deposition on plant diversity, highlighting the need for targeted studies in low background areas and G200 ecoregions.This paper assesses the effects of atmospheric nitrogen (N) deposition on terrestrial plant diversity across various ecosystems, from Arctic and boreal systems to tropical forests. The authors review field experiment evidence and current understanding of N deposition mechanisms, global ecoregion distribution, and future N deposition rates to identify risks to plant diversity. Key findings include: 1. **Global Impact of N Deposition**: N accumulation is the primary driver of changes in species composition, leading to shifts in plant diversity. Other effects, such as direct toxicity of nitrogen gases and aerosols, long-term negative impacts of increased ammonium availability, and soil-mediated effects like acidification, are ecosystem-specific. 2. **Ecosystem-Specific Effects**: - **Arctic and Alpine Ecosystems**: Lichens and bryophytes are highly sensitive to increased N inputs, with a threshold effect of 5-15 kg N ha⁻¹ yr⁻¹. - **Boreal Forests**: N addition affects understorey vegetation, with a threshold of 5-10 kg N ha⁻¹ yr⁻¹. Bryophytes and dwarf shrubs are particularly sensitive. - **Temperate Forests**: Excess N can reduce herb layer richness, with a threshold of <20 kg N ha⁻¹ yr⁻¹ for significant species changes. Epiphytic species are also affected, with a threshold of >20 kg N ha⁻¹ yr⁻¹. - **Temperate Non-forest Ecosystems**: Heathlands and grasslands are sensitive to N enrichment, with thresholds of 15-20 kg N ha⁻¹ yr⁻¹ for heathlands and 15-20 kg N ha⁻¹ yr⁻¹ for grasslands. 3. **Mediterranean Vegetation**: N accumulation enhances the spread of nitrophilous and invasive species, leading to biodiversity loss. A threshold of >10 kg N ha⁻¹ yr⁻¹ is identified for significant impacts. 4. **Future Projections**: The study highlights vulnerable regions outside Europe and North America, including parts of Asia, the Mediterranean, and subtropical and tropical areas in Latin America and Africa. 5. **Policy Implications**: The critical load approach has helped European governments reduce N loads on sensitive ecosystems. More efforts are needed in Europe and North America, especially for ecosystems of high conservation importance. The paper emphasizes the widespread and potentially severe impacts of increased atmospheric N deposition on plant diversity, highlighting the need for targeted studies in low background areas and G200 ecoregions.