Effects of Long-Term Nitrogen Addition on Plant Diversity in a Cold-Temperate Coniferous Forest

As a major driver of global change, increasing nitrogen deposition is affecting community structure and biodiversity in forest ecosystems. However, the responses of different plant life forms to long-term nitrogen inputs and their associations with environmental changes remain poorly understood. In this study, we investigated the effects of long-term nitrogen addition on α- and β-diversity and their components (species turnover and nestedness) across different plant life forms in a cold-temperate coniferous forest, based on a 14-year nitrogen addition experiment. We further examined the relationships between plant diversity changes and soil environmental variables. The results showed that long-term nitrogen addition had limited effects on tree-layer α-diversity, whereas shrub- and herb-layer α-diversity decreased. Nitrogen addition significantly altered shrub-layer β-diversity, whereas no significant changes were detected in tree- and herb-layer β-diversity. β-diversity partitioning indicated that nestedness contributed more to shrub-layer compositional differences, whereas species turnover contributed more to herb-layer compositional dissimilarity among plots. Changes in understory plant diversity were associated with soil pH and inorganic nitrogen concentrations. These findings demonstrate contrasting responses of plant diversity among vegetation layers under long-term nitrogen enrichment and highlight the importance of considering life-form-specific responses when assessing the impacts of atmospheric nitrogen deposition on forest biodiversity.