From Oligotrophic to Eutrophic States: Floristic Responses to Long-Term Organic and Mineral Fertilization in Mountain Grasslands

High-Nature-Value (HNV) mountain grasslands are highly sensitive to nutrient enrichment, and understanding their responses to contrasting fertilization regimes is essential for safeguarding biodiversity and ecosystem functioning. This study compared the long-term effects of organic and mineral fertilization on floristic composition, vegetation types, and diversity in HNV grasslands in the Apuseni Mountains. After 15 years of differentiated inputs, moderate organic fertilization preserved a floristic structure closely related to oligotrophic grasslands and maintained a high degree of structural heterogeneity typical of habitat type 6520, while supporting a diverse assemblage of stress-tolerant species. In contrast, mineral fertilization induced marked shifts toward mesotrophic and eutrophic communities, reduced species richness, and promoted the dominance of nitrophilous grass. Multivariate analyses (PCoA, MRPP, and ISA) consistently distinguished the treatments, revealing distinct successional pathways shaped by nutrient sources and input intensity. Overall, the findings demonstrate the high sensitivity of HNV grasslands to fertilization and underscore the ecological importance of moderate organic inputs in maintaining community stability and characteristic floristic patterns in nutrient-limited mountain ecosystems.