The Effect of Fertilization on Floristic Composition and Biodiversity of Montane Grasslands (HNV) in the Eastern Carpathians

High Nature Value (HNV) mountain grasslands in the Eastern Carpathians are highly sensitive to changes in management intensity, particularly fertilization. This study assessed the effects of contrasting organic and mineral fertilization regimes on floristic composition, vegetation types, and diversity in an oligotrophic Nardus stricta grassland within an experimental framework established in 2021. The analysis is based on vegetation data collected over three consecutive years (2022–2024) from nine treatments, including an unfertilized control, organic fertilization with manure (10–30 t ha−1 applied in autumn or spring), and mineral fertilization with nitrocalcar (Nitrocalc_20—200 kg ha−1 calcium ammonium nitrate and Nitrocalc_30—300 kg ha−1 calcium ammonium nitrate). Vegetation responses were evaluated using hierarchical cluster analysis, principal coordinates analysis (PCoA), multi-response permutation procedures (MRPP), indicator species analysis (ISA), and α-diversity indices. Six floristic types were identified along a pronounced trophic gradient ranging from oligotrophic to eutrophic communities. Low to moderate organic fertilization (10–20 t ha−1) maximized species richness, diversity, and community evenness, maintaining a stable assemblage of oligotrophic and mesotrophic species. In contrast, high manure inputs (30 t ha−1) and mineral fertilization resulted in rapid floristic simplification, loss of oligotrophic indicators, and dominance of competitive grasses. These results indicate that moderate organic fertilization represents an effective adaptive management option for conserving HNV mountain grasslands, whereas intensive mineral fertilization is incompatible with biodiversity conservation objectives.