The Impact of Fertilization Type and Intensity on the Structure and Diversity of High Nature Value (HNV) Mountain Grasslands

High Nature Value (HNV) grasslands in mountain areas are important ecosystems for biodiversity maintenance and offer a multitude of ecosystem services but constantly threatened by abandonment or intensive fertilization. The aim of this study was to assess the effects of organic and mineral fertilization, under mulching and abandonment scenarios, on the floristic composition and diversity of Nardus stricta-dominated grasslands located in the North-Eastern Carpathians (Romania). The field experiment included 11 variants (control, low, moderate and high inputs), analyzed as communities with cluster, ordinations, indicator species and α indices (specific richness, Shannon–Wiener, equity, Simpson). The results showed a clear separation of communities along the input gradient, from the oligotrophic grassland dominated by Nardus stricta (control variant) to mesotrophic/eutrophic communities dominated by Dactylis glomerata, Festuca pratensis and Trifolium pratense at moderate and high inputs. Moderate fertilization (10–20 t ha⁻¹ manure; N50P50K50–N100P100K100) maximized species richness (37–38 species), Shannon diversity (H′ = 2.5–2.6) and evenness (E = 0.70–0.75), confirming the Intermediate Disturbance Hypothesis. In contrast, high inputs reduced diversity and favored competitive grasses. Indicator species analysis highlighted a multitude of species that show plant communities response to the adaptive management. Moderate fertilization provides a viable trade-off between productivity and biodiversity, while abandonment or overfertilization accelerates biodiversity loss.