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 sensitive to fertilization intensification. This study evaluated the effects of organic and mineral inputs on the floristic composition, vegetation types, and diversity of an oligotrophic grassland with Nardus stricta in a long-term experiment established in 2002, with results presented here from three recent experimental years (2022–2024) with nine variants (unfertilized control, 10–30 t ha⁻¹ manure applied in autumn/spring, and 200–300 kg ha⁻¹ nitrocalcar). Vegetation composition was analyzed using hierarchical classification, PCoA, MRPP, Indicator Species Analysis, and α-diversity indices (species richness, Shannon, Simpson, and evenness). The results revealed six floristic types arranged along a clear trophic gradient, ranging from oligotrophic to eutrophic communities. Specifically, low-moderate manure treatments (10–20 t ha⁻¹) simultaneously maximized community diversity and balance, maintaining a rich core of oligotrophic and mesotrophic species. Conversely, high doses of manure (30 t ha⁻¹) and mineral fertilization led to a marked reduction in species richness and the dominance of a few competitive-grass species. Ultimately, this study demonstrates that moderate organic fertilization is compatible with the adaptive management of HNV grasslands, whereas intensive mineral fertilization is incompatible with conservation objectives.