Long-Term Effects of Organo-Mineral Fertilization on Floristic Composition and Biodiversity in High Nature Value Mountain Grasslands of the Apuseni Mountains (Romania)

This study evaluated the long-term effects of organo-mineral fertilization on floristic diversity, species diversity, and vegetation structure in an HNV grasslands of the Apuseni Mountains. The experiment included five fertilization variants (control, organic, organo-mineral, mineral, and intensive organo-mineral), applied over a period of more than 15 years. Floristic diversity was assessed using a modified Braun–Blanquet method and multivariate methods—cluster analysis, principal coordinate analysis (PCoA), MRPP procedure, and indicator species analysis (ISA). Our analysis showed a trophic gradient, from oligotrophic Festuca rubra grasslands to mesotrophic (Agrostis capillaris–Trisetum flavescens) and eutrophic (Agrostis capillaris–Centaurea pseudophrygia) communities, depending on the intensity of organo-mineral fertilization applied. Moderate organo-mineral fertilization maintained a balanced floristic diversity and higher Shannon and Simpson indices compared to variants fertilized only with mineral inputs. Organo-mineral inputs improved soil fertility and ecosystem resilience, supporting soil microbiota activity and reducing nutrient losses. Intensive mineral fertilization led to a reduction in floristic richness and the dominance of nitrophilic species. This study demonstrates that moderate organo-mineral fertilization (≤10 t ha−1 manure combined with N50P25K25) provides an optimal balance between grassland productivity and biodiversity conservation, offering practical guidance for the sustainable management of High Nature Value mountain grasslands.