Impact of nitrogen inputs on multiple facets of plant biodiversity in mountain grasslands: does nutrient source matter?

Questions: What is the impact of nitrogen (N) inputs on the vegetation of mountain permanent grasslands? Beyond the effect of fertilizer quantity (total and available N supply), is there a specific effect of fertilizer quality (the various N sources) on the multiple biodiversity facets of plant communities? Location: Jura Mountains (France). Methods: Based on a vegetation and agronomic survey of 29 mown grasslands, we considered taxonomic, phylogenetic and multi-trait functional diversity indices as well as community-weighted means of seven functional traits (CWMs) including CSR strategies. Using partial redundancy analysis (pRDA) and Spearman correlations, we assessed the response of these biodiversity metrics to (1) the medium-term total N input and the short-term available N input of any origin, and (2) the relative contribution of industrial fertilizers, solid manure, liquid manure, livestock restitutions and atmospheric deposition to the amount of available N added to the soil during the year preceding vegetation sampling. Results: We report significant negative rank correlations between total or available N input and two diversity metrics: species richness and functional richness. Among CWMs, high N inputs were positively correlated with specific leaf area (SLA), competitive and ruderal strategies, and negatively with seed mass and stress-tolerant strategy. Results also revealed various trends according to the N source: when the proportion of available N from livestock restitutions was high, plant communities showed higher taxonomic and phylogenetic diversity and lower leaf dry matter content. In contrast, communities receiving mainly industrial fertilizers showed lower taxonomic and phylogenetic diversity and higher SLA. Moreover, pRDA suggested contrasting impacts of the main N source on life strategies: R strategy appeared more favoured by a high proportion of industrial fertilizers, while C strategy was enhanced by liquid or solid manure. Conclusions: Our observations support the common finding that a high N supply impacts plant functional composition and is associated with a decrease in species richness through the loss of stress-tolerant species. In addition, they show that biodiversity patterns mostly depend on fertilizer origin. In particular, industrial fertilizers appear to be the most detrimental sources of nutrients for both taxonomic and phylogenetic diversity in mountain grasslands.