Effects of fertilization and irrigation on vascular plant species richness, functional composition and yield in mountain grasslands.

Land-use intensification is a major threat to biodiversity in agricultural grasslands and fertilization is one of the main drivers. The effects of fertilization on biodiversity and plant functional composition (community-weighted mean traits and mean ecological indicator values) are well studied in lowland regions, but have received less attention in mountain grasslands. Moreover, in inner-alpine dry valleys, fertilizer is often applied in combination with irrigation, and irrigation effects are less well known. We experimentally tested the effects of fertilization and irrigation on vascular plant species richness and the functional composition of mountain grasslands in the Swiss Alps. After five years, fertilization increased yield but the relationship was quadratic with maximum yield reached at intermediate fertilizer levels (58 kg N ha-1year-1). The species richness of all vascular plants and forbs decreased, on average, by 6 and 5 species respectively, per 50 kg N of extra fertilizer (ha-1 year-1) applied. Fertilization also favored fast-growing plants (increased mean specific leaf area) and plants typically found in productive environments (increased mean indicator values for soil productivity and moisture). In contrast, we found no effects of irrigation on plant community composition, which suggests that irrigation does not affect vascular plant diversity to the same extent as fertilization in these mesic mountain hay meadows, at least in the mid-term. Our finding that maximum yield can be achieved at intermediate fertilizer levels is very important from an applied, agronomical and conservation point of view. It suggests that without loss of yield, farming costs and at the same time environmental pollution and negative effects on biodiversity can be reduced by applying less fertilizer. We therefore recommend maintaining non-intensive land use and keeping fertilizer inputs as low as possible to maintain the high plant diversity of mountain grasslands.