Contrasting effects of irrigation and fertilization on plant diversity in hay meadows

Extensive grassland management practices are crucial drivers to maintain biodiversity. However, it is challenging to combine biodiversity conservation with high agricultural yields. Traditional lowland meadow irrigation used to be a common management practice to improve hay production. However, it has been widely abandoned throughout Europe due to land use intensification. In an observational field study we examined the long-term impact of traditional irrigation, fertilization and biomass production on plant diversity of hay meadows. Traditional meadow irrigation enhanced plant alpha diversity (Simpson diversity and Evenness) and beta diversity (heterogeneity of multivariate dispersions). However, we found annual differences in the strength and significance of these effects, possibly due to different weather conditions. In contrast, plant species richness was unaffected by irrigation but consistently negatively influenced by the amount of applied N fertilizer. Moreover, we found significant relations between plant alpha diversity and biomass which were either unimodal (plant species richness and Simpson diversity) or negative (Evenness). Our results confirm the generally negative effects of fertilization on plant species richness. The moderately higher plant alpha and beta diversity in irrigated meadows may be a result of the heterogeneous within and between site environmental conditions induced by the annually repeated irrigation events. We conclude that traditional meadow irrigation is compatible with the conservation of plant diversity. Even stronger conservation benefits could be expected from diversified irrigation schemes that include longer-term inundation to favor even more hygrophilic plant communities.