Nitrogen fertilisation is a common form of agricultural intensification, aimed at increasing
biomass, which can affect plant species diversity and ecosystem functioning. Using a systematic
review and meta-analysis of nitrogen fertilisation studies in European permanent grasslands, we
asked: (i) what relationship form exists between nitrogen application rate and change in plant
diversity, compared to zero fertilisation controls; and (ii) how grassland, management and study
characteristics affect this relationship. Meta-analysis of 34 control-treatment effects from 14 studies
conducted across nine European countries revealed a negative linear relationship between nitrogen
fertilisation rate and change in plant species richness, equivalent to approximately 1.5 species/m2
lost for every 100 Kg ha−1 yr−1 of nitrogen added. Fertilisation induced reductions in plant species
richness were greater when defoliation rates were lower. We found some evidence that grasslands
with a higher baseline plant diversity lost more species when fertilised compared to more species
poor grasslands, although uncertainty was high. Due to the diverse grassland types included in the
analysis, the variability in fertilisation-driven changes in plant diversity was high. We identified
several remaining limitations to our understanding, including uncertainty about non-linear effects,
which could aid efforts to optimise the trade-off of plant diversity and increasing grassland yields