A trait‐based analysis of the role of phosphorus vs. nitrogen enrichment in plant species loss across North‐west European grasslands

Summary 1. Both nitrogen (N) and phosphorus (P) enrichment have been identified to drive plant species losses from nutrient‐poor semi‐natural grasslands. The relative contribution of N vs. P to species loss remains unclear, however. 2. We investigated how soil N and P availability affect the occurrence of 61 grassland species across North‐western Europe. We selected 132 study sites, located in Great Britain, Belgium and France, along a soil fertility gradient based on variability in atmospheric N deposition and on nutrient input from adjacent agricultural land. To gain insight into the underlying ecological mechanisms of species loss, we examined the role of a suite of plant traits that may mediate a species’ response to increased N or P availability. 3. Mixed logistic regression showed that the occurrence of 24 plant species (39·3%) was affected by soil nutrient availability. Of these species, 18 were negatively affected by increased P (29·5%) and five by increased N (8·2%). Regionally declining plant species were absent from both P‐rich and N‐rich grasslands. Higher susceptibility to elevated P was associated with stress tolerance, low maximum canopy height and symbiosis with arbuscular mycorrhizae. 4. Synthesis and applications . Although we also identified negative effects on plant diversity through N enrichment, our results strongly suggest that P enrichment is a more important driver of species loss from semi‐natural grasslands. Species in symbiosis with mycorrhizae and with low canopy height are especially at risk. Because detrimental effects of P enrichment are very difficult to mitigate due to the persistence of P in the soil, nature management should give absolute priority to preventing P input in grasslands through fertilization, agricultural run‐off or inundation with P‐polluted surface water. To restore species‐rich grasslands on P‐enriched soils, top soil removal appears crucial and more research regarding alternative removal strategies is essential.