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

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.