Species ecological strategy and soil phosphorus supply interactively affect plant biomass and phosphorus concentration

Excess soil phosphorus often constrains ecological restoration of degraded semi-natural grasslands in Western-Europe. Slow-growing species, often target of restoration (measures), are at a disadvantage because they are outcompeted by fast-growing species. Gaining insight into the responses of plant species and communities to soil phosphorus availability will help understanding restoration trajectories of grassland ecosystems. We set up two pot experiments using twenty grassland species with contrasting growth forms (i.e. grasses versus forbs) and nutrient use strategies (i.e. acquisitive versus conservative nutrient use). We quantified the nutrient use strategy of a species based on the stress-tolerance value of the CSR framework (StrateFy et al. 2017). We grew these species (1) as monocultures and (2) in mixtures along a soil phosphorus gradient and measured the aboveground biomass and plant phosphorus concentrations. Plant phosphorus concentration generally increased with soil phosphorus supply and biomass increased with soil phosphorus supply only in conservative communities. Forbs had higher plant phosphorus concentrations compared to grasses both in monocultures and mixtures. The species' nutrient use strategy had contrasting effects on plant tissue phosphorus concentrations, depending on soil phosphorus supply (interaction effect) and vegetation biomass (dilution effect). Our findings contribute to the knowledge required for successful ecological restoration of species-rich grasslands. Our results suggest that under specific conditions (i.e. nitrogen limitation, no dispersal limitation, no light limitation), slow-growing species can survive and even thrive under excess soil phosphorus availability. In the field, competition by fast-growing species may be reduced by increased mowing or grazing management.