Nutrient enrichment is one of the main drivers of the degradation of semi-natural grasslands in Western Europe (Janssens et al., 1998; Klaus et al., 2011; Pywell et al., 2007). Increased soil nutrient availability causes a decline in plant species richness because it promotes a few fast-growing species that monopolise light and outcompete slow-growing species (Hautier et al., 2009) and it reduces niche dimensionality and thus opportunities for many species to co-exist (Harpole & Tilman, 2007). Ecological restoration of degraded grasslands hence involves lowering the soil nutrient availability (Marrs, 1993), often by extracting nutrients with plant biomass by reinstating a no-fertilization mowing management (Walker et al., 2004). For many specialist and endangered species, limiting phosphorus availability is key (Ceulemans et al., 2011; Fujita et al., 2014; Janssens et al., 1998; Wassen et al., 2005). However, phosphorus removal may take decades as phosphorus is a persistent element in the soil that represents long-lasting legacies of past fertilization (Dupouey et al., 2002; Schelfhout et al., 2017). Insight into the response of plant species and communities to soil phosphorus availability will help in understanding restoration trajectories of grassland ecosystems.
Plants use phosphorus for growth by building it into nucleic acids, but species differ in their ability to acquire, use and conserve nutrients (Reich et al., 2003). Generally, graminoids have lower tissue phosphorus concentrations compared to forbs (Güsewell, 2004), because leaf growth in grasses is localised in the basal meristems, which reduces phosphorus requirements for nucleic acids in the remaining part of the leaf (Halsted & Lynch, 1996). The leaf economic spectrum, which runs from fast to slow return on investments of nutrients and dry mass in leaves, classifies species along a resource-acquisition versus resource-conservation gradient (Díaz et al., 2016; Reich, 2014; Wright et al., 2004). High leaf nutrient concentrations are associated with the acquisitive side of the spectrum. Species exhibiting acquisitive traits, such as fast aboveground growth and short-lived tissues (i.e., high specific leaf area and low leaf dry matter content), are assumed to grow best when resources are abundant. Their high relative growth rate is positively correlated with a high concentration of phosphorus-rich ribosomal RNA required for growth, which explains their high tissue phosphorus concentrations (also referred to as the ‘growth rate hypothesis’; Elser et al., 2010; White & Hammond, 2008). On the other side of the spectrum, we find species with lower potential relative growth rates that are characterized by structurally tougher tissues (i.e. low specific leaf area and high leaf dry matter content) containing less nutrients. The species that exhibit conservative traits, tend to internally recycle nutrients and dominate in vegetation on oligotrophic soils (Dayrell et al., 2018; Lambers & Poorter, 1992).
Next to the species characteristics, the amount of phosphorus available in the soil may also determine the plant responses. High soil phosphorus availability may lead to increased phosphorus uptake by the plant and hence higher tissue phosphorus concentrations. Alternatively, the extra phosphorus can also be used for increased biomass production, and such growth response leads to a dilution of phosphorus in the tissue (the ‘dilution effect’, Jarrell & Beverly, 1981). On the other hand, plants are also able to store a surplus of phosphorus in their tissue that they do not need for biomass production (‘luxury consumption’, Chapin et al., 1990). The role of this surplus phosphorus is yet debated (Ågren, 2008).
In order to understand the plant-ecological basis for the linkages between soil phosphorus supply, plant tissue phosphorus concentrations and aboveground biomass production in plant species and communities, we set up a pot experiment with twenty grassland species having contrasting growth forms (i.e., grasses versus forbs) and nutrient use strategies (i.e., acquisitive versus conservative). We grew the species as monocultures and in 4-species mixtures along a soil phosphorus gradient. We hypothesized that plant phosphorus concentrations and aboveground biomass increase with increasing soil phosphorus supply, yet depending on the species’ identity (i.e., growth form and nutrient use strategy). We expected (1) higher plant phosphorus concentrations in forbs compared to grasses and (2) higher plant phosphorus concentrations in acquisitive species compared to conservative species (3) higher biomass in acquisitive species compared to conservative species and (4) a trade-off between the responses of tissue phosphorus concentration and aboveground biomass, with species responding with growth to increasing soil phosphorus supply having lower tissue phosphorus concentrations because of a dilution effect.