Contrasting responses of nitrogen: Phosphorus stoichiometry in plants and soils under grazing: A global meta-analysis

Grazing by ungulate herbivores can greatly alter nitrogen ([N]) and phosphorus ([P]) concentrations in plants and soils. It is not clear, however, how grazing might affect N:P co-limitation in grasslands depending on soil N and P availability. Here we selected 173 peer-reviewed studies, which measured 12 key variables associated with changes in [N], [P] and N:P ratios (i.e. N:P stoichiometry) in soils and plants in the presence or absence of herbivore grazing. Subsequently, we addressed the magnitude and direction of grazing effects on these variables using a meta-analysis approach. Grazing increased leaf [N] and [P] but decreased total and available soil [N] and [P]. Grazing also increased leaf N:P ratios while decreasing root and total soil N:P ratios. The response ratio (RR) of leaf N:P was negatively correlated with RR of plant-available soil [P] and positively correlated with RR of available soil N:P ratio (rather than with RR of total soil N:P). Intensive grazing (e.g. heavy grazing or long-term grazing) had in general more positive effects on plant N:P stoichiometry and negative effects on soil N:P stoichiometry than light grazing. Responses of plant-soil N:P stoichiometry to grazing greatly varied depending on plant functional group identity, plant organizational level (i.e. species and community) and grassland type. Synthesis and applications. Our study suggests that understanding changes in available soil N:P stoichiometry (rather than total soil N:P) in response to grazing is crucial to predict nutrient co-limitation in grassland biomes. Our findings show that P is more important for plant growth than generally thought due to greater reduction of plant-available soil [P] under grazing. A better mechanistic understanding of the relationships between plant and available soil N:P stoichiometry under grazing will greatly help improve the sustainability of natural and semi-natural grassland ecosystems.