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

1. 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.

2. 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.

3. 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.

4. 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).

5. Intensiv...