Are N, P, and N:P stoichiometry limiting grazing exclusion effects on vegetation biomass and biodiversity in alpine grassland?

Abstract

Nitrogen (N), phosphorus (P), and N:P stoichiometry have been proposed as an index to assess plant nutrient limitation in grassland. The alpine grasslands in Tibet were investigated to evaluate the relationships between N, P, N:P ratio and vegetation biomass and species biodiversity by comparing free grazing (FG) and grazing exclusion (GE) grasslands. Grazing exclusion had no significant effect on N, P concentrations and N:P ratio of both plant shoot and root, but significantly reduced soil N, P concentrations by 15.5% and 12.6%, respectively. In general, both biomass and biodiversity indicators positively correlated with shoot P, negatively correlated with shoot N:P ratio. Nevertheless, they had different correlation patterns with soil stoichiometry, the biomass indicators positively correlated with soil N and N:P ratios, but the biodiversity indicators negatively correlated with them. Furthermore, the effect sizes of above-ground biomass were significantly correlated with the ratio of soil N:P between GE and FG plots and the effect sizes of biodiversity indicators were significantly correlated with the ratio of soil P between GE and FG plots. Therefore, how extent of grazing exclusion effects on biomass allocation depends on how difference of soil N:P ratio between inside and outside of fencing, while how extent of grazing exclusion effects on biodiversity depends on how difference of soil P ratio between them.