Nitrogen inputs and mowing do not alter N:P stoichiometry in grassland insect herbivores.

Abstract

Nitrogen (N) inputs result in stoichiometric imbalances between N and phosphorus (P) in soils and plants. There are increasing concerns about the transmission of such stoichiometric imbalances to higher trophic levels through the bottom-up effect. However, most studies focus on the species-level responses, leaving community-level responses largely unexplored. We examined the responses of N:P stoichiometry in plant community, dominant insect groups (Arcypteridae, Cicadellidae, and Lygaeidae), and insect community in both mown and unmown grasslands at the end of 8-year N addition. Nitrogen addition significantly increased plant community N concentrations and N:P by 27.4% and 30.5%, respectively. Nitrogen addition and mowing did not alter the concentrations and stoichiometry of N and P in herbivore communities. Nitrogen addition significantly increased the relative abundance of Lygaeidae by 13.8%. There was no nutritional association between plant and herbivore communities, suggesting no detectable bottom-up transfer. Those results suggest that herbivores in this grassland are insensitive to N inputs, which may be attributed to high stoichiometric homeostasis. These findings support the theory that organisms from higher trophic levels possess relatively stronger stoichiometric homeostasis compared to those from lower trophic levels.