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Abstract

Nutrient cycling in terrestrial ecosystems is aff ected by various factors such as plant diversity and insect herbivory. While several studies suggest insect herbivory to depend on plant diversity, their interacting eff ect on nutrient cycling is unclear. In a greenhouse experiment with grassland microcosms of one to six plant species of two functional groups (grasses and legumes), we tested the infl uence of plant species richness (diversity) and functional composition on plant community biomass production, insect foliar herbivory, soil microbial biomass, and nutrient concentrations in throughfall. To manipulate herbivory, zero, three or six generalist grasshoppers ( Chorthippus parallelus ) were added to the plant communities. Increasing plant species richness increased shoot biomass and grasshopper performance, without signifi cantly aff ecting root biomass or insect herbivory. Plant functional composition aff ected all of these parameters, e.g. legume communities showed the highest shoot biomass, the lowest grasshopper performance and suff ered the least herbivory. Nutrient concentrations (dissolved mineral N, PO 4 -P, SO 4 -S) and pH in throughfall increased with herbivory. PO 4 -P and pH increases were positively aff ected by plant diversity, especially under high herbivore pressure. Plant functional composition aff ected several throughfall variables, sometimes fully explaining diversity eff ects. Increasing plant diversity tended to increase soil microbial biomass, but only under high herbivore pressure. Faeces quantities strongly correlated with changes in pH and PO 4 -P; frass may therefore be an important driver of throughfall pH and a main source of PO 4 -P released from living plants. Our results indicate that insect herbivory may signifi cantly infl uence fast nutrient cycling processes in natural communities, particularly so in managed grasslands