Insect herbivory has been hypothesized to increase the rate of nutrient cycling by making nutrients fixed in plant biomass available through faeces deposition and leaching from damaged plant tissue. Studies have mainly been restricted to forest ecosystems and to outbreak situations, and have not considered potentially modifying factors such as plant productivity or plant diversity. We studied the effects of foliar herbivory by the acridid grasshopper Chorthippus parallelus on nutrients in throughfall and soil water along a gradient of plant species richness in a glasshouse experiment and quantified the response of the soil microbial community. We used a species pool of six plant species, three grasses and three legumes, to create plant communities consisting of twelve plant individuals of one, two, three or six species that were subjected to three levels of grasshopper density. Throughfall and soil water was collected over a four-week period and analysed for ammonium, nitrate and phosphate concentrations. At the end of the experiment, plant biomass was harvested, percentage herbivory estimated and soil samples were analysed for microbial carbon.
Aboveground plant biomass increased with plant species richness while belowground biomass did not. Herbivory significantly decreased with increasing plant community biomass and differed between functional groups. Plant root biomass tended to respond negatively to herbivory. Nutrients (e.g. ammonium) were enriched in throughfall when herbivores were present. Ammonium in soil water decreased with herbivory in the six species mixtures only. Microbial carbon was positively related to root biomass and seemed little affected by plant species richness and herbivory. Samples are still being analysed for further nutrients and analysis of existing data continues. Our preliminary results show that nutrient input (ammonium) through insect herbivory can be substantial although consequences for the soil microbial community were not clear. As plant productivity and herbivory are affected by plant species richness and plant species composition, the effects of insect herbivory on nutrient cycling may be more pronounced at higher levels of plant diversity.