Above- and belowground insect herbivores mediate the impact of nitrogen eutrophication on the soil food web in a grassland ecosystem

Insect herbivores are important drivers of ecosystem processes in grasslands, and can mediate the grassland's response to environmental change. For example, recent evidence shows that above- and belowground herbivory, individually and in combination, can modify how a plant community responds to nitrogen (N) eutrophication, an important driver of global change. However, knowledge about how such effects extend to the associated soil food web is lacking. In a mesocosm experiment, we investigated how communities of soil nematodes - an abundant and functionally important group of soil organisms - responded to above- and belowground insect herbivory at contrasting N levels. We found that the strongest influence of above- and belowground herbivory on the nematode community appeared at elevated N. The abundance of root-feeding nematodes increased when either above- or belowground insect herbivores were present at elevated N, but when applied together the two herbivore types cancelled out one another's effect. Additionally, at elevated N aboveground herbivory increased the abundance of fungal-feeders relative to bacterial-feeders, which indicates changes in decomposition pathways induced by N and herbivory. Belowground herbivory increased the abundance of omnivorous nematodes. The shifts in both the herbivorous and detrital parts of the soil food web demonstrate that above- and belowground herbivory does not only mediate the response of the plant community to N eutrophication, but in extension also the soil food web sustained by the plant community. We conclude that feedbacks between effects of above- and belowground herbivory mediate the response of the grassland ecosystem to N eutrophication.