Abstract
Herbivorous insects can influence grassland ecosystem functions in several ways, notably by altering primary production and nutrient turnover. Interactions between above- and belowground herbivory could affect these functions; an effect that might be modified by nitrogen (N) addition, an important global change driver. To explore this, we added above- (grasshoppers) and belowground (wireworms) insect herbivores and N into enclosed, equally composed, grassland plant communities in a fully factorial field experiment. N addition substantially altered the impact of above- and belowground herbivory on ecosystem functioning. Herbivory and N interacted such that biomass was reduced under above ground herbivory and high N input, while plant biomass remained stable under simultaneous above- and belowground herbivory. Aboveground herbivory lowered nutrient turnover rate in the soil, while belowground herbivory mitigated the effect of aboveground herbivory. Soil decomposition potential and N mineralization rate were faster under belowground herbivory at ambient N, but at elevated N this effect was only observed when aboveground herbivores were also present. We found that N addition does not only influence productivity directly (repeatedly shown by others), but also appears to influence productivity by herbivory mediated effects on nutrient dynamics, which highlights the importance of a better understanding of complex biotic interactions.
Highlights
Herbivorous insects can influence grassland ecosystem functions in several ways, notably by altering primary production and nutrient turnover
We found striking interactive effects on nutrient turnover related variables, which displayed inverted responses to above- and belowground herbivory at contrasting N levels (Figs. 2, 3), suggesting that effects of insect herbivory on nutrient dynamics is strongly context dependent
We observed higher aboveground biomass production when above- and belowground herbivory were combined, but only at elevated N. We suggest that this is a consequence of the enhanced nutrient turnover caused by combined below- and aboveground herbivory that we observed (Figs. 1, 2b)
Summary
Herbivorous insects can influence grassland ecosystem functions in several ways, notably by altering primary production and nutrient turnover. Interactions between above- and belowground herbivory could affect these functions; an effect that might be modified by nitrogen (N) addition, an important global change driver. In addition to plant community composition, herbivory can influence ecosystem functions such as nutrient d ynamics[5,9,11], this is true in N-limited e nvironments[12]. Links between above- and belowground trophic interactions are likely to influence ecosystem process rates over time[17], and have been suggested as being critical for understanding and predicting the ecosystem-level impact of global c hange[18], but have so far not been thoroughly explored in controlled experiments. There is little experimental evidence of the separate and combined effects of above- and belowground herbivory on ecosystem functioning, and even less is known about how such interactions modulate impacts of N addition on the ecosystem
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