Cross-site comparison of herbivore impact on nitrogen availability in grasslands: the role of plant nitrogen concentration

We tested whether there is a relationship between plant N concentration and herbivore impact on soil N availability (measured with resin bags) with a study of replicate 6-8 year old exclosures (with an unfenced control) of vertebrate herbivores (> 1 kg) established at each of seven grassland sites in North America and Europe. Contrary to model predictions, we found a negative relationship between the effect of herbivores on resin bag soil N availability and plant N concentration. Our study confirms the importance of plant N concentration as a predictor of herbivore effect on soil N availability across grasslands, but contradicts the models. A possible explanation may be that the results represent a transient situation as the exclosures were relatively young whereas the models may refer to an equilibrium state. Simultaneous measurements of both plant N concentration and herbivore effect on soil N availability from more grassland sites, preferably with contrasting plant N concentrations and including exclosures of different ages, should resolve the contrast between model predictions and our field measurements.