Frank and Evans investigated the effects of native ungulates on grassland N cycling in Yellowstone National Park by examining natural 15N abundance of soils and plants inside and outside long-term exclosures. Across six topographically diverse sites, grazers increase isotope 15N of soil by 0.7%, which was substantial considering that values for ungrazed soil ranged 2.4%. The magnitude of grazer 15N enrichment was positively related to the intensity of herbivore activity during the study, indexed by the amount of dung deposited at the sites. These findings indicate that native grazers increased N loss from this north-temperate grassland as a result of accelerated losses on urine- and dung-affected microsites and, potentially, from elevated N loss throughout the grazed landscape due to grazers promoting N cycling. Furthermore, these results suggest that herbivores increase plant NO3- assimilation, which may positively affect primary productivity in this grazed ecosystem.
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