Effects of native grazers on grassland N cycling in Yellowstone National Park

We investigated the effects of native ungulates on grassland N cycling in Yellowstone National Park by examining natural ¹⁵N abundance (δ ¹⁵N) of soils and plants inside and outside long-term (32-36 yr) exclosures. Across six topographically diverse sites, grazers increased δ¹⁵N of soil (0-20 cm) by 0.27‰, which was substantial considering that values for ungrazed soil ranged 2.4‰ (2.4-4.8‰). The magnitude of grazer ¹⁵N enrichment was positively related (r² = 0.70) to the intensity of herbivore activity during the study, indexed by the amount of dung (g/m²) deposited at the sites. We also found that soil δ¹⁵N of ungulate urine and dung patches was significantly higher than that of control areas. Grazers probably increased soil δ¹⁵N by promoting N loss from the soil via leaching, ammonia volatilization, and/or denitrification. Each of these processes results in the removal of ¹⁵N depleted products from the soil and, consequently, ¹⁵N enrichment of the remaining soil. In contrast to soil results, grazers reduced plant ¹⁵N by an average of 0.7‰, probably due to isotopically light, soil NO₃^- (compared to soil NH₄⁺) constituting a more important N source for plants in grazed grassland relative to those in ungrazed grassland. 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 NO^3- assimilation, which may positively affect primary productivity in this grazed ecosystem.