Effects of Native Grazers on Grassland N Cycling in Yellowstone National Park

Frank, Douglas; Evans, R

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 δ^15N of soil (0-20 cm) by 0.7 per mil, which was substantial considering that values for ungrazed soil ranged 2.4 per mil (2.4-4.8 per mil). The magnitude of grazer ^15N enrichment was positively related (r^2 = 0.70) to the intensity of herbivore activity during the study, indexed by the amount of dung (g/m^2) deposited at the sites. We also found that soil δ^15N of ungulate urine and dung patches was significantly higher than that of control areas. Grazers probably increased soil δ^15N 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 per mil, 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 NO3- assimilation, which may positively affect primary productivity in this grazed ecosystem.

Effects of Native Grazers on Grassland N Cycling in Yellowstone National Park

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