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 N-15 abundance (del ta(15)N) of soils and plants inside and outside long-term (32-36 yr) e xclosures. Across six topographically diverse sites, grazers increased delta(15)N Of soil (0-20 cm) by 0.7 parts per thousand, which was sub stantial considering that values for ungrazed soil ranged 2.4 parts pe r thousand (2.4-4.8 parts per thousand). The magnitude of grazer N-15 enrichment was positively related (r(2) = 0.70) to the intensity of he rbivore activity during the study, indexed by the amount of dung (g/m( 2)) deposited at the sites. We also found that soil delta(15)N Of ungu late urine and dung patches was significantly higher than that of cont rol areas. Grazers probably increased soil delta(15)N by promoting N l oss from the soil via leaching, ammonia volatilization, and/or denitri fication. Each of these processes results in the removal of N-15 deple ted products from the soil and, consequently, N-15 enrichment of the r emaining soil. In contrast to soil results, grazers reduced plant N-15 by an average of 0.7 parts per thousand, 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 lo ss from this north-temperate grassland as a result of accelerated loss es on urine-and dung-affected microsites and, potentially, from elevat ed N loss throughout the grazed landscape due to grazers promoting N c ycling. Furthermore, these results suggest that herbivores increase pl ant NO3-assimilation, which may positively affect primary productivity in this grazed ecosystem.