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We investigated the effects of native ungulates on grassland N cycling in Yellowstone National Park by examining natural 15N abundance (δ15N) 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 0.7‰, 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 NO3− assimilation, which may positively affect primary productivity in this grazed ecosystem.
Frank et al. (Wed,) studied this question.