This study assessed effects of voles (Microtus) on nitrogen dynamics using exclosures in two riparian meadows (Crystal Bench and Blacktail Deer Creek), in Yellowstone National Park. Results showed that voles affected nitrogen dynamics directly and indirectly. Direct mechanisms included alterations in litter quantity and quality that increased the pool of soil organic nitrogen that was readily mineralizable. However, after several years, and contrary to the authors initial hypothesis, indirect effects of voles became important to nitrogen dynamics. The removal of chronic grazing by voles increased the biomass of preferred food species, which were the most readily decomposable, inside exclosures. The balance of direct and indirect effects of herbivores on nitrogen cycling appears to be a key to understanding whether herbivores accelerate or decelerate nitrogen cycling. Direct effects of voles on nitrogen cycling occurred as pulses through time, being strong when populations were high, as they were in 1992 and 1993. Indirect effects, resulting from longer term changes in plant community composition, may overshadow direct effects, as the authors found at the Blacktail site, when vole populations were low.
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