Defoliation by grazers can initiate compensatory growth, increased root exudation, and increased nutrient flows that enhance aboveground and belowground feedbacks between plants and microbes. This study examined the effect of defoliation on microbial biomass and community composition in a Yellowstone National Park (YNP) grassland during 2006 using substrate induced respiration and operational taxonomic units (OTU’s) were determined by terminal restriction fragment length polymorphism analysis using a universal 16s rDNA bacterial primer and a primer specific to ammonia oxidizing bacteria (AOB). We compared abundance and community composition changes at a winter and summer range site over four sampling times during  a two week period in clipped plots (grazed; G) and unclipped plots (ungrazed; UG). Grazing increased microbial biomass at the winter range site within 24 h of clipping and the increase was maintained (15-20%) until biomass returned to UG levels after 14 days. At the summer range site microbial biomass was not effected by grazing, but microbial biomass was higher (12%) in G plots 6 and 12 days after clipping compared to UG plots for the same time period. Grazing influenced bacterial community composition at both sites. At the winter range site OTU’s increased from 30 to 38 in G plots with a total of 12 new OTU’s identified in G plots.  In the winter range G plots there were 8 AOB OTU’s and only 3 in the UG plots. At the summer range site OTU’s increased from 42 to 53 in G plots with a total of 8 new OTU’s identified in G plots and AOB OTU's increased from 2 to 6 in UG and G plots respectively. These results suggest that grazing has not only positive effects on microbial abundance but can also shift bacterial community composition and stimulate bacterial communities associated with specific nitrogen cycle processes.