Tracy and Frank compared microbial biomass size and activity, as measured by in situ net N mineralization, inside and outside 35- to 40-year old exclosures across a topographic gradient in Yellowstone National Park to determine the relative effect of topography and large grazers on microbial biomass and nitrogen mineralization. Microbial biomass varied across topographic features (which determine plant productivity) but was unaffected by chronic grazing on the northern winter range of Yellowstone National Park, as evident by identical soil C levels in exclosures (147 g/m2 plant biomass available for decomposition) relative to plots grazed by large ungulates (27 g/m2 plant biomass available for decomposition). Grazing did effect microbial activity, increasing mineral N fluxes (40% higher net N mineralization rate), particularly in mesic, topographic depressions containing high plant biomass. The authors conclude that topography mainly influence microbial biomass size, while mineral N fluxes are affected more by grazing in this grassland ecosystem.
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