Large herbivore vs topographic control of soil carbon storage

Herbivores and topo-edaphic gradients can alter grassland carbon (C) assimilation and metabolism in distinct ways, and, therefore, may differentially influence the rate of soil C  storage.  To examine the effects of herbivores and topographic position on soil C age and storage, we compared the amount of radiocarbon (¹⁴C) the organic matter of soils collected inside and outside exclosures erected in 1958 and '62 across topographically variable sites in Yellowstone National Park.  We also compared those radiocarbon measurements with Net Ecosystem Production (NEP) rates that were measured with environmentally controlled Plexiglas chambers at the sites in 2005 to determine if the short-term rate of C storage, i.e., NEP, was associated with longer-term, 45 yr storage.  We found that average C was youngest at sites that were intermediate along the topo-edaphic gradient from dry hilltop to wet slope - bottom.  Herbivores had no effect on soil C age or the amount of C stored in the system.   In addition, there was no relationship of NEP with C age or 45 yr storage.  These results indicate that large herbivores had no influence on soil organic C age or 45 yr storage, but that the topographic gradients in Yellowstone grasslands influenced the relative importance of C assimilation and respiration in determining 45 yr soil C pools.