Native Ungulates As Regulators of Belowground Processes in Yellowstone National Park.

The effects of large herbivores on grasslands is largely driven by how grazers influence belowground processes.  In Yellowstone National Park (YNP), herds of migratory elk, bison, and pronghorn stimulate shoot and root productivity via two belowground pathways.  First, grazers increase the availability of nitrogen, the major limiting nutrient in YNP, by (a) converting nitrogen in forage to forms in urine and dung that are more available to soil microbes, (b) increasing the quality of litter and soil organic material, and (c) stimulating rhizospheric processes of grazed plants.  Second, Yellowstone grazers alter the  composition of the arbuscular mycorrhizae fungal community in grazed grassland, which promotes forage production.  The combined effects that Yellowstone grazers have on plant and soil processes have major impacts on soil carbon stability and the capacity for these grasslands to sequester carbon during the 21^st century.