THE BIOGEOCHEMISTRY OF A NORTH-TEMPERATE GRASSLAND WITH NATIVE UNGULATES - NITROGEN DYNAMICS IN YELLOWSTONE-NATIONAL-PARK

Nutrient dynamics of large grassland ecosystems possessing abundant mi gratory grazers are poorly understood. We examined N cycling on the no rthern winter range of Yellowstone National Park, home for large herds of free-roaming elk (Cervus elaphus) and bison (Bison bison). Plant a nd soil N, net N mineralization, and the deposition of ungulate fecal- N were measured at five sites, a ridgetop, mid-slope bench, steep slop e, valley-bottom bench, and riparian area, within a watershed from May , 1998 to April, 1992. Results indicated similarities between biogeoch emical properties of Yellowstone grassland and other grassland ecosyst ems: (1) landscape position and soil water affected nutrient dynamics, (2) annual mineralization was positively related to soil N content, a nd (3) the proportion of soil N mineralized during the year was negati vely related to soil C/N. Grazers were a particularly important compon ent of the N budget of this grassland. Estimated rates of N flow from ungulates to the soil ranged from 8.1 to 45.6 kg/ha/yr at the sites (a verage = 27.0 kg/ha/yr), approximately 4.5 times the amount of N in se nescent plants. Rates of nitrogen mineralization for Yellowstone north ern range grassland were higher than those measured in other temperatu re grassland ecosystems, possibly due to grazers promoting N cycling i n Yellowstone.