AMMONIA VOLATILIZATION FROM A SEASONALLY AND SPATIALLY-VARIABLE GRAZED GRASSLAND - YELLOWSTONE-NATIONAL-PARK

We measured ammonia volatilization at three topographic positions (hil ltop, midslope, slope-bottom) on three grassland landscapes at three t imes during 1995 (April, May, July) on the northern winter range of Ye llowstone National Park that supports large herds of native ungulates. Percent ammonia-N lost from all sites during the study ranged 1-24% o f urea-N applied. Volatilization among sites was negatively related to soil cation-exchange capacity (r = -0.85) and rates were highest in J uly. We used the relationship between soil CEC and percent N volatiliz ed from urea-amended plots to estimate annual ammonia-N volatilization from 5 sites for which annual ungulate urine inputs were previously d etermined (Frank et al. 1994). Estimated mean annual ammonia-N volatil ized from those sites was 1.4 kg/ha/yr, which was less than a previous ly reported regional atmospheric deposition rate (2 kg/ha/yr; Swank 19 84). Results indicate the need to understand the interaction between ( 1) spatially heterogeneous patterns of soil processes, and 2) nonunifo rm patterns of ungulate use of landscapes to determine rates of ecosys tem-level N-gaseous loss. Findings also suggest that ammonia-N volatil ized from urine patches should not lead to a decline in soil N in this ecosystem.