Ammonia volatilization from a seasonally and spatially variable grazed grassland: Yellowstone National Park

We measured ammonia volatilization at three topographic positions (hilltop, midslope, slope-bottom) on three grassland landscapes at three times during 1995 (April, May, July) on the northern winter range of Yellowstone National Park that supports large herds of native ungulates. Percent ammonia-N lost from all sites during the study ranged 1-24% of urea-N applied. Volatilization among sites was negatively related to soil cation-exchange capacity (r = 0.85) and rates were highest in July. We used the relationship between soil CEC and percent N volatilized from urea-amended plots to estimate annual ammonia-N volatilization from 5 sites for which annual ungulate urine inputs were previously determined (Frank et al. 1994). Estimated mean annual ammonia-N volatilized from those sites was 1.4 kg/ha/yr, which was less than a previously reported regional atmospheric deposition rate (2 kg/ha/yr; Swank 1984). Results indicate the need to understand the interaction between (1) spatially heterogeneous patterns of soil processes, and 2) nonuniform patterns of ungulate use of landscapes to determine rates of ecosystem-level N-gaseous loss. Findings also suggest that ammonia-N volatilized from urine patches should not lead to a decline in soil N in this ecosystem.