PARENT SESSION
Symposium #8: Nitrogen deposition in the Western U.S.... Is there cause for concern?.
Sponsored by ESA Rocky Mountain Chapter
Organized by: H.M. Rueth and J.S. Baron.
Tuesday, August 7, 2001. 8:00 AM to 12:00 PM. Madison Ballroom A
Nitrogen budgets in Sierran Forests: Relative roles of atmospheric deposition, fire and nitrogen fixation.
Johnson, Dale1, Susfalk, Richard2, Dahlgren, Randy3, Caldwell, Todd4, Miller, Wally1, 1 2 3 4
ABSTRACT- The relative roles of atmospheric deposition, fire and nitrogen fixation in the long-term nitrogen budget of a semiarid, snowmelt-dominated forest in Little Valley, Nevada are evaluated. Variations in deposition and leaching data collected from 1995 to 1999 were used to examine the potential effects of snowpack amount and duration on N concentrations and fluxes. Soil solution NO3- and NH4+ were uniformly low, and the variations in concentration bore no relationship to snowmelt water flux inputs of these ions. In contrast to other studies, the highest concentration and the majority of ion flux from the snowpack in Little Valley occurred in the later phases of snowmelt. Possible reasons for this include sublimation of the snowpack and dry deposition of organic matter during the later stages of snowmelt. This pattern has implications for the possibility of biological N uptake in low vs high snow years. Nitrogen losses due to a wildfire and subsequent N gains due to N fixation were also evaluated. N losses by wildfire, when expressed on an annualized basis, far exceeded N losses by leaching (by > 1000 fold). During the 16 years that followed, the site was dominated by snowbush (Ceanothus velutinus Dougl.) Nitrogen accretion in vegetation and litter in the snowbush stand over 16 years equaled that lost to fire. Fire and post-fire N fixation, when they occur, clearly dominate the long-term N status of these sites.
KEY WORDS: Nitrogen, Snowmelt, Fire, N-fixation