PARENT SESSION
Thursday, August 10, 8:00-11:30 am
COS 78 - Nitrogen cycling I
L-4, Lobby Level, Cook Convention Center
Presiders: K Buckeridge

Comparison of wildfire and prescribed burn effects on nitrogen dynamics in headwater ecosystems in central Idaho.

Stephan, Kirsten^*,1, Koyama, Akihiro¹, Kavanagh, Kathleen¹, ¹ University of Idaho, Moscow, ID

ABSTRACT- Fire can have profound effects on nitrogen (N) dynamics, however little is known about the simultaneous effects on N in soil, plants and streamwater within watersheds. This replicated study compares wildfire with prescribed burn effects on watershed ecosystems in central Idaho Rocky Mountain headwaters. We measured N concentrations and ¹⁵N values in major ecosystem N pools (foliage, soil, streamwater) during the two growing seasons following the burns. Soil ammonium concentrations were increased about 10-fold in burned watersheds relative to controls (6 g NH₄⁺-N kg^-1) in the first growing season post fire, regardless of fire type. Soil nitrate concentrations followed a similar trend, even though the absolute concentrations were considerable lower than ammonium (wildfire: 9.4 g NO₃^--N kg^-1, prescribed burn: 1.2g NO₃^--N kg^-1, unburned: 0.1 g NO₃^--N kg^-1). Resprouting shrubs and forbs had higher foliar N concentrations (burned 2.4%, unburned 1.5%) and were enriched in ¹⁵N (average increase in ¹⁵N by 3.3 ‰(wildfire) and 1.2 ‰(prescribed burn)) in the first post-fire season in both fire types. Streamwater nitrate concentrations increased on average about 5-fold (burned: 141 g l^-1, unburned: 30 g l^-1) in wildfire sites, and no increase occurred in prescribed fire sites. During the second post-fire season wildfire effects on streamwater nitrate concentrations and foliar isotopic values persisted whereas fire effects on soil and plant foliage N concentrations were not detectable anymore. No fire effects could be detected in prescribed burn sites in the second season. In sum, wildfire effects strongly influenced N dynamics both locally (i.e. sampling plots) and also across the entire watershed (i.e. streamwater biochemistry as integrator of watershed processes). Prescribed burn effects are shorter lived and localized. The integrated study of watershed ecosystems can provide greater insights into N dynamics than studying individual ecosystem components separately

Key words: nitrogen cycling and fire, watershed nutrient biogeochemistry, coniferous forest