HR: 0800h
AN: B11D-0766    [Abstracts]
TI: Influence of Fire Severity on Watershed Nitrogen Cycling Using ¹⁵N Natural Abundance in Terrestrial and Aquatic Ecosystem Components
AU: * Stephan, K
EM: stephank@lincolnu.edu
AF: Lincoln University, Department of Agri., Biol., Chem., & Phys., Jefferson City, MO 65101,
AU: Kavanagh, K
EM: katyk@uidaho.edu
AF: University of Idaho, Department of Forest Resources P.O. Box 441133, Moscow, ID 83843,
AU: Koyama, A
EM: akoyama@wsu.edu
AF: University of Idaho, Department of Forest Resources P.O. Box 441133, Moscow, ID 83843,
AB: Fire is an integral component of ecosystem nitrogen (N) cycling in coniferous ecosystems of the Rocky Mountains. The objective of this study was to use N stable isotopes at natural abundance to study post-fire N cycling in small watersheds that experienced different fire severities. Within four wildfire, one spring test burn, and three spring prescribed burn sites we quantified and interpreted the N concentrations and N isotopic signatures of soil, plants, streamwater and in-stream moss, and the NO₃^- use by plants. We found short-term (1-3 y) post-fire increases of δ¹⁵N in these ecosystem N pools to be correlated with fire severity. After wildfires, δ¹⁵N significantly increased in all of the studied N pools (P < 0.05), whereas after spring burns only plant foliage δ¹⁵N significantly increased (P < 0.05), although with a smaller magnitude than after wildfire. For example, the δ¹⁵N of foliage of upland plants was enriched by 2.9 ‰\ (absolute difference between burned and unburned watersheds) in the first two years after wildfire, but only 1.3 ‰\ after spring burns. The simultaneous enrichments of both shoots and roots in wildfire-burned areas indicated that isotopic enrichment was caused by uptake of enriched soil N. This was corroborated by absolute increases in soil NH₄⁺ δ¹⁵N by on average 4.6 ‰\ for the first two post-fire years relative to unburned areas (P < 0.05). In-stream moss δ¹⁵N in wildfire-burned watersheds was increased by 1.3 ‰\ relative to unburned watersheds, but there was no response in prescription-burned watersheds. A major difference between wildfire and spring prescribed burn effects was the substantial (two orders of magnitude), sustained (3 yr), and significant (P < 0.05) increase in streamwater NO₃^- concentrations after wildfire and the lack thereof after spring burns. The smaller or lacking isotopic response after spring prescribed burns likely reflected less volatilization of ¹⁴N during the lower-temperature burns and less altered N cycling processes (i.e., minor increases in net nitrification and subsequent lack of nitrate leaching) relative to wildfire. Thus, isotopic shifts in terrestrial plant foliage or in-stream moss after fire are a useful indicator of the magnitude and duration of fire effects and the fate of post-fire available N.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0439 Ecosystems, structure and dynamics (4815)
DE: 0469 Nitrogen cycling
SC: Biogeosciences [B]
MN: 2007 Fall Meeting