PARENT SESSION
Contributed Oral Session 148: Biogeochemistry: Nutrient Dynamics
Friday, August 12, 8:00 AM - 11:30 AM, Meeting Room 511 B, Level 5, Palais des congrès de Montréal

Fire effects on nitrogen dynamics in headwater systems.

Stephan, Kirsten¹, Kavanagh, Kathleen¹, ¹ University of Idaho, Moscow, ID

ABSTRACT- The spatial extent of headwater systems makes up a major proportion of total catchment area. Yet, in those systems there is a lack of integration between aquatic and terrestrial biogeochemistry in response to disturbance. This study investigates the effects of severe wildfires on nitrogen (N) dynamics in four independent headwater systems in central Idaho, USA. We measured N concentrations and ¹⁵N values in selected biotic and abiotic N pools of both hillslopes and streams following the first or second spring run-off after the burns. Depending on the individual headwater system, available soil ammonium and nitrate concentrations were increased 12-30fold and 2-15fold, respectively, in burned systems compared to controls (2.2 mg NH₄⁺-N kg^-1; 0.7 mg NO₃^--N kg^-1 in controls) in June. Upslope shrub and graminoid foliar N concentrations were increased on average by 50 % relative to controls, and foliar ¹⁵N values were 2.9 ‰ higher in burned systems. Stream water nitrate concentrations in burned headwater systems were highest in June and 5-20fold higher than in controls (2-63 g/ l in controls). Stream water ammonium concentrations (10.5 g/ l) and dissolved organic N concentrations (76 g/ l), did not differ between control and burned headwater systems. Aquatic moss showed an increase in N concentration of 50 % but no consistent trend in ¹⁵N values was apparent. While we are still unsure of the cause of the increased soil N pool, the N is readily assimilated by the resprouting vegetation with some leaching of nitrate into the streams.. This is consistent with other studies. Changes in soil N dynamics are reflected in terrestrial foliar ¹⁵N values making it possible to track the fate of the increased N post-fire. Biogeochemical linkage between terrestrial and aquatic systems has been demonstrated at the scale of headwater systems; however, more research is necessary at smaller scales to elucidate the complex biogeochemical relationships between soil, plants and streams.

Key words: N cycling, N isotopes, forest, streams