Export Of Carbon, Nitrogen And Major Solutes From A Boreal Forest Watershed: The Influence Of Fire And Permafrost

Abstract

Detailed observations of stream, soil, and groundwater chemistry were used to determine the role of fire, permafrost and snowmelt processes on the fluxes of carbon, nitrogen and major solutes from interior Alaskan catchments. We examined an experimentally burned watershed and two reference watersheds that differ in permafrost coverage (high, 53%; medium-burn, 18%; and low, 4%) during the FROSTFIRE prescribed burn in July 1999. The fire elevated stream nitrate concentrations for a short period during the first post-fire storm, but nitrate declined thereafter, suggesting that less severe fires that leave an intact riparian zone may have only a short-term effect on stream chemistry. Nevertheless, we found fundamental differences in hydrochemical differences between watersheds due to the presence of permafrost. Flowpaths in the low-permafrost, likely from the riparian zone, depleted stream nitrate levels while flowpaths in the high permafrost watershed, generated from more distant hillslopes, were a source of nitrate. All watersheds were sources of organic solutes during snowmelt and summer storms. On an annual basis, watersheds were net sources of every individual ion or element (Cl-, PO42- , SO42-, DOC, DON, NO3 -, Na+, K+ Mg2+, Ca2+) except NH4+, which was a small fraction of the total N concentration in streams. The concentration of NO 3- was high for an ecosystem with low atmospheric N deposition and compared to non-Alaskan boreal and temperate watersheds, resulting in net N loss. These findings suggest that boreal watersheds in the discontinuous region of interior Alaska may be fundamentally different in their capacity to retain N compared to ecosystems with net N retention.