The influence of fire and permafrost on sub-arctic stream chemistry during storms.

Permafrost and fire are important regulators of hydrochemistry and landscape structure in the discontinuous permafrost region of interior Alaska. We examined the influence of permafrost and a prescribed burn on concentrations of dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and other solutes (NO^-₃, Ca²⁺, K⁺, Mg²⁺, Na⁺) in streams of an experimentally burned watershed and two reference watersheds with varying extents of permafrost in the Caribou-Poker Creeks Research Watershed in interior Alaska. The low-permafrost watershed has limited permafrost (3%), the high-permafrost watershed has extensive permafrost (53%), and the burn watershed has intermediate permafrost coverage (18%). A three end-member mixing model revealed fundamental hydrologic and chemical differences between watersheds due to the presence of permafrost. Stormflow in the low-permafrost watershed was dominated by precipitation and overland flow, whereas the high-permafrost watershed was dominated by flow through the active layer. In all watersheds, organic and groundwater flow paths controlled stream chemistry: DOC and DON increased with discharge (organic source) and base cations and SO^2-₄ (from weathering processes) decreased. Thawing of the active layer increased soil water storage in the high-permafrost watershed from July to September, and attenuated the hydrologic response and solute flux to the stream. The FROSTFIRE prescribed burn, initiated on 8 July 1999, elevated nitrate concentrations for a short period after the first post-fire storm on 25 July, but there was no increase after a second storm in September. During the July storm, nitrate export lagged behind the storm discharge peak, indicating a flushing of soluble nitrate that likely originated from burned soils. [ABSTRACT FROM AUTHOR]