Soil nitrogen dynamics after fire in a boreal forest of Interior Alaska

Abstract

This study is part of the FROSTFIRE experiment, a controlled bum of black spruce (BS) and mixed hardwood (MH) forest in the boreal zone of Interior Alaska in 1999. As part of the examination of post-burn changes in soil carbon (C) and nitrogen (N) dynamics, this study reports on active soil C and net N mineralization and nitrification in-situ over one year (beginning the year after the burn) in BS and MH stands. Total soil C and N concentrations and pools were higher in burned BS (BS-B) than in controls (BS-C), whereas MH stands (MH-B and MH-C) showed no fire effect. MH stands exhibited substantially higher net N mineralization and nitrification rates than equivalent BS stands. Annual net N mineralization was lower and annual net nitrification was much higher in MH-B stands than in controls, whereas it did not differ between BS-B Band BS-C. Potentially mineralizable soil C was substantially higher in BS than in MH and was lower in burned soils relative to controls. Findings indicate that post-fire soil N dynamics in Interior Alaska are highly dependent on forest type and that MH stands, but not BS, may be a large source of soil nitrate (NO₃⁻) after fire.