Tuesday, November 6, 2007 - 1:15 PM
214-2

Effects of Fire on Soil Carbon in Siskiyou National Forest, Oregon.

Christopher Swanston, US Forest Service, 410 MacInnes Drive, Houghton, MI 49931, Heath Powers, Los Alamos National Lab, Atmospheric and Environmental Dynamics Group, Los Alamos, NM 87545, and Beverly Law, Forest Science, Oregon State University, 328 Richardson Hall, Oregon State University, Corvallis, OR 97331.

Fire is a fundamental part of western US forest carbon (C) cycles.  We sought to characterize the effects of a large fire in the Siskiyou National Forest in Oregon, USA on the forms and flow of C in those soils.  After the fire, we sampled soils from 0-5 and 5-15 cm of burned and unburned plots, then separated the soils into light and heavy density fractions (LF, HF), followed by elemental and radiocarbon analysis of the fractions, soils, and charcoal.  There was a 15% net loss of soil C from the upper soil, which was not evenly distributed across the fractions: the LF sustained a 20% net C loss, while the HF-C showed a net gain of 5%.  There was a 20% net C gain in 5-15 cm, reflecting a 15% net C loss from the LF, and a 58% net increase in HF-C.  Radiocarbon values indicated that C loss from LF was not responsible for the net gain in HF-C at either depth.  There was a significant decrease (50 ‰) in the Δ14C of upper-soil HF from the burned plots, but no significant change in the 5-15 cm HF-14C.  Coarse charcoal Δ14C from the burned-plot soils indicated large inputs of wood >50 years old.  Mineral stabilization of fine charcoal from the surface could be responsible for C gains in the HF, and for observed 14C-depletion in the upper-soil burned-plot HF.  The Δ14C of 5-15 cm HF was similar to coarse charcoal, possibly explaining the lack of change in Δ14C even with large C gains.  Because charcoal is one of the more chemically recalcitrant forms of C in soil, and mineral stabilization tends to increase the mean residence time of soil C, it's likely that the C in these post-fire soils is substantially more stable than the pre-fire C distribution.