HR: 1340h
AN: B23A-0943    [Abstracts]
TI: Seasonal Changes of Soil Respiration Sources in a Boreal Forest
AU: * Czimczik, C I
EM: czimczik@uci.edu
AF: University of California, Department of Earth System Science 2103 Croul Hall , Irvine, CA 92697-3100 United States
AU: Trumbore, S E
EM: setrumbo@uci.edu
AF: University of California, Department of Earth System Science 2103 Croul Hall , Irvine, CA 92697-3100 United States
AB: We studied how seasonal changes in soil temperature and moisture during the growing season (May-September) control soil respiration and potential sources (roots and microbial) at different forest recovery stages after fire. We investigated six black spruce stands in the BOREAS Northern Study Area in Canada (55N, 98W) on clay soils with underlying permafrost covering 0-150 yrs since fire. We measured the rate of CO$_{2}$ respired at the soil surface and CO$_{2}$ concentrations at various soil depths and its isotopic composition with dynamic chambers and soil probes. We sampled monthly, in 2004 we started automated continuous measurements. We determined the $^{14}$C signature of root-respired CO$_{2}$ in field incubations and that of microbial-respired CO$_{2}$ in laboratory incubations. The $\Delta$$^{14}$C signature of CO$_{2}$ was determined with accelerator mass spectrometry. In both years CO$_{2}$ fluxes from the soil surface increased from 6-21 mg C m$^{-2}$hr$^{-1}$ in spring to 35-135 mg in summer, and decreased to 25-65 mg in autumn. Fluxes were highest 40 yrs since fire. In early recovery stages burning intensity is a confounding factor to stand age, affecting the available C source and soil temperature and moisture. The $^{14}$C signature of the soil respired CO$_{2}$ followed the same trend with time since fire in both years. In early spring, fluxes were dominated by microbial respiration, the fraction of root respiration increased throughout the growing season. In autumn of the warm year 2003, soils of early recovery stages became a source of old mineral soil carbon to the atmosphere, while root respiration dominated in older stands. The incubations suggest that microorganisms within certain depth of the organic layer respire CO$_{2}$ with a constant $\Delta$$^{14}$C signature. However, increases in CO$_{2}$ fluxes at each depth with seasonal increases in temperature and moisture change the $\Delta$$^{14}$C of the CO$_{2}$ measured at the soil surface. Microbial respiration and its temperature and moisture response were highest in fresh litter. In young recovery stages, the dominant source of respiration was none of the mass dominating fractions (needles, moss, wood, roots, amorphous org. matter), and could potentially be dissolved organic C.
DE: 1600 GLOBAL CHANGE (New category)
DE: 0315 Biosphere/atmosphere interactions
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting