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The Influence of fire on the Radiocarbon Signature and Character of Soil Organic Matter in the Siskiyou National Forest, Oregon, USA

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https://ir.library.oregonstate.edu/concern/articles/0z708x025

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  • Forest fires contribute a significant amount of CO₂ to the atmosphere each year, and CO₂ emissions from fires are likely to increase under projected conditions of global climate change. In addition to volatilizing aboveground biomass and litter layers, forest fires have a profound effect on belowground carbon (C) pools and the cycling of soil organic matter as a whole. However, the influence of fire on belowground organic matter cycling is not well defined and varies widely with fire severity. We measured soil organic matter (SOM) characteristics across a range of fire severities two years after the 2002 Biscuit Fire in southwest Oregon, USA, to address the following questions: (1) Which C pools are preferentially volatilized or transformed to charcoal under low-severity and high-severity fire? (2) How does fire change the distribution of SOM among density fractions and depths? (3) How does fire affect the general character of SOM including such variables as abundance, C:N ratio, C¹³ abundance, and C¹⁴ abundance? We examined soils from a mixed hardwood-evergreen forest across a range of burn severities: unburned, low severity, mixed severity, and high severity. Results indicated that increasing burn severity led to progressive loss of forest floor mass, but not to progressive loss of C from the mineral soil. Although fire significantly increased the charcoal content of the soils, fire did not significantly change the distribution of soil organic matter between heavy and free or light fractions. Other significant changes in soil organic matter characteristics included a progressive increase in nitrogen (N) with increasing burn severity, possibly due to the encroachment of N-fixing shrubs following the loss of native vegetation. Although qualitative changes in total root abundance following fire were noted, differences among burn severity treatments were not statistically significant. Increased concentrations of rock fragments in burned areas may be suggestive of erosion in these areas, consistent with previous studies documenting varying degrees of soil erosion following fire. In addition, although C¹³ abundances were similar among severely burned and unburned plots, soils from severely burned plots were significantly depleted in C¹⁴ in comparison to soils from unburned plots. This C¹⁴ depletion is most likely the combined result of erosion and preferential combustion of organics enriched in C¹⁴ relative to the bulk soil, perhaps reflecting a historical pattern of fire occurrence and severity across the landscape.
  • Keywords: charcoal, density separation, Biscuit Fire, wildfire, radiocarbon, soil organic matter
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  • Heckman, K., Campbell, J., Powers, H., Law, B., & Swanston, C. (2013). The influence of fire on the radiocarbon signature and character of soil organic matter in the siskiyou national forest, oregon, usa. Fire Ecology, 9(2), 40-56. doi:10.4996/fireecology.0902040
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  • 9
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  • 2
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  • This research was supported by the Office of Science (Biological and Environmental Research), US Department of Energy (DF-FG02-04ER63917, and DE-FG02-07ER64361). A portion of this work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, LLNL-JRNL-567353.
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