We used radiocarbon dating of the organic soil surface remaining post-fire to examine whether the fire burned into ancient and likely irreplaceable soil C pools. Suprisingly, it did not; all radiocarbon dates from burned soil surfaces contained bomb carbon, setting the maximum age of the burned soil surfaces at ~50 years.
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We used radiocarbon dating of the organic soil surface remaining post-fire to examine whether the fire burned into ancient and likely irreplaceable soil C pools. Because the moss and organic soil layer accumulates vertically through time, the radiocarbon values of the remaining organic matter can provide some indication of how long it took to accumulate the organic matter above it that was consumed during the fire. Two cores were selected from each of ten sites randomly selected from the 20 sampling sites and sliced into 1 cm depth increments. Moss macrofossils were removed from the top layer under a dissecting scope. Cellulose was extracted from the macrofossils45 and the sample was converted to graphite at 650C with an iron catalyst in a hydrogen atmosphere46. Our primary standard for 14C analysis was NIST oxalic acid II (SRM 4990C, National Institute of Standards and Technology). For a secondary standard, we used IAEA-C6 sucrose standard (International Atomic Energy Agency, Vienna, Austria). Our blank was anthracite coal cleaned with a standard acid–base–acid treatment. All standards and blanks used for 14C measurements were combusted and purified similarly to the samples. The 14C content of the graphite was measured at the W. M Keck Carbon Cycle Accelerator Mass Spectrometry Laboratory at the University of California–Irvine.
Radiocarbon data are expressed as 14C, which is the per mil (‰) difference between the 14C/12C ratio of the sample and an international standard. Each sample was corrected for the effects of mass-dependent isotope fractionation of 14C values by correcting to a constant 13C value of -25 ‰ and normalized to sample-specific measured 13C values. 13C was measured prior to graphitizing the CO2 at the University of Florida on a Delta XL isotope ratio mass spectrometer with a gas bench peripheral (Thermo Scientific, Waltham, MA), using a
-10.46 ‰ PDB CO2 standard from Oztech (Stafford, Arizona, USA). In some circumstances, radiocarbon values during the bomb period (1950 to present) do not return unequivocal date estimates because of the shape of the atmospheric bomb curve (ref. 47). In a subset of those cases, we also analyzed the second, deeper 1 cm increment of organic soil for radiocarbon in order to determine the appropriate location of the surface date relative to the bomb peak. All samples had radiocarbon values above zero, so samples where the deeper segment was more enriched in radiocarbon then the surface segment were assigned to the ascending slope of the bomb peak (<1966), while samples where the deeper segment was more depleted than the surface segment were assigned to the descending slope (>1966).
45Gaudinski, J. B., Dawson, T. E., Quideau, S., Schuur, E. A. G., Roden, J. S., Trumbore, S. E. et al., Comparative analysis of cellulose preparation techniques for use with C-13, C-14, and O-18 isotopic measurements. Analytical Chemistry 77 (22), 7212-7224 (2005).
46Vogel, J. S., Southon, J. R., and Nelson, D. E., Catalyst and binder effects in the use of filamentous graphite for AMS. Nuclear Instruments and Methods in Physics Research Section B-Beam Interactions with Materials and Atoms 29 (1-2), 50-56 (1987).
47Levin, I. and Hesshaimer, V., Radiocarbon - a unique tracer of global carbon cycle dynamics. Radiocarbon 42 (1), 69-80 (2000).
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