M. Derek MacKenzie1, Jan O. Skjemstad2, Robert Graham3, and Sylvie Quideau1. (1) Renewable Resources, 442 Earth Science Building, Univ. of Alberta, Edmonton, AB T6G 2E3, Canada, (2) CSIRO Land and Water, PMB No. 2, Glem Osmond, Australia, (3) Dept. of Soil & Environ. Sci., Univ. of California, Riverside, CA 92521-0424
Most forest ecosystems of western North America are pyrogenic and have burned at different intervals since the last glacial period (ca 15 K yrs. BP). However, recent forest management and fire exclusion have effectively removed fire from the landscape and many stands have missed multiple disturbance events. Fire has been shown to increase N availability, but the exact mechanism by which this occurs remains elusive. It is believed to be mediated by charcoal, the long-term residue of fire, and not the short-term thermal pulse of the disturbance itself. Charcoal also represents a highly recalcitrant sink for C, but very little is known of the amount and spatial distribution of charcoal on the landscape. We collected soil samples with a spatially explicit sampling protocol from three different forest ecosystems in the Sierra Nevada of California. Samples were collected along an elevation gradient from oak woodlands at low elevation (300-500 m), to mixed conifer forest at mid elevation (500-1500 m), to pure red-fir stands at high elevation (1500-2500 m). We compared a technique using mid-infrared diffuse reflectance Fourier transform (DRIFT) spectra and partial least squares (PLS) regression, which determines a host of parameters including charcoal, with a digestion that uses 30% H2O2 and 1 M HNO3 acid, which determines charcoal C. NMR analysis was also used for calibration. Preliminary results indicate that DRIFT PLS analysis is less time consuming than wet chemistry and NMR, and extremely precise. The charcoal content increases with elevation, likely as a result of higher biomass and more frequent burning. Opposite to our expectations, there was no spatial autocorrelation on the oak woodland or mixed conifer sites, but there was significant autocorrelation on the red-fir sites, giving a patchy distribution reminiscent of the forest structure.
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