Mapping Soil Organic Carbon in Wildfire-Affected Areas of the McKenzie
River Basin, Oregon, USA
Large-scale wildfires are increasing in frequency and are likely to
become more severe under future Pacific Northwest climate scenarios. The
effects of wildfires on soil organic carbon (SOC) remain difficult to
estimate because soil heterogeneity limits generalizations. We mapped
fired severity within the footprint of the Holiday Farm Fire (McKenzie
River, Oregon, 2020) and sampled a burn severity gradient (unburned,
low, high) in a detailed scheme to account for inter- and intra-site
variation (20 soil profiles/half-hectare for burned sites, 9/hectare for
unburned) at three depths (0-2 cm, 2-20 cm, 20-40 cm). We measured total
SOC, mineral-associated organic carbon (MAOC), particulate organic
carbon (POC), and pyrogenic carbon (PyC). We found significant SOC
differences in the high severity fire in most carbon pools and depths,
with the largest total SOC decrease of 6.48% (56% change) in 0-2 cm.
Compared to unburned, the low severity site had higher MAOC (0-2 cm:
+0.48%, 22% change; 2-20 cm: +0.28%, 17% change) and significantly
lower POC (0-2 cm: -5.12%, 54% change; 2-20 cm: -1.73%, 48% change).
We found lower PyC in burned sites, indicating combustion of this pool.
SOC stocks at 0-20 cm were higher in low severity (total SOC: +7.45
kg/m2, 71% change; MAOC: +4.81 kg/m2, 153% change) compared to
unburned. There was remarkable variation within each site, but the
consistent high levels of MAOC in low severity area support prescribed
burning as a technique to mitigate wildfire risk while limiting losses
or increasing SOC compared to high severity fires.