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.