Mapping Soil Organic Carbon in Wildfire-Affected Areas of the McKenzie River Basin, Oregon, USA

Abstract

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.

The influence of burn severity on dissolved organic carbon concentrations across a stream network differs based on seasonal wetness conditions post-fire

“…So, a decrease in soil organic matter and loss of vegetation post-fire would likely mute this flushing behavior, resulting in decreased DOC during the wetting period. A post-fire soil study in the McKenzie sub-basin found that total soil organic carbon was decreased along a burn severity gradient (Katz et al, 2023). Other studies have also proposed decreased soil organic stocks as the cause of decreased DOC observed post-fire in California and Alaska, USA (Betts and Jones, 2009;Santos et al, 2019).…”

“…However, this was likely not the case in our basin due to the mixed severity burn of the Holiday Farm fire, including ~63 % of the area burning at moderate to high burn severity. Moreover, research in high burn severity sites across the Holiday Farm fire area, found evidence of decreases in the percentage of soil organic carbon in the first 2 cm of soil (56 % change) and in the particulate organic carbon fraction (61 % change, Katz et al, 2023). The lack of a fire signal on the DOC concentrations may be related to the thick and active organic horizons and extremely high rates of saturated hydraulic conductivity in soils of the Pacific Northwest (Jarecke et al, 2021), which would have enabled infiltration and vertical percolation of water and DOC through the soil profile despite the burn.…”