EROSION RATE INFLUENCE ON SOIL ORGANIC CARBON FRACTIONS IN A BURNED MONTANE LANDSCAPE (Invited Presentation)

The significance of soil organic carbon (SOC) in the carbon cycle and the use of SOC in climate solutions and Earth system models have been well established. With increasing wildfire frequency and severity in the western USA quantifying the degree to which these perturbations influence soil organic carbon stocks and hillslope processes is integral in understanding landscape evolution and ecological response to climate change. Previous work demonstrates that wildfire results in the creation of pyrogenic carbon (PyC), combustion and loss of SOC, and redistribution of SOC. On hillslopes, SOC stocks are composed of varying proportions of particulate organic carbon (POC), which is more sensitive to fire, and mineral associated organic carbon (MOC), which is longer-lived and less sensitive. Soil properties established over millennia appear to impart a strong control on these SOC stocks, yet, we lack a mechanistic way to map the relative proportions of MOC and POC, and thus gauge the influence of fire in forested steeplands. A prominent knickzone in the 2013 Douglas Fire area in southern Oregon separates gentle (slow-eroding) and steep (fast-eroding) terrain creating a novel opportunity to analyze how topography and soil properties set up by soil production, weathering, and erosion influence the relative proportions of MOC and POC along a strong erosional gradient. Sites below the knickzone, with fast eroding steep hillslopes, have sharp hilltops and thin, coarse, relatively unweathered soils not conducive to storing MOC. By contrast, hilltops above the knickpoint are broad with gentle slopes and thick soils with greater accommodation space for SOC (and integration of PyC). Furthermore, the abundance of secondary minerals, clay, and soil aggregates on slow-eroding sites facilitates MOC storage. Additionally, in bare earth DEMs of difference, we see greater surface lowering on steep slopes and sediment excavation events in steep, unchanneled valleys, indicating these regions may redistribute more SOC following fire than gentle regimes above the knickzone. Quantifying how SOC socks vary with erosion rate is essential for increasing the accuracy of global carbon models and assessing the role of wildfire.