Alterations to Soil and Eroded Sediment Carbon after the Rim Fire, Yosemite National Park

Abstract:

The soil system is a critical global carbon (C) pool that is under threat from both fire and erosion perturbations. In the Western United States, forecasted increases in average temperatures, as well as for extended summer growing seasons, indicate that large fuel loads and ideal fire conditions may generate more high intensity wildfires. Understanding how wildfires control soil C storage is critical for both projecting losses of soil C and how to better manage fire regimes to increase soil C storage. This research addresses a topic that has only been briefly addressed by the current body of literature – the erosion of pyrogenic C, or C which has undergone some combustion, which has the potential to affect storage of C within the soil system. The Rim Fire was a wildfire that consumed over 250,000 acres of land in Yosemite National Park and Stanislaus National Forest in 2013. After the fire, sediment traps were established on a hillslope under three treatment conditions: 1) high burn severity — high slope; 2) high burn severity — moderate slope, and; 3) moderate burn severity — high slope. Sediments were collected from these traps after every major precipitation event following the Rim Fire, Additionally, representative soils were collected from the source areas of the eroded material. Differences in chemical composition of organic matter and concentration of pyrogenic matter were determined using ¹³C Cross-Polarization Magic Angle Spinning (CPMAS) Nuclear Magnetic Resonance (NMR) spectroscopy, in addition to elemental and stable isotope analysis of carbon and nitrogen. Our results show that carbon eroded from areas of higher burn intensity generally had higher concentration of aromatic functional groups, compared to moderate burn intensity areas. Differences in the form of C eroded from areas of different burn intensity and slope steepness can be used as a proxy for determining how fire severity and geomorphology dictate the amount and nature of C eroded from burned soils as sites recover.