Pyrogenic carbon erosion after the Rim Fire, Yosemite National Park: The Role of Burn Severity and Slope

Abstract

AbstractPyrogenic carbon (PyC) is an incomplete combustion by‐product with longer soil residence times compared with nonpyrogenic components of the soil carbon (C) pool and can be preferentially eroded in fire‐affected landscapes. To investigate geomorphic and fire‐related controls on PyC erosion, sediment fences were established in three combinations of slope (high 13.9–37.3%; moderate 0–6.7%) and burn severity (high; moderate) plots within the perimeter of the Rim Fire in 2013, Yosemite National Park, California, USA. After each major precipitation event following the fire, we determined transport rates of total sediment, fine and coarse sediment fractions, and C and nitrogen (N). We measured stable isotope (δ13C and δ15N) compositions and 13C‐nuclear magnetic resonance spectra of soils and eroded sediments. The highest total and fine (<2 mm) sediment transport in high severity burned areas correlated with initial discharge peaks from an adjacent stream, while moderate burn severity sites had considerably more of the >2 mm fraction transported than high burn severity sites. The δ13C and δ15N values and 13C‐nuclear magnetic resonance analyses indicated that sediment eroded from moderate severity burn areas included fresh organic matter that was not as significantly affected by the fire, whereas sediments from high severity burn areas were preferentially enriched in PyC. Our results indicate that along a single hillslope after the Rim Fire, burn severity acted as a primary control on PyC transport postfire, with slope angle likely playing a secondary role. The preferential erosion of PyC has major implications for the long‐term persistence of PyC within the soil system.