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The role of erosion in soil organic matter and pyrogenic carbon dynamics in fire-prone temperate forests


Wildfire and erosion are major perturbations to the global carbon cycle in dynamic, fire-affected ecosystems around the world, including temperate forest ecosystems in the Sierra Nevada. As a byproduct of fires, pyrogenic carbon (PyC) is formed due to incomplete combustion of biomass. PyC constitutes an important component of the soil carbon pool and has been noted for its long residence time in soil and its susceptibility to erosion. As part of my dissertation research, I determined the rate of PyC, bulk soil carbon, and other soil constituents erosion after two wildfires: the Gondola Fire that occurred in South Lake Tahoe in 2002, and the Rim Fire that affected parts of Yosemite National Park in 2013. I found significant and preferential erosion of PyC, and vertical mobilization of PyC down into the soil profile after the fires. The preferential erosion of PyC, and overall quality of the soil and eroded sediments were controlled by burn severity, with PyC from higher burn severity sites being more preferentially eroded. To assess the fate of PyC post-fire in dynamic landscapes, I incubated chars formed at different temperatures in soils from eroding and depositional landform positions. Both charring temperature and landform position played significant roles in controlling soil respiration, with the lower temperature chars and the soil from the depositional landform position having much higher respiration than higher temperature chars and the soil from the eroding landform position. The difference in breakdown rates of PyC in soil from different landform positions demonstrates the importance of considering landform position as a control on PyC persistence in soil and that the interaction between charring temperature and landform position plays a significant role in the persistence of PyC. The post-fire erosional transport of PyC may act in a feedback to enhance or decrease overall PyC and bulk carbon stocks in soil. In a modeling exercise, I showed that explicit consideration for erosional loss (from eroding slope positions) and depositional gain (in lower-lying depositional landform positions) of PyC in soil can have its mean residence time in soil. I found that ignoring the role of erosional lateral distribution on PyC dynamics can introduce error in estimated turnover times of up to 150 years. Among the major accomplishments of my dissertation project include the realistic integration of biogeochemical and geomorphological approaches to derive improved representation of mechanisms that regulate soil carbon persistence in dynamic landscapes that routinely experience more than one perturbation. Findings from my dissertation research will have far reaching implications for improving our understanding of fate of terrestrial carbon after it enters streams and other aquatic systems. Furthermore, results of this project will play important role in establishing how the interaction of fire and erosion will play out under anticipated climate change scenarios, and the implications of these interactions on biogeochemical cycling of essential elements in a warmer world with intensified hydrologic cycle.

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