WILDFIRE IMPACTS ON SOIL CHEMISTRY AND THE ROLE OF PYROGENIC CARBON IN SOIL MINERAL TRANSFORMATION

Climate change drives an increase in the frequency, intensity, and extent of wildfire in the Western United States. Of the approximately 2.2 Pg of C emitted annually by wildfires, an estimated 12% is offset by deposition of pyrogenic organic carbon (PyOC). PyOC is suggested to be considerably resistant to degradation, a property of PyOC that has sparked interest regarding its ability to act as a carbon-sink. However, a relatively unexplored factor in this consideration is the effect PyOC has on soil mineral properties. Due to its high redox activity, a number of studies have suggested that PyOC is utilized by microbes in soil to reduce Fe(III) and induce changes to soil Fe-mineralogy.

Here, we investigate the impacts of fires on soil reactive components by linking post-burn soil and water chemistry from the Kwis and Beachie Creek Fires in the Oregon Cascades. Soils were collected across a burn severity gradient controlling for vegetation, lithology, and climate. Surface waters were collected within burnt watersheds, or upstream and downstream of burnt areas. Soils were analyzed for organic carbon content, δ13Corg, major element geochemistry, and the distribution of soil reactive metals were quantified using a three-step leaching method. Waters were analyzed for major and trace metals, major anions, alkalinity, and dissolved organic carbon (DOC). Our results suggest clear evidence of enhanced microbial activity and the reduction and subsequent leaching of ferric oxyhydroxides from soils, corroborated by waters with elevated levels of DOC and arsenic. With recent advances suggesting the importance of soil mineralogy on the ability of soils to sequester carbon, these results highlight the need to account for post-fire changes to soil reactive minerals in considerations of PyOC as a C-sink.