IMPACT OF WILDFIRE SEVERITY ON SOIL BIOGEOCHEMISTRY FOLLOWING WILDFIRE IN THE CALIFORNIA SIERRA NEVADA

Wildfire has cascading impacts on the biogeochemistry of affected landscapes and ecosystems. As the frequency and size of high severity wildfire increases, further research is needed to understand how fire severity affects the biogeochemical impact of wildfire. Here, we explore the impact of fire severity on burnt landscapes by analyzing ash (where present), litter, and soil (0-5 cm and 5-10 cm depths) from high, moderate, and low severity burns, and unburned sites within two recent wildfires in the California Sierra Nevada. The KNP Complex fire burned 88,307 acres in Sequoia National Park in fall of 2021 and samples were collected immediately following the fire and 6 months postfire. The French Fire burned 26,535 acres in the Sequoia National Forest in the Fall of 2021; samples were collected immediately following the fire, one month postfire, and 6 months postfire. Using samples from both fires, we quantified carbon and nitrogen content (litter and soil), carbon and nitrogen stable isotopes (from ash, litter, and soil), elemental concentration of major and minor trace elements using X-Ray fluorescence (from ash and soil), and changes in chemical composition of soil using Fourier transformed infrared spectroscopy. Our results show an inverse relationship between fire severity and carbon and nitrogen content. Impacts of wildfire on soil carbon and nitrogen content and stable isotope compositions persisted for at least six months postfire. Wildfire impacts the elemental composition of topsoils (0-5 cm), yet these impacts vary between the two fires; for example Ca is depleted in burnt soils relative to unburned soils in the French Fire, yet shows the opposite trend within the KNP Complex Fire. Outcomes of this work will inform our understanding of biogeochemical change across a gradient of fire severity and identify the impacts of wildfire and postfire precipitation on the geochemistry of forest soils.