Forest ecosystems are important reservoirs for long term carbon (C) storage. Forests of the western United States account for 20-40% of total U.S. carbon C sequestration, and nearly half of the total C in these forests is stored in soil. However, many forests in the western U.S are experiencing wildfire conditions that diverge from historical fire regimes. Prior to Euro-American settlement, California’s mixed-conifer forests typically experienced frequent surface fires of low to moderate burn severity, but, due to the combined effects of altered forest structure and climate change, now experience fires that are larger and more severe than historical conditions. Fires have numerous direct and indirect effects on the soil biological, chemical, and physical characteristics that influence the soil C cycle. Understanding how altered soil characteristics influence the cycling and persistence of soil C, and how they vary with severity, is important for managing forests for C storage and for predicting fire-climate feedbacks. My dissertation work incorporates observational and manipulative experiments to understand the direct and indirect effects of burn severity on soil C cycling and microbial communities over the short to intermediate term, with a particular focus on the distribution of soil C between active and slow cycling pools. Soil C can be conceptualized as discrete pools of variable persistence in soil. The active C pool is quickly decomposed, contributing to the return of CO2 to the atmosphere, whereas the non-active C pool is more stable and contributes to long term C storage. I leveraged a burn severity gradient resulting from a wildfire in a California mixed-conifer forest to determine the structure and kinetics of these C pools at an intermediate time point in post-fire recovery (i.e. three years). I found that the size of the non-active C pool was smaller in burned areas than unburned areas, and the kinetic rate of the non-active C pool was negatively related to burn severity. I also characterized the soil microbial communities across this severity gradient and identified the environmental characteristics responsible for differences. I found that fungal-to-bacterial ratio and oligotroph-to-copiotroph bacteria ratio decreased with burn severity, and these effects were driven by differences in live and dead tree basal area, soil nutrients, and pH. Leveraging another burn severity gradient, I then determined whether differences in microbial communities and soil C pools were related one-year post-fire in a mixed-conifer forest. I again found lower non-active C pool kinetic rates, and higher abundances of copiotrophic bacteria in burned compared to unburned areas. Differences in soil C pool kinetics were related to tree basal area, soil nutrients, and bacterial communities. I determined the short-term impacts of fire on soil C pools and cycling using lab experiments in which I manipulated soil heating intensity and pyrogenic organic matter (PyOM) additions. I found that high intensity soil heating can decrease microbial biomass C (MBC) accumulation, whereas PyOM had minimal effects on MBC in the short-term. Finally, I found that the size of the active C pool increased with soil heating intensity, while the kinetic rate of the non-active C pool decreased; PyOM primarily increased the size of the non-active C pool. Taken as a whole, my research suggests that fire induces short-term soil C losses by increasing the size of the active C pool, but, over the intermediate-term, residual soil C is more persistent. Fire severity is predicted to increase globally throughout the 21st century, and my research contributes to understanding how forest C storage will be affected by disrupted wildfire regimes.
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