Altered fire regime impacts on the soil biogeochemistry and microbial community structure of mixed conifer and ponderosa pine forests

Fire exclusion in the early 1900s altered forest composition and structure by increasing shade-tolerant species and creating a continuous fuel structure. This increased fire severity and extent throughout many forests. Climate change models suggest that fire severity and extent will continue to rise. To mitigate this risk, managers are conducting burns during wetter seasons. The effects of these atypical fires on ecosystem structure and function are unknown.; This dissertation assesses the impact of fire season and severity on soil biogeochemistry and microbial community of two forests. To assess effects of fire season on soil systems, I studied sites subjected to early season and late season fire in a mixed-conifer forest of Sequoia National Park, CA. Results showed that late season fires had larger, more persistent impacts on the soil than early season fires, altering the soil environment and decreasing biotic activity for 2-3 years.; Inorganic nitrogen is difficult to measure, due to complex processes impacting net mineralization rates and methodological artifacts. To identify some problems associated with two techniques and to assess the sensitivity to environmental variables, I evaluated fire treatment effects on net nitrogen mineralization rates as measured by the soil core incubation method and the ion exchange membrane (IEM) method. The methods were not significantly correlated. The core incubation method showed no change in net mineralization and a two-fold increase in net nitrification with fire while the IEM technique showed a 3-fold increase in net mineralization and a 4-fold increase in nitrification with fire. The IEM method was more sensitive to the soil environment than the core incubation method.; To evaluate fire severity impacts on soil microbial communities, I established sites in ponderosa pine forests subjected to low severity, high severity, and no fire in Pike National Forest, CO. The different severities differentially impacted the soil environment (pH, temperature, moisture). Soil microbial communities from low and high severity sites were not distinct, however, they were both different from the unburned sites. Microbial biomass and richness were not different among treatments, suggesting a quick recovery of a structurally distinct microbial community in both low and high severity burn sites.