UC Irvine

Understanding the response of soil microbial communities and decomposition to global environmental changes is central to our ability to accurately forecast future terrestrial carbon (C) storage and atmospheric CO₂ levels. Increases in the frequency and severity of disturbance events are one element of global change in terrestrial ecosystems. The goal of this dissertation was to measure the response of soil microbial communities and decomposition to disturbance events and to examine the mechanisms underlying post-disturbance changes in decomposition. In the first part of my dissertation work I explored these questions within the context of wildfires in boreal forests. Chapter 1 characterized soil microbial communities and the rate of decomposition across a fire chronosequence in interior Alaska. I found that boreal forest fires reduced soil microbial abundance, altered fungal community composition, and suppressed litter decomposition. Chapter 2 investigated whether soil microbial responses to boreal forest fires differ as a function of fire severity. I demonstrated that higher severity fires elicited greater reductions in soil microbial biomass and larger shifts in fungal community composition than lower severity fires. Chapter 3 tested the mechanisms through which boreal forest fires alter decomposition processes. I discovered that decomposition rates were slower in recently burned forests because of post-fire reductions in soil moisture and C substrate quality.

In the second part of my dissertation I expanded my findings to other types of disturbance events using meta-analysis. Chapter 4 reviewed the response of soil microbial biomass to fires. I found that soil microbial biomass was significantly lower in recently burned ecosystems, but the response of microbial biomass to fire differed by fire type and biome. Chapter 5 examined soil microbial responses to abiotic (fire, harvesting, storms) and biotic (insect infestation, pathogen outbreaks) disturbances in forests. I observed that abiotic disturbances significantly reduced soil microbial biomass, while changes in microbial biomass were non-significant following biotic disturbance events. Collectively, these findings suggest that reductions in soil microbial biomass and decomposition rates following abiotic disturbances are likely to slow the transfer of C from soils to the atmosphere and provide a negative feedback to rising atmospheric CO₂ concentrations and global change.