Microbes and Post-Burn Carbon Cycling in Boreal Forest Soils.

다국어 초록 (Multilingual Abstract)

An ongoing shift in wildfires regimes in many areas of the boreal forests of North America towards more frequent and severe fires raises questions about the fate of the vast reservoirs of soil carbon (C) held in these ecosystems. Wildfires have a wide range of effects on boreal forest ecosystems including combustion of organic matter during the fire itself, shifts in aboveground plant community composition, changes in soil C substrate chemistry and availability, and shifts in microbial community composition. There is a growing interest in how fire-induced changes in microbial community composition may impact functions, such as respiration and carbon use efficiency, and post-fire soil C cycling. Disentangling the impacts of fire and varying fire severity on soil microbial respiration and carbon use efficiency is important for both understanding the mechanisms by which fire affects soil C cycling and for improving our model-based predictions of post-fire C emissions. In Chapter 2, I investigate the effect of fire and fire-induced changes in soil properties on post-fire soil respiration and explore the implications of reduced post-fire respiration rates on long term soil C stocks. Burning caused a decrease in respiration per gram total (post-burn) C driven by fire-induced changes in the soil C pool, indicating that soil C lost during a burn may be partially offset by burn-induced decreases in respiration rates. In Chapter 3, I use metrics of resistance and resilience to explore the effects of burning on soil microbial community composition. Microbial community resistance and resilience to burning varied across soil types and between fungi vs. bacteria highlighting the complexity of burn effects on microbial communities. In Chapter 4, I explore how burning and varying burn duration impact microbial community function as measured by changes in substrate-specific carbon use efficiency in boreal forest soils. Burning caused large decreases in glucose-specific carbon use efficiency primarily driven by decreases in microbial growth. The effect of burning on the carbon use efficiency of ground pine roots was smaller, which suggests that the effects of fire on community-level carbon use efficiency will depend on the availability of various C substrates. In Chapter 5, I use the Carbon, Organisms, Rhizosphere, and Protection in the Soil Environment (CORPSE) model to investigate the role of microbial functional groups in modulating post-fire soil respiration. I expanded the CORPSE model by splitting the microbial biomass pool into two discrete microbial functional groups - fast-growing and slow-growing taxa - each with unique decomposition abilities - and added a pyrogenic C pool. While adding multiple microbial functional groups did not improve model agreement with laboratory and field data, the 2-functional pool model allowed us to observe temporal changes in average community carbon use efficiency. Variation in microbial carbon use efficiency may be an important part in linking post-fire microbial community recovery to the fate of post-fire soil C stocks and fluxes. In conclusion, these findings imply that C storage in boreal forests following wildfires will be driven by the combination of C losses during the fire itself, fire-induced changes to the soil C pool that modulate post-fire respiration rates, and shifts in microbial community function, all of which are dependent to some degree on fire severity. Moving forward, more work is needed to expand this work beyond fire severity to other changes in predicted future fire regimes in the boreal forest, such as decreased fire return intervals and larger fire size.