Permafrost is prone to thawing under disturbances resulting from frequent wildfires in boreal forests due to climate change, increasing the risk of the release of carbon (C) from it. Although the decomposition of organic C is mainly determined by the activity of soil microorganisms, productions of pyrogenic material after fire may offset this process. To evaluate the C dynamics related to wildfire disturbance in permafrost regions, this study examined the postfire changes in soil organic matter (SOM) decomposition, and the microbial community composition and its potential functions during a > 100-year chronosequence of burnt boreal forests.
Based on the kinetic theory, the temperature sensitivity of slowly decomposing SOM tends to be higher than that of easily decomposing SOM. Consistently, we found the decomposition of SOM in burnt surface soils containing less-decomposable SOM generated by fire, was more sensitive to temperature than that in old-growth forests. Fire also decreased the microbial biomass and the fungal-to-bacterial ratio of the surface soils. Despite this, soil heterotrophic respiration and the microbial C:N:P ratio in burnt forests remained similar level to that in old-growth forests regardless of the changing SOM quality and quantity. This suggests the notion of a lower microbial C use efficiency following a fire. Unexpectedly, permafrost thaw did not alter the microbial biomass and the fungal-to-bacterial ratio, but increased the microbial metabolic quotient.
Illumina Miseq sequencing of bacterial 16S rDNA revealed that the bacterial community composition in recently burnt surface soils differed from it in old-growth forest soils. Permafrost thaw, however, showed little effect on the bacterial community composition. Bacterial communities of burnt surface-soil exhibited higher abundance of Ktedonobacteria (Chloroflexi) but lower abundance of Betaproteobacteria. Functional gene compositions (DNA-based) of the burnt surface soil differed from those of the unburnt ones; particularly for genes coding for C degradation and the nitrogen cycle. Yet, the difference in the frequency of genes responsible for C degradation between thawed and frozen permafrost was not statistically significant.
This thesis provides further evidence for effects of wildfire on the microbial biomass, microbial community composition, and its potential functions on the C and N cycles in boreal permafrost regions. To estimate the real pattern of soil C cycles following a fire, we must understand how fire affects the metabolic processes of soil microorganisms.Permafrost is prone to thawing under disturbances resulting from frequent wildfires in boreal forests due to climate change, increasing the risk of the release of carbon (C) from it. Although the decomposition of organic C is mainly determined by the activity of soil microorganisms, productions of pyrogenic material after fire may offset this process. To evaluate the C dynamics related to wildfire disturbance in permafrost regions, this study examined the postfire changes in soil organic matter (SOM) decomposition, and the microbial community composition and its potential functions during a > 100-year chronosequence of burnt boreal forests.
Based on the kinetic theory, the temperature sensitivity of slowly decomposing SOM tends to be higher than that of easily decomposing SOM. Consistently, we found the decomposition of SOM in burnt surface soils containing less-decomposable SOM generated by fire, was more sensitive to temperature than that in old-growth forests. Fire also decreased the microbial biomass and the fungal-to-bacterial ratio of the surface soils. Despite this, soil heterotrophic respiration and the microbial C:N:P ratio in burnt forests remained similar level to that in old-growth forests regardless of the changing SOM quality and quantity. This suggests the notion of a lower microbial C use efficiency following a fire. Unexpectedly, permafrost thaw did not alter the microbial biomass and the fungal-to-bacterial ratio, but increased the microbial metabolic quotient.
Illumina Miseq sequencing of bacterial 16S rDNA revealed that the bacterial community composition in recently burnt surface soils differed from it in old-growth forest soils. Permafrost thaw, however, showed little effect on the bacterial community composition. Bacterial communities of burnt surface-soil exhibited higher abundance of Ktedonobacteria (Chloroflexi) but lower abundance of Betaproteobacteria. Functional gene compositions (DNA-based) of the burnt surface soil differed from those of the unburnt ones; particularly for genes coding for C degradation and the nitrogen cycle. Yet, the difference in the frequency of genes responsible for C degradation between thawed and frozen permafrost was not statistically significant.
This thesis provides further evidence for effects of wildfire on the microbial biomass, microbial community composition, and its potential functions on the C and N cycles in boreal permafrost regions. To estimate the real pattern of soil C cycles following a fire, we must understand how fire affects the metabolic processes of soil microorganisms
