abstract
Climate warming is projected to increase the frequency and severity of wildfires in boreal forests, and
increased wildfire activity may alter the large soil carbon (C) stocks in boreal forests. Changes in boreal
soil C stocks that result from increased wildfire activity will be regulated in part by the response of
microbial decomposition to fire, but post-fire changes in microbial decomposition are poorly understood.
Here, we investigate the response of microbial decomposition to a boreal forest fire in interior Alaska and
test the mechanisms that control post-fire changes in microbial decomposition. We used a reciprocal
transplant between a recently burned boreal forest stand and a late successional boreal forest stand to
test how post-fire changes in abiotic conditions, soil organic matter (SOM) composition, and soil microbial
communities influence microbial decomposition. We found that SOM decomposing at the burned
site lost 30.9% less mass over two years than SOM decomposing at the unburned site, indicating that
post-fire changes in abiotic conditions suppress microbial decomposition. Our results suggest that
moisture availability is one abiotic factor that constrains microbial decomposition in recently burned
forests. In addition, we observed that burned SOM decomposed more slowly than unburned SOM, but
the exact nature of SOM changes in the recently burned stand are unclear. Finally, we found no evidence
that post-fire changes in soil microbial community composition significantly affect decomposition. Taken
together, our study has demonstrated that boreal forest fires can suppress microbial decomposition due
to post-fire changes in abiotic factors and the composition of SOM. Models that predict the consequences
of increased wildfires for C storage in boreal forests may increase their predictive power by incorporating
the observed negative response of microbial decomposition to boreal wildfires.