Abstract. Following a wildfire, organic carbon (C) accumulates in
boreal-forest soils. The long-term patterns of accumulation as well as the
mechanisms responsible for continuous soil C stabilization or sequestration
are poorly known. We evaluated post-fire C stock changes in functional
reservoirs (bioreactive and recalcitrant) using the proportion of C
mineralized in CO2 by microbes in a long-term lab incubation, as well
as the proportion of C resistant to acid hydrolysis. We found that all soil
C pools increased linearly with the time since fire. The bioreactive and
acid-insoluble soil C pools increased at a rate of 0.02 and 0.12 MgC ha−1 yr−1, respectively,
and their proportions relative to total soil C stock remained constant with
the time since fire (8 % and 46 %, respectively). We quantified direct and
indirect causal relationships among variables and C bioreactivity to
disentangle the relative contribution of climate, moss dominance, soil
particle size distribution and soil chemical properties (pH, exchangeable
manganese and aluminum, and metal oxides) to the variation structure of in vitro
soil C bioreactivity. Our analyses showed that the chemical properties of
podzolic soils that characterize the study area were the best predictors of
soil C bioreactivity. For the O layer, pH and exchangeable manganese were
the most important (model-averaged estimator for both of 0.34) factors
directly related to soil organic C bioreactivity, followed by the time since
fire (0.24), moss dominance (0.08), and climate and texture (0 for both). For
the mineral soil, exchangeable aluminum was the most important factor
(model-averaged estimator of −0.32), followed by metal oxide (−0.27), pH
(−0.25), the time since fire (0.05), climate and texture (∼0 for
both). Of the four climate factors examined in this study (i.e., mean annual
temperature, growing degree-days above 5 ∘C, mean annual
precipitation and water balance) only those related to water availability –
and not to temperature – had an indirect effect (O layer) or a marginal indirect
effect (mineral soil) on soil C bioreactivity. Given that predictions of the
impact of climate change on soil C balance are strongly linked to the size
and the bioreactivity of soil C pools, our study stresses the need to
include the direct effects of soil chemistry and the indirect effects of
climate and soil texture on soil organic matter decomposition in Earth
system models to forecast the response of boreal soils to global warming.
Time since fire strongly and variously influences anthophilous insects in a fire‐prone landscape