Alteration of the Black Carbon pool shortly after a fire under dry conditions at the boreal southern border

Fires belong to the most intensive disturbances in ecosystems, but do have different effects on the soil depending on their intensity and fuel materials. Taiga ecosystems contain significant reserves of potentially fire-prone materials, and as temperatures rise in the circumpolar region and precipitation patterns change, an increase in the frequency and intensity of fires is observed. In these fires, incomplete combustion processes result in the formation of black carbon (BC), which is known as a long-term carbon sink due to its chemical properties. As the majority of forest fires are ground fires burning at a rather low intensity in terms of duration and temperature, it is discussed that the BC species formed under these circumstances are chemically less stable than those formed at high temperatures and should therefore only be considered as temporary carbon sinks.

Here we studied the effects of low intensity ground fire shortly after the event and tracked changes in BC within the first four years after the fire event at the southern edge of the boreal forest. We analysed a fire transect running through the two main forest types of this region, focusing on the BC species that we could quantify using the BPCA method. Our results indicate a decline in BC after the fire within the four years of observations, which mainly mainly occurred for the low condensed BPCAs. This finding is independent of the forest typ. Since the precipitation within the experimental period was also negligible and only occurred in very small amounts, we exclude leaching as well as a possible significant aeolian losses, since the trees remained unaffected by the fire and covered the soil against strong wind. We therefore deduce that in situ degradation of the BC must have occurred.
Concluding, the general assumption that BC is a stable, long-term carbon sink needs to be questioned more critically. Together with other studies, our results show a quite fast decrease in the concentration of low-condensed BC species in soil over time, indicating a potential for degradation.