Post fire Black Carbon alteration: Rapid changes in a supposedly inert pool

Forest fires are among the most influential disturbances in ecosystems and have varying effects on the soil depending on fire intensity and biomass consumption. The significant decline in biodiversity in European forests due to centuries of non-sustainable forest management, combined with worsening drought from climate change, has greatly increased vulnerability to wildfires. Incomplete combustion during fires leads to the formation of black carbon (BC), a group of substances known for their persistence in soil. However, studies suggest that medium-condensed BC species may have lower chemical and spatial stability and are therefore potentially more mobile and consequently only serve as temporary carbon sinks.

In order to assess the mobilization of BC, we investigate short-term changes in BC under field conditions, particularly of the low-condensed BC, and call into question the established concept of the general stability of BC pools. We investigated the dynamics of BC alterations during the post-fire period within one winter, following a late summer forest fire. We selected two comparable sites featuring spruce-dominated forest stands with different geologic parent material and weather conditions, particularly with respect to the amount of precipitation during the observation period. We sampled soil profiles down to 40 cm depth shortly after the fire event in late summer and after a 6-month period in late spring. After performing density fractionation to separate the mineral associated organic matter (MAOM) from particulate organic matter (POM), we analysed the BC content in the MAOM fraction using benzene polycarboxylic acids (BPCA) analysis.

The results show a high content of low to medium condensed BPCAs directly after fire, which decreased, especially the medium condensed BPCA marker, during the observation period. Taking into account the fast change in medium BPCA values in the MAOM fraction, we conclude that the general assumption that BC is in principle a stable, long-term carbon sink needs to be addressed more carefully.