The effect of dissolved char on microbial activity in an extract from the forest floor

Climate change is associated with an increased risk in the occurrence of wildfires. Forests store large amounts of carbon (C), which are threatened by these wildfires. Pyrogenic material produced after a wildfire constitutes an important part of the soil organic carbon pool in forest soils. Microorganisms play an important role in the cycling of C. This study investigated microbial activity in dissolved char from burned wood from two tree species in different stages of decay. The char from branches of beech and Norway spruce was produced under laboratory fire conditions and extracted in water after which microbial activity was measured for a 4-week period. Further stages of decay resulted in increased flammability with higher peak temperatures and combustion completeness. For the beech samples, further decay also resulted in a decrease of extractable C but a higher proportion of stable C. Further decay resulted in less respiration for beech and more for Norway spruce. With less C being respired, this points towards an increased C sequestration potential in the form of microbial C and microbial derived products for beech compared to Norway spruce. This study provided a workflow to assess the effects of dissolved char on microbial activity by mimicking natural fire conditions. It also indicated the need for future research to further elucidate the underlying mechanisms explaining why fire-originated dissolved char of wood in different decay stages influences microbial respiration with diverging effects per species.