Wildfire-derived pyrogenic carbon modulates riverine organic matter and biofilm enzyme activities in an in-situ flume experiment

Wildfires produce large amounts of pyrogenic carbon (PyC), including charcoal, known for its chemical recalcitrance and sorption affinity for organic molecules. Wildfire-derived PyC can be transported to fluvial networks. Here it may alter dissolved organic matter (DOM) concentration and composition as well as microbial biofilm functioning. Employing in-stream flumes with a control vs treatment design (PyC pulse addition), we present evidence that field-aged PyC inputs to rivers can increase dissolved organic carbon (DOC) concentration, and alter DOM composition. Decreased DOM aromaticity indicated by lower SUVA245 (-0.31 units), and higher pH (0.25 units) were associated with changes in enzymatic activities in benthic biofilms, including a lower recalcitrance index (β-glucosidase/phenol oxidase), suggesting preferential usage of recalcitrant over easily available DOM by biofilms. Particulate PyC deposition onto biofilms may further modulate the impacts of PyC due to direct contact with the biofilm matrix.