Photochemical and Microbial Degradation of Deadwood Leachate

During the last decades, intensive forest dieback due to drought events and bark beetle infestation was globally observed leading to accumulation of deadwood. However, data on molecular composition of deadwood DOM, of its bacterial and photo‐transformation, and of the interaction of these processes are scarce. Here, we investigate the fate of DOM leached from deadwood into streams. We hypothesized that (a) bacterial degradation dominates quantitatively over photodegradation in stream water, (b) bacterial degradation is further promoted by labile and easily degradable photoproducts, and (c) DOM compositional changes reflect both the bacterial and light transformation. A leachate of spruce branches and bark in pure water was used for a degradation experiment in a 2 × 2 factorial design without and with stream bacteria and light, respectively. Dissolved organic carbon concentration did not change in dark incubation without bacteria but decreased slightly (3%) in the light. The decrease with bacteria in the dark was stronger (9%), that is, photodegradation of spruce leachate was less important than bacterial degradation (a). Photodegradation and bacterial degradation added in the light plus bacteria treatment (12%), and bacterial degradation was similar in light and dark, indicating no quantitative priming by easily available photoproducts but some qualitative modifications were detected (b). Light induced the production of mostly small and polar molecules, mainly from stream water DOM, while bacteria preferentially degraded nonpolar molecules from dead‐wood leachate (c). Our results indicate distinct transformation pathways and high microbial availability for deadwood‐derived DOM as compared to stream water DOM that may stimulate heterotrophic processes in headwater streams. Plain Language Summary Forest dieback accumulated large amounts of dead wood during the last decades. We investigated the fate of leached dissolved organic compounds in streams. Bacterial degradation was more important than photodegradation, and photoproducts did not further promote microbial processes. Light induced the production of water‐soluble components while bacteria degraded less water‐soluble components. The dead wood leachate was reactive to both light and bacterial degradation, and its transformation was dominated by oxidation rather than decarboxylation. Key Points Photodegradation of spruce leachate was lower than bacterial degradation Similar bacterial degradation in light a