Moisture content modulates DOM thermodynamic regulation of oxygen consumption in drying streambed sediments

Abstract Dissolved organic matter (DOM) thermodynamic properties regulate biogeochemical processes (e.g., O 2 consumption rates) in riverbed sediments. This connection has been established under saturated conditions, yet most streams and rivers experience variable inundation in parafluvial zones and go dry at least one day per year. To evaluate how drying and re-inundation influence sediment DOM thermodynamics and their link with O 2 consumption, we conducted wet/dry laboratory experiments using riverbed sediments from 33 sites across the contiguous United States. DOM thermodynamic responses to treatments varied widely by site and were not linked to large-scale or within-sample variables, highlighting the potential importance of additional variables or site-specific processes. Our results further showed that sediment moisture modified the link between DOM thermodynamic favorability and O 2 consumption rates. We found a moisture threshold (~ 46%) below which thermodynamic regulation of O 2 consumption rates was no longer observed. Together, these results imply that drying effects on DOM thermodynamics are not yet predictable, but that temporal shifts in the relative influence of DOM thermodynamics on O 2 consumption may be generalizable across streams experiencing wet-dry cycles. We propose a conceptual model integrating these results to guide development of process-based models of hydrology and biogeochemistry in variably inundated ecosystems.