Permafrost thaw exposes old dissolved organic carbon to photo- and biodegradation

The transformation of permafrost-derived dissolved organic carbon (DOC) to CO₂ is mediated by interacting photodegradation and biodegradation processes that influence DOC lability and persistence in Arctic aquatic systems. Here, we experimentally evaluated the relative roles of photodegradation and biodegradation pathways on DOC concentration and age along a hydrologic continuum from soil pore water to a first-order stream to a second-order stream draining permafrost soils during spring freshet. Spring freshet is characterized by pulses of DOC, resulting from interactions of snowmelt with modern surface soils and vegetation as well as pulses of older DOC, stored overwinter. In our study, we incubated spring freshet waters and exposed these waters to photodegradation, biodegradation, and combined photo-biodegradation pathways. We observed a 7%–72% DOC loss at all sites within the 2 month incubation. Photodegradation produced the greatest losses and strongest radiocarbon (Δ^1⁴C) shifts, particularly when combined with biodegradation. Radiocarbon signatures revealed three DOC pools: young, labile carbon; old, labile carbon; and old, stable carbon. Soil pore waters released from a warming permafrost landscape exhibited rapid DOC losses and large radiocarbon shifts within 24 h, indicating the presence of a highly labile, old DOC pool that may be released with increased permafrost thaw. Contrary to expectations, photodegradation remained important in second-order streams, with greater DOC losses observed over time than from biodegradation alone. These results suggest that photodegradation enhances microbial access to otherwise resistant DOC, especially older, permafrost-derived fractions. Our findings provide mechanistic insight into how light and microbial processes interact to transform permafrost DOC, with implications for understanding DOC fate following permafrost thaw.