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Abstract The thawing of ancient organic carbon stored in arctic permafrost soils, and its oxidation to carbon dioxide (CO2, a greenhouse gas), is predicted to amplify global warming. However, the extent to which organic carbon in thawing permafrost soils will be released as CO2is uncertain. A critical unknown is the extent to which dissolved organic carbon (DOC) from thawing permafrost soils is respired to CO2by microbes upon export of freshly thawed DOC to both dark bottom waters and sunlit surface waters. In this study, we quantified the radiocarbon age and13C composition of CO2produced by microbial respiration of DOC that was leached from permafrost soils and either kept in the dark or exposed to ultraviolet and visible wavelengths of light. We show that permafrost DOC most labile to microbial respiration was as old or older (ages 4,000–11,000 a BP) and more13C‐depleted than the bulk DOC in both dark and light‐exposed treatments, likely indicating respiration of old,13C‐depleted lignin and lipid fractions of the permafrost DOC pool. Light exposure either increased, decreased, or had no effect on the magnitude of microbial respiration of old permafrost DOC relative to respiration in the dark, depending on both the extent of DOC oxidation during exposure to light and the wavelength of light. Together, these findings suggest that photochemical changes affecting the lability of permafrost DOC during sunlight exposure are an important control on the magnitude of microbial respiration of permafrost DOC in arctic surface waters.
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This content will become publicly available on February 23, 2027
Permafrost thaw exposes old dissolved organic carbon to photo- and biodegradation
Abstract The transformation of permafrost-derived dissolved organic carbon (DOC) to CO2is 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 (Δ1⁴C) 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.
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- Award ID(s):
- 1754839
- PAR ID:
- 10710576
- Publisher / Repository:
- IOP Science
- Date Published:
- Journal Name:
- Environmental Research: Ecology
- Volume:
- 5
- Issue:
- 1
- ISSN:
- 2752-664X
- Page Range / eLocation ID:
- 015003
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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