Figure - available from: Journal of Geophysical Research: Biogeosciences
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δ¹³C‐CO2 produced from microbial respiration of DOC kept in the dark (circle symbols), exposed to ultraviolet light (UV; 305 nm, diamond symbols), and exposed to visible light (405 nm, square symbols) increased with (a) increasing respiratory quotient and (b) decreasing age of the CO2 from: Imnavait wet sedge tundra (orange symbols), Toolik tussock tundra A (dark blue symbols), Toolik tussock tundra B (yellow symbols), and LTER 395 thermokarst (teal symbols). In panel (a), data were fit using a least‐squares regression where R² = 0.94 and p < 0.01. All values on the x‐axis are shown as the average ±1 SE (n = 3). All values on the y‐axis are shown as the average ±1 SE (n = 2). In panel (b), data were fit using a least‐squares regression where R² = 0.57 and p < 0.05. All values on the x‐ and y‐axes are shown as the average ±1 SE (n = 2).
Source publication
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 CO2 is uncertain. A critical unknown is the extent to which dissolved organic carbon...
Citations
... Taken together these findings suggest that higher background DOC concentrations (in 2022) caused light limitation of autotrophs (similar to dark treatments in Rober et al. 2023), allowing heterotrophs to use available nutrients. The extent to which autotrophic biofilms are able to buffer peatlands against net heterotrophy more broadly may depend on the composition of resources delivered to surface waters with variable hydrology and warming soil conditions (Kendrick et al. 2018;Wickland et al. 2018;Weaver and Jones 2022;Rieb et al. 2024) as well as the changing physical aspects of northern peatlands (Euskirchen et al. 2024), all of which influence concentrations of dissolved organic matter (Kane et al. 2010;Cory and Kling 2018). We might expect greater light attenuation associated with increasing levels of dissolved organic matter to overwhelm the stimulatory effect of nutrients on autotrophic microbes by constraining photosynthesis, favoring heterotrophy and increasing CO 2 emissions. ...
... Interestingly, the raised water table treatment was the strongest sink for CO 2 among experimental plots (taking up on average 32.7 g CO 2 m −2 y −1 ), which is notable because the raised treatment has also shifted toward sedge dominance (e.g., Carex), which we show to support autotrophy. It is important to note that other primary producers (e.g., plants) or processes such as photomineralization (Cory et al. 2014) or microbial transformation of DOC (Rieb et al. 2024) could have influenced CO 2 flux in this study and it would be interesting to consider their contribution in future investigations. In this region of Alaska, open-water areas are expanding (Douglas, Turetsky, and Koven 2020), resulting in the release of between 13 and 59 g CO 2 m −2 y −1 (Euskirchen et al. 2024). ...
To better understand linkages between hydrology and ecosystem carbon flux in northern aquatic ecosystems, we evaluated the relationship between plant communities, biofilm development, and carbon dioxide (CO 2 ) exchange following long‐term changes in hydrology in an Alaskan fen. We quantified seasonal variation in biofilm composition and CO 2 exchange in response to lowered and raised water table position (relative to a control) during years with varying levels of background dissolved organic carbon (DOC). We then used nutrient‐diffusing substrates (NDS) to evaluate cause–effect relationships between changes in plant subsidies (i.e., leachates) and biofilm composition among water table treatments. We found that background DOC concentration determined whether plant subsidies promoted net autotrophy or heterotrophy on NDS. In conditions where background DOC was ≤ 40 mg L ⁻¹ , plant subsidies promoted an autotrophic biofilm. Conversely, when background DOC concentration was ≥ 50 mg L ⁻¹ , plant subsidies promoted heterotrophy. Greater light attenuation associated with elevated levels of DOC may have overwhelmed the stimulatory effect of nutrients on autotrophic microbes by constraining photosynthesis while simultaneously allowing heterotrophs to outcompete autotrophs for available nutrients. At the ecosystem level, conditions that favored an autotrophic biofilm resulted in net CO 2 uptake among all water table treatments, whereas the site was a net source of CO 2 to the atmosphere in conditions that supported greater heterotrophy. Taken together, these findings show that hydrologic history interacts with changes in dominant plant functional groups to alter biofilm composition, which has consequences for ecosystem CO 2 exchange.
