Weak mineralization despite strong processing of dissolved organic matter in Eastern Arctic tundra ponds

Abstract

Permafrost thawing mobilizes large quantities of organic carbon that was sequestered in Arctic regions over the last glacial cycle. Processes involved in the oxidation of this carbon need to be further assessed to estimate the fraction to be released into the atmosphere. Shallow tundra ponds are sites of active carbon turnover on the landscape and significant sources of greenhouse gases. Dissolved organic matter (DOM) leached from thawing peat into these ponds is exposed to sunlight, with the potential to accelerate its mineralization directly into CO2 or through the production of more labile molecules. We tested the catalytic effect of sunlight on DOM mineralization in tundra ponds formed on organic‐rich polygonal landscapes originating from syngenetic permafrost, including a pond exposed to active permafrost erosion. Microbial decay rates, measured as the loss of chromophoric DOM, were similar to photodecay rates (1%–3% d−1). Groups of fluorescing molecules were formed through microbial transformation or lost through photolysis at differing rates among studied ponds, with the erosive trough pond presenting a unique response suggesting the involvement of soil microbes. Despite the stimulation of microbial growth under sunlight and the dynamic response of DOM optical properties, the loss of dissolved organic carbon was not significant under any treatment. This suggests that microbial and photochemical mineralization of DOM was slow and potentially substrate‐limited during the dry period when ponds were sampled. The static nature of tundra ponds, with their long water retention time, may thus constrain hot moments when water moves and transports carbon on the landscape.

Environmental effects of stratospheric ozone depletion, UV radiation, and interactions with climate change: UNEP Environmental Effects Assessment Panel, Update 2021

“…In Arctic waters and boreal lakes experiencing browning, UV radiation, however, has a limited role in DOM mineralisation (e.g. release of CO 2 ) compared to biological processes [135][136][137]. It is possible that previous research has overestimated the extent of photomineralisation in Arctic rivers because data have been modelled with a gas transfer velocity typical of lakes, an assumption that recent studies proved inaccurate [136].…”

The dominant role of sunlight in degrading winter dissolved organic matter from a thermokarst lake in a subarctic peatland

“…broadly observed in boreal and arctic lakes (e.g. Berggren et al, 2020;Laurion et al, 2021). The study of Berggren et al (2020) on 101 boreal lakes suggests the reciprocal dependence between microbial processes and DOM optical properties.…”

“…This difference suggests a variability in peatland DOM reactivity among regions, potentially related to different historical processing of the organic matter. Historical processing could differ prior to permafrost inception (Tank et al, 2020), or prior to sampling (Laurion et al, 2021). The sunlight dose received (historical 440 dose in situ, or during an experiment) needs to be closely taken into account when comparing studies.…”

Element cycling and aquatic function in a changing Arctic

“…Such changes in residence time and associated land‐water connectivity can have broad‐scale biogeochemical implications: for example, changing the composition of dissolved organic matter (DOM) in lakes (Kellerman et al 2015) as a result of increases in DOM processing enhanced by increasing residence time. In this issue, Laurion et al (2021) suggest that even in lakes affected by direct inputs of permafrost thaw substrates, dry (i.e., longer residence time) periods may lead to DOM substrate limitation for both microbial and photochemical mineralization. As Richardson et al (2021) show, longer water residence times are also associated with lower lake water concentrations of total and methyl mercury, and decreases in the ratio of mercury to dissolved organic carbon (DOC), which is an important predictor of the bioavailability of mercury within food webs.…”