Assessing the Potential for Mobilization of Old Soil Carbon After Permafrost Thaw: A Synthesis of ¹⁴C Measurements From the Northern Permafrost Region

Abstract

The magnitude of future emissions of greenhouse gases from the northern permafrost region depends crucially on the mineralization of soil organic carbon (SOC) that has accumulated over millennia in these perennially frozen soils. Many recent studies have used radiocarbon (14 C) to quantify the release of this "old" SOC as CO 2 or CH 4 to the atmosphere or as dissolved and particulate organic carbon (DOC and POC) to surface waters. We compiled~1,900 14 C measurements from 51 sites in the northern permafrost region to assess the vulnerability of thawing SOC in tundra, forest, peatland, lake, and river ecosystems. We found that growing season soil 14 C-CO 2 emissions generally had a modern (post-1950s) signature, but that well-drained, oxic soils had increased CO 2 emissions derived from older sources following recent thaw. The age of CO 2 and CH 4 emitted from lakes depended primarily on the age and quantity of SOC in sediments and on the mode of emission, and indicated substantial losses of previously frozen SOC from actively expanding thermokarst lakes. Increased fluvial export of aged DOC and POC occurred from sites where permafrost thaw caused soil thermal erosion. There was limited evidence supporting release of previously frozen SOC as CO 2 , CH 4 , and DOC from thawing peatlands with anoxic soils. This synthesis thus suggests widespread but not universal release of permafrost SOC following thaw. We show that different definitions of "old" sources among studies hamper the comparison of vulnerability of permafrost SOC across ecosystems and disturbances. We also highlight opportunities for future 14 C studies in the permafrost region.

Radiocarbon Data Reveal Contrasting Sources for Carbon Fractions in Thermokarst Lakes and Rivers of Eastern Canada (Nunavik, Quebec)

“…This could suggest that the increase of vegetation growth in permafrost free areas, which increases the input of labile DOC from plant exudates into aquatic environments, will strongly influence diffusive CO2 and CH4 emissions from thermokarst lakes of Nunavik in Northern Quebec. These results are similar to other studies highlighting that diffusive CH4 and CO2 fluxes from lakes incorporate large proportions of modern carbon (Cooper et al, 2017;Dean et al, 2020;Elder et al, 2019).…”

“…In contrast, the palsa lakes dissolved CH4 and DIC contained inputs of centennial aged permafrost carbon, and the palsa lakes ebullition CH4 and SAS river DIC contained inputs from even older carbon sources. The difference in CH4 and DIC between the peatland and non-peatland lakes is broadly consistent with previous studies (Elder et al, 2018;Dean et al, 2020) that have found that the geological substrate and soil type exerts a strong influence on the age of carbon emitted from lacustrine CH4 and CO2 fluxes.…”

“…carbon sources cycling on decadal timescales, such as shallow soil organic matter, likely predominated over carbon from autochthonous primary productivity, which would have a contemporary 14 C signature. These results are consistent with evidence from Arctic rivers, which shows that DOC does transport some pre-modern organic carbon, but it is dominantly derived from young carbon reservoirs (Dean et al, 2020;Guo et al, 2007;Raymond et al, 2007;Wild et al, 2019).…”

“…• Soil type influences the age of carbon fractions, with a greater contribution of millennia-aged carbon in peatland systems • In peatland systems diffusive methane (CH 4 ) and CO 2 mostly contain centennial-old carbon but ebullition CH 4 largely contains millennia-old carbon • In peatland systems, dissolved and particulate carbon fractions contained older carbon in winter, but CH 4 as variations in landscape relief, permafrost extent, ice content, composition and degree of processing of permafrost material, vegetation, and hydrological factors all influence how aquatic system carbon-cycling responds to thaw (Tank et al, 2020). Radiocarbon ( 14 C) measurements are used to estimate the contribution of permafrost carbon to DOC, POC, CH 4 , and CO 2 , as the storage of soil carbon in permafrost for hundreds to thousands of years results in distinct 14 C ages (Dean et al, 2018;Estop-Aragonés et al, 2020;Elder et al, 2018Elder et al, , 2019. The 14 C content of the carbon fraction indicates the weighted sum of the 14 C signals of the carbon sources, which means that the approximate relative contribution from each carbon source can be estimated with a mass balance if the 14 C content of the endmembers is known.…”

Stream Dissolved Organic Matter in Permafrost Regions Shows Surprising Compositional Similarities but Negative Priming and Nutrient Effects

“…For example, Arctic and Boreal surface waters receive ~100 Tg of dissolved organic carbon (DOC) each year from terrestrial ecosystems, a third of which (~35 Tg C yr −1 ) they deliver to the Arctic Ocean and surrounding seas (Abbott, Jones, et al, 2016; Kicklighter et al, 2013; McGuire et al, 2009). Radiocarbon measurements suggest that more than 80% of this DOC is modern—fixed since the 1950s (Qu et al, 2017; Raymond et al, 2007; Wild et al, 2019)—and even under extreme warming scenarios, DOM from degrading permafrost will likely remain a small proportion of total DOM flux (Abbott et al, 2015; Abbott, Jones, et al, 2016; Estop‐Aragonés et al, 2020; Laudon et al, 2012). However, when biolabile DOC (BDOC) and nutrients from permafrost mix with modern DOM, they could influence mineralization rates and alter the net ecosystem carbon balance of the permafrost zone (Abbott et al, 2014; Larouche et al, 2015; Textor et al, 2019), potentially resulting in greater CO 2 efflux from permafrost ecosystems to the atmosphere.…”