Heterogeneous Patterns of Aged Organic Carbon Export Driven by Hydrologic Flow Paths, Soil Texture, Fire, and Thaw in Discontinuous Permafrost Headwaters

Abstract

Climate change is thawing and potentially mobilizing vast quantities of organic carbon (OC) previously stored for millennia in permafrost soils of northern circumpolar landscapes. Climate‐driven increases in fire and thermokarst may play a key role in OC mobilization by thawing permafrost and promoting transport of OC. Yet, the extent of OC mobilization and mechanisms controlling terrestrial‐aquatic transfer are unclear. We demonstrate that hydrologic transport of soil dissolved OC (DOC) from the active layer and thawing permafrost to headwater streams is extremely heterogeneous and regulated by the interactions of soils, seasonal thaw, fire, and thermokarst. Repeated sampling of streams in eight headwater catchments of interior Alaska showed that the mean age of DOC for each stream ranges widely from modern to ∼2,000 years B.P. Together, an endmember mixing model and nonlinear, generalized additive models demonstrated that Δ14C‐DOC signature (and mean age) increased from spring to fall, and was proportional to hydrologic contributions from a solute‐rich water source, related to presumed deeper flow paths found predominantly in silty catchments. This relationship was correlated with and mediated by catchment properties. Mean DOC ages were older in catchments with >50% burned area, indicating that fire is also an important explanatory variable. These observations underscore the high heterogeneity in aged C export and difficulty of extrapolating estimates of permafrost‐derived DOC export from watersheds to larger scales. Our results provide the foundation for developing a conceptual model of permafrost DOC export necessary for advancing understanding and prediction of land‐water C exchange in changing boreal landscapes.

Cascading effects of climate change and wildfire on a subarctic lake: A 20‐year case study of watershed change

“…We observed a slow steady rise in solute concentrations in the lake prior to 2019 (Figures 6 and 9). Others have observed similar increases in solute concentrations, which are attributed to deeper hydrologic flow through thickening active layers (Keller et al, 2010; Koch et al, 2022; Kokelj et al, 2009). This slow rise was followed by an abrupt increase in solutes in 2019, coincident with the appearance of groundwater seeps at the base of the footslope (Figure 4B).…”

“…The return to prefire concentrations we observed may be due to complex interactions taking place in the warming soils and solutes entering the lake. Koch et al (2022) recently demonstrated DOC concentrations increase with seasonal active layer thickening, and that DOC age was older in catchments with >50% burn, indicating that carbon trapped in thawing permafrost was contributing DOC to the aquatic environment. Permafrost‐derived DOC is likely entering our study lake, as evidenced by our observations on water clarity, radiocarbon, and SUVA.…”

“…Permafrost-derived DOC is likely entering our study lake, as evidenced by our observations on water clarity, radiocarbon, and SUVA. Radiocarbon data taken in 2020 date the DOC to approximately 185 years old, suggesting ancient sources of carbon are entering the lake (Koch et al, 2022). Furthermore, relatively low SUVA values from 2020 to 2021 indicate that the DOC is of low molecular weight, characteristic of bioavailable DOC typical of permafrost porewater and ice (Lim et al, 2022).…”

The Impact of Climatic Conditions, Human Activities, and Catchment Characteristics on the Propagation From Meteorological to Agricultural and Hydrological Droughts in China

“…This division paradigm was based on municipal regions and allowed us a larger amount of data, thus reducing the impact of spatial heterogeneity and enhancing the accuracy and reliability of the results. Based on the new division mode, we imported the generalized additive model (GAM) to fit the complicated non‐linear relationships between selected indicators and the PT (Kim et al., 2019; Koch et al., 2022; Peng et al., 2023). Then we further explored the effects of various factors on drought propagation and the interaction between these factors from a spatial perspective by applying the geographic detector (GD) method (Luo et al., 2016; J. F. Wang et al., 2010).…”

Complexity of nutrient enrichment on subarctic peatland soil CO2 and CH4 production under increasing wildfire and permafrost thaw

“…in western Canada (Gibson et al, 2018), Siberia (Talucci et al, 2022), and Alaska (Mekonnen et al, 2022). Increases in the number and extent of fire events result not only in the immediate carbon loss through biomass burning but also an increase of downstream export of particulate and dissolved organic carbon (DOC) from the burned areas as summarized in Table 1 (Burd et al, 2018;Burd et al, 2020;Ackley et al, 2021;Koch et al, 2022;Mekonnen et al, 2022).…”