Long-term soil redox dynamics of intact and degraded permafrost in Interior Alaska

Permafrost degradation, freezing and thawing processes, and poor drainage due to underlain frozen ground have far-reaching consequences on soil hydrology and biology and, thus, on the redox dynamic in soils of the Arctic. Assessing the redox status of these soils is essential for understanding soil organic matter decomposition processes and can be done by temporal measurements in the field, analyses of redox-sensitive elements, or identification of microbial species or enzymes in redox process chains. While such approaches provide snippets of the complex redox dynamic, publications reporting long-term in-situ redox potential (E_H) measurements in arctic permafrost soils are scarce. Limited accessibility to study sites and technical limitations in measuring the redox potential in a frozen environment may be two reasons for this research gap.

But how does the redox potential develop in permafrost soils at different depths in the active layer during the summer? What happens during freezing and thawing? Finally, do thawing/degrading permafrost soils show different patterns compared to intact permafrost?

We approached these research questions by installation of a unique soil monitoring setup at 3 sites near Fairbanks, Alaska, in August 2021. An intact permafrost soil (active layer depth about 50 cm) was equipped with 3 redox electrodes (for E_H) and 3 hydra probes (for water content and soil temperature) in the topsoil and subsoil, respectively, and connected to a logger unit allowing continuous measurement of these parameters in both depths every 15 minutes. In addition, two sites with advanced permafrost degradation (permafrost level below 100 cm) were equipped in the same way. One degraded site featured large water contents, representing a wet thaw scenario, while the other site was well-drained, representing a dry thaw scenario, thus representing different endmembers of the ongoing climate-change induced permafrost thaw.

Here, we present the first 2 years of soil monitoring in a discontinuous permafrost area in Interior Alaska from 09/2021 to 09/2023. Overall, pH values of all soils varied between 4.5-6.3. The dry thaw scenario showed oxic conditions (i.e., E_H >600 mV) in top- and subsoil, while water contents were low. The wet thaw scenario exhibited high topsoil redox potentials (i.e., E_H >500 mV), while subsoil redox potential was lower (i.e., E_H <500 mV). High water contents in both intact permafrost and wet thaw scenario demonstrated a pronounced zero curtain effect over several months during the long winter season due to the release of latent heat during freezing. We further detected a strong 200-600 mV decrease in E_H in the topsoil of the intact permafrost active layer during the summer season, reaching reducing conditions 1-3 months after seasonal thaw. Redox measurements in the subsoil of the intact permafrost active layer, which was about 25 cm below the topsoil measuring depth and about 5 cm above the frozen ground, revealed E_H of >400 mV in the summer period (August to October), suggesting less oxygen consumption in this recently thawed permafrost subsoil.