Permafrost soil cores were collected on the North Slope of Alaska during the ice-free summer months of June-August 2018 near the Toolik Field Station. Soil cores were collected from within the permafrost layer (at 85 cm below the surface) of Imnavait Creek wet sedge tundra and Toolik Lake tussock tundra soils. In June 2022, soil was sampled from a thermokarst failure on the shore of Lake LTER 395 on the North Slope of Alaska, where an abrupt collapse of thawing soil exposed deeper permafrost soil. Soil was sampled from the permafrost layer exposed in the headwall of the thermokarst failure (> 80 cm below the surface) using MilliQ-rinsed pickaxes. The permafrost and thermokarst soil samples were collected as previously described in detail (Bowen et al., 2020). All soils were stored in freezers at the Toolik Field Station until overnight shipment to Woods Hole Oceanographic Institution (WHOI). All samples were frozen upon arrival at WHOI and immediately placed into freezers until leachate preparation.
DOC was leached from the permafrost and thermokarst soils as previously described (Bowen et al., 2020). Briefly, frozen soil and MilliQ water were mixed in 5-gallon, MilliQ-rinsed HDPE buckets and allowed to leach in the dark for up to 48 hours at 4 °C. Both the soil-to-water ratio of soil leachates and the leaching time were adjusted to achieve a final concentration of ~1500 μM DOC in the leachates, as estimated from the absorbance of chromophoric dissolved organic matter at 305 nm (a<sub>305</sub>). All leachates were passed through MilliQ-rinsed 60 μm mesh screens to remove the largest particulates, then through MilliQ-rinsed 5 μm high-capacity Whatman cartridge filters, and finally through 0.2 μm high-capacity Whatman cartridge filters. Soil leachates were stored at 4 °C until further use. Supporting soil leachate chemistry analyses (pH, specific conductivity, DOC, and chromophoric and fluorescent dissolved organic matter) were performed as previously described (Cory et al., 2013, 2014; Kling et al., 2000).
Two separate soil leachates were prepared from the thermokarst soil on different dates (20-Oct-2022 and 9-Nov-2022), using the same soil and leaching conditions. Two separate soil leachates (tussock tundra A and tussock tundra B) were prepared from the tussock tundra soil on different dates (19-Feb-2022 and 18-Oct-2022), using the same soil and leaching conditions.
Water chemistry analyses (pH, specific conductivity, DOC, and chromophoric and fluorescent dissolved organic matter) were performed on each of the permafrost leachates, as previously described (Kling et al., 2000; Cory et al., 2013). DOC concentrations were measured using a Shimadzu TOC-V analyzer (CV < 5% on duplicate analyses; Kling et al., 2000). Chromophoric and fluorescent dissolved organic matter (CDOM and FDOM, respectively) were measured for each permafrost leachate (Cory et al., 2014), and then the spectral slope ratio (S<sub>R</sub>), specific UV absorbance at 254 nm (SUVA<sub>254</sub>), and fluorescence index were calculated as previously described (Cory et al., 2014).
References:
Bowen, J. C., Ward, C. P., Kling, G. W., & Cory, R. M. (2020). Arctic amplification of global warming strengthened by sunlight oxidation of permafrost carbon to CO2. Geophysical Research Letters, 47(12), 0–3. https://doi.org/10.1029/2020GL087085
Cory, R. M., Crump, B. C., Dobkowski, J. A., & Kling, G. W. (2013). Surface exposure to sunlight stimulates CO2 release from permafrost soil carbon in the Arctic. Proceedings of the National Academy of Sciences USA, 110(9), 3429–3434. https://doi.org/10.1073/pnas.1214104110
Cory, R. M., Ward, C. P., Crump, B. C., & Kling, G. W. (2014). Sunlight controls water column processing of carbon in arctic fresh waters. Science, 345(6199), 925–928. https://doi.org/10.1126/science.1253119
Rieb, E. C., Polik, C. A., Ward, C. P., Kling, G. W., & Cory, R. M. Controls on the respiration of ancient permafrost carbon in sunlit arctic surface waters. In review.
Kling, G. W., Kipphut, G. W., Miller, M. M., & O’Brien, W. J. (2000). Integration of lakes and streams in a landscape perspective: The importance of material processing on spatial patterns and temporal coherence. Freshwater Biology, 43(4), 477–497. https://doi.org/10.1046/j.1365-2427.2000.00515.x
