Soils were collected from the frozen permafrost layer (> 60 cm below the surface) at five sites underlying moist acidic tussock or wet sedge vegetation, and on three glacial surfaces on the North Slope of Alaska during summer 2018.  Soil cores were collected at Imnavait wet sedge tundra using a SIPRE corer, and the permafrost layer (1.0 – 1.3 m below the surface) was separated from the soil core using a knife.  At the other four sites, 1 m x 1 m x 1 m soil pits were excavated using a jack hammer, shovels, and pickaxe.  Soil sampling at each site took place over the course of one day.  From each site, an equal mass of soil (~2.5 kg) was placed in four Ziploc bags (1 gallon) and then each soil sample was quintuple-bagged.  Following collection, soil samples were immediately transferred to coolers in the field and then stored in freezers at the Toolik Field Station for ≤ 4 weeks until overnight shipment on dry ice to the Woods Hole Oceanographic Institution (WHOI).  All soil samples were frozen upon arrival at WHOI and immediately placed into freezers until leachate preparation.

Dissolved organic carbon (DOC) was leached from each permafrost soil at WHOI as described in the following five steps.  First, frozen soil in one or two Ziploc bags was broken into smaller pieces inside the bag using a clean chisel.  Second, 0.8 to 3.3 kg of frozen soil was transferred to a new Ziploc bag (1 gallon) and then thawed in a chest cooler at 4 °C for up to 20 hours.  Third, the thawed permafrost soil was added to five liters of MilliQ water (Millipore Simplicity ultraviolet, UV, system) in a MilliQ-rinsed high density polyethylene (HDPE) bucket (5 gallon).  Each bucket was covered with a HDPE lid and allowed to leach at 4 °C for 24 hours.  Fourth, the permafrost leachate was filtered through a sieve with 60 micrometer nylon mesh screening (Component Supply) into a new, MilliQ-rinsed 5 gallon HDPE bucket and then placed in the chest cooler at 4 °C for ≤ 1 day to allow suspended particles to settle before additional filtration.  Fifth, the 60-micrometer filtered leachate was filtered through 10 micrometer (Geotech Environmental Equipment, Inc.) and then finally through 0.2 micrometer  (Whatman), MilliQ-rinsed high-capacity cartridge filters.  Four liters of the final 0.2-micrometer filtered permafrost leachate (now referred to as permafrost leachate) were then transferred to a precombusted (450 °C; 4 h) 4 L glass amber bottle and kept at 4 °C prior to light exposure experiments. 

Photo-oxidation:

The O2 consumed from photo-oxidation of permafrost DOC was measured as a function of sunlight wavelength, defined as the apparent quantum yield spectrum of photo-oxidation (O2 consumed per mol photon absorbed by DOC: φPO,λ).  φPO,λ of permafrost DOC were measured with a custom-built high-powered (≥ 100 mW), narrow-banded (± 10 nm) light-emitting diode (LED) system for each soil.  Each permafrost leachate was equilibrated to room temperature (~24 hrs) and then placed in 20 gas-tight, flat-bottomed 12 mL quartz vials with butyl rubber septa and GL-18 caps (light-exposed vials) and four gas-tight precombusted (450 °C; 4 hrs) 12 mL borosilicate exetainer vials (dark control vials; Labco, Inc.).  Vials were placed in an inner aluminum housing (painted black matte to minimize light scattering), with the flat bottom facing upward toward the light source, and then exposed to ≥ 100 mW, narrow-banded (± 10 nm) LEDs at 278, 309, 348, 369, and 406 nm alongside the dark controls for 12 or 15 hours.  Each permafrost leachate was exposed to light for the same amount of time at each wavelength.  After LED exposure, light-exposed and dark control waters were immediately analyzed for dissolved O2 on a membrane inlet mass spectrometer (Bay Instruments) and for chromophoric dissolved organic matter (CDOM) as described previously (Cory et al., 2014).  At each LED wavelength, φPO,λ was calculated as the concentration of O2 consumed divided by the light absorbed by CDOM.  The amount of light absorbed by CDOM (mol photon m-2 nm-1) was quantified for each vial exposed to a LED using absorption coefficients of CDOM (aCDOM,λ) and the photon flux spectrum (Cory et al., 2014).  The photon flux spectrum was quantified from the solar irradiance spectrum from each LED source, which was measured by radiometry and chemical actinometry.  The photon dose was calculated as the sum of the total photon flux spectrum across all wavelengths and is reported as the average ± 1 standard error (SE) of replicate measurements (n = 4).  The amount of light absorbed by CDOM and φPO,λ are reported as the average ± 1 SE of experimental replicate vials (n = 4).

Photomineralization:

The CO2 produced from photomineralization of permafrost DOC was measured as a function of sunlight wavelength, defined as the apparent quantum yield spectrum of photomineralization (CO2 produced per mol photon absorbed by DOC: φPM,λ).  φPM,λ of permafrost DOC were measured with a ≥ 100 mW, narrow-banded (± 10 nm) LED system for each soil.  Each permafrost leachate was equilibrated to room temperature (~24 hrs) and then placed in 20 gas-tight, flat-bottomed 12 mL quartz vials with butyl rubber septa and GL-18 caps (light-exposed vials) and four gas-tight precombusted (450 °C; 4 hrs) 12 mL borosilicate exetainer vials (dark control vials; Labco, Inc.).  Vials were placed in an inner aluminum housing (painted black matte to minimize light scattering), with the flat bottom facing upward toward the light source, and then exposed to ≥ 100 mW, narrow-banded (± 10 nm) LEDs at 278, 309, 348, 369, and 406 nm alongside the dark controls for 12 or 30 hours.  Each permafrost leachate was exposed to light for the same amount of time at each wavelength.  After LED exposure, light-exposed and dark control waters were immediately analyzed for dissolved inorganic carbon (DIC) using a DIC analyzer (Apollo SciTech, Inc.) and for CDOM as described previously (Cory et al., 2014).  At each LED wavelength, φPM,λ was calculated as the concentration of DIC produced divided by the light absorbed by CDOM.  The amount of light absorbed by CDOM (mol photon m-2 nm-1) was quantified for each vial exposed to a LED using absorption coefficients of CDOM (aCDOM,λ) and the photon flux spectrum (Cory et al., 2014).  The photon flux spectrum was quantified from the solar irradiance spectrum from each LED source, which was measured by radiometry and chemical actinometry.  The photon dose was calculated as the sum of the total photon flux spectrum across all wavelengths and is reported as the average ± 1 SE of replicate measurements (n = 4).  The amount of light absorbed by CDOM and φPM,λ are reported as the average ± 1 SE of experimental replicate vials (n = 4).

References: 

Bowen, J.C.,  C. P. Ward, G. W. Kling, R. M. Cory..  Arctic amplification of global warming strengthened by sunlight oxidation of permafrost carbon to CO2.    In review.

Cory, R. M., C. P. Ward, B. C. Crump, G. W. Kling.  2014. Sunlight controls water column processing of carbon in arctic fresh waters. Science, 10.1126/science.1253119