Soils were collected from the frozen permafrost layer (greater than 60 cm below the surface) at six sites underlying tussock or wet sedge vegetation, and on three glacial surfaces on the North Slope of Alaska during the summers of 2015 and 2018. Dissolved organic carbon (DOC) was leached from each permafrost soil and the water chemistry was analyzed.
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Soils were collected from the frozen permafrost layer (> 60 cm below the surface) at six sites underlying tussock or wet sedge vegetation, and on three glacial surfaces on the North Slope of Alaska during the summers of 2015 and 2018.
Sampling and preparation of the permafrost leachates from soils collected in 2015 are described in Ward et al. (2017). Two permafrost leachates were prepared from each permafrost soils collected in 2015, except for Toolik moist acidic tundra, which did not have an experimental replicate.
Here we describe how permafrost leachates were prepared from soils collected in the summer of 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 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. Two permafrost leachates were prepared from permafrost soils collected at each site, except for Sagwon moist acidic tundra, which did not have an experimental replicate. 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 mm 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-mm filtered leachate was filtered through 10 mm (Geotech Environmental Equipment, Inc.) and then finally through 0.2 mm (Whatman), MilliQ-rinsed high-capacity cartridge filters. One liter of the final 0.2-mm filtered permafrost leachate (now referred to as permafrost leachate) was transferred to amber HDPE bottles for water chemistry analyses.
Water chemistry analyses (pH, specific conductivity, iron, DOC, and cations) were performed on each of the permafrost leachates, as previously described (Kling et al., 2000; Cory et al., 2013). Dissolved total and reduced iron concentrations were quantified using the Ferrozine method (Stookey, 1970) immediately after permafrost leachate preparation (detection limit = 1 µM; coefficient of variance, CV, < 2% on triplicate analyses; Cory et al., 2015). 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 (SR), specific UV absorbance at 254 nm (SUVA254), and fluorescence index were calculated as previously described (Cory et al., 2014). Permafrost leachates prepared from soils collected in 2018 were analyzed for cations on a Thermo Scientific 2 High-Resolution inductively coupled plasma mass spectrometer (ICP-MS; CV < 2% on triplicate analyses; Linge & Jarvis, 2009). DOC leached from soils collected in 2015 was isolated by solid-phase extraction, freeze-dried, and analyzed by solid-state 13C nuclear magnetic resonance (NMR; Ward & Cory, 2015, 2016). The percentages of aromatic and carboxyl carbon (C) within the DOC pool were calculated as previously described (Cory et al., 2007).
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