Abstract

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Dataset Citation

Michael Philben, David Graham, Baohua Gu. 2020. Greenhouse Gas Production and Soil Chemistry in Anaerobic Soil Microcosm Incubations after Nitrogen Addition, Teller Road Site, Seward Peninsula, 2018-2019. Next Generation Ecosystem Experiments Arctic Data Collection, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee, USA. Dataset accessed on [INSERT_DATE] at https://doi.org/10.5440/1529003.

Dates

2017-06-01 - 2017-06-01

Geographic Location

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Methodology

Study site and soil collection Soil cores were collected from two locations within the same watershed in the Teller Road mile 27 site of the Next Generation Ecosystem Experiment (NGEE)-Arctic. One site (hereafter “Plateau”) is located on a peat plateau on the top of the hillslope (64.74514°N, 165.966551°W), and the other site (“Toeslope”) is located at the base of the hillslope (64.729193°N, 165.944072°W). Both sites are characterized by tussock tundra, sedge-dominated vegetation, and a water table at or near the soil surface. Cores from the two sites were collected in spring of 2017 prior to the thawing of the active layer. The core from the plateau was 76 cm in length, and the toeslope core was 84 cm. The cores were shipped frozen to Oak Ridge National Laboratory and stored frozen until the start of the incubation. The frozen cores were transferred to an anaerobic chamber and separated into organic and mineral soil layers based on visual inspection. The uppermost layers containing intact vegetation were removed. The 0-38 cm and 0-34 cm intervals were characterized as organic for the toeslope and the plateau soils, respectively. Core sections from 38-84 cm for the toeslope and 61-76 cm for the plateau were used as the mineral soil. Microcosm construction The separated cores were cut into small (<0.5 cm3) pieces using an oscillating cutting tool and mixed with a spoon, creating four homogenized samples (organic and mineral soils for the toeslope and the plateau). Soil microcosms were prepared by adding 7 g (wet soil) to 60 mL serum bottles. 1 mL of either MilliQ water (control treatment) or NH4Cl solution containing 32 mM N (+N treatment) was added to each microcosm. Three replicate microcosms were prepared for the control and +N treatments to be incubated at -2°C and 8°C for 55 days. In addition, three replicates were constructed for destructive sampling after 15 and 30 days for the 8°C treatment only. There were therefore 96 microcosms in total. The microcosms were sealed with blue rubber septa, crimped with aluminum caps, flushed with N2 for 10 minutes, and transferred to incubators at the appropriate temperature. Greenhouse gas and chemical analysis Concentrations of CO2 and CH4 were measured in the headspace of the microcosms every two days for the first two weeks, then every five days thereafter, following Roy Chowdhury et al. (2015). On each sampling day, 0.5 mL of the headspace was analyzed using an SRI 8610C gas chromatograph with a flame ionization detector (FID). CO2 was converted to CH4 using a methanizer for analysis by FID. The microcosms incubated at -2°C were kept in a cooler filled with ice packs during analysis to reduce temperature change during the incubation. Headspace CO2 and CH4 concentrations were converted to total gas production using Henry’s Law based on the temperature of incubation and measured soil pH (Sander, 2015). Microcosms were destructively sampled after 15, 30, and 55 days of incubation. In an anaerobic chamber, 2 g of each soil was extracted with 10 mL of degassed water or 0.1 M KCl in a 15 mL plastic tube and placed on a reciprocal shaker for 90 minutes. The soil extracts were centrifuged at 3000 RPM for 10 minutes and filtered through a 0.2 µm syringe filter. Aliquots of the KCl extracts were analyzed immediately for pH and ferrous Fe using the 1,10-phenanthroline method (Hach method 8146). NH4-N concentrations were also analyzed in the KCl extracts using the colorimetric salicylate and cyanurate method (Hach method 10031). The water extracts were analyzed for major anion content, low-molecular weight organic acid concentration, UV-visible absorbance, and water-extractable organic C (WEOC). Samples were either analyzed within three days of collection or frozen until analysis. Anions (Cl-, Br-, NO3-, and SO42-) and organic acids (formate, acetate, propionate, butyrate, and oxalate) were analyzed in the water extracts using ion chromatography following Herndon et al. (2015). The ions were separated using a 4µm Dionex IonPac AS11-HC column and gradient elution. The eluent was 1 mM KOH from 0-7 min, ramping to 15 mM from 7-16 min, 30 mM at 25 min, and 60 mM at 33 min. Ions were detected using a Dionex suppressed conductivity detector. Water-extractable organic carbon (WEOC) concentration in the soil extracts were analyzed using a Shimadzu TOC-L analyzed after acidification with 0.1% HCl. Ultraviolet-visible (UV-Vis) spectroscopy was also conducted on the water extracts in a quartz cuvette over the range 200–800 nm on a Hewlett-Packard 8453 spectrophotometer.

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