Gf d filters

Grain size distribution was analyzed by dispersing permafrost sediment in 0.01 M tetrasodium pyrophosphate and demineralized H₂O by 2 min of ultrasound and then analyzed by laser diffraction on a Mastersizer 2000 (Malvern Instruments Ltd, Malvern, UK). The pH-value was measured in soil slurries of 1:5 w/v ratio of soil to demineralized H₂O using a PHM 80 pH-meter (Radiometer, Copenhagen, Denmark). Water/ice content was determined gravimetrically by drying a permafrost subsample to a constant mass at 105 °C. Total C and N were determined by combustion of dry soil at 1200 °C and 800 °C, respectively, followed by analyses on a TrueSpec CN determinator (LECO Corporation, St Joseph, MO, USA). Extracts were made for the analysis of dissolved organic carbon (DOC), nitrate (NO₃⁻) and ammonium (NH₄⁺) by shaking 1:5 w/v of sediment to demineralized H₂O for 5 hours followed by centrifugation at 4300 RPM for 10 min and filtering the supernatant through GF/D filters (Whatman™, Maidstone, UK). The concentration of DOC was measured on the filtered soil extracts by first removing inorganic C by acidifying sample extracts with 2 M HCl, and then analyzing them on a TOC-5000A/SSM-5000A (Shimadzu, Kyoto, Japan). Concentrations of NO₃⁻ and NH₄⁺ were determined by flow injection analysis using a FIAstar™ 5000 (FOSS, Hillerød, Denmark) following the manufacturer′s instructions.

Soluble nitrogen species (NO₃⁻, NO₂⁻, and NH₄⁺) and gas components including C₂H₆, CO₂, and N₂ in the headspace were determined as described previously [16 (link)]. The butane, ¹³C-labelled butane, ¹³C-labelled ethane, ¹³CO₂, ²⁹N₂, and ³⁰N₂ in gaseous samples were quantified using a GC (7890A, Agilent, USA) coupled to a quadrupole mass spectrometer (MS, 5957C inert MSD, Agilent, USA). The GC–MS was operated as described in the supplementary text.
The isotopic fractions of ¹⁵N-labelled nitrogen-oxyanions (NO₃⁻ + NO₂⁻) were analysed using a Thermo Delta V isotope ratio mass spectrometer (IRMS; Thermo Fisher Scientific, USA) following conversion to N₂O via the denitrifier protocol [38 (link)]. To measure ¹⁵N-labelled NO₃⁻, NO₂⁻ was removed from the liquid samples with 4% (wt/vol) sulfamic acid in 10% HCl as described previously [39 (link)]. The fraction of ¹⁵N in NO₂⁻ was calculated according to the difference between ¹⁵N fraction in nitrogen-oxyanions (NO₃⁻ + NO₂⁻) and that in NO₃⁻. To analyse ¹⁵N-labelled NH₄⁺, NH₄⁺ was trapped in GF/D filters (Whatman, UK) with a microdiffusion method [40 (link)] and then combusted before IRMS analysis.