| Summary: | This dataset contains soil respiration, water chemistry, and soil gas data for thermokarst and reference tundra sites on the North Slope of Alaska. Data were collected around the Toolik Field Station (68.63°N, 149.60°W) in May-August of 2009-2012. Additional samples were collected from Feniak Lake (68.27°N, 158.34°W) in and Kelly River (67.94°N, 162.39°W). To test the impact of thermokarst on the hydrologic export of carbon and nitrogen, we measured carbon (DOC, DIC, and dissolved CH4), nitrogen, and major ions in outflows of thermokarst features and reference waters. For thermokarst features with surface flow, water samples were collected above, throughout and below the impacted area. We also sampled nearby reference water tracks and soil water unimpacted by thermokarst formation to serve as controls. Samples were analyzed for DIC and DOC using a Shimadzu 5000 TOC analyzer. Total dissolved nitrogen was measured with an Antek 7050 nitric oxide chemiluminescent detector plumbed in line with the TOC analyzer. A Dionex Ion Chromatograph was used to analyze samples for anions and cations (Cl-, NO3-, NO2-, SO42-, Ca+, Na+, NH4+, Mg2+, and K+). 18O was analyzed on a Picarro L1102-i isotopic analyzer. Dissolved CO2, CH4, and N2O was extracted from solution and analyzed on a Varian 3300 gas chromatograph. To characterize the temporal progression of thermokarst from formation to stabilization and recovery we classified features on a five point activity scale defined as follows: 0-No apparent present or past thermokarst impact
 1-Stabilized and revegetated, vegetated stream bed (clear outflow) 2-Limited thermo-degradation, cobble stream bed (clear outflow) 3-Moderate thermo-degradation with (somewhat turbid outflow) 4-Active thermo-degradation, (turbid outflow)
 5-Very active thermo-degradation (totally turbid outflow) To test the impact of thermokarst on gaseous carbon flux, we measured the gaseous release of CO2 and CH4 from control and impacted sites using a Li-COR Li-8100 automatic soil respiration monitoring system and static chambers. Because thermokarst disturbance creates a jumble of vegetation and mineral soil we classified sites by ground cover type. Divisions are defined as follows: Undisturbed tundra-More than 5 m outside any visible disturbance with no apparent past or present thermokarst activity Margins-Within 5 m of visible disturbance but have not experienced subsidence Drapes-Subsided but vegetation is still attached to surrounding tundra Tundra rafts-Subsided and detached from surrounding tundra Exposed-Surfaces where vegetation has been removed exposing bare mineral soil We measured soil temperature and soil moisture in situ at each site with a soil temperature probe and a ThetaProbe ML2x soil moisture sensor calibrated to local soils. We also sampled soil gas to characterize the distribution of CO2, CH4 and N2O production on the landscape. We measured soil gases with a 40 cm long 0.3 cm diameter hollow stainless steel tube with intake ports drilled in the final 5 cm. After inserting the sipper to a depth of 15 cm, we drew up and flushed 5 ml of soil gas through the tube and then collect the sample with an airtight syringe attached to the end of the sipper tube. Gas samples were analyzed on a Varian 3300 gas chromatograph for CO2, CH4, and N2O. To test the impact of thermokarst on the hydrologic export of carbon and nitrogen, we measured carbon (DOC, DIC, and dissolved CH4), nitrogen, and major ions in outflows of thermokarst features and reference waters. For thermokarst features with surface flow, water samples were collected above, throughout and below the impacted area. We also sampled nearby reference water tracks and soil water unimpacted by thermokarst formation to serve as controls. Samples were analyzed for DIC and DOC using a Shimadzu 5000 TOC analyzer. Total dissolved nitrogen was measured with an Antek 7050 nitric oxide chemiluminescent detector plumbed in line with the TOC analyzer. A Dionex Ion Chromatograph was used to analyze samples for anions and cations (Cl-, NO3-, NO2-, SO42-, Ca+, Na+, NH4+, Mg2+, and K+). 18O was analyzed on a Picarro L1102-i isotopic analyzer. Dissolved CO2, CH4, and N2O was extracted from solution and analyzed on a Varian 3300 gas chromatograph. To test the impact of thermokarst on gaseous carbon flux, we measured the gaseous release of CO2 and CH4 from control and impacted sites using a Li-COR Li-8100 automatic soil respiration monitoring system and static chambers. Because thermokarst disturbance creates a jumble of vegetation and mineral soil we classified sites by ground cover type. We measured soil temperature and soil moisture in situ at each site with a soil temperature probe and a ThetaProbe ML2x soil moisture sensor calibrated to local soils. We also sampled soil gas to characterize the distribution of CO2, CH4 and N2O production on the landscape. We measured soil gases with a 40 cm long 0.3 cm diameter hollow stainless steel tube with intake ports drilled in the final 5 cm. After inserting the sipper to a depth of 15 cm, we drew up and flushed 5 ml of soil gas through the tube and then collect the sample with an airtight syringe attached to the end of the sipper tube. Gas samples were analyzed on a Varian 3300 gas chromatograph for CO2, CH4, and N2O. We also measured rates of nitrogen transformation and size of nitrogen pools at multiple thermo-erosion gullies. Extractable pools of inorganic N were determined by extraction in 2 M KCl. NH4 concentration was determined using the phenol-hypochlorite method, and NO3- via cadmium reduction on a Bran+Luebbe Autoanalyzer 3. Soil pH was determined on slurries in deionized water that had equilibrated with the atmosphere for 30 min. Total C and N were measured following acidification of samples to remove inorganic C on a Costech 4010 elemental analyzer. Net rates of N mineralization and nitrification were estimated as net change in inorganic N or NO3- pools, respectively, following aerobic incubation for 7 days at 20 °C. Potential rate of nitrifi- cation was measured in aerobic slurries supplemented with 0.5 mM NH4? and 1 mM PO43- that were sub-sampled four times in 24 h. CaCl2 was added to each sub-sample as a flocculant, and solids were separated using a centrifuge. NO3-concentration of the supernatant was analyzed as previously described. Potential rate of nitrification was calculated as the change in NO3- concentration over the incubation time. We assayed potential denitrification enzyme activity using the acetylene block method. Media containing 722 mg NO3--N/L, 100 mg dextrose/L, and 10 mg chloramphenicol/L was purged of O2 using N2 and added to soils in gas-tight jars equipped with a stopcock, followed by purging with N2 for 2 min. Acetylene was added to the sample headspace (10 % v/v) to prevent the reduction of N2O to N2, and samples were vented to bring pressure to ambient. Following vigorous shaking, headspace gas was sampled and stored in evacuated containers. Headspace was sampled again after 4 h of incubation at 20 °C. Headspace N2O concentration was analyzed on a Varian CP-3800 gas chromatograph via electron-capture detection. Bunsen coefficients were applied to determine the mass of N2O dissolved in the slurry, and total N2O produced by each sample was used to calculate production of N2O over the incubation period. For complete methods see Harms et al. 2013 Thermo-erosion gullies increase nitrogen available for hydrologic export.
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