Abstract

Authors

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Dataset Citation

Jeff Heikoop, Heather Throckmorton. 2015. Nitrogen concentration and isotope dataset for environmental samples from 2012 and 2013, Barrow, Alaska. 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/10.5440/1179130.

Dates

2012-04-12 - 2013-09-09

Geographic Location

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GCMD Keywords

N/A

Methodology

2.2 Water Samples from Polygonal Terrain and Drainages A total of 184 surface water and active layer pore water samples were collected from an area of significant polygon development and from drainages on the broader surrounding landscape for determination of NO3- concentrations during the summers of 2012 and 2013. Active layer pore water sample depths ranged from approximately 5-57 cm from the soil surface. A subset of 24 samples active layer pore water samples in polygonal terrain and surrounding drainages yielded sufficient sample volume and NO3- concentrations (see Supporting Information) for analysis of 15N and 18O of NO3- (Table S1, Figure 1). These isotopic results are the primary focus of the study. Two sets of surface and active layer pore water samples were collected. Samples were collected from troughs and centers of high- and low-centered polygons (Figure 2), herein defined as polygonal samples. High- and flat-centered polygons are not differentiated, as both tend to have slightly elevated, more oxic centers. Additionally, in 2013, surface and active layer pore water samples were collected from the periphery of drainages (i.e. streams, ponded areas in channels, etc.) in and around the BEO (Figure 1 and Table S1), herein defined as drainage samples. Samples from drainages were typically collected from three depths at each location, including, 1) adjacent surface waters, 2) shallow subsurface pore waters (up to 15 cm depth from the surface; commonly organic-rich), and 3) at the frost table (bottom of the unfrozen active layer at the time of sampling; often mineral-rich and silty in texture; < 60 cm from the surface, Table S1). Surface waters were collected as grab samples in 1 L bottles (HDPE, Nalgene). Subsurface active layer pore waters from polygonal terrain and drainages were collected using a combination of techniques. For saturated shallow organic-rich soils, stainless steel drive points (Solinst; 2.1 cm inner diameter) were installed and water was pumped into 1 L bottles (HDPE, Nalgene) using a hand-pump vacuum or peristaltic pump (Masterflex). Macro-rhizons [Seeberg- Elverfeldt et al., 2005] were more effective for extracting water from deeper saturated soils at the frost table. Macro-rhizon samplers were installed into soils and pore waters were extracted within 2 h into 60-mL syringes. Passive capillary fiberglass samplers (wick samplers) [Frisbee et al., 2010a; Frisbee et al., 2010b], along with macro-rhizon samplers, were used to sample pore waters from the unsaturated soil in the centers of high-centered polygons. Wicks were installed at 5 cm depth in the organic horizon and 20 cm depth in the mineral horizon. Additionally, active layer wells made from slotted PVC pipes (10 cm diameter) were installed in the BEO intensive sampling area during the summer of 2012. Multilevel samplers (MLS; Margan, Netanya, Israel) were deployed in the PVC wells. The MLS consisted of a solid polyvinyl chloride (PVC) rod, which was drilled to receive horizontally oriented 40 mL cylindrical diffusion cells every 10 cm depth. The cells were filled with deionized water and sealed with 0.2 1/4 m nylon membranes. The cells were isolated by depth in the well casing with neoprene seals at 10 cm depth intervals [Crenshaw et al., 2010]. Solute equilibration was allowed to proceed for a minimum of one month. Subsurface samples for NO3- isotopic analyses were collected by macro-rhizons, diffusion cells, or passive wicks (Table S1), which have been shown to provide representative soil pore water chemistry [Gee et al., 2003; Feaga and Selker, 2004; Seeberg-Elverfeldt et al., 2005; Frisbee et al., 2010a; Frisbee et al., 2010b; Crenshaw et al., 2010]. Quantitative recovery of solutes reduces or eliminates the potential for isotopic fractionation associated with sampling. Surface and pore water samples were filtered (0.45 1/4 m) and stored on ice packs in the field then frozen until analyzed at the Geochemistry and Geomaterials Research Laboratory (GGRL), Los Alamos National Laboratory (LANL) (Los Alamos, NM, USA). NO3- and NH4+ concentrations were measured where sufficient sample volume was available (see Supporting Information for analytical details). Similarly, NO3- and NH4+ isotopes (15N and 18O, and 15N, respectively) were measured for samples with sufficient sample volume and NO3- and NH4+ concentrations (see Supporting Information for analytical details). The 15N of DON was also measured on a subset of samples. Water isotopes (2H) were measured on all samples except those collected by the MLS, the cells of which are initially filled with deionized water, thereby rendering such samples unusable for water isotopic analysis. 2.3. Snowmelt Freshly melted, ponded snowmelt and snowmelt runoff samples were collected as daily grab samples from 11 locations for 14 consecutive days during the brief two week snowmelt period (from 30 May 2013 through 13 June 2013); with the exception of one location where samples were not collected on 7 June 2013). One additional snow sample collected in May of 2013 prior to snowmelt was also analyzed for NO3- concentrations and isotopes. A total of 153 snow samples were measured for NO3- concentrations. 11 samples with the highest NO3- concentrations (representing 7 of the 11 locations; and the additional snow sample) were measured for NO3- isotopes (Table S1). The isotopic composition of dry or wet deposition of atmospheric NO3- during the growing season has not been determined for the site. 2.4. Permafrost and Active Layer SOM Permafrost soil cores were excavated from a range of microtopographic features (see Polygonal-Type and Location in Table 1) in and around the BEO (cores were obtained as described by Roy Chowdhury et al. [2015] or by using a hand held SIPRE auger system) during April of 2012 and April and May of 2013. Ice NO3- concentrations and isotopes were measured on a subset of these samples. Details on core subsampling and sample preparation are provided in the Supporting Information. Water isotopes were also determined for those permafrost samples amenable to NO3- isotopic analysis. Water isotope from massive ice samples were measured by conventional isotope ratio mass spectrometry (IRMS) while water isotopes from textural ice were measured by equilibration of thawed core samples with dry air in plastic bags and subsequent analysis by cavity ring down spectroscopy [Lis et al., 2008]. 15N of SOM was measured for 50 samples collected from soil cores taken from the active layer as part of NGEE vegetation studies performed by Oak Ridge National Laboratory (ORNL) researchers. 2.5 Chemical and Isotopic Analysis Detailed descriptions of chemical and isotopic analysis methodology for all sample types are provided in the Supporting Information.

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