Study site
Disko Island is located off the coast of Western Greenland. The southern part of the island consists of Early Tertiary basaltic plateau mountains cut through by ancient glaciers forming U-shaped valleys such as the study site valley Blæsedalen (69°18′40.9″N; 53°30′40.9″W).
The climate in Blæsedalen is Arctic Maritime with mean annual air temperatures of -3 ± 1.8°C (1991–2017) and an increase of 0.16°C y− 1 in the same period (Zhang et al. 2019). The average precipitation (1991–2017) was 418 ± 131 mm y− 1, with about 40 % falling as snow (Hansen et al. 2006; Zhang et al. 2019).
The area is affected by permafrost with active layer depths largely related to drainage conditions. On the well-drained tundra slopes, maximum active layer depths reach 2–3 m, whereas the water-saturated fen areas have maximum thaw depths of 40–80 cm (D’Imperio et al. 2017). On the eastern slope with aspect towards the west, a snow accumulation area supplies meltwater from a semi-permanent snow fan throughout most of the summer. The slope is made up of colluvial stones to boulders, intersected by alluvial and nival deposits and with a soil depth of 40–90 cm.
In 2014, a study site was established on the slope, covering an area of 35×50 m. The slope site represents a vegetation gradient from top to bottom, with gradually different environmental conditions (Table 1). The growing season is longest at the footslope (> 10 weeks) and shorter upslope as a snow drift melts back. Some years, the snow drift persist the entire summer, some years it is gone by mid-July. This affects the vegetation types along the slope; with upslope dominated by Salix herbacea and downslope featuring dwarf shrubs (e.g. Salix arctica, Betula nana, Cassiope tetragona and Empetrum nigrum). The shrubs are interlaid with mesic tundra mosses such as Tomentypnum nitens, Racomitrium lanuginosum and Sphagnum sp. The active layer development is delayed upslope with later snowmelt, causing generally lower temperatures and higher soil moisture upslope in the early thaw season. In the peak and later parts of the growing season, snow meltwater drains downslope through deeper active layers, restricted by underlying frozen ground. The resulting conditions are generally warmer and drier upslope compared to downslope (Table 1). Water table depth at the footslope moves downward with the thawing of the active layer and fluctuates over the growing season from ~ 50–100 cm depending on evaporation and precipitation.
N deposition is low (~ 1 kg N ha− 1 y− 1), and N fixation supplies between 1 and 2 kg N ha− 1 y− 1 to the ecosystem with little variation (< 10 %) between years (Hobara et al. 2006; Rousk et al. 2017).
Table 1
General relationship between position on the slope and different environmental parameters.
Environmental parameter
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Top
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Bottom
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Growing season length
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Shorter
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Longer
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Soil moisture level
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Drier
|
Wetter
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Soil temperature, growing season
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Warmer
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Cooler
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Dominating plant species
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Salix Herbaceae
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Salix arctica, Betula nana, Cassiope tetragona, Empetrum nigrum
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N mineralization rate
|
Lower
|
Higher
|
Experimental design
At the foot of the slope site, five plots were established in June 2018 for monitoring of the physical and chemical ambient conditions (figure 1). The plots were positioned on a line perpendicular to the slope on mesic tundra heath with 2-5 m apart and had similar vegetation community (figure 1A). The plots thus represent the same position on the slope based on the vegetation composition. The study site receives meltwater from the above-lying snow drift area and is henceforth referred to as the Reception area. Water from the study site drains further into a lower-lying fen area 100 m further away.
In July 2018, plots (Fig. 1B) were equipped with sensors for hourly measurements of volumetric soil moisture and temperature at 2, 10 and 20 cm depth, in one plot also at 40 and 60 cm depth (Tinytag, Gemini Datalogger, UK; Decagon Devices, METER Group, USA).
Soil water suction cups (Prenart Equipment Aps, DK) were installed in all plots at 10–20 and 20–30 cm depth and water extracted twice a week during the growing seasons 2018 and 2019. Water was analyzed for Total Organic Carbon (TOC) (Shimadzu TOC analyzer; Kyoto, Japan), NH4+-N, NO3−-N and Total Dissolved Nitrogen (TDN) (FIAstar 5000; Höganäs, Sweden). Soil moisture may affect the volume, from which water is extracted, but the method is regarded a reasonable measure of soil water solute concentration in natural ecosystems (Singh et al. 2018). In immediate vicinity of the plots, a HOBO-20 pressure transducer (Onset Computer Corporation, MA, USA) measured water table depth during the thaw season.
Soil cores from 0–30 cm taken near the plots were split 0–10, 10–20 and 20–30 cm and analyzed for grain size distribution, total C and N content (Flash 2000, Thermo Scientific, Bremen, Germany), root biomass, root C and N, gravimetric water content (GWC%), and microbial C and N using chloroform-fumigation direct-extraction (Brookes et al. 1985). Aboveground vegetation was sampled in a 10×20 cm area in each plot; leaves and stems were ground and analyzed for total C and N.
Vertical soil hydraulic conductivity was measured near monitoring plots at depths 1–7, 12–18, 23–29, 35–41 and 51–57 cm using HYPROP/KSAT rings (Meter Group, CA, USA) carefully excavated for complete, undisturbed samples. Similarly, horizontal soil hydraulic conductivity was measured at 1–9, 12–20, 23–31, 35–43 and 51–59 cm depth.. Samples were analyzed using the falling head method (e.g. Angulo-Jaramillo et al. 2016).
In all plots, the surface exchange of CO2 and N2O was measured weekly from late June to late August using static chambers (Ambus et al. 1993). Chambers with fans to ensure air mixing and temperature loggers were placed on pre-installed frames in the plots for three hours, and every 45 minutes, an air sample was withdrawn and sent for analysis for content of trace gases (Agilent 7890A, CA, USA). On 30th June and 4th August 2018, six top soil samples (4 cm depth, 5.5 cm diam.) were taken adjacent to the plots, wrapped in plastic foil and bags, and frozen during transport. The N2O and N2 emissions were determined at in-situ water content at the Austrian Research Centre for Forests in Vienna using the gas-flow-core method, where gas exchange is measured in the headspace atmosphere, where N2 has been replaced prior to the accumulation phase with He/O2, such as described in Butterbach-Bahl et al. (2002).
15 N tracer experiment.
In order to test the fate of NO3- moving laterally, ten plots were established downstream of the Reception area plots (a view of one from above in figure 1C). 3rd of July, when active layer had reached a depth of 30 cm, and 3rd of August with an active layer depth of 90 cm, a 15NO3- solution was applied on to the top of the frozen surface in a line perpendicular to the slope above the tracer experiment plots so that the solution would move downslope and into the plot with the lateral water input on the frozen surface (cross-sectional view (A) and conceptual view from above (B) in figure 2). The solution consisted of KNO3-N (99% 15N) dissolved in 1.1 L H2O (0.085 g N L-1), corresponding to 0.15 g N m-2. A 110 mL solution was injected through a thin hollow rod at the top of the frozen surface for each 10 cm in a line of 125 cm perpendicular to the slope for each of the five replicates (figure 2B). Due to the low concentrations, the added N is not considered to have any fertilization effect, but is used only as a highly enriched tracer. To investigate the fate of the added 15N, soil samples 0-30 cm were taken and vegetation was harvested in an area of 10×20 cm on day 1, 3, 7 and 25 after injection, and the following year on the day with thaw depth corresponding to the thaw depth at injection (figure 2). The horizontal distance between injection and upper limit of sampling areas was 5-10 cm.
The following procedure was adopted and the following samples were obtained inside the plot and analyzed for % C, % N and 15N recovery using elemental analysis (Flash 2000, Thermo Scientific, Bremen, Germany) coupled to an isotope ratio mass spectrometer (Thermo Delta V Advantage IRMS, Thermo Scientific, Bremen, Germany). Soil samples were split in depths 0–10, 10–20 and 20–30 cm. As many roots as were feasible were manually removed from the samples, which were subsequently homogenized and analyzed for grain size distribution, gravimetric water content, bulk C and N and δ15N of the bulk soil N. Soil bulk density was measured on a subset of samples. Furthermore, subsamples were obtained and analyzed for microbial C, N and 15N recovery by comparing water extractable N and C with replicates subject to chloroform fumigation (Brookes et al. 1985) for 24 h prior to extraction. Root dry weight was quantified, and the roots were crushed and analyzed for total C, N and δ15N. In an area of 10×20 cm (Fig. 2), all aboveground biomass was harvested and separated based on plant species before air drying. Leaf and stem was separated, and each fraction was crushed and analyzed for % C, % N and δ 15N.
The ratio of the occurrence of stable isotopes 14N/15N compared to the reference 14N/15N ratio in Air-N2 is expressed as the δ15N value. The δ15N value of all sample materials was calculated as:
In order to achieve the recovery, the measured δ15N relative to the amount of N g− 1 in the background samples was converted to atom percentage of 15 N and subtracted from the measured amount in the experiment samples, and all pools were scaled to plot size using dry soil bulk density and the known size of the sampling areas. The scaled amount of 15N was subtracted from the known added 15N to obtain recovery in each pool, respectively, and the total recovery in percentage of added 15N.