Carbon and nitrogen pools in thermokarst-affected permafrost landscapes in Arctic Siberia

Ice rich Yedoma-dominated landscapes store considerable amounts of organic carbon (C) and nitrogen (N) and are 10 vulnerable to degradation under climate warming. We investigate the C and N pools in two thermokarst-affected Yedoma landscapes – on Sobo-Sise Island and on Bykovsky Peninsula in the North of East Siberia. Soil cores up to three meters depth were collected along geomorphic gradients and analysed for organic C and N contents. A high vertical sampling density in the profiles allowed the calculation of C and N stocks for short soil column intervals and enhanced understanding of within-core parameter variability. Profile-level C and N stocks were scaled to the landscape level based on landform 15 classifications from five-meter resolution, multispectral RapidEye satellite imagery. Mean landscape C and N storage in the first meter of soil for Sobo-Sise Island is estimated to be 20.2 kg C m -2 and 1.8 kg N m -2 and for Bykovsky Peninsula 25.9 kg C m -2 and 2.2 kg N m -2 . Radiocarbon dating demonstrates the Holocene age of thermokarst basin deposits but also suggests the presence of thick Holocene aged cover layers which can reach up to two meters on top of intact Yedoma landforms. Reconstructed sedimentation rates of 0.10 mm yr -1 0.57 mm yr -1 suggest sustained mineral soil accumulation across all 20 investigated landforms. Both Yedoma and thermokarst landforms are characterized by limited accumulation of organic soil layers (peat). We further estimate that an active layer deepening by about 100 cm will increase organic C availability in a seasonally thawed state in the two study areas by ~5.8 Tg (13.2 kg C m -2 ). Our study demonstrates the importance of increasing the number of C and N storage inventories in ice-rich Yedoma and thermokarst environments in order to account 25 for high variability of permafrost and thermokarst environments in pan-permafrost soil C and N pool estimates.

Abstract. Ice-rich yedoma-dominated landscapes store considerable amounts of organic carbon (C) and nitrogen (N) and are vulnerable to degradation under climate warming. We investigate the C and N pools in two thermokarst-affected yedoma landscapes -on Sobo-Sise Island and on Bykovsky Peninsula in the north of eastern Siberia. Soil cores up to 3 m depth were collected along geomorphic gradients and analysed for organic C and N contents. A high vertical sampling density in the profiles allowed the calculation of C and N stocks for short soil column intervals and enhanced understanding of within-core parameter variability. Profile-level C and N stocks were scaled to the landscape level based on landform classifications from 5 m resolution, multispectral RapidEye satellite imagery. Mean landscape C and N storage in the first metre of soil for Sobo-Sise Island is estimated to be 20.2 kg C m −2 and 1.8 kg N m −2 and for Bykovsky Peninsula 25.9 kg C m −2 and 2.2 kg N m −2 . Radiocarbon dating demonstrates the Holocene age of thermokarst basin deposits but also suggests the presence of thick Holoceneage cover layers which can reach up to 2 m on top of intact yedoma landforms. Reconstructed sedimentation rates of 0.10-0.57 mm yr −1 suggest sustained mineral soil accumulation across all investigated landforms. Both yedoma and thermokarst landforms are characterized by limited accumulation of organic soil layers (peat).
We further estimate that an active layer deepening of about 100 cm will increase organic C availability in a seasonally thawed state in the two study areas by ∼ 5.8 Tg (13.2 kg C m −2 ). Our study demonstrates the importance of increasing the number of C and N storage inventories in icerich yedoma and thermokarst environments in order to account for high variability of permafrost and thermokarst environments in pan-permafrost soil C and N pool estimates.

Introduction
Yedoma regions store large amounts of soil organic carbon (SOC) and are highly vulnerable to permafrost thaw under climate warming Strauss et al., 2013). Organic C, freeze-locked for millennia in these permafrost deposits, may become available for increased microbial activity and be released in the form of the greenhouse gases CO 2 and CH 4 after thaw (Gruber et al., 2004;Tarnocai, 2006;Dutta et al., 2006;Schuur et al., 2008Schuur et al., , 2015. Carbon-rich yedoma regions therefore have the potential to contribute significantly to greenhouse gas emissions in a rapidly warming Arctic (Schneider von Deimling et al., 2015).
The term yedoma describes late Pleistocene, ice-rich, silty deposits covering vast areas in the northern permafrost regions that were not glaciated during the last ice age (Schirrmeister et al., 2011a. These deposits can reach a thickness of up to 50 m, store large amounts of C and are still present today in an area of approximately 625 000 km 2 in the Siberian and North American permafrost region . Since deglacial warming, large areas M. Fuchs et al.: Carbon and nitrogen pools in thermokarst-affected permafrost landscapes of the former yedoma surfaces, expanding across several million km 2 of northern and eastern Siberia, Alaska, northwestern Canada, and the adjacent shelf regions, were eroded by thermokarst processes. These processes affected the landscape by permafrost thawing and ice wedge melting leading to surface lowering, lake formation, and lake drainage. Resulting landscape features include thermokarst mounds (also called baydzherakhs), thermo-erosional gullies, thermokarst lakes, and thermokarst depressions (drained thaw lake basins or alas) which now are dominant features in the yedoma terrain (Jorgenson and Shur, 2007;Veremeeva and Gubin, 2009;Kokelj and Jorgenson, 2013;Morgenstern et al., 2013;Ulrich et al., 2014).
Ice-rich yedoma and thermokarst deposits are prone to thaw after disturbances that change the surface thermal regime (e.g. from changing air temperatures, fire, or anthropogenic disturbances). Since yedoma as well as thermokarst deposits contain large, potentially vulnerable C pools in permafrost regions Strauss et al., 2013;Walter Anthony et al., 2014) they are of global importance for SOC accumulation, degradation, and release. Whereas degrading yedoma surfaces and thermokarst features lead to thaw-induced remobilization of organic C, the drainage of thermokarst lakes and the following permafrost aggradation lead to SOC accumulation and stabilization (Hinkel et al., 2003;Jorgenson and Shur, 2007;Jones et al., 2012;Grosse et al., 2013). Olefeldt et al. (2016) estimated that landscapes potentially vulnerable to thermokarst contain up to 50 % of the total SOC stored in the permafrost region. In the yedoma region, 60 % of the C is stored in drained and refrozen thermokarst basins (Strauss et al., 2013). However, these estimates are based on a few data points from the yedoma region and it remains important to enhance our understanding of the C storage, distribution, and vulnerability in these environments with detailed landscape-scale assessments.
Several permafrost C pool estimates have been carried out in different areas within the Arctic region (e.g. Michaelson et al., 1996;Kuhry et al., 2002;Ping et al., 2008;Tarnocai et al., 2009;Horwath Burnham and Sletten, 2010;Hugelius et al., 2010Hugelius et al., , 2011Fuchs et al., 2015;Siewert et al., 2015;Palmtag et al., 2015). A synthesis for the circum-Arctic by Hugelius et al. (2014) merged a large number of data sets into the Northern Circum-polar Soil C Database (NC-SCD) resulting in an estimate of 1035 ± 150 Pg of organic C for 0-3 m for soils in the permafrost region. Several additional estimates for SOC storage in the yedoma region exist, some of which take into account slightly different components of the yedoma region. Zimov et al. (2006) calculated 450 Pg C for the C pool for the entire yedoma region (including both yedoma and thermokarst deposits). Walter Anthony et al. (2014) estimated the total Holocene and Pleistocene soil C pools of the yedoma region with 429 ± 101 Pg C, while Hugelius et al. (2014) calculated 181 ± 54 Pg C for all deposits in the yedoma region below 3 m depth and Strauss et al. (2013) calculated 211 + 160/ − 153 Pg C for the entire yedoma deposits including the top 3 m. Despite the variation in these estimates they all suggest a very large C pool of several hundred Pg for this region and confirm that these ice-rich deep deposits are a globally important C pool in the northern circumpolar permafrost region. Detailed local studies for particular parts of the yedoma region are scarce so far but suggest significant landscape-scale and interregional variation in SOC stocks that warrant further local studies and regional syntheses (Schirrmeister et al., 2011b, c;Strauss et al., 2012;Siewert et al., 2015Siewert et al., , 2016Webb et al., 2017).
However, the yedoma and thermokarst landscapes not only host important C pools but potentially store a significant amount of N. Even though there are several permafrost soil C studies, only very few report N stocks (Ping et al., 2011;Harden et al., 2012;Michaelson et al., 2013;Zubrzycki et al., 2013;Obu et al., 2017;Palmtag et al., 2016). In tundra environments, N often is the limiting factor for plant growth (Shaver et al., 1986;Chapin et al., 1995;Mack et al., 2004;Beermann et al., 2015). As a result, thawing permafrost does not only have the potential to release large amounts of organic C but can also increase the availability of N which may increase primary production and partly offset increased greenhouse gas emissions from permafrost soils (e.g. Keuper et al., 2012Keuper et al., , 2017Natali et al., 2012;Salmon et al., 2016).
A potential indicator of the potential C loss upon thaw and the decomposability of C stored in permafrost soils is the carbon to nitrogen (C / N) ratio (Schädel et al., 2014). Generally, a higher C / N ratio suggests less degraded organic C, while a lower C / N ratio points towards already partly degraded C before incorporation into permafrost (Weiss et al., 2016). Even though the C / N ratio cannot be taken as a single variable to estimate the recalcitrance of the soil organic matter, it provides a first indication of the potential degradability of C stored in permafrost soils (Kuhry and Vitt, 1996;Hugelius and Kuhry, 2009;Hugelius et al., 2012).
In general, ice-rich landscapes are underrepresented in pan-Arctic permafrost C pool inventories and the variability of these environments is poorly accounted for. In particular, the deep C and N stocks below 1 m depth remain associated with high uncertainties due to low sample numbers. In addition, thermokarst areas are temporally dynamic and heterogeneous environments with characteristics that are difficult to generalize and characterize. A wide range of local case studies are needed to capture spatial variability and to improve C pool estimations. This study presents new data on C and N storage in two yedoma regions in the north of eastern Siberia and analyses the variability of C and N contents along landscape gradients extending from yedoma uplands into adjacent drained thaw lake basins (DTLBs) investigating whether C and N storage is significantly higher in DTLBs than in yedoma soils as proposed in regional studies by Strauss et al. (2013) or Walter Anthony et al. (2014).  Santoro and Strozzi, 2012), and (c) overview map including the permafrost zonation in Siberia (after Brown et al., 1997).
In this study, sediment cores up to 3 m depth from typical thermokarst-affected yedoma regions were analysed (1) to quantify the organic C and N variability with depth and along geomorphologic gradients in thermokarst-affected terrain; (2) to understand depositional chronologies as well as C accumulation rates; and (3) in combination with a satelliteimage-based terrain classification to quantify landscapescale SOC and N inventories for the study areas Bykovsky Peninsula and Sobo-Sise Island.
2 Material and methods

Study area
The two study areas are situated in the north of eastern Siberia in the Lena River delta region (Fig. 1). The first study site, Sobo-Sise Island (72 • 29 N, 128 • 16 E), is a yedoma remnant within the eastern Lena Delta. These remnants are commonly referred to as the third geomorphological river delta terrace (Schwamborn et al., 2002). According to Morgenstern et al. (2011), Sobo-Sise Island accounts for almost 16 % of the entire yedoma coverage in the Lena Delta. It is characterized by yedoma uplands but also features permafrost degradation landforms such as thermokarst lakes, drained thaw lake basin, and thermo-erosional gullies. Sobo-Sise is located between two of the main channels of the Lena Delta (Sardakhskaya and Bykovskaya channels), elongated in east-west direction (45 km) and has an area of 336 km 2 . Very few studies have been conducted on the depositional characteristics of Sobo-Sise. For example, Grigoriev (1993) dated a mammoth bone from a yedoma cliff on Sobo-Sise and Grigoriev (2007) investigated the shore erosion on Sobo-Sise's coastline. Morgenstern et al. (2011) as well as Nitze and Grosse (2016) included Sobo-Sise in regional remote sensing studies aiming at the quantification of thermokarst lakes and basins and surface landscape changes, respectively.
The second study site, Bykovsky Peninsula (71 • 51 N, 129 • 19 E), is similar to Sobo-Sise and also dominated by yedoma uplands that are intersected by thermokarst lakes, drained thermokarst lake basins and thermo-erosional gullies. The Bykovsky Peninsula is located to the south-east of the Lena River delta at the Laptev Sea coast and represents an erosional remnant of a late Pleistocene yedoma accumulation plain blanketing the foreland of low mountain ridges (Grosse et al., 2007). The peninsula is a narrow tongue of land with a width between 1 and 4 km and an area of 167 km 2 . In contrast to Sobo-Sise Island in the Lena Delta, the Bykovsky Peninsula is strongly affected by rapid coastal erosion from all sides and by coastal inundation, manifested in several thermokarst lagoons. Studies on the Bykovsky Peninsula focused mostly on palaeoenvironmental reconstructions using late Quaternary deposits at the exposure Mamontovy Khayata (Kunitsky, 1989;Slagoda 1991Slagoda , 1993Grigoriev, 1993;Siegert et al., 2002;Meyer et al., 2002;Schirrmeister et al., 2002a). Kholodov et al. (2006) described organic matter characteristics in Alas and yedoma deposits, Grigoriev et al. (1996) and Lantuit et al. (2011) determined coastal erosion rates on the coasts of Bykovsky Peninsula, and Grosse et al. (2005) mapped the periglacial geomorphology based on remote sensing data.
The climate of Sobo-Sise Island and Bykovksy Peninsula is continental, despite their proximity to the coast. Both sites are dominated by polar tundra climate (Peel et al., 2007). The mean annual air temperature (MAAT; period 1998-2011) on Samoylov Island (the closest weather station to Sobo-Sise, around 50 km to the west) is −12.5 • C with an approximate mean annual precipitation of 180-200 mm (including mean summer rainfall and mean snow water equivalent, period 1998-2011) . On Bykovsky, the climate is similar with a MAAT of −12.9 • C (period 1933-2013) at the closest weather station in Tiksi (Günther et al., 2015), but a higher mean annual precipitation with around 240-260 mm (Grigoriev, 1993). The higher precipitation can be explained by the influence of the Laptev Sea and the mountain ridge nearby in the hinterland . At both study sites, permafrost is continuous and is assumed to have a thickness of about 500-650 m (Griogriev, 1993). Boike et al. (2013) reported a mean annual permafrost temperature at Samoylov Island to be −8.6 • C in 10.7 m depth.
At both study sites, the tundra vegetation is sparse and is limited by the short growing season (June to September). The vegetation composition is dominated by sedges, grasses, mosses, lichen, and sporadic dwarf shrubs (Salix species). According to Schneider et al. (2009) most of Sobo-Sise Is-land is classified as moist grass-and moss-dominated tundra and wet sedge-and moss-dominated tundra. In the Circum Arctic Vegetation Map (Walker et al., 2005), large parts of the Lena River delta including Sobo-Sise Island are classified as sedge, moss, dwarf shrub wetland and Bykovksy Peninsula is classified as non-tussock sedge, dwarf-shrub, moss tundra.
Following the USDA Soil Taxonomy (Soil Survey Staff, 2014) the soil types at both study sites belong to the Gelisol order with its suborders Turbels and Orthels. Only two soil profiles sampled in thermokarst deposits in this study contain thicker organic layers and can be considered as Histels. Permafrost occurs at almost all sampled sites within the first metre. Active layer thickness ranges from a minimum of 18 cm in thermokarst deposits to a maximum of 84 cm in sandy fluvial deposits with a mean of all sampled sites of 40 cm (median 41 cm). Only at two sites in non-vegetated, sandy, temporally flooded Lena River floodplain deposits of Sobo-Sise, permafrost was not present near the surface and these soils can be classified as Entisols.

Field work
Soil samples were collected in August 2014 along four transects of 500 (BYK14-T3) and 600 m (BYK14-T2, SOB14-T1, SOB14-T2) length. We chose transects to cover key geomorphologic gradients of the thermokarst-affected landscapes starting with yedoma uplands and passing through different stages of yedoma degradation (including yedoma slopes) and stages of the thermokarst lake cycle. Similar transect-based approaches were used in other regions of Siberia (Siewert et al., 2015;Palmtag et al., 2015), Canada (Hugelius et al., 2010), and Alaska (Jorgenson, 2000;Kanevskiy et al., 2014). We determined the starting points as well as the direction of each transect. To include randomness, the sampling points along the transects were then chosen at equidistant intervals in alignment with the scale of landscape features at a site. A 150 m distance between sampling points was chosen on the first transect (SOB14-T1) and a 100 m distance on the second transect (SOB14-T2). The first transect on Sobo-Sise extended from a yedoma upland into the adjacent DTLB. The second transect extended from a yedoma upland, crossing a DTLB and ending in fluvial deposits of the floodplain. Additional non-random sample points were collected on a baydzherakh (erosional remnants of polygon centres forming thermokarst hills) and two DTLBs. No cores were taken from extant thermokarst lakes or thermoerosional gullies. On Bykovsky Peninsula, transects were chosen with the same approach with a 150 m distance on the first transect (BYK14-T2) and 100 m on the second transect (BYK14-T3) between the sampling points. BYK14-T2 runs from one DTLB over a yedoma remnant covered with baydzherakhs into another adjacent DTLB. BYK14-T3 was again similar to the Sobo-Sise transects, running from the top of a yedoma upland into an adjacent DTLB.
Along the transects, soil pits were excavated down to the bottom of the active layer. A soil profile description was made and fixed-volume samples were collected with a metal cylinder of known volume. After sampling the active layer, permafrost cores and samples were collected with a SIPRE (Snow, Ice and Permafrost Research Establishment) auger barrel drill (Jon's Machine Shop, Fairbanks, USA) with a diameter of 7.62 cm (3 ). Total sampling depths ranged from 45 to 318 cm. As a result, at 23 sites the whole soil profile was sampled and characterized including active, transient, and permafrost layers and at five additional sites only the active layer was sampled. A permafrost core description was made and the cores were subsampled in the field at 5 to 10 cm intervals depending on facies horizons. The visual core description included cryostratigraphy according to French and Shur (2010) as well as a description of the lithology and plant macrofossils.

Laboratory analysis
In total 455 samples were processed for total carbon (TC), total nitrogen (TN), and total organic carbon (TOC). Samples were freeze-dried, homogenized by grinding prior to a measurement of 5-8 mg of the samples with a Vario EL III elemental analyser for TC and TN. Both %C and N were measured in one run. We measured two replicates of each sample where we accept a < 5 % deviation for the two measurements. The sensitivity of the elemental analyser is < 0.1 %. Afterwards, samples were measured for TOC (15-100 mg, depending on the TC content) with a Vario Max C elemental analyser. Again, we allow a deviation of < 5 % for the double measurements. Total inorganic carbon (TIC) is then calculated as the difference between TC and TOC. In addition, the C / N ratio was calculated as quotient between TOC and TN contents.
Based on Hugelius et al. (2010), the (SOC) storage was calculated for each sample using Eq. (1): Here, TOC is the total organic C content derived from the elemental analysis, BD is the dry bulk density of the sample, CF is the coarse fragment fraction (∅ > 2 mm) (which was zero because of the absence of coarse fragments in the sampled soils), and length is the actual sample length. The samplespecific SOC contents were added up to the reference depths of 0-30, 0-100, 0-200 cm. If the cores were not recovered completely, missing core intervals or missing SOC contents were interpolated between adjacent samples or from samples with the same characteristics following the field notes. The ice content in a sample is reflected in the dry bulk density and therefore included in the calculations. The SOC contents for the different reference depths, however, do not include ice wedge volumes. Ice wedge volume was included in a later step when scaling site-specific SOC stocks to the landscape level. Likewise the N storage was calculated for individual samples and for the different reference depths. In addition, 25 subsamples from various depths of seven different sampling sites were chosen for radiocarbon dating of organic macrofossils. All samples were wet-sieved with a 2 µm sieve and plant macro remains (mostly moss leaves or sedge stems) were hand-picked under a microscope. In a few cases with insufficient macro remains, bulk samples were selected. Samples were then submitted to the Radiocarbon Laboratory in Poznan, Poland, where the samples were analysed and dated with the accelerated mass spectrometer (AMS) dating method (Goslar et al., 2004). The obtained radiocarbon ages were eventually calibrated with the Calib 7.1 software to calibrated radiocarbon years before present (cal. a BP) (Stuiver and Reimer, 1993;Stuiver et al., 2017).

Landform classification and upscaling C and N pools
All geospatial analyses were performed in the ESRI ArcGIS 10.1 and ENVI 5.3 software. For both Sobo-Sise Island and Bykovsky Peninsula, multispectral RapidEye Images (pixel resolution 5 m) in combination with high-resolution digital elevation models (DEMs) were used to classify the landscape into the dominant landscape features. For Sobo-Sise Island two scenes from the same date (27 July 2014) were orthorectified and mosaicked together to cover the entire island. To improve the data basis, the RapidEye mosaic from the 27 July 2014 was stacked with another RapidEye scene (acquisition: 30 June 2014) which covers the entire island. The same scenes (27 July and 30 June 2014 images) were also used for Bykovsky Peninsula, stacked together with an additional scene acquired on 9 September 2014 since the first scene does not cover the entire peninsula. This procedure ensures that additional landform variability from the phenological stages of the tundra vegetation at both study sites is captured entirely by at least two RapidEye scenes. A maximum likelihood supervised classification relying on training areas was used to classify the landscape into the predominant landscape features. Identification of training areas has also been facilitated through near-simultaneous acquisition of RapidEye imagery to our field work. Training areas were chosen based on field notes and field knowledge. For Sobo-Sise Island the different RapidEye images were stacked with a DEM (pixel resolution 2 m) that had been derived from photogrammetric processing of three overlapping GeoEye stereo image pairs (acquisition dates range from 27 July to 15 August 2014). For Bykovsky Peninsula, a DEM was derived from overlapping WorldView-1 and WorldView-2 stereo image pairs (acquisition period: 25 to 29 August 2015) and again combined with the 15-band deep multispectral RapidEye image stack to run the maximum likelihood classification. Adding the DEM allowed enhanced classification of yedoma uplands, since plant communities on yedoma uplands cannot be entirely distinguished from those in DTLBs based only on spectral signatures. This advantage was already demonstrated by Grosse et al. (2006) and Siewert et al. (2016), who showed that, by including a DEM, non-degraded yedoma uplands and partly degraded yedoma uplands could be better discriminated compared to image classification only.
The two landform classifications for Sobo-Sise and Bykovsky Peninsula initially included the main geomorphological units yedoma upland, partly degraded yedoma (yedoma slope), and DTLBs of different generations. However, due to the small amount of collected sampling sites in DTLBs of different generations, the landform classes of DTLBs were merged to a single-class thermokarst for the upscaling. The final landform classification used for upscaling included the following classes: yedoma uplands, degraded yedoma/yedoma slope, thermokarst, and lakes. The areas of lakes were excluded from upscaling since this study focuses on terrestrial soil C storage and no lake cores were collected.
The landform classification accuracy was based on fieldbased ground truthing points complemented with data points extracted from high-resolution imagery. The high-resolution imagery include a GeoEye (Sobo-Sise Island), WorldView2 (Bykovsky Peninsula) and aerial photography to assess the correctness of classification and overlap with the sampled field sites. For each study area, 300 randomly selected points and an additional 20 points for each landform class were manually classified and then compared with the landform classification based on the RapidEye satellite imagery.
The total SOC and N storage for Sobo-Sise and Bykovsky Peninsula was based on mean C stocks of the collected sampling sites for the reference depths 0-30, 0-100, and 0-200 cm of each class. The mean stocks were upscaled based on the areal extent of the corresponding landform class. This approach allows a first estimate of the potential C and N storage in the study areas for the first 2 m of soil. Confidence intervals for the mean SOC and N landscape stocks were calculated according to . However, these confidence intervals do not include uncertainties evolving from the landform classification-based upscaling.
To avoid overestimating the C and N stocks in the upscaling, we accounted for the ice wedge volume in the landscape carbon and nitrogen calculations. Ice wedge contents for thermokarst deposits were adopted from the study of Ulrich et al. (2014) as a mean value derived from the maximum ice wedge contents from their three north-eastern Siberian study areas Ebe-Basyn-Sise, Cape Mamontov Klyk and Buor Khaya Peninsula. This resulted in a mean ice wedge volume of 9 ± 3 % (standard deviation) for thermokarst deposits which is similar to what Kanevskiy et al. (2013) found for drained lake basins in Arctic Alaska with 8 %. For the estimation of yedoma upland ice wedge volume, the GIS-based approach from Günther et al. (2015) was applied to Sobo-Sise. We orthorectified the GeoEye image using our highresolution DEM in order to ensure consistent mapping in an image free of geometrical distortions. We mapped more  Table S2.

Sedimentological results
The total nitrogen (TN) contents show a similar pattern to the TOC values. There is less TN in the samples from Sobo-Sise. There are, however, only very small differences in TN between the different landform types for both study areas. Whereas there is slightly more TN in yedoma upland samples than in thermokarst samples on Sobo-Sise, there is less in yedoma uplands compared to thermokarst on Bykovsky Peninsula. However, when looking at the median, the sample series (yedoma upland on Sobo, yedoma upland on Bykovsky and thermokarst on Bykovsky) tend to be similar (see Table S2 with the median values).
Volumetric ice contents are very similar for all the sampled cores. For both study sites as well as for yedoma and thermokarst the mean values range between 61 and 67 %. The only exceptions are the samples from fluvial sandy deposits on Sobo-Sise Island with a mean value of 45 % (see Table 1). Active layer samples were excluded from this analysis.
C / N ratios also show a similar pattern across all classes, with mean C / N ratios ranging from 10.7 to 13.2 for the different geomorphological units. However when separating C / N ratios into active layer and permafrost layer samples, active layer samples show a higher C / N ratio at both study sites and in all classes. Mean C / N ratios for active layer samples (samples from cores of same landform units combined) are 15.8 for Sobo-Sise yedoma uplands, 20.1 for Sobo-Sise thermokarst, 12.5 for Bykovsky yedoma uplands, and 17.0 for Bykovsky thermokarst. For permafrost samples mean C / N ratios are 11.1 for Sobo-Sise yedoma uplands, 10.3 for Sobo-Sise thermokarst, 10.1 for Bykovsky yedoma upland and 11.7 for Bykovsky thermokarst. In all cases, active layer samples have higher C / N ratios and there is a moderate correlation of decreasing C / N ratio with increasing depth for three classes, and one weak correlation (Bykovsky thermokarst) of decreasing C / N ratio with increasing depth (Fig. 2).

Sampling site SOC and N stocks
The TOC and bulk density values were used to estimate the site SOC stocks which were averaged for the different landform types. Mean sampling site SOC stocks (excluding ice wedge volume) were higher for yedoma upland sites than for thermokarst sites for the reference depths of 0-30 and 0-100 cm for both study sites (Fig. 3). SOC storages for 0-100 cm for Sobo-Sise are 25.3 ± 8.0 kg C m −2 for yedoma upland and 19.2 ± 5.9 kg C m −2 for thermokarst sites. For fluvial deposits only one profile down to 1 m depth was collected with an SOC stock of 11.2 kg C m −2 . Also for 0-200 cm, more SOC is stored in yedoma upland soils. The findings for Bykovsky Peninsula are similar with more C stored at yedoma uplands sites in the first metre of soil than at thermokarst sites, despite the fact that a higher mean TOC content was found in the thermokarst samples. For 0-100 cm yedoma upland sites store 29.7 ± 12.9 kg C m −2 and thermokarst sites store 23.9 ± 9.7 kg C m −2 . For 0-200 cm there is more organic C stored in thermokarst than in yedoma upland soils. However, this estimation is only based on one relatively C-rich core (BYK14-T3-3), since this is the only  When reaching an ice wedge in a collected site, this was included in the extrapolation as well, assuming no N for ice wedge layers. Soil N data for this graph are presented in the supplementary material, Tables S1 and S5. core reaching a depth of 2 m for thermokarst on Bykovsky Peninsula. Therefore the carbon estimation of thermokarst on Bykovsky Peninsula for the soil interval 0-200 cm has to be interpreted carefully.
The mean sampling site soil N stock (excluding ice wedge volume) for yedoma upland sites on Sobo-Sise Island is 2.3 ± 1.1 kg N m −2 (0-100 cm) and for thermokarst sites it is 1.4 ± 0.4 kg N m −2 (Fig. 4)  sula are 2.6 kg N m −2 ± 0.9 for yedoma upland sites and 1.9 kg N m −2 ± 0.6 for thermokarst sites.

Upscaling: landscape SOC and N stocks
The landform classification (Fig. 5) shows that 43 and 51 % of the landscape on Sobo-Sise and Bykovsky, respectively, are yedoma or partly degraded yedoma. Thermokarst depressions cover approximately 43 % on Sobo-Sise and 38 % on Bykovsky (excluding lakes and lagoons which cover 14 and 11 % of the landscapes, respectively). Based on the landform classification and the sampling site C contents the total C storage for the two study regions was calculated. In total, 5.81 Tg of organic C are stored in the first metre of soil on Sobo-Sise (288 km 2 ) of which around 57 % are stored within the active layer. These calculations include a landscape-wide ice wedge volume of 40 % for yedoma and 9 % for thermokarst, which is always applied to deposits below the active layer. This results in an average SOC storage for non-lake areas on Sobo-Sise of 20.2 ± 2.9 (95 % confidence interval) kg C m −2 for 0-100 cm. For Bykovsky Peninsula (154 km 2 ) the results are similar. In the first metre of soil, 3.98 Tg of organic C are stored of which 58 % are stored in the active layer, including an ice wedge volume of 44 % for yedoma and 9 % for thermokarst. This results in a landscape average of 25.9 ± 9.3 kg C m −2 (excluding lakes) for 0-100 cm.
Besides the organic C, there is a considerable amount of N stored in the soils of Sobo-Sise Island and Bykovsky Peninsula. About 0.53 Tg of N is stored on Sobo-Sise, resulting in a mean N storage of 1.8 ± 0.2 kg N m −2 (0-100 cm, excluding lakes). On Bykovsky, a total of 0.34 Tg of N is stored in the first metre of soil. This results in a mean N stock of 2.2 ± 0.5 kg N m −2 for 0-100 cm. Mean N and organic C storage for the reference depths and the two study areas are summarized in Table 2 and the total landscape stocks are presented in Table S3.

Radiocarbon dates
In general, the radiocarbon dates (Table 3) indicate that organic matter in the first 2 m (and at one thermokarst site down to 3 m) in both study areas is predominantly of Holocene age. Only one yedoma upland site (BYK14-T3-6B) clearly has late Pleistocene ages around 1 m depth. The two other dated yedoma upland sites BYK14-T2-3 and SOB14-T2-2 indicate the presence of a thick Holocene cover layer exceeding 1.67 and 2.23 m, respectively. In general, only one age-depth inversion was found (SOB14-T2-5). At all other sites, organic matter age increased with depth.
In addition, cumulative SOC storage and radiocarbon dates were combined to calculate organic C accumulation rates (Table 4). Figure 6 shows the radiocarbon ages plotted against the cumulative SOC for each sampling site, indicating the C accumulation rates. The plots show that the C accumulation rate was fairly linear in all of the cores, especially when removing one age-inversion from a core (SOB14-T2-5 with 5517 cal yr BP) and one outlier (the exceptional old date from BYK14-T2-3 with 45 203 cal yr BP); however the accumulation rates vary for both the two classes thermokarst and yedoma upland soils as well as for single sampling sites. The highest mean SOC accumulation rate is found at the thermokarst site SOB14-T2-5 with 49.7 g C m −2 yr −1 (and 300 cm of sediment accumulation), which is almost 20 times higher than SOB14-T1-5, another thermokarst site from Sobo-Sise which has a mean organic C accumulation rate of 2.7 g C m −2 yr −1 (and 200 cm of sediment accumulation). This is also reflected in the different sediment accumu- lation rates (Table 4). This high variability in sediment and C accumulation rates reveals that even within a small area (the sites are located within 3 km) high heterogeneity exists in soil forming and C accumulation processes.

Site specific soil organic C and N stock characteristics
We found that particularly DTLBs contain less C than estimates from other studies (Strauss et al., 2013;Walter Anthony et al., 2014). However, Strauss et al. (2013) and Walter Anthony et al. (2014) also included samples from greater soil depths and partially included sites from boreal regions with higher net primary productivity, whereas our study focused on the first 3 m of the soils in a high-latitude tundra region. Strauss et al. (2013) did a C inventory for the entire yedoma region resulting in a SOC storage of 10 + 17/ − 6 kg m −3 for yedoma and 31 + 23/ − 18 m −3 for thermokarst deposits. Thermokarst sites on Sobo-Sise Island are especially more depleted in SOC and store less N than the yedoma upland sites. In contrast to several previous studies investigating drained thermokarst lake basin peat accumulation in Alaska, (Bockheim et al., 2004;Hinkel et al., 2003;Jones et al., 2012) and the Kolyma region in Siberia (Walter Anthony et al., 2014), it is clear that the investigated DTLB soils in Sobo-Sise and Bykovsky do not show signs of increased peat formation and contain only thin organic layers. Organic layer depths of the studied DTLBs on Sobo-Sise and Bykovsky Peninsula are largely less than 10 cm with only two sites having thicker organic-rich peaty layers at the top. This indicates that the conditions for peat accumulation in these DTLBs were not favourable. Also for yedoma upland soils, organic layers are relatively shallow (< 10 cm). This is especially important when considering that the organic layer which insulates the ice-rich yedoma deposits from warming and thawing is only thin, rendering the yedoma in this region vulnerable to active layer deepening and permafrost degradation. Due to the lack of thick organic or peaty layers, most of the profiles were classified as mineral-dominated Orthels or Turbels. This is consistent with the argumentation in Hugelius et al. (2016) which emphasized that DTLBs do not always contain peaty C-rich deposits (Histels).
A key reason for the rather low SOC content in DTLBs is the low primary productivity of the study sites at ∼ 72 • N latitude. In addition, the sampled DTLBs represent only a fraction of all the basins in the study areas and may not be representative of the full range of basin ages. Previous studies from Alaska indicated that older basins contained thicker organic layers than younger basins (Hinkel  (Stuiver et al., 2017). Table 4. Sediment and organic carbon accumulation rates (OC accumulation rate). Sediment accumulation rates are based on the depth of the sample and the calibrated radiocarbon date. Organic carbon accumulation rates are based on cumulative soil organic carbon (cumulative SOC) storage at a specific depth and the calibrated radiocarbon date at the corresponding depth. Mean sediment and organic carbon accumulation rates are calculated, always referring to the soil surface (depth = 0 cm and cumulative SOC = 0 kg C m −2 ). Relative sediment and organic carbon accumulation rates are calculated always referring to the sample above a particular sample.

Sample
Age * Cumulative SOC Depth Mean Relative Mean OC Relative OC (cal yr BP) (kg C m −2 ) (cm) sediment sediment accumulation accumulation accumulation accumulation rate rate (mm yr −1 ) (mm yr −1 ) (g C m −2 yr −1 ) (g C m −2 yr −1 ) SOB14-T1-1-3 842 10.29 19.5-23.5 0.26 0.26 12.2 12.2 SOB14-T1-5-2 modern 1.21 10-11 n/a n/a n/a n/a SOB14-T1- SOB14-T2-5-2 modern 0.54 5-6 n/a n/a n/a n/a SOB14-T2-5-  (Stuiver et al., 2017). n/a = not applicable. et al., 2003;Jones et al., 2012). Other reasons may include the topographic gradient that impacts how well the DTLBs are drained and whether these basins remain waterlogged peat-forming landscapes or become dry environments that are not favourable for peat formation. Several of the studied DTLBs were eroded by the Lena River (Sobo-Sise) or the sea (Bykovsky Peninsula) and some have deeply incised drainage channels, all of which caused rather strong drainage gradients and enhanced landscape drying. Our mean landscape SOC stocks for Sobo-Sise Island and Bykovsky Peninsula are, however, in the same range as previous studies in similar settings. For example,  found a mean of 19.2 kg C m −2 (0-100 cm) in another yedoma-dominated landscape in the central Lena River delta. Zubrzycki et al. (2013) investigated the SOC character-istics of the Holocene river terrace and the active floodplain in the Lena River delta and found mean SOC stocks of 29.5 and 13.6 kg C m −2 , respectively. The C storage of the active floodplain is therefore very similar to the 11.2 kg C m −2 for fluvial deposits in our study area. Ping et al. (2011) also investigated C storage along the Alaska Beaufort Sea coastline and found a normalized mean landscape storage of 38 kg C m −2 . However, this number is based on profiles only and not on a landscape-based upscaling.  investigated sites in the Kolyma yedoma region and found a C storage of 17.0 ± 51.1 kg C m −3 (note that it is kg C m −3 ) for the Holocene cover layer, 16.2 ± 31.3 kg C m −3 for the Alas (thermokarst) deposits and 14.0 ± 23.5 kg C m −3 for yedoma deposits. These values, however, refer not only to the first 2 m of soil cover-ing Pleistocene Ice Complex deposits but to the upper 25 m of Yedoma Ice Complex. Also, Webb et al. (2017) investigated deep (15 m) C stocks in a larch-dominated yedoma area in the Kolyma River basin and found more organic C at the Alas site than at the yedoma site. Based on a landscape upscaling, Siewert et al. (2015) calculated the SOC storage for the Kytalik region, a yedoma-and thermokarstdominated tundra landscape in the Yana-Indigirka Lowland, to 25.8 ± 9.9 kg C m −2 for the first metre of soils which falls in the range spanned by the Sobo-Sise Island and Bykovsky Peninsula calculations.
Aside from organic C, a significant amount of N is stored in the soils of Bykovsky Peninsula and Sobo-Sise Island, almost twice as much as what has been found in the study by Zubrzycki et al. (2013) for the Holocene river terrace (1.2 kg m −2 ) and the active floodplain (0.9 kg m −2 ) of the Lena Delta. Obu et al. (2017) reported higher N storages from the western Canadian Arctic, where 3.4 kg N m −2 is stored in cryoturbated or recently disturbed type soils or sediments and Michaelson et al. (2013) calculated a mean N storage of 2.7 kg m −2 for Arctic Alaska pedons. The normalized average N storage from the Alaskan Beaufort Sea coast is in the same range with 1.9 kg N m −2 (Ping et al., 2011) and N storages from two study sites on Taymir Peninsula are also in the same range, with 1.0 and 1.3 kg N m −2 (Palmtag et al., 2016).
Even though the N storages are an order of magnitude lower than the organic C storages, a large amount of N is present in these soils. Since N is the limiting factor for plant growth in Arctic environments (Shaver et al., 1986;Chapin et al., 1995;Mack et al., 2004;Beermann et al., 2015), permafrost thawing will affect the N stocks in the soils. The N could partially become available to plants upon permafrost thawing. However, the role of N and whether it can offset an increased organic carbon release through increased plant growth needs further exploration. In a recent study, Keuper et al. (2017) found that plant-available N from thawing permafrost is an additional source of deep-rooting subarctic plants and can increase their biomass production. Also, Salmon et al. (2016) reported that increased N from thawed permafrost enhances plant growth and biomass, but that it might not offset C from deep deposits. An increase in N has, however, the potential to change the plant productivity and the species composition (Keuper et al., 2012). Keuper et al. (2012) show that not only dissolved plant-available N becomes available with permafrost thawing but also organically bound N can be mineralized at faster rates in thawed near permafrost soils.
However, an increase in N availability might both stimulate vegetation growth and increase microbial activity (Nowinski et al., 2008). Also, Koven et al. (2015) report that near-surface N released from thawing permafrost might reduce nutrient limitations, even though the same study shows limited importance of the deep soil N to offset deep C release. In an expert assessment Abbott et al. (2016) stated that even an increase in Arctic and boreal biomass might not offset permafrost carbon release. Since we only focused on N stocks, we cannot derive conclusions on potential plant-available N in the soils of Sobo-Sise Island and Bykovsky Peninsula and its effect on the primary production or C release. Nevertheless, this first estimation of total N in the soils of Sobo-Sise Island and Bykovsky Peninsula will be relevant for future climate models.

Upscaling of C and N pools
The total C stocks of our study sites are in the range of other permafrost C studies and confirm previous high C stock estimates from northern permafrost regions (e.g. Hugelius et al., 2014). The landform classification proved to be adequate for the upscaling. The overall accuracies for the classification are 71.5 and 71.1 % for Sobo-Sise and Bykovsky, respectively. Additional field data in a greater variety of subclasses and with more diversified classification would further increase the precision of the upscaling. Nevertheless, by including a high-resolution DEM, classifying remotely sensed images into geomorphological landform types results in an accurate map of a first estimation of SOC and N stocks on the landscape level. In this context, of particular note is the areal fraction of yedoma uplands we found on Sobo-Sise Island and the Bykovsky Peninsula of 43 and 51 %, respectively. While this is within the range Morgenstern et al. (2011) described for the third terrace of the Lena Delta, on a larger regional level yedoma coverage is generally lower. For example Veremeeva and Glushkova (2016) calculated 16 % of yedoma coverage for the entire Kolyma Lowland. However, our higher values are a combination of yedoma uplands and partly degraded yedoma slopes. Excluding areas with slopes, 19 % of Sobo-Sise and 22 % of Bykovsky Peninsula are covered by intact yedoma uplands not yet significantly affected by thermokarst or erosion.
For upscaling deep C and N pools, the determination of ice wedge volumes is important. In our study we assessed ice wedge volume using a combination of literature values and own values derived form a GIS-based analysis of highresolution satellite data. This analysis was based on more than 1500 mapped baydzherakhs and resulted in a mean ice wedge volume of 40 % for Sobo-Sise Island with a standard deviation of ±8 %. Conducting additional calculations with an ±8 % ice wedge volume for yedoma uplands and ±3 % for thermokarst areas would lead to a ±4 % higher respective lower landscape SOC stock (±5 % for N) for 0-100 cm and ±7 % of SOC and total N for 0-200 cm for Sobo-Sise Island. While these results are still in the same range, this analysis reveals the importance of assessing ice wedge volumes correctly within a landscape for SOC and N upscaling, especially for deep deposit calculations. For a more detailed upscaling and to capture the entire variability of these heterogeneous environments, additional and deeper soil cores are needed as well as a more detailed estimation of the land-

Sediment and organic C accumulation rates
Most of the analysed soil C was of Holocene age. Even for yedoma upland soils, radiocarbon dates indicated a large number of Holocene ages. Hence, the yedoma uplands appear to be blanketed by Holocene cover material sometimes exceeding a thickness of 2 m, which needs to be accounted for in carbon pool inventories, because these 2 m cover deposits cannot be considered as yedoma. In DTLBs, the Holocene age of soil C fits well with the findings from previous studies, suggesting that the accumulation of lacustrine sediments, drainage of thermokarst lakes, and accumulation of soils and organic layers in the basins occurred mostly during the Holocene (Kaplina, 2009;Grosse et al., 2013;Walter Anthony et al., 2014). Based on the radiocarbon dates and the cumulative SOC storages, the accumulation rates for the soil cores were calculated. The mean (linear) C accumulation rate of SOB14-T2-5 with 49.7 g C m −2 yr −1 is very high, even higher than what has been found by Jones et al. (2012) for palaeo peat accumulation rates (9-35.2 g C m −2 yr −1 ) in thermokarst basins on the Seward Peninsula. Nevertheless, it has to be considered that this site location is close to the Lena River with only approximately 5 m above river water level. It is likely that this location is affected by spring flood events which can deposit large amounts of sediments. On the other side, the site SOB14-T1-5 with 2.7 g C m −2 yr −1 has very low accumulation rates and is most certainly not affected by the Lena River flood. For comparison, Kurganova et al. (2014) find that modern C accumulation on arable land in Russia was on average 9.6 g C m −2 yr −1 over a 20-year period after abandonment. Hicks Pries et al. (2012) found a mean Holocene C accumulation rate of 25.8 g C m −2 yr −1 for surface soils and 2.3 g C m −2 yr −1 for deep soils in subarctic tundra in central Alaska. Bockheim et al. (2004) found a mean long-term accumulation rate of 13 g C m −2 yr −1 for the last 5500 years in DTLBs in Arctic Alaska, but referring to the organic layer only. Sediment accumulation rates are all of the same order of magnitude, excluding SOB14-T2-5, which might be affected by the Lena River, with mean accumulation rates for the different thermokarst cores ranging between 0.13 and 0.26 mm yr −1 and mean accumulation rates for yedoma up-land cores between 0.10 and 0.57 mm yr −1 (Table 4). For comparison, Murton et al. (2015) found sediment accumulation rates between 0.75 and 2.00 mm yr −1 for yedoma silt at Duvanny Yar in the Kolyma Lowland. However, these rates refer to a time period between 38 700 and 23 600 years BP. Also, Schirrmeister et al. (2002a, b) found a similar accumulation rate for a yedoma deposit (Mamontovy Khayata) on Bykovsky Peninsula of about 0.75 mm yr −1 for the time period between 60 000 and 6000 years BP. These rates are slightly higher than the rates calculated in our study. In addition, all presented sediment accumulation rates (Table 4) will be lower when taking into account the ice content within the deposits. Sediment accumulation rates corrected for soil core ice content are presented in the Table S6.

Characterizing soil organic carbon
The rather low C / N ratio in our study is common to all sampled soils. Only individual samples showed higher C / N ratios and in general there is a trend of decreasing C / N ratios with increasing soil depth. Even though the permafrost organic matter is already partly degraded, these finding suggests that organic matter in the top permafrost layer may be remobilized and decomposed when thawed out as the result of fluctuation of the permafrost table due to climate change. Also, Strauss et al. (2015) found relatively small C / N ratios for yedoma and thermokarst samples (median values below 8 and 10), although they looked at samples from deeper deposits. The mean values from our study might be lower too when incorporating samples from greater depths; this is indicated by the decreasing C / N ratio with increasing depth. Higher C / N values in the upper metre of soil were found by Zubrzycki et al. (2013) with mean values between 20 and 42 from the Holocene river terrace and mean values between 13 and 21 for the active floodplain level of the Lena Delta. This indicates fresher material in these deposits compared to yedoma uplands and DTLB deposits.

The fate of organic carbon in thermokarst-affected yedoma in Siberia
Permafrost soil layers beneath the active layer and below 1 m depth are important for future C remobilization, because models suggest permafrost degradation and thaw well beyond 1 m depth by end of the 21st century (Lawrence et al., 2012;Koven et al., 2013). The cores and the high sam-ple resolution in this study provide detailed information on the C stored in the soil beneath the active layer in the study areas which will be thawed first by future warming. This study provides additional soil C and N data for multiple cores deeper than 100 cm for thermokarst-affected yedoma landscapes. Studies with such deep cores are rare and even the NCSCD contains three times more profiles for the 0-100 cm (1778 profiles) interval than for estimations exceeding 100 cm depth (Hugelius et al., 2014). Our upscaling suggests that the study sites contain significantly more C than soils in temperate climate zones (e.g. Wiesmeier et al., 2012). Both study areas could become sources of organic C and N if permafrost thaw continues in a warming Arctic. An estimation based on the sampled cores and the landform classification shows that with an overall deepening of the active layer of 10 cm 700 000 t C (1.6 kg C m −2 ) will thaw out in both study areas combined (Table 5). A regional study of Siberian permafrost dynamics (Sazonova et al., 2004) includes scenarios where the active layer deepens by more than 100 cm in north-eastern Siberia at the end of the 21st century. This would result in an additional pool of available SOC of 5 830 000 t (13.2 kg C m −2 ) in the two study areas combined. In addition, different forms of permafrost thaw other than active layer deepening would further increase the amounts of SOC thawed and mobilized. For example, lake shore erosion rapidly degrades permafrost around lakes and releases organic C to the aquatic environment. Shore erosion not only affects the active layer soils from the top but does affect deeper permafrost soil layers (Walter Anthony et al., 2016). This vulnerability of large currently frozen C pools to thaw highlights the importance of deep permafrost organic C to be considered in future C cycle models.

Conclusions
This study presents the first SOC and N inventories for Sobo-Sise Island and Bykovsky Peninsula, two yedoma-dominated and thermokarst-affected landscapes in the north of eastern Siberia for the first 2 m of soil. These ice-rich permafrost landscapes are vulnerable to climate warming and have the potential to release large amounts of SOC and total N through active layer deepening and permafrost thaw.
Sampling sites in DTLBs were found to contain less organic C than soils in the yedoma upland. Permafrost soils in DTLBs were all of Holocene age and soils in the upper 2 m on the yedoma uplands largely were all part of a Holocene cover layer above late Pleistocene yedoma deposits. The mean upscaled landscape SOC storage for 0-100 cm is 20.2 ± 2.9 kg C m −2 for Sobo-Sise and 25.9 ± 9.3 kg C m −2 for Bykovsky Peninsula, which results in a total storage of 9.8 Tg C across both study areas for the first metre of soil. Based on our high sample density, detailed C estimations for active layer deepening were derived, suggesting that 5.8 Tg (13.2 kg C m −2 ) of SOC may become available for microbial degradation due to thaw if the active layer deepens by 100 cm in the two study areas. The N stocks are an order of magnitude lower; nevertheless a mean of 1.8 ± 0.2 kg N m −2 is stored on Sobo-Sise and 2.2 ± 0.5 kg N m −2 on Bykovsky Peninsula for the 0-100 cm depth interval. Therefore, as a consequence of permafrost thaw not only SOC but also additional N may become available for plants and microorganisms.
This study confirms the importance of yedoma and thermokarst landscapes for the permafrost C pool and adds important shallow and deep C and N data to the permafrost region soil databases. It also shows the high variability of SOC and N distribution in thermokarst-affected yedoma environments. Our study particularly underlines the benefits of soil cores beyond 1 m depth when capturing the high variability both within the soil and the entire landscape. Our result on C and N storage and availability to permafrost thaw in the upper subsurface points towards the fact that not only does the amount of C and N require more attention in the yedoma region but so does the potential for remobilization and the fate of freshly thawed organic matter. Even though Sobo-Sise Island and the Bykovsky Peninsula do not contain extraordinarily high SOC stocks in the near-surface permafrost at soil profile level, they nevertheless have a large potential for rapid mobilization of significant amounts of C due to their large proportion of thaw-vulnerable juvenile yedoma and thermokarst-affected landscape units.
Author contributions. MF designed the study; MF, GG, JS, FG, MG, GMM conducted the field work during the expedition Lena 2014; MF carried out the laboratory analysis; FG generated the high-resolution DEMs; and MF wrote the publication with input from all co-authors.
Competing interests. The authors declare that they have no conflict of interest.
with the laboratory work. RapidEye imagery was kindly provided by the German Aerospace Center (DLR) and BlackBridge AG through the RapidEye Science Archive. The authors thank the editor and three anonymous reviewers for their constructive comments.
Edited by: Elise Pendall Reviewed by: three anonymous referees