Soil organic matter composition along altitudinal gradients in permafrost affected soils of the Subpolar Ural Mountains

Catena 131 (2015) 140–148

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Soil organic matter composition along altitudinal gradients in permafrost affected soils of the Subpolar Ural Mountains A.A. Dymov a,b,⁎, E.V. Zhangurov a, F. Hagedorn c a b c

Institute of Biology Komi SC UrD RAS, Kommunisticheskay 28, Syktyvkar, Russian Federation Syktyvkar State University, Octabrskii 55, Syktyvkar, Russian Federation Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Zürcherstrasse 111, Birmensdorf CH-8903, Switzerland

a r t i c l e

i n f o

Article history: Received 13 February 2014 Received in revised form 2 March 2015 Accepted 6 March 2015 Available online xxxx Keywords: Ural Mountains Humic substances 13 C NMR Amino acids Permafrost affected soils

a b s t r a c t Soil organic matter (SOM) in high-latitude soils is assumed to be highly vulnerable to climate changes. Relative little information exists from soils of mountain ecosystems which might respond differently to permafrost melt than in flat terrain due to a better drainage. In this study, we measured SOM composition of six typical soils along an altitudinal gradient of the remote Subpolar Urals, reaching from alpine tundra to the forest zone. The SOM characteristics was estimated by applying 13C nuclear magnetic resonance (13C NMR) spectroscopy, electron paramagnetic resonance (ESR), elemental analysis and amino acid composition of humic acid (HA) extracts from soils. Result showed that SOM stocks ranged between 8 and 13 kg C m−2 but reached up to 40 kg C m−2 in a Stagnic Podzol in the alpine tundra. In the mineral soil, 13C NMR indicated that the contribution of alkyl-C was 60% in the forest and 50% in the tundra, while aromaticity was 5% in the tundra, but 19% in the forest. This shows that SOM of mineral soils in alpine tundra was more aliphatic but less aromatic than in the Podzols of the forested zone. In contrast to mineral soils, SOM characteristic in organic layers was very similar among all soil types despite different vegetation types. Consequently, we suggest that the large difference in SOM quality in the mineral soil between tundra and forest can primarily be attributed to abiotic soil conditions in the deeper soil with a stronger waterlogging and a lower permafrost depth in the tundra soils. The low status of oxidative SOM degradation in the mineral soil of the tundra is also an indication that SOM of tundra is highly vulnerable to an improved aeration associated with permafrost melt in drained mountain soils. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Soils of high latitudes play an important role in the global carbon cycle. In the northern permafrost region, soils store about 40% of the global soil organic carbon on 17% of the area (Hiederer and Köchy, 2011; Jones et al., 2010; Tarnocai et al., 2009). Mountain soils with permafrost contain 66 Pg of soil organic carbon, which constitutes 4.5% of global pool (Bockheim and Munroe, 2014). These large carbon reserves are assumed to be highly vulnerable, because thawing of ‘locked’ carbon potentially releases large amounts of CO2 and CH4 to the atmosphere (Lal, 2005; Waldrop et al., 2010; Zimov et al., 2006). In addition, climate change scenarios predict a particular strong climate warming at high latitudes, strongly suggesting that permafrost melt will become reinforced potentially affecting world's climate in the near future (Harden et al., 2012). Soil organic matter is a complex mixture of different organic substances. The chemical and functional properties of SOM are determined by the interaction of a number of factors such as the amount and ⁎ Corresponding author at: Institute of Biology Komi SC UrD RAS, Kommynisticheskay 28, Syktyvkar, Russian Federation. E-mail address: [email protected] (A.A. Dymov).

http://dx.doi.org/10.1016/j.catena.2015.03.020 0341-8162/© 2015 Elsevier B.V. All rights reserved.

chemical composition of plant detritus, the soil microbial community structure, microclimatic conditions and the mineralogical soil properties (Kögel-Knabner and Amelung, 2014; Schmidt et al., 2011). The chemical compounds contained within SOM differ in their bioavailability and in their affinity to the soil mineral phase (Kögel-Knabner and Amelung, 2014). Hence, the analysis of the chemical properties of soil organic matter helps to understand SOM vulnerability and to predict the responses of SOM to climate change (Xu et al., 2009). Soil humic acids (HAs) reflect the principle SOM properties and allow the assessment of SOM compounds involved in the biogeochemical cycle (Abe and Watanabe, 2004; Kholodov еt al., 2011). One powerful approach to characterize SOM composition is nuclear magnetic resonance (13C NMR), which quantifies the relative contribution of different chemical groups in humic substances (Kögel-Knabner, 2000). Based on NMR spectra, for instance, Sjögersten et al. (2003) showed that tundra soils of Scandes contained large fractions of labile SOM, suggesting that these soils may respond sensitive to climatic warming. Most studies on permafrost soils have been conducted in the plains, primarily in North America (Dai et al., 2001; Gittel et al., 2014; Zimov et al., 2006). However, due to the overarching effect of anaerobicity upon permafrost melt (Harden et al., 2012; Waldrop et al., 2010), it seems likely that SOM cycling may respond fundamentally different to

A.A. Dymov et al. / Catena 131 (2015) 140–148

permafrost melt in mountains than in the plains because mountain soils are generally much better drained (Hagedorn et al., 2010). So far, mountains soils of the high Eurasian Arctic have hardly been studied except in the Scandes where permafrost is not widely spread (e.g. Sjögersten et al., 2003). In Russia, mountain soil studies have primarily focused on morphological and physicochemical soil properties (Dymov and Zhangurov, 2011; Firsova and Dedkov, 1983; Lesovaya et al., 2012; Semikolennykh et al., 2013; Vladychenskii, 1998). However, the few assessments of the carbon pools showed that mountain soils from mountain-tundra and mountain-forest belts contain high carbon stocks of 5 to 40 kg m−2 in the upper 50-cm soil layer (Pereverzev, 2011; Pereverzev and Alekseeva, 1980); partly exceeding the soil carbon stocks in Mid- and Northern Europe. In the South Ural Mountains, Kammer et al. (2009) have shown that although SOM pools remained almost constant with altitude and hence with climatic conditions, SOM composition strongly changed with thinner, more decomposable organic layers in the subalpine forest than in the tundra above. Forest–tundra ecotones are a temperature sensitive vegetation boundary (Körner, 2012). Treeline advances and hence forest expansion have documented along the Ural mountain range and in other regions of Siberia (Devi et al., 2008; Hagedorn et al., 2014; Kirdyanov et al., 2012), very likely due to climatic warming and/or improved snow conditions. The climate change driven increase in forests will affect soil carbon cycling indirectly by changing microclimatic conditions and organic matter inputs into soil (Hagedorn et al., 2010; Kammer et al., 2009; Sjögersten-Turner et al., 2011). Hence, there is a complex interplay of climate change with other biotic and abiotic factors, which makes prediction on SOM cycling and characteristics difficult (Saenger et al., 2013; Zollinger et al., 2013). Here, in our study, we assessed the pools and composition of SOM in high-latitude soils in one of the most remote and least studied mountain regions in Europe, the Subpolar Urals. We sampled soils from typical ecosystem types reaching from the alpine tundra above treeline on permafrost, the forest zone on drained soils on the slopes to mountain tundra on flat terraces with ice lenses. In these soils, we determined the concentration, chemical properties of total SOM and the structure of humic acids in soils. The major aim of this study was to determine how SOM composition and pools differ among typical soils of the

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Subpolar Urals in order to evaluate how vulnerable they might respond to expected climatic changes. 2. Materials and methods 2.1. Site description and soil sampling The study was conducted in the Yugyd Va National Park in the northern part of the Subpolar Urals with untouched, natural, and undisturbed ecosystems (Patova, 2010). The study area is located on the Southern limit of perennial icy rocks in the European Subarctic region (Oberman, 1998). The Subpolar Urals are characterized by а strong continental climate with the site specific conditions depending on the local orographic features and the slope aspect. The mean annual temperature varies from − 3 to − 7 °С. The annual precipitation is 800–1000 mm, with the highest amounts falling in May–October (Taskaev, 1997). The parent materials are acidic rocks (quartzite–sericite schist, porphyric rhyolite) and moraine deposits. We sampled soils along an altitudinal gradient on the Maldynyrd ridge (65°20′N, 60°40′E) reaching from 400 to 730 m a.s.l. (Fig. 1) and encompassing the following three typical vegetation zones: alpine tundra above 600 m a.s.l.; forest zone with Larix sibirica at an altitude of 500–580 m a.s.l.; and mountain tundra on permafrost icy rocks close to the soil surface on gentle slopes and flat terraces at 400–450 m a.s.l. close to the valley bottom. In each of the vegetation zones, we have sampled soils from two soil profiles according to soil horizons. Soil profile and vegetation cover photos are presented in Fig. 2. A short characterization of the study plots and a description and classification of soil according to the World Reference Base for Soil Resources (FAO, 2014) are provided in Table 1. More information on the vegetation and morphological characteristic can be found in Dymov et al. (2013). Soil sampling was carried out in 2011 by taking mixed samples from each horizon at five locations on an area of approximately 2 m2. Soils were sampled down to the bedrock or to ice lenses which occurred at less than 50 cm depth. Bulk density of soils was determined by taking soil cores of 15 cm in diameter for organic horizons and 5 cm in diameter for mineral horizons. The bulk density values in gravelly horizons were determined according to Zaidel'man (2008). Stone and ice

Fig. 1. Location of study sites on the Maldynyrd ridge (65°20′N, 60°40′E) in the northern part of the Subpolar Ural reaching from 400 to 730 m a.s.l.

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A.A. Dymov et al. / Catena 131 (2015) 140–148

Alpine tundra

Stagnic Entic Podzol

Mountain tundra with permafrost

Haplic Gleysol

Turbic Cryosol

Histic Cryosol

Forest zone

Albic Podzol I

Albic Podzol II

Fig. 2. Soil profiles and vegetation in the alpine tundra, mountain tundra and forest zone in the northern part of the Subpolar Ural.

contents were quantified by weighing. The carbon pools were calculated according to Hiederer and Köchy (2011). Samples were air-dried and sieved to 2 mm and roots and stones were removed. Organic layer samples were milled using a Waring WCG75 mill. 2.2. Chemical analysis of soils Contents of exchangeable bases, cation exchange capacity (CEC), and base saturation (BS) were determined according to Van Reeuwijk (2002) on a Sapletex 24 VE mechanical extractor with pre-measured concentrations of Ca, Mg, K, and Na on ICP Spectro Сiros. The values of pH in H2O and KCl were determined potentiometrically using a glass electrode ES-11.7 (Akvilon, Russia) using a soil:solute ratio of 1:2.5 for mineral and 1:25 for organic horizons, respectively. The concentrations of C and N in the soil samples were measured using a Carlo Erba EA-

1100 CHN analyzer. Measurement of dithionite-soluble iron was done with a modified method according to Mehra and Jackson (1960). Briefly, 1 g soil was treated in a water bath at 80 °C with two portions of 10 ml 0.3 M Na3C6H5O7 + 1 M NaHCO3 and 0.5 g Na-dithionite in 2.5 ml 1 N NaHCO3. Dithionite-soluble iron (Fedit.) was measured by ICP Spectro Ciros after centrifugation at 1000 g and was filtrated. Oxalateextractable Fe (Feox.) and Al (Alox.) were determined according to Van Reeuwijk (2002). Particle-size fractionation was done according to Van Reeuwijk (2002). 2.3. Isolation and chemical analysis of humic acids Humic acids (HAs) were extracted of according to Swift (1996) using aqueous 0.1 N NaOH with a soil:solution ratio of 1:10 for 24 h, followed by precipitation of HAs at a pH 1.0. Finally, HAs were cleaned

Table 1 The main soil-forming factors and morphological structure of soils. Soil type

Absolute Inclination, Soil profile altitude, grad. m a.s.l.

Alpine tundra vegetation Stagnic Entic Podzol 610 (Turbic, Skeletic) Haplic Gleysol (Skeletic)

730

5–7

О (0–10 cm)–Вh (10–20 cm)–Bg (20–40 cm)–Cg (40–60 cm)

5–7

O (0–4 cm)–Bg1 (4–22 cm)–Bg2 (22–50 cm)

Vegetation

Shrub–dwarf shrub–green moss tundra (Betula nana L.; Carex arctisibirica (Jurtz.); Empetrum Hermaphroditum (Lange) Hagerup Czer; Dicranum; Cladonia lichens) Grass–sedge tundra (Betula nana L.; Vaccinium uliginosum L.; Avenellaflexuosa (L.) Schur; Festuca ovina L.; Carex arctisibirica (Jurtz.) Czer)

Mountain-tundra vegetation with permafrost icy rocks Turbic Cryosol 400 2–3 O (0–8 cm)–Bg (8–18 cm)–BСg (18–50(60) cm)–Сgf (50(60)–↓cm) (Dystric, Thixotropic) Histic Cryosol 450 1–2 Mosses (0–5 cm)–H (5–20 cm)–Bh (Dystric, (20–25 cm)–Bgf (25–↓cm) Reductaquic)

Sedge–lichen–green moss tundra (Betula nana L.; Carex arctisibirica (Jurtz.) Czer; Rubus chamaemorus L.; Empetrum Hermaphroditum (Lange) Hagerup; Dicranum; Cladonia lichens) Moss–lichen tundra (Betula nana L.; Rubus chamaemorus L.; Empetrum Hermaphroditum (Lange) Hagerup; Dicranum; Cladonia lichens)

Mountain-forest zone Albic Podzol I (Skeletic)

Dwarf shrub–green moss larch forest (Larix sibirica L.; Betula nana L.; Vaccinium myrtillus L.; Polytrichum commune)

Albic Podzol II (Skeletic)

580 510

5–7

O (0–10 cm)–E (10–22 cm)–Bs (22–40

6–7

cm)–C (40–60 cm) О (0–10 cm)–Eh (10–20 сm)–Bs (20–35 cm)–C (35–60 cm)

Dwarf shrub–green moss larch forest (Larix sibirica L.; Betula nana L.; Vaccinium myrtillus L.; Polytrichum commune)

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by dialysis though cellulose dialysis film (PR, Russia) for seven days against distilled water and dried at 40 °C. The concentrations of C, N, and H in HAs were estimated using a Carlo Erba EA-1100 CHN analyzer. Cross-Polarization Magic Angle Spinning (CPMAS) 13C NMR spectra of HAs were recorded on a JNM-ECA 400 spectrometer (JEOL, Japan) (100.53 MHz), with a frequency rate of 6 kHz, a contact time of 5 min and a 5-s recycle delay. Chemical shifts of fractions were determined relative to a tetramethylsilane shift (0 ppm). A peak of adamantine was used as a standard in the weak field, and approximately 5.000 scans were performed for each spectrum's accumulation. For quantitative analysis, numerical integration using the v. 4.3.6 Delta program (JEOL, Japan) was applied to estimate the fields of functional groups and molecular fragments. Signals from aromatic structures (AR) were recorded in the 105–164 and 183–190 ppm fields; signals from aliphatic structures (AL) were recorded in the 0–105 and 164–183 ppm fields. The aromaticity values were calculated according to 100 × AR / (AR + AL) (Liang et al., 1996). The electron paramagnetic resonance (ESR) spectra were recorded on a JES FА 300 JEOL spectrometer (Japan) with a microwave power in the cavity of 1 mW and a free radical (FR) modulation amplitude of 0.06 mT using manganese as a reference with fixed radical concentration. The concentration of paramagnetic centers in the samples was determined by comparing the relative signal intensities of the sample with the standard ones using JES-FA sw ESR v. 3.0.0.1 program (JEOL, Japan). The amino acids were extracted with 6 N HCl from HAs. The composition of amino acids (AAs) in HAs was estimated using an AAA T339 analyzer (Microtechna, Praha). 3. Results 3.1. General soil properties The physico-chemical properties of the mountain-tundra soils were relatively similar (Table 2). All soils except Gleysol had a loamy texture and were acidic with minimum pH values in the organic layer. In the

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mineral horizons, the pH values tended to increase with soil depth. While the Haplic Gleysol and the Stagnic Entic Podzol had a modertype organic layer, all other soil types were characterized by mor-type organic layer (Ponge, 2003). As expected the organic carbon contents decreased with depth from the organic layers to the mineral soils (Table 2). Along with SOC contents, the cation-exchange capacity (CEC) was highest in the organic layers. Oxalate and dithionite soluble forms Fe and Al increased from the E horizons to the Bs horizons of Albic Podzols. In contrast, the waterlogged soil horizons of the Cryosols on ice lenses were depleted in Fe and Al indicating reducing conditions. The Albic Podzols of the forest zone had low pH values in both organic layer and Albic (E) horizons. SOM stocks were highest in the Stagnic Entic Podzol in the alpine tundra (40 kg C m− 2) and smallest in the Histic Cryosol (8 kg C m−2; Fig. 3). 3.2. Distribution and elemental composition of HAs The contents and elemental composition of HAs extracted from the soils differed strongly among soil types and horizons (Table 3). In the organic horizons, contents of HAs varied from 3.8 to 25.5 g kg−1 with the lowest concentration of HAs in the O horizon of the Turbic Cryosol and the highest one in the H2 horizon of the Histic Cryosol. In the mineral soils, the highest HA content occurred in the illuvial horizons of mountain-tundra soils (Bh horizon of Stagnic Entic Podzol and Bh horizon of Histic Cryosol). The lowest HA content was found in the reduced Bg horizon of the Turbic Cryosol. The concentration of N in HAs ranged between 4.0 and 5.6% in the tundra and forest soils and the C:N ratio of HAs varied from 11.6 to 16.1. The highest C:N ratio in HAs was found in the organic horizons of Albic Podzols II (Table 3). In general, the H:C ratios of HAs decreased from the organic layer to the mineral soil. While the H:C ratios of HAs in the organic horizons were similar in all soil types, the ratios of the mineral soils were higher in the tundra soils than in the Podzols of the forest. Based on the oxidation degree, which was close to zero, the humic substances studied were weakly reduced compounds. The highest oxidation degree (ω) belonged to the HA extracted from the Stagnic Entic Podzol in the alpine tundra.

Table 2 Chemical and physical characteristics of soils. Soil type

Horizon

Depth, cm

рН H2O

Ctot KCl

Ntot

g kg−1

Oxalate-soluble

Dithionite-soluble

Feox.

Fedit.

Alox.

BS

Aldit.

CEC cmol/kg soil

% Alpine tundra vegetation Stagnic Entic Podzol О Вh Вg Сg Haplic Gleysol O Bg1 Bg2

4.2 4.0 4.8 4.9 3.9 4.6 4.9

3.3 3.3 3.9 4.0 2.8 3.6 3.8

374 ± 12 105 ± 3 10.7 ± 1.9 8.7 ± 2.0 302 ± 10 7.3 ± 1.7 6.5 ± 1.5

12.4 ± 2.2 6.5 ± 1.2 0.85 ± 0.22 0.71 ± 0.18 11.8 ± 2.1 0.62 ± 0.16 0.56 ± 0.14

n.d. 0.49 0.42 0.38 n.d. 0.09 0.14

Mountain-tundra vegetation with permafrost icy rocks Turbic Cryosol O 0–8 3.7 Bg 8–18 5.2 BCg1 18–25 5.1 BCg2 25–50 5.1 Histic Cryosol H1 5–10 4.1 H2 10–20 4.5 Bh 20–25 4.6

2.6 3.6 3.5 3.5 3.1 3.7 3.6

439 ± 14 9.9 ± 2.3 6.0 ± 1.4 6.2 ± 1.4 421 ± 13 386 ± 12 144 ± 5

8.8 ± 1.6 0.86 ± 0.22 0.64 ± 0.17 0.69 ± 0.18 10.3 ± 1.8 13.0 ± 2.3 8.0 ± 1.4

2.8 3.5 3.8 3.9 2.8 2.9 3.4 3.6

423 ± 14 13.6 ± 2.4 7.9 ± 1.4 3.8 ± 1.1 412 ± 13 23 ± 4 12.9 ± 2.3 9.8 ± 2.2

15.3 ± 2.8 1.18 ± 0.2 0.71 ± 0.18 0.40 ± 0.10 18 ± 3 1.36 ± 0.23 0.76 ± 0.20 0.46 ± 0.12

n.d. 0.46 0.91 0.19 n.d. 0.16 1.19 0.44

Mountain-forest zone Albic Podzol I

Albic Podzol II

O Е Bs C O Eh Bs C

Note: n.d. — not determined.

0–10 10–20 20–40 40–60 0–4 4–22 22–50

0–10 10–22 22–40 40–60 0–10 10–20 20–35 35–60

3.7 4.4 4.6 4.8 3.8 3.8 4.4 4.6

Clay content (b2 μm) %

0.38 0.14 0.18

1.09 0.84 0.84

0.42 0.16 0.18

0.08 0.08

0.13 0.26

0.06 0.06

n.d. 0.39 2.13 1.49 n.d.

0.24 0.31 0.25

0.45 2.25 1.96

0.12 0.19 0.17

1.29

0.44

1.31

0.29

0.2 0.34 0.18

0.79 1.29 0.38

0.16 0.27 0.13

0.19 0.37 0.38

0.21 1.49 0.66

0.11 0.27 0.26

44 10 8 4 29 51 7

24.9 17.2 2.2 2.8 37.9 0.1 1.1

n.d. 37 35 20 n.d. 14 14

21 24 38 43 31 41 29

27.0 12.7 9.9 11.7 84.0 71.2 39.9

n.d. 40 37 38 n.d. n.d. n.d.

12 4 9 6 34 7 4 3

57.8 4.6 3.9 1.0 53.7 7.8 7.5 4.9

n.d. 35 31 23 n.d. 42 42 29

kg m-2

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45

3.4. ESR spectroscopy of HAs

Organic horizons

40

Mineral horizons

The ESR spectroscopy showed that the contents of free radicals (FRs) in the HAs extracted from organic horizons of the soils varied from 3.1 to 7.8 × 1015 spin g−1 (Fig. 5). The content of paramagnetic centers in the HAs from Albic horizons was more than twice as high as that in HAs extracted from the organic horizons. The HAs of the Stagnic Entic Podzol and Histic Cryosol illuvial horizons had the lowest FR concentrations, which amounted 0.4–0.7 × 1015 spin g−1.

35 30 25 20 15 10

3.5. Amino acid composition of HAs

5 0 Stagnic Entic Podzol

Haplic Gleysol

Turbic Cryosol

Histic Cryosol

Albic Podzol I

The analysis of the hydrolysates (6n HCl) of HAs identified 17 amino acids (AAs) in their composition, which were classified into four groups (non-polar, polar, acidic, base amino acids) (Fig. 6). The HAs of different soil types were similar in their total amino acid (AA) content. The AA content of the HAs was high and comprised 13.3–18.4% of the total weight of the samples. All the HA samples were dominated by nonpolar and acidic amino acids (Fig. 6). Aspartic and glutamic acids had the highest concentrations.

Albic Podzol II

Fig. 3. Carbon pools in the studied soils.

3.3. 13C NMR spectra of HAs The HAs of organic horizons contained a large portion of aliphatic or O-alkyl-C fragments (36–42%) and of twice-substituted aliphatic moieties (24–30%) (Table 4). The soil extracts consisted of few aromatic moieties, many C, H-substituted aliphatic moieties, as well as those that were twice-substituted by heteroatoms (substituted double heteroatoms), including carbon heteroatoms (Fig. 4). Their contribution decreased with soil depth in the tundra soils as the content of carboxylic groups in HAs decreased. In contrast, carboxylic groups in the HA extracted from the mountain-forest soils increased within increasing soil depth. A similar pattern existed for aromatics of HAs, with the aromatic compounds decreasing from the O horizon to the mineral soil in the tundra soil. By comparison, the aromatic compounds in HAs of the Podzols in the forest increased from the organic layer to the mineral horizons.

4. Discussion Permafrost soils are considered to be highly vulnerable due to their large stocks of labile carbon and the ongoing climate warming, potentially unlocking large amounts of carbon to the atmosphere (Harden et al., 2012; Zimov et al., 2006). A number of factors such as the indirect effects via changes in vegetation and soil moisture are still highly uncertain, making prediction of future changes rather difficult (Waldrop et al., 2010). Our results show that in the mountainous permafrost landscape of the Subpolar Ural, SOM pools and composition and hence its vulnerability vary at a small scale, particularly in the mineral soil. Surprisingly, in the organic layers, the chemical composition of humic acids measured by 13C NMR, ESR and extractable amino acids hardly differs

Table 3 Distribution and elemental composition Humic Acids (HAs), atomic ratio, oxidation degree (ω) of HAs (weight proportion and atomic portion are given above and under the line, respectively; all results are given for ash-free preparations). Soil type

Alpine tundra zone Stagnic Entic Podzol

Haplic Gleysol

Horizon

Depth, cm

Mountain-forest zone Albic Podzol

Albic Podzol

a

Ash, %

O

0–10

18.11

2.60

Bh

10–20

53.89

14.75

O

0–4

24.72

1.55

3.77

0.01

Mountain-tundra zone with permafrost icy rocks Turbic Cryosol O 0–8

Histic Cryosol

HA content, g∗kg soil−1

Bg

8–18

0.42

2.38

H2

10–20

25.51

1.98

Bh

20–25

39.83

5.54

O

0–10

22.90

2.62

E

10–22

1.73

4.35

O

0–10

19.41

1.75

Eh

10–20

4.13

3.18

Bs

20–35

0.89

3.31

Content, %

Atomic ratio

Oxidation degree

С

N

H

O

H:C

Н:Сcor.a

O:C

C:N

54.21 36.08 54.88 37.29 55.66 39.59

4.79 2.74 5.10 2.97 4.77 2.91

5.49 43.44 5.19 41.97 4.59 38.82

35.51 17.74 34.82 17.76 34.99 18.68

1.20

1.86

0.49

13.20

−0.22

1.13

1.76

0.48

12.56

−0.17

0.98

1.61

0.47

13.61

−0.04

53.83 38.90 57.92 43.61 55.48 37.99 55.04 42.48

4.21 2.42 5.64 3.05 4.34 3.27 4.76 2.91

5.10 39.90 4.44 38.28 4.87 40.72 5.14 37.47

36.86 18.77 32.01 15.07 35.31 18.02 35.06 17.14

1.13

1.82

0.51

14.91

−0.10

0.91

1.47

0.41

11.99

−0.08

1.04

1.68

0.48

14.92

−0.09

1.11

1.75

0.48

13.49

−0.15

55.49 40.75 61.90 40.84 54.88 36.92 59.12 41.51 58.26 37.55

4.03 3.06 5.04 3.15 5.51 2.48 4.72 3.47 5.25 2.79

4.79 39.50 4.57 39.28 4.95 41.62 4.38 37.81 4.71 41.70

35.69 16.69 28.49 16.72 34.67 18.98 31.78 17.22 31.78 17.96

1.03

1.67

0.48

16.06

−0.06

0.88

1.34

0.35

14.32

−0.19

1.07

1.71

0.47

11.62

−0.12

0.88

1.42

0.40

14.62

−0.07

0.96

1.51

0.41

12.96

−0.14

H:Ccor. = (H / C) + 2 ∗ (O/C) ∗ 0.67; ω = (2O − H / С), where O, H, C are numbers of these atoms (Orlov, 1995).

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Table 4 Percentage of carbon in the main structural fragments of HAs (according to 13С NMR). Soil type

Horizons

Chemical shift, ppm 0–47

47–60

60–105

105–144

144–164

40.16 61.60 41.99

10.32 7.46 11.87

30.00 20.01 24.41

8.49 2.80 10.39

0.61 0.78 1.03

Mountain-tundra zone with permafrost icy rocks Turbic Cryosol O 35.73 Histic Cryosol H2 41.17 Bh 58.52

10.85 11.03 9.63

29.85 24.69 17.40

9.66 9.27 4.19

11.19 7.54 11.89 8.45

23.45 11.70 21.87 14.01

10.99 15.28 11.26 15.68

Alpine tundra Stagnic Entic Podzol Haplic Gleysol

Mountain-forest zone Albic Podzol I Albic Podzol II

O Bh O

O E O Eh

40.08 51.23 41.68 46.33

164–183

AR AL

Aromaticity, %

0.93 0.03 0.57

0.10 0.04 0.14

9.5 3.8 11.9

0.48 0.18 0.07

0.61 1.49 1.05

0.15 0.14 0.05

13.3 12.2 4.95

0.29 1.21 0.50 1.60

0.66 0.83 0.35 0.98

0.16 0.23 0.18 0.23

13.5 18.7 15.2 18.5

183–190

190–204

9.28 7.07 9.28

0.23 0.24 0.46

3.12 2.60 0.64

9.70 9.57 8.50

3.80 1.87 1.71 1.25

9.52 10.34 10.74 11.70

Note: 0–47 ppm — alkyl C structures; 47–60 ppm — methoxyl and O,N-substituted aliphatic moieties; 60–105 ppm — aliphatic moieties, substituted heteroatom double, methyne group of ethers; 105–144 ppm — C,H-substituted aromatic moieties; 144–164 ppm — O,N-substituted aromatic fragments; 164–183 ppm — carboxylic groups; 183–190 ppm — quinoid groups; 190–204 ppm — aldehyde and ketone groups.

between tundra and forested sites and among sites with different permafrost depths despite the large difference in vegetation. The plant communities spanned a gradient from tundra with a dominance of shrubs (Betula nana, Empetrum Hermaphroditum, Vaccinium myrtillus, Vaccinium uliginosum), Cladonia lichens and green mosses to the sparse larch forest. Nevertheless, the composition of the organic layers was rather similar, suggesting that also the quality of the C input from the organic layer into the mineral soil was comparable for all soil types. Consequently, the large difference in SOM quality observed in the mineral soil between forest und tundra soils can primarily be attributed to abiotic soil conditions in the deeper soil such as waterlogging and permafrost depth and not to a different vegetation type. Both the 13C NMR spectra and the H:C ratio indicated that humic acids (HAs) in tundra soils and in soils with a shallow active layer are more aliphatic than HAs in forested soils. By comparison forested soils are characterized by a higher aromaticity (Fig. 4; Table 4). This finding is consistent with the 13C NMR data of Dai et al. (2001) in Arctic Alaska and in the Russian Subarctic in Umbric Albeluvisols (Kholodov et al., 2011), Haplic Albeluvisols and Stagnic Albeluvisols (Lodygin and Beznosikov, 2010), as well as in ‘young’ soils formed at ditch banks

(Abakumov, 2009), Cryosols (Lodygin et al., 2014; Vasilevich et al., 2014) and Khibini Mountains (Vladychenskii et al., 2006). White et al. (2004) related a smaller SOM aromaticity in arctic soils to their severe climatic conditions, in particular to high moisture contents, anaerobic conditions and a shorter period of biological activity. In line with this conclusion, Waldrop et al. (2010) found that the alkyl/o-alkyl ratio increased during anaerobic conditions and Pedersen et al. (2011) observed that this ratio decreased with soil depth in permafrost soils, which indicates that waterlogging in permafrost soils leads to high contents of aliphatic groups. Large proportion of aliphatic compounds may be associated with algal and fungal sources (Dai et al., 2001) concentrated in the upper horizons. Recent analysis of microbial community structures indicated small contributions of fungi in anaerobic cryoturbated soils with particularly low fractions of mycorrhizal fungi (Gittel et al., 2014). An additional difference between tundra and forested soils was the contributions of carboxylic groups and aldehyde groups (Table 4). These fractions increased within the Podzols under forests reflecting an increasing oxidative SOM degradation with soil depth and the leaching of dissolved organic matter (Rumpel et al., 2002). In contrast,

Albic Podzol II

Stagnic Entic Podzol O horizon

Eh horizon

O horizon

Bh horizon

Fig. 4. Examples of solid-state CPMAS 13C NMR spectra of HAs from Stagnic Entic Podzol and Albic Podzol (Skeletic) II.

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Concentration of free radicals, N*1015 spin/g

18

Alpine tundra

Mountain tundra (with permafrost)

16

Forest zone

14 12 10 8 6 4 2 0 O

Bh

O

O

H2

Bh

O

E

O

Eh

Stagnic Entic Haplic Turbic Histic Cryosol Albic Podzol I Albic Podzol II Podzol Gleysol Cryosol Fig. 5. The content of free radicals in the HAs.

carboxylic and aldehyde groups decreased from the organic layer to the mineral soils in the waterlogged tundra soils, very likely as a result of anaerobic conditions impeding oxidative degradation. The vertical distribution of SOM in the Podzols in the forested zone of the Subpolar Ural observed in this study differed from Podzols in the East European plain of Northern Russia (Dymov et al., 2014). In the Albic Podzols studied here, the albic horizons had relatively high concentrations of carbon (14–23 g kg−1) and there was no carbon enrichment in the illuvial spodic (Bs) horizons (8–13 g kg−1 C), although oxalate and dithionite-soluble Fe and Al clearly indicated eluviation from the E to the Bs horizons (Table 2). By comparison, Podzols of the sandy plains show typically pronounced vertical SOM distributions with low C concentrations in the E horizon (1.5–2.0 g kg−1) and reincreases of C concentrations in the illuvial horizons of up to 1.3– 40 g kg−1 (Dymov and Gabov, 2015; Dymov et al., 2014). The smaller degree of podzolization in the Subpolar Urals than in the sandy plains is in agreement with observations in the Polar Urals, where the lower podzolization in mountain soils was attributed to a stronger lateral and a smaller vertical translocation of organic compounds than in the plains (Firsova and Dedkov, 1983). An important characteristic of HAs is the concentration of free radicals (FRs) in macromolecules of HAs, largely depending on the degree of aromaticity of HAs and on the content of stable semiquinone free radicals (Hammel et al., 2002; Rammer, 2006). The concentration of FRs in the HA clearly reflected the site conditions and soil development (Fig. 5). The contents of paramagnetic centers were particularly

low in the illuvial horizons (Bh) of the permafrost-affected tundra soils, which can be related to their deactivation in mineral soil horizons by the formation of strong organic–mineral complexes. At the same time, the illuvial horizons have relative high Н:Сcor. ratios as compared to the Podzols (Table 3) showing that there are structurally dominated by aliphatic moieties. The concentrations of paramagnetic centers in the HAs of soils of the Subpolar Urals (5–8 × 1015 spin × g−1) were higher than those of organic horizons in Albeluvisols (3–4 × 1015 spin × g− 1; Lodygin et al., 2007). This finding is an indication that the HAs in the Subpolar Ural might have a higher potential for polymerization reactions and to form condensed macromolecules. The contents of amino acids comprised between 13 and 18 % of HAs, which is higher than in soils from temperate regions where values range between 5 and 11% (Orlov, 1995). The likely reason for the higher contents of AAs in northern soils is the limited biological activity because a higher metabolic activity in plants and microorganisms generally leads to lower AA contents (Amelung et al., 2006). In addition, Jones and Kielland (2012) presented the hypothesis that a high concentration of amino acids in the northern soils could be associated with a high concentration of polyphenols. We suggest that the AAs in the studied HAs are primarily poorly condensed macromolecules, because most AAs were extracted during acid hydrolysis. There was no apparent difference in total contents and in specific fractions of AAs in HAs among the soil types along the altitudinal gradient.

Mountain tundra (with permafrost)

Alpine tundra

Forest zone

100%

100%

80%

80%

80%

60%

60%

60%

polar

40%

40%

40%

non-polar

20%

20%

20%

100%

0%

base amino acids

0%

0% O

Bh

Stagnic Entic Podzol

O Haplic Gleysol

acidic

O

Bg

Turbic Cryosol

H2

Bh

Histic Cryosol

O

E

O

Eh

Albic Podzol I Albic Podzol II

Fig. 6. Fractions of amino acid groups in total amino acids extracted from humic acids.

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Our C pools estimate showed that C stocks varied at a small scale with the Stagnic Entic Podzol of the alpine tundra having a substantially higher C stock as compared to all other soil types. The measured 40 kg C m− 2 in the Entic Podzol corresponds to average C stocks of Gelisols reported by Tarnocai et al. (2009) in the Northern Circumpolar Soil Carbon Database. The soil C stocks of all other soils of the Subpolar Urals ranging between 8 and 13 kg C m−2 are typical for upland soils of other mountain areas (Hoffmann et al., 2014; Sjögersten-Turner et al., 2011; Ward et al., 2014). Integrating among all soils, we interpret our pool estimates as an indication that permafrost-affected mountain soils have smaller C stocks than soils from the plains unless they are strongly waterlogged or cryoturbated. In conjunction with the relatively high status of oxidative degradation in the aerobic soils, this finding suggests that the well-drained mountain soils have a smaller potential to lose SOM upon permafrost melt than anaerobic soils of the plains and on ice lenses. 5. Conclusion Our study showed that SOM stocks and characteristics varied strongly among typical soil types of the Subpolar Urals. The chemical composition measured in humic acids using 13C NMR, ESR and the H:C ratio indicated that the mineral soils of mountain tundra ecosystems were more aliphatic than in the forested zone which were characterized by a higher aromaticity. These different characteristics very likely resulted from an impeded oxidative degradation by at least temporarily anaerobic conditions in the tundra soils. In comparison to the large differences among soil types in the mineral soil, organic layers had very similar SOM characteristics. This suggests that despite the large difference in vegetation spanning a gradient from moss, dwarf shrubs to trees, the quality of the C input from the organic layer into the mineral soil was similar for all soil types. Consequently, we suggest that the large difference in SOM quality in the mineral soil between forest and tundra can primarily be attributed to abiotic soil conditions in the deeper soil such as waterlogging and permafrost depth. Overall, SOM stocks in the permafrost-affected mountain soils of the SubPolar Ural were in the lower range of values reported for permafrost soils, very likely due to a better drainage. We therefore suggest that mountain soils have to be considered specifically in larger scale estimates of C cycling in permafrost-affected soils, but more surveys are needed. Despite the relatively small soil C stocks in the mountainous tundra soil, we interpret our results of SOM composition with high contents of amino acids and a low oxidative SOM degradation in the mineral soil as an indication that SOM of tundra soils is highly vulnerable to an improved aeration associated with permafrost melt in these mountain soils. Acknowledgments This work was supported by the Russian Fund for Basic Research (RFBR) under grant nos. 11-04-00885а and 13-04-00570a and by the President of the Russian Federation under grant MK-2905.2015.4, and by the Presidium of the RAS under program no. 12-P-4-1018. F. Hagedorn acknowledges the support by the ERA.Net RUS STProject-207. References Abakumov, E.V., 2009. Elemental composition and structural features of humic substances in young Podzols developed on sand quarry dumps. Eurasian Soil Sci. 6, 616–622. Abe, T., Watanabe, A., 2004. X-ray photoelectron spectroscopy of nitrogen functional groups in humic acids. Soil Sci. 169, 35–43. Amelung, W., Zhang, X., Flach, K.W., 2006. Amino acids in grassland soil: climatic effects on concentration and chirality. Geoderma 130, 207–217. Bockheim, J.G., Munroe, J.S., 2014. Organic carbon pools and genesis of alpine soils with permafrost: a review. Arct. Antarct. Alp. Res. 46, 987–1006. Dai, X.Y., Ping, C.L., Candler, L., Haumaier, L., Zech, W., 2001. Characterization of soil organic matter fractions of tundra soils in arctic Alaska by carbon 13 nuclear magnetic resonance. Soil Sci. Soc. Am. J. 65, 87–93.

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