Carbon Stocks and Microbial Activity in the Low Arctic Tundra of the Yana–Indigirka Lowland, Russia

Abstract

Arctic warming is expected to alter permafrost landscapes and shift tundra ecosystems from greenhouse gas sinks to sources. We quantified plant biomass and necromass, carbon stocks, and microbial activity across five Low-Arctic tundra sites in the Yana–Indigirka Lowland (Chokurdakh, NE Siberia) during the 2024 growing season. Above- and below-ground plant biomass was measured by harvest adjacent to 50 × 50 m permanent plots; total C and N were determined by dry combustion on an elemental analyzer. Total organic carbon (TOC) stocks were calculated by horizon from TOC (%), bulk density, and thickness. Microbial basal respiration (BR), substrate-induced respiration (SIR), microbial biomass C (MBC), and the metabolic quotient (qCO₂) were assessed in litter/organic (O), peat (T), and mineral gley horizons. Mean above-ground biomass was 15.8 ± 1.5 t ha⁻¹; total living biomass averaged 43.1 ± 1.6 t ha⁻¹. Below-ground biomass exceeded above-ground by 1.73×. Carbon in above-ground, below-ground, and necromass pools averaged 7.8, 12.2, and 12.5 t C ha⁻¹, respectively. Surface organic horizons dominated ecosystem C storage: litter–peat stocks ranged from 234 to 449 t C ha⁻¹, whereas 0–30 cm mineral layers held 18–50 t C ha⁻¹; total (surface + 0–30 cm) stocks spanned 258–511 t C ha⁻¹ among sites. Key contributors to biomass and C storage were deciduous shrubs (Salix pulchra, Betula nana), bryophytes (notably Aulacomnium palustre), and the graminoids (Eriophorum vaginatum). BR and MBC were highest in O and T horizons (BR up to 21.9 μg C g⁻¹ h⁻¹; MBC up to 70,628 μg C g⁻¹) and declined sharply in mineral soil; qCO₂ decreased from O to mineral horizons, indicating more efficient C use at depth. These in situ data show that Low-Arctic tundra C stocks are concentrated in surface organic layers while microbial communities remain responsive to warming, implying high sensitivity of carbon turnover to thaw and hydrologic change. The dataset supports model parameterization and remote sensing of shrub–tussock tundra carbon dynamics.

1. Introduction

Primary production is a fundamental property of terrestrial ecosystems. This function is especially significant in tundra biomes, where biomass accumulation and organic matter decomposition occur at slow rates under extreme conditions. Increasing anthropogenic pressure and ongoing global climate shifts are pushing vulnerable tundra landscapes toward disequilibrium and degradation, which could trigger the release of vast volumes of greenhouse gases (GHGs) stored in permafrost soils into the atmosphere.

The Arctic is warming 2–2.5 times faster than the global average [1]. According to some estimates, the linear trend of the mean annual temperature in this region showed an increase of about 2.64 °C over 30 years (1991–2020) [2]. These trends highlight the need for a quantitative assessment of the structure, productivity, and carbon balance of tundra ecosystems and for forecasting their responses to changing climate and landscape properties. Of particular scientific interest are the functional groups of tundra plants that create the local microclimate, stabilize permafrost, and accumulate the largest reserves of organic matter (OM).

Among the most common foundation species in the Siberian sector of the southern tundra subzone are the deciduous shrubs Betula nana L. and Salix pulchra Cham. The dense upper canopy they form provides a thermal insulating function, shading the ground cover, cooling the soil surface, and thereby substantially retarding permafrost thaw. Numerous studies have shown that rising air temperatures in recent decades have promoted the expansion of these shrubs, enabling them to assume dominant positions in tundra across the globe [3,4,5,6,7,8,9,10]. Reports of increased growth in these species, as well as in graminoids and other plant groups, require field verification to assess the rate of change in their functional state under rapidly changing conditions. In this context, we aimed to conduct a detailed in situ study of the composition and structure of tundra communities, the life strategies and productivity of their dominant and ecosystem-structuring plant groups, and the extent of their carbon and nitrogen storage.

Many aspects of tundra plant productivity remain unresolved due to a scarcity of field studies involving direct measurements of biomass for key species crucial to carbon storage. This highlights the growing importance of in situ studies to quantify OM and carbon stocks within functional plant groups. Field studies determining biomass and necromass stocks in the Yana–Indigirka Lowland are exceedingly rare [11]. Our research helps fill this knowledge gap by providing detailed field measurements that improve the understanding of OM distribution and carbon storage patterns in the plants and communities of the southern shrub tundra.

Changes in the Earth’s climate system [12,13] are driving the transformation of permafrost landscapes across vast northern territories, particularly in northern and eastern Siberia. These landscapes are a key component of the global carbon cycle; the permafrost region covers 22% of the Northern Hemisphere’s land area and contains nearly twice as much carbon as the atmosphere [14,15]. Currently, the stability of carbon in permafrost largely depends on the rate of its degradation and the extent of anthropogenic impacts. In Yakutia, the most hazardous areas for carbon release are degraded sections of the ice complex, where thermokarst processes are actively developing as ground ice thaws and loses volume. This, combined with rising permafrost temperatures, creates a cause-and-effect relationship leading to the loss of total organic carbon (TOC). However, global models that account for abrupt permafrost thaw do not yet exist [16], which complicates the assessment of true carbon emissions and changes in landscape components (e.g., soil cover, water bodies, relief), including the response of biogenic elements to current climate fluctuations.

The circumpolar region contains approximately 1300–1395 Gt of terrestrial organic carbon [16], and tundra ecosystems hold 30% of the world’s TOC stock. About 850 Gt are stored within permafrost [17]. The total Arctic carbon stock in sediments up to 3 m deep is estimated at 1400–1850 Gt [18]. The thawing of Late Pleistocene ice complex deposits, which are highly saturated with organic material and silt, could transform these areas from a carbon sink into a significant carbon source. Late Pleistocene deposits contain 83 Gt [19,20] of organic carbon, while Holocene thermokarst formations contain 130 Gt [21]. The risk of losing organic matter from the upper soil layer and permafrost is increasing, which will undoubtedly accelerate anaerobic and aerobic microbial decomposition [22,23]. This could potentially lead to an increase in greenhouse gas emissions, creating a positive feedback loop with rising air temperatures [24] and altering regional hydrology [25]. Experiments [26] have shown that TOC in the ice complex, cryoturbated soils, and peatlands has low relative mobility. The stability of organic matter decreases in the following order: eolian and alluvial deposits, wetland peatlands, the ice complex, cryoturbated soils, and peat. Therefore, quantifying sequestered carbon is a key element in assessing potential losses in northern permafrost regions.

Early research focused on assessing carbon stocks in deep permafrost layers (0–100 to 0–300 cm), the impact of permafrost thaw on the carbon cycle, and thermokarst processes in response to climate change [27].

The research presented here represents the initial phase of a systematic monitoring program for the southern tundra of the Yana–Indigirka Lowland. This program will track climate-induced changes in the abundance of key dominant plant species, overall vegetation cover, soils, and fluctuations in their carbon content and microbial respiration. The objective of this study is to empirically assess the contribution of plant and soil systems to the accumulation of total organic carbon (TOC) in the southern Low Arctic shrub tundra of the Yana–Indigirka Lowland, northeastern Siberia. Our specific tasks were: 1. To assess the above- and below-ground biomass and necromass of dominant plant species and communities; 2. To analyze the carbon and nitrogen content in plants and lichens, and their C/N ratios; 3. To quantify carbon pools and stocks in the litter and active soil layer, developed under complex cryo-climatic conditions; 4. To statistically analyze the relationship between vegetation and soil carbon; and 5. To conduct a detailed analysis of microbial activity in the litter, peat, and mineral soil horizons.

This research will facilitate a quantitative and qualitative assessment of production and decomposition processes at local and regional scales and will provide a foundation for databases on the productivity of southern shrub tundra ecosystems. These data can also be used for remote monitoring of vegetation dynamics in the Low Arctic.

2. Materials and Methods

2.1. Study Area Characteristics

To assess the structural state, primary productivity, and carbon-nitrogen balance of tundra ecosystems, we conducted studies in the lower Indigirka River basin (Yana–Indigirka Lowland), a significant area in the eastern Russian Arctic. The studied tundra types belong to the Lena-Kolyma Low Arctic tundra biome [28] and represent dominant zonal lowland tundra types in this area.

Fieldwork was conducted during the 2024 growing season at the “Chokurdakh” Tundra Research Station (YA-Ckd), located 30 km northwest of Chokurdakh village in the Allaikhovsky District (70.62239708° N, 147.89648227° E), and 167 km south of the East Siberian Sea. The station is situated within Kytalyk National Park, managed by the Ministry of Ecology, Nature Management and Forestry of the Republic of Sakha (Yakutia). It is part of the regional SakhaFluxNet network, which is integrated into the global FLUXNET and the all-Russian RuFlux networks for monitoring ecosystem greenhouse gas fluxes (Figure 1). Site descriptions: Chok-1 Shrub-sphagnum-green moss tundra (70.828864° N, 147.487575° E); Chok-2 (70.828478° N, 147.486033° E) and Chok-5 (70.829556° N, 147.475229° E) Shrub-lichen-green moss tundra; Chok-3 (70.828722° N, 147.479973° E) and Chok-4 (70.830210° N, 147.478900° E) Shrub-lichen-green moss tundra with Eriophorum vaginatum L.

The climate is classified as arctic and subarctic continental [29], characterized by long, cold winters and short, cool summers. According to data from the Chokurdakh meteorological station, the mean annual air temperature is −12.7 °C, with a mean January temperature of −33.4 °C and a mean July temperature of 10.6 °C. Mean daily air temperatures typically rise above 10.0 °C in early to mid-June. There are approximately 65 days per year with a mean daily temperature >5 °C. Annual precipitation is 237 mm. The average maximum snow depth ranges from 50 to 80 cm.

2.2. Methods for Assessing Carbon Stocks and Microbial Activity

To assess tundra productivity, we used methods described in [30,31,32,33], adapted and approved by the Center for Forest Ecology and Productivity of the Russian Academy of Sciences (Moscow). This work was part of the state project “Development of a System for Ground-Based and Remote Monitoring of Carbon Pools and Greenhouse Gas Fluxes in the Russian Federation, Ensuring the Creation of a Data Accounting System for Fluxes of Climatically Active Substances and the Carbon Budget in Forests and Other Terrestrial Ecosystems” (https://ritm-c.ru/, accessed on 29 June 2025).

Latin species names follow the Royal Botanic Gardens, Kew, Plants of the World Online (POWO) database (https://powo.science.kew.org/, accessed on 29 June 2025), and lichen names follow the ‘Plants and Lichens of Russia and Adjacent Countries’ online atlas (https://www.plantarium.ru/, accessed on 3 July 2025).

Plant community classification was based on prevailing life forms, dominant species, vertical stratification, fine-scale heterogeneity, structure, and microsite characteristics, following the principles of V.I. Perfilieva et al. [34] and G.N. Ogureeva et al. [28]. At each permanent sample plot (PSP), geobotanical descriptions were conducted following the ‘Arctic Vegetation Archive’ (AVA) project protocols, an international CAFF initiative for standardizing geobotanical research in Arctic plant communities [35]. The protocol records information on relief, the upper soil horizon, moisture, geomorphological processes, percent cover of the community, strata, and individual species (%), average heights of strata and species (cm), and any vegetation disturbances.

To monitor the dynamics of productivity and C and N fluxes in plant communities and soils, we established five replicate 50 × 50 m PSPs along a transect on a low-lying depositional plain. On each PSP, we performed detailed geobotanical descriptions, including floristic composition, horizontal and vertical structure, plant vitality assessment, and measurements of height and percent cover.

Above-ground biomass stocks were determined using the harvest method [30]. Above- and below-ground biomass were measured in mid-July, during the period of maximum seasonal plant development. Sampling plots were established in a buffer zone adjacent to the PSPs, at sites designated for subsequent soil pit excavation. The buffer zone was defined as an area within 50–100 m of the PSP with homogeneous conditions. Plant samples were collected from sampling plots in the buffer zone of each PSP using 25 × 25 cm monoliths excavated to a depth of 20 cm [30,32,33]. Herbaceous plants and dwarf shrubs were clipped at the boundary between the green and brown parts of the moss layer, and the entire moss-lichen turf was removed. For dwarf shrubs, leaves and stems were separated into current-year growth and perennial parts, including fruits and seeds. For herbaceous plants, we distinguished between vegetative and generative parts. For mosses, we separated current-year shoots from perennial ones. Dead (brown) parts of mosses and lichens were separated from living parts and classified as necromass [30,32,33]. Dead grass, senesced plants, and discolored (yellowed and brown) plant parts were classified as litter. Underground plant parts were washed from mineral soil horizons using sieves with a 0.25 mm mesh. Rhizomes were collected separately [32,33].

Living and dead plant and lichen samples were first air-dried, then oven-dried at 105–110 °C, and weighed on an electronic balance with 0.001 g precision. The hygroscopic coefficient was determined for each sample to convert mass to an oven-dry basis for total C and N analysis. All biomass and necromass figures in this article are reported on an oven-dry mass basis. Total biomass and necromass were calculated by summing the oven-dry, site-weighted mean mass of all plant and lichen fractions (for convenience, lichen thalli are included in the term ‘biomass’). In total, approximately 350 samples of above- and below-ground fractions and organs from various plant and lichen species were collected and processed.

The site-weighted mean values of biomass and carbon for each significant species were calculated using their percent cover within the PSP. This method allowed us to determine the proportional contribution of each species to the productivity and carbon content of the studied tundra types. The structural and functional unit of the ecosystem is the patch type (or microhabitat). It serves as the elementary unit for ecosystem functions, such as soil fertility formation and biogeochemical cycling. The areas, shapes, and names of the patch types were determined by the dominant and co-dominant plants. Within these identified elements of the fine-scale heterogeneity, we calculated the percent cover of the main dominant and subdominant species. In the buffer zone outside the PSPs, within similar patch types, we selected plots for collecting monoliths and excavating soil pits. To study ecosystem productivity, we identified the following patch types: Salix pulchra, Betula nana, Eriophorum vaginatum tussock, sedge tussock, lichen-green moss, and sphagnum.

We determined the amount of carbon stored in above- and below-ground fractions of plants and lichens, as well as in annual production and necromass. We calculated site-weighted mean values, accounting for landscape position and horizontal heterogeneity (patch types). We also assessed the total nitrogen (TN) content and the C/N ratio in all fractions of biomass and dead OM.

Annual production in the tundra ecosystems was assessed using the method from [31], which sums the current-year biomass stocks of individual life forms, plus the mass of fruits and seeds. Total annual production was calculated using the formula: Pannual = M-annuals + Mdeciduous + Mevergreen + Mmosses + Mherbs + Mfruits + 1/10Mlichens, where:

Mannuals = total mass of leaves and stems of annual plants,

Mdeciduous = total mass of current-year stems and all leaves of deciduous perennial dwarf shrubs,

Mevergreen = total mass of leaves and stems of current-year shoots of evergreen perennial dwarf shrubs,

Mmosses = mass of current-year moss shoots,

Mherbs = mass of living green parts of herbaceous plants,

Mfruits = total mass of all fruits and seeds,

Mlichens = mass of lichens.

On the PSPs, we collected ground cover samples (litter) using a 25 × 25 cm frame, with five replications. Soil samples were taken from full-profile soil pits excavated to the depth of seasonal thaw (i.e., the active layer). After collection, plant and soil samples were oven-dried at ≤40 °C to halt decomposition. We performed a morphological description of each soil profile and measured the bulk density of each genetic horizon in triplicate. Density was sampled using 100 cm³ steel cylinders. Carbon stocks were calculated for each genetic horizon and then summed to obtain total carbon stocks. Total organic carbon (TOC) stocks were calculated using Equation (1):

$$\mathrm{S}=\mathrm{X}\times \mathrm{p}\times \mathrm{H}$$

(1)

where S is the carbon stock (t C ha⁻¹); X is the TOC content (%); p is the soil bulk density (g cm⁻³); and H is the thickness of the genetic horizon (cm).

To determine the proportion of fine earth (<2 mm) in mineral horizons, samples were passed through a 2 mm sieve. The fine earth fraction was 100% in all mineral layers; therefore, this indicator was not included in the TOC stock calculation.

Analyses were performed at the Federal Research Center “V.V. Dokuchaev Soil Science Institute” (Moscow). Each sample was analyzed in triplicate for the following:

• Total C and N in plants, necromass, and soil were determined by dry combustion using an ECS 8020 elemental analyzer (NC Technologies, Bussero, Italy).
• The pH of an aqueous extract was measured in the organic, peat, and mineral soil horizons. The pH was determined potentiometrically in a soil: water suspension using a 1:25 ratio for organic horizons and a 1:2.5 ratio for mineral horizons.
• The particle size distribution of the parent material was determined in accordance with ISO 11277:2020 [36].

Microbiological analyses were performed at the V.N. Sukachev Institute of Forest, SB RAS (Krasnoyarsk, Russia). CO₂ released by microorganisms was measured on a Picarro 2201-i gas analyzer (Picarro, Inc., Santa Clara, CA, USA). Substrate-induced respiration (SIR), basal respiration (BR), and microbial biomass C (MBC) were measured using a glucose-mineral mixture.

A fresh 100 g sample was air-dried, ground with a mortar and pestle, and passed through a 2 mm sieve. The prepared samples were stored in airtight containers at 20–25 °C for no more than 3 months before analysis.

Prior to microbiological assays, the hygroscopic moisture and field capacity (FC) were determined for all soil samples gravimetrically to ensure uniform moisture content during analysis. To determine FC, glass tubes (~50 mm diameter, 150–200 mm length) were used. A paper filter was placed at the bottom of the tube and covered with gauze. The tube was filled with water, allowed to drain, and then packed with prepared dry soil to a height of 100–150 mm. The top of the tube was covered with film, and the tube was placed in a container of water so that the water level was 1–2 cm above the soil surface, ensuring complete saturation. The tubes remained in the water for 24 h, then were removed and placed on a mesh tray to drain freely. After one hour, the tubes were wiped dry and weighed. The FC value was calculated as a percentage of dry soil using Equation (2):

$$\mathrm{F}\mathrm{C}={\displaystyle \frac{\left({\mathrm{M}}_{\mathrm{w}\mathrm{s}}\mathrm{ }-\mathrm{ }{\mathrm{M}}_{\mathrm{d}\mathrm{s}}\right)}{{\mathrm{M}}_{\mathrm{d}\mathrm{s}}}}\times 100$$

(2)

where FC is the field capacity (%); M_ws is the mass of the wet soil (g); and M_ds is the mass of the dry soil (g).

The prepared sample was divided into three analytical replicates, each undergoing a rewetting and pre-incubation procedure: the air-dry soil was moistened to 60 ± 3% of FC and pre-incubated in glass vials. The vials were sealed with Parafilm and placed in a dark incubator for 7 days at 22 ± 0.5 °C. During pre-incubation, sample moisture was monitored and adjusted as necessary to maintain 60 ± 3% of FC.

After pre-incubation, a soil subsample was placed in a glass vial. The ratio of soil volume to vial volume did not exceed 1:10. Concurrently, another subsample was taken in triplicate to determine moisture content gravimetrically. To determine SIR, 0.2 cm³ of a 5% glucose solution per gram of dry soil was added to each vial. To determine BR, 0.2 cm³ of distilled water per gram of dry soil was added. To accelerate measurements, BR and SIR were determined sequentially in the same vial. After adding glucose or water, the vial was sealed with a rubber stopper, and the time was recorded.

Three identical empty glass vials served as controls to measure the ambient CO₂ concentration. For SIR determination, the sample and control vials were incubated in the dark at 22 ± 0.5 °C for 3–5 h. For BR determination, the vials were incubated in the dark at 22 ± 0.5 °C for 20–24 h.

The SIR rate was calculated using Equation (3) [37]:

$$\mathrm{S}\mathrm{I}\mathrm{R}={\displaystyle \frac{\left({\mathrm{C}\mathrm{O}}_2\mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l}\mathrm{ }-\mathrm{ }{\mathrm{C}\mathrm{O}}_2\mathrm{a}\mathrm{i}\mathrm{r}\right)\times {\mathrm{V}}_{\mathrm{v}\mathrm{i}\mathrm{a}\mathrm{l}}\times 60\times 10}{∆\mathrm{t}\mathrm{ }\times \mathrm{ }{\mathrm{m}}_{\mathrm{a}\mathrm{d}\mathrm{s}}}}$$

(3)

where SIR is the rate of substrate-induced respiration (μL CO₂ g⁻¹ h⁻¹); CO₂ soil is the CO₂ concentration in the vial headspace with soil (vol %); CO₂ air is the CO₂ concentration in the empty control vial (vol %); V_vial is the headspace volume in the vial with the soil sample (cm³); 60 is the conversion factor from minutes to hours; 10 is a constant derived from 1000 (cm³ to μL)/100 (vol % to units); Δt is the incubation time (min); and mads is the mass of the oven-dry soil sample (g).

The mass of the oven-dry soil sample was calculated using Equation (4):

$${\mathrm{m}}_{\mathrm{a}\mathrm{d}\mathrm{s}}={\displaystyle \frac{{\mathrm{m}}_{\mathrm{s}\mathrm{m}}\times 100}{100+\mathrm{W}}}$$

(4)

where m_ads is the mass of the oven-dry soil sample (g); m_sm is the mass of the moist soil sample (g); and W is the gravimetric soil moisture (%).

The microbial biomass carbon (MBC) was calculated using Equation (5) [38]:

$$\mathrm{M}\mathrm{B}\mathrm{C}=\mathrm{S}\mathrm{I}\mathrm{R}\times 40.04+0.37$$

(5)

where MBC is the microbial biomass carbon (μg C g⁻¹); SIR is the rate of substrate-induced respiration (μL CO₂ g⁻¹ h⁻¹); and 40.04 and 3.7 are coefficients for estimating MBC from SIR.

The basal respiration rate was calculated using Equation (6) [39]:

$$\mathrm{B}\mathrm{S}={\displaystyle \frac{{(\mathrm{C}\mathrm{O}}_2\mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l}-{\mathrm{C}\mathrm{O}}_2\mathrm{a}\mathrm{i}\mathrm{r})\times {\mathrm{V}}_{\mathrm{v}\mathrm{i}\mathrm{a}\mathrm{l}}\times 60\times 10}{∆\mathrm{t}\times {\mathrm{m}}_{\mathrm{a}\mathrm{d}\mathrm{s}}}}\times 0.272\times 1.8177$$

(6)

where BS is the basal respiration rate of the soil (μg C g⁻¹ h⁻¹); CO₂ soil is the CO₂ concentration in the vial headspace with soil (vol %); CO₂ air is the CO₂ concentration in the empty control vial (vol %); V_vial is the headspace volume in the vial with the soil sample (cm³); 60 is the conversion factor from minutes to hours; 10 is a constant derived from 1000 (cm³ to μL)/100 (vol % to units); Δt is the incubation time (min); m_ads is the mass of the oven-dry soil sample (g); 0.272 is the carbon content in carbon dioxide (mass ratio of C to CO₂: 12/44); and 1.8177 is the density of CO₂ at 22 °C (g L⁻¹).

The specific respiration of microbial biomass, or the metabolic quotient (qCO₂), was calculated as the ratio of the basal respiration (BR) rate to the microbial biomass, according to Equation (7):

$${\mathrm{q}\mathrm{C}\mathrm{O}}_2={\displaystyle \frac{\mathrm{B}\mathrm{S}}{\mathrm{M}\mathrm{B}\mathrm{C}}}\times 1000$$

(7)

where qCO₂ is the metabolic quotient (μg C mg⁻¹ Cmic h⁻¹); BR is the basal respiration rate of the soil (μg C g⁻¹ h⁻¹); MBC is the microbial biomass carbon (μg C g⁻¹); and 1000 is the conversion factor from μg to mg of MBC.

Primary statistical data processing was performed in Microsoft Excel for Windows 10. Data in the manuscript are presented as arithmetic means with the standard error of the mean (SE). Correlations between site-level mean values (n = 5) of C stocks in various ecosystem components were assessed using Pearson’s correlation coefficient. The level of statistical significance for the obtained data is p ≤ 0.05.

2.3. Permafrost and Landscape Conditions

The study areas are located in the Yana-Kolyma region, within the northern geocryological zone characterized by continuous permafrost with a mean annual rock temperature of −9 to −11 °C [40]. Geologically, they belong to the Indigirka-Kolyma synclinal zone. The territory is underlain by continuous permafrost. The ground temperature at the depth of zero annual amplitude ranges from −2 to −12 °C. The region’s Quaternary deposits are composed of sandy-clay and silty material. Global climatic and landscape changes during the Pleistocene led to the development of Low Arctic landscapes and the onset of long-term ground freezing. In the initial stage of permafrost development in the Pliocene-Lower Pleistocene, a continental stratum of aleurites with interlayers of sand and peat was formed. This stratum includes several tiers with pseudomorphs along polygonal ice veins, reflecting a complex history of geocryological development. The Middle and Upper Pleistocene strata of unconsolidated deposits are relief-forming over a significant portion of the region and are exposed on the surface of the ancient lacustrine-alluvial plain in the continental part [40]. These deposits generally have a uniform composition (silty and peaty sandy loams, loams, and fine-grained sands). The Ice Complex (IC), formed during the most severe epochs of the Upper Pleistocene, reached the maximum ice content (up to 80–100%) for the entire permafrost zone. The thickness of IC deposits reaches 80 m, and the total thickness of the permafrost zone under typical zonal conditions can range from 200 to 700 m. Within IC sections, alas complexes of sediments (taberal, lacustrine, bog, and alas proper) are found embedded in the Middle and Upper Quaternary lacustrine-alluvial deposits [41]. Alas plains are erosional-thermokarst basins formed on the surface of an ancient lacustrine-alluvial plain, with their floors typically featuring a polygonal-ridged microrelief. In some areas of the lowlands, alases may cover a larger area than all other geomorphological levels combined. These alas floors can have different absolute and relative elevations with respect to depositional interfluves. Within the PSPs, depositional plains are particularly prominent. They occupy areas of tectonic subsidence and accumulation of unconsolidated Quaternary deposits (alluvial, lacustrine, marine, and glacial) and are characterized by a slightly dissected relief with minor fluctuations in relative heights. On these plains, landforms associated with their permafrost origin, high ice content, and the presence of massive ground ice are widespread. These include thermokarst basins, frost mounds (pingos), frost cracks, and polygons [42].

According to permafrost-landscape zoning, the PSPs are located in the Nizhneindigirskaya lacustrine-thermokarst province. The locality type is classified as alas, represented by flat surfaces of lacustrine-alluvial plains and their remnants (yedoma). The research sites occupy a depositional position in the landscape, with elevations ranging from 5 to 15 m above sea level. The stratigraphic-genetic complex is composed of lacustrine-alluvial deposits. Typologically, the landscape is a typical tundra on continuous permafrost. The lithological composition consists mainly of sandy loams and loams. The permafrost conditions of the PSPs are manifested in the development of the IC, which is covered with a mantle of loam and sandy loam and is penetrated by syngenetic ice wedges (SIWs). Periglacial processes such as frost cracking, thermokarst, and ground heaving are highly active at the PSPs. The predominant cryogenic textures and ground ice formations in the alas locality type are layered, lenticular, and reticulated, along with a system of massive SIWs. The volumetric ice content varies widely, from 0.2 to 0.8 [43,44].

2.4. Cryogenic Processes and Their Impact on Soil Carbon

The periglacial processes identified in our study are associated with the high ice content of the IC permafrost and yedoma uplands. These processes manifest as polygonal frost cracking of soils, accompanied by the growth of syngenetic ice wedges (Figure 2A). This results in the mechanical movement and redistribution of organic material in the upper soil layers. TOC accumulated in peat horizons can be transported to deeper soil layers, where it becomes less accessible to microbial decomposition. This process aids in the long-term sequestration of carbon in permafrost, which is especially important in the context of global warming, as permafrost degradation can release significant amounts of carbon into the atmosphere. During summer, solifluction (slow downslope movement of the soil-plant layer) is observed on ridge slopes. Solifluction microrelief forms (lobes, small terraces, and frontal scarps) are evident only on certain sections of these slopes. These areas typically coincide with outcrops of argillite rock, which weathers rapidly into sandy-loam material (Figure 2B). This process can contribute to carbon accumulation in topographic depressions, such as thermokarst lakes or alases. In these settings, organic carbon can be buried under a layer of mineral deposits, slowing its decomposition and promoting long-term storage. However, with permafrost degradation, solifluction can initiate soil erosion, leading to the loss of organic material and its transport into river systems as dissolved organic carbon. Additionally, widespread thermokarst—the process of ground subsidence resulting from the thawing of ground ice—leads to the formation of thermokarst lakes and basins. In some cases, thermokarst can promote carbon burial under water or mineral deposits, which slows decomposition.

Thus, thermokarst can either enhance carbon emissions or contribute to carbon sequestration, depending on specific landscape conditions. An important mechanism of physical carbon loss is the thermal erosion of the Beryolyokh River banks (Figure 2C), where the flow of warm water and wave action erodes unconsolidated soils, washing away significant masses of plant (organic) and soil material. Polygonal-ridged relief is a characteristic feature of the southern tundra of the Yana–Indigirka Lowland. It is a system of polygons formed by cryogenic frost cracking of the ground. The polygons are separated by ridges that form from the accumulation of syngenetic ice wedges in the cracks. This relief is typical for permafrost areas with seasonal freeze-thaw cycles. Hydrolaccoliths, or frost mounds (pingos), also result from cryogenic processes. They form from the freezing of subsurface water and the subsequent heaving of the ground surface (Figure 2D). These cryogenic and exogenous processes indicate permafrost degradation and have a significant impact on the distribution and dynamics of TOC in the landscape.

2.5. Morphological and Physicochemical Characteristics of the Soil Cover

The intrazonal soils of the PSPs are located in the Eurasian polar geographical belt and belong to the subzone of tundra gley soils (Gleysols) [45] of the Yana–Indigirka-Kolyma district (

Table 1. Characteristics of soils in the hypoarctic southern shrub tundra of the Yana-Indigirka Lowland, Northeastern Siberia.

Site (PSP)	Horizon (cm)	Soil Type (WRB, 2022 [45])	Soil Profile	Total N (%)	TOC (%)	C:N	Bulk Density (g cm−3)	pH (H2O)	Sand (2–0.063 mm, %)	Silt (0.063–0.002 mm, %)	Clay (<0.002 mm, %)
Chok-1	Litter O (0–1)	Gleysol		0.78 ± 0.16	39.62 ± 4.16	51.0 ± 10.31	0.31 ± 0.01	5.20 ± 0.15	-	-	-
	Peat T (1–20)			1.84 ± 0.12	12.74 ± 6.29	6.9 ± 4.33	0.92 ± 0.02	5.20 ± 0.09	-	-	-
	Sandy loam GC⊥ (20–29)			0.11 ± 0.01	1.34 ± 0.17	12.0 ± 3.04	1.37 ± 0.02	6.0 ± 0.25	11.22 ± 2.97	74.29 ± 2.85	14.49 ± 0.44
Chok-2	Litter O (0–2)	Gleysol		0.62 ± 0.10	46.60 ± 6.37	75.0 ± 14.28	0.28 ± 0.01	5.10 ± 0.12	-	-	-
	Peat T (2–17)			1.53 ± 0.15	31.60 ± 3.24	21.0 ± 8.40	0.87 ± 0.02	4.90 ± 0.05	-	-	-
	Sandy loam GC⊥ (17–30)			0.06 ± 0.01	0.96 ± 0.12	17.0 ± 3.57	1.41 ± 0.02	5.60 ± 0.30	14.59 ± 3.26	72.90 ± 3.00	12.51 ± 0.58
Chok-3	Litter O (0–1.5)	Gleysol		1.30 ± 0.17	23.8 ± 2.34	18.0 ± 7.02	0.26 ± 0.01	5.50 ± 0.12	-	-	-
	Peat T (1.5–19)			1.32 ± 0.10	36.80 ± 4.05	28.0 ± 3.07	0.90 ± 0.02	5.30 ± 0.07	-	-	-
	Sandy loam GC⊥ (19–32)			0.10 ± 0.01	1.91 ± 0.18	18.0 ± 4.02	1.47 ± 0.02	5.10 ± 0.19	22.24 ± 1.32	64.66 ± 1.61	13.10 ± 0.52
Chok-4	Litter O (0–3)	Gleysol		0.79 ± 0.10	46.50 ± 6.05	59.0 ± 11.63	0.28 ± 0.01	5.80 ± 0.14	-	-	-
	Peat T (3–12)			1.81 ± 0.23	46.80 ± 3.77	26.0 ± 6.82	0.84 ± 0.02	5.40 ± 0.11	-	-	-
	Sandy loam GC⊥ (12–24)			0.09 ± 0.01	1.46 ± 0.11	16.0 ± 3.70	1.37 ± 0.02	6.10 ± 0.29	26.80 ± 3.56	59.40 ± 3.93	13.80 ± 0.37
Chok-5	Litter O (0–1.1)	Gleysol		1.45 ± 0.19	39.90 ± 4.64	28.0 ± 6.33	0.24 ± 0.01	5.80 ± 0.14	-	-	-
	Peat T (1.1–17)			1.96 ± 0.22	17.10 ± 4.61	8.7 ± 3.51	0.96 ± 0.02	5.10 ± 0.08	-	-	-
	Sandy loam GC⊥ (17–41)			0.08 ± 0.01	1.38 ± 0.17	17.0 ± 3.11	1.41 ± 0.02	4.80 ± 0.13	24.33 ± 3.73	63.65 ± 3.99	12.02 ± 0.53

Note: Values are mean ± SE (n = 5 sites).

). They consist of polygonal-fissured complexes of tundra gley peaty, tundra gley peaty-humus, and humus-gley soils, along with soils of bare spots and frost cracks. They also include polygonal-ridged complexes of peaty-gley bog soils, tundra gley peaty soils, and soils of frost cracks. These soils are predominantly loamy, with frequent changes in texture, showing a predominance of sands and sandy loams on lacustrine-alluvial and loess-ice deposits [46]. The soil cover is characterized by: (1) predominantly acidic soils; (2) ubiquitous development of gley soils on sandy loam parent materials; (3) absence of salt and carbonate accumulations; (4) a high degree of decomposition of organic horizons; (5) widespread formation of bare spots in watershed uplands, but a small contribution of these spots to the overall soil cover due to rapid revegetation; and (6) a lesser degree of gleying on elevated meso- and microrelief elements compared to other tundra facies, with gleying manifested as a supra-permafrost gley horizon [47]. In the sandy loam-loam Quaternary deposits on the surface of the ice complex (IC), the soil thaw depth is 0.4–0.6 m. In peaty sandy loams, loams, peat, and the moss litter of alases and low-lying swampy areas, the thaw depth varies from 0.15 to 0.4 m, while on higher, drier areas it ranges from 0.2 to 0.45 m. The soils of the PSPs are characterized by a high degree of water saturation due to the shallow depth of ice-rich permafrost, which also results in thixotropy in the mineral horizons. The proportion of fine earth (<2 mm) is 100%.

The TOC content in the organic horizon (O) reaches 23.8–46.6%. In the peat horizon (T), this indicator is highly variable, ranging from 12.7–46.8%. The change in TOC content as plant matter transforms into peat indicates the extent of the peat formation process. As biomass in the organic horizon decomposes, TOC accumulates and TN decreases; conversely, an increase in the TN content in the peat horizon promotes its decomposition. Such a wide range was not observed in the mineral gley horizon (GC⊥), where values ranged from 0.96% to 1.91%. This pattern is characteristic of all studied tundra types, regardless of vegetation. The low carbon content in the gleyed mineral horizon (GC⊥) may indicate mineral weathering and a relatively mature stage of soil formation. The mineral portion of the soil is nitrogen-poor due to permafrost and cold soil conditions, which prevent the complete conversion of organic compounds into nitrates, leaving nitrogen primarily in ammonium form. The C/N ratio plays a crucial role in the decomposition of plant material and the activity of heterotrophic microorganisms. The high TOC and low TN content in the litter and soil contribute to the slow decomposition of both fresh and preserved organic material. This is due to a reduced rate of ammonification on nitrogen-poor substrates and the presence of a recalcitrant nitrogen fraction. Therefore, it can be assumed that organic matter with a high TOC content results in a high C/N ratio and leads to nitrogen immobilization.

The particle size distribution of the parent material significantly impacts the soil’s hydro-physical, physicochemical, and thermal properties, as well as its redox conditions, sorption capacity, and C and N accumulation. In the Yana–Indigirka Lowland, silt fractions prevail, constituting 67% of the composition, but they have a low TOC content compared to the upper horizons. The proportions of sand and clay particles are 20% and 13%, respectively. The bulk density of the litter and soil is the ratio of the mass of dry material in its natural state to its volume. This parameter is essential for calculating the pools and stocks of carbon and other elements. Bulk density values increase with depth from the organic horizon down to the parent material.

3. Results

3.1. Geobotanical Analysis and Content of Biogenic Elements in Biomass and Necromass

The research area belongs to the southern subzone of Low Arctic tundras and, according to geobotanical zoning [33], is part of the Khromo-Beryolyokhsky district of the Yana–Indigirka okrug of the Low Arctic Yana-Kolyma subprovince, within the East Siberian province of the Russian tundra zone. According to the floristic zoning of Yakutia, the territory is assigned to the Arctic floristic region [48,49].

The vegetation cover of the region reflects the main features of the Low Arctic tundra biome of northeastern Siberia. The high heterogeneity of the vegetation and the complex landscape differentiation of communities are due to the spatial distribution of environmental factors along meso- and microrelief gradients. The studied tundra communities are developed on flat, inter-ridge depositional plains and are classified as Low Arctic shrub (Salix pulchra, Betula nana, S. glauca L.) dwarf shrub-green moss (Vaccinium vitis-idaea L., Ledum decumbens (Aiton) Lodd. ex Steud., Aulacomnium palustre (Hedw.) Schwägr.) tundra. These occur in a complex with sedge–tussock cottongrass (Carex bigelowii Torr. ex Schwein., Carex appendiculata (Trautv. & C.A. Mey.) Kük, Eriophorum vaginatum) groupings that include Sphagnum patches (S. warnstorfii Russow).

The total taxonomic list of the surveyed tundra ecosystems includes 24 families, 30 genera, and 43 species. Of these, there are 27 species of higher vascular plants, eight bryophytes, and eight lichens. Among vascular plants, the families Cyperaceae, Ericaceae, Salicaceae, and Poaceae are dominant. The biomorphological spectrum of the surveyed plant communities is characterized by a predominance of herbaceous hemicryptophytes (53.85%), among which long-rhizomatous graminoids dominate (30.77% of all species). Dense-tussock and loose-tussock graminoids each account for 7.69%. Shrubs are represented by hemiprostrate forms (15.38%), and dwarf shrubs (19.23%) by erect and prostrate forms. With respect to soil moisture, hygrophilic species predominate (30.77%), primarily sedges, grasses, and cottongrasses. Mesophytic species and eurytopic species with wide ecological amplitudes each account for 26.93% (e.g., Salix, Betula, dwarf shrubs).

In the ecosystems examined, the upper shrub layer is formed by the Low Arctic, circumpolar deciduous shrub Salix pulchra (35–70 cm in height), and the second layer by Betula nana (15–40 cm), an arctoboreal, deciduous phanerophyte. Both species adopt a hemiprostrate form under tundra conditions and co-dominate in various combinations, reaching 40–65% of the total cover. On flat peaty mounds, Low Arctic dwarf shrub-shrub and Flavocetraria cucullata (Bellardi) Kärnefelt & A. Thell patch types are formed. In the herb-dwarf shrub layer, Vaccinium vitis-idaea and Ledum decumbens are found with high constancy and abundance, while the constancy and cover of Vaccinium uliginosum L. are substantially lower. Small dwarf shrubs, such as Dryas octopetala L. and Arctous alpina (L.) Nied., as well as semi-shrubs and various tundra forbs, are sparse and have insignificant cover. The green moss cover is quite dense, formed by Aulacomnium palustre and A. turgidum (Wahlenb.) Schwägr., with an admixture of Dicranum elongatum Schleich. ex Schwägr. and species of Polytrichum. Mosses grow on micro-elevations, in wet hollows, on the edges of waterlogged depressions, and between tussocks. In deep depressions where water stagnates, hygrophilic sedges and grasses dominate, and patches of Sphagnum warnstorfii occur. Shallow, saucer-shaped micro-depressions host cottongrass and sedge tussock communities. Fruticose lichens (e.g., Cladonia arbuscula (Wallr.) Flot., C. cornuta (L.) Hoffm.) colonize micro-elevations and flat mounds, while foliose lichens are confined to micro-depressions and exposed mineral soil.

We determined the carbon content in above- and below-ground fractions and organs of plants and lichens in the studied tundra ecosystems. The highest mean C content was found in the woody fraction of above-ground shoots of evergreen and deciduous shrubs and dwarf shrubs, such as Ledum decumbens, Betula nana, and Vaccinium vitis-idaea (

Table 2. Mean C and N content (%) in various organs and fragments of plants and lichens in the southern Low Arctic shrub tundra of the Yana–Indigirka Lowland, Northeastern Siberia.

Plant Organs and Fragments	Mean C Content, %	Mean N Content, %
Salix pulchra
Woody stems	49.68 ± 4.51	0.53 ± 0.12
Woody twigs	48.52 ± 2.33	0.65 ± 0.10
Current shoots	48.21 ± 1.89	1.01 ± 0.27
Leaves	46.94 ± 1.38	1.78 ± 0.38
Underground rooting shoots	45.85 ± 1.81	0.65 ± 0.11
Betula nana
Woody stems	50.31 ± 3.56	0.51 ± 0.05
Woody twigs	52.45 ± 3.21	0.57 ± 0.13
Current shoots	53.56 ± 3.04	1.17 ± 0.23
Leaves	49.42 ± 0.99	1.78 ± 0.24
Underground rooting shoots	47.45 ± 1.50	0.87 ± 0.12
Ledum decumbens
Woody stems	54.49 ± 2.51	0.52 ± 0.11
Woody twigs	54.48 ± 2.49	0.52 ± 0.09
Current shoots	46.13 ± 1.25	0.78 ± 0.12
Current leaves	48.37 ± 1.12	1.25 ± 0.20
Perennial leaves	47.93 ± 1.32	0.89 ± 0.15
Underground rooting shoots	47.14 ± 1.27	0.53 ± 0.13
Vaccinium vitis-idaea
Perennial shoots	48.36 ± 1.42	0.50 ± 0.07
Current shoots	48.58 ± 1.32	0.57 ± 0.10
Current leaves	49.08 ± 1.98	0.50 ± 0.06
Perennial leaves	49.63 ± 3.87	0.72 ± 0.06
Poaceae
Vegetative annual shoots	44.83 ± 0.33	1.19 ± 0.01
Carex
Vegetative annual shoots	43.30 ± 1.87	1.74 ± 0.42
Eriophorum vaginatum
Vegetative annual shoots	42.78 ± 2.11	1.18 ± 0.22
Mosses
Aulacomnium palustre
Current leafy shoots	41.42 ± 2.10	0.84 ± 0.30
Perennial leafy shoots	44.13 ± 1.13	0.76 ± 0.22
Aulacomnium turgidum
Current leafy shoots	36.85 ± 1.15	1.21 ± 0.50
Perennial leafy shoots	39.28 ± 1.17	1.13 ± 0.45
Sphagnum warnstorfii
Current leafy shoots	39.54 ± 1.20	0.85 ± 0.31
Perennial leafy shoots	39.45 ± 1.19	0.83 ± 0.35
Lichens
Thallus	32.71 ± 1.30	0.96 ± 0.81
Flavocetraria cucullata
Thallus	31.65 ± 0.72	0.24 ± 0.04

). Among graminoids, the C content was lower in Eriophorum vaginatum. The lowest C concentrations were found in bryophytes.

The process of humification requires an optimal carbon-to-nitrogen ratio in plant residues, typically ranging from 20:1 to 30:1. In the studied tundra, different plant groups, life forms, and plant parts showed marked differences in their C/N ratios. We found that the highest mean C/N values occurred in the stem mass of shrubs, the perennial woody shoots of dwarf shrubs, evergreen leaves, and lichen thalli (Figure 3). The maximum value was recorded for the stems of Salix pulchra (178), and for the thalli of Flavocetraria cucullata (169). In graminoids, the C/N ratio was, on average, close to optimal: in sedges with annual summer-green shoots, it was 28, while in the winter-green cottongrass with biennial leaves, it was higher at 40.

Our results show that in these southern tundra types, the most important primary producers accumulating the maximum biomass—both above- and below-ground—are shrubs, bryophytes, and Eriophorum vaginatum. The mean above-ground living biomass in these tundra ecosystems reached 15.8 ± 1.5 t ha⁻¹, with an annual production of 2.6 ± 1.6 t ha⁻¹ yr⁻¹ (oven-dry mass). Overall, the mean total living biomass, including above- and below-ground components, was 43.1 ± 1.6 t ha⁻¹. The total amount of carbon in above-ground biomass was 7.76 ± 0.12 t C ha⁻¹, in below-ground biomass was 12.22 ± 0.73 t C ha⁻¹, and in necromass it was 12.47 ± 1.15 t C ha⁻¹ (Figure 4).

We found that Salix pulchra accumulates the largest site-weighted mean above-ground biomass, accounting for 40.5% of the total above-ground plant mass in the studied ecosystems (Figure 5). The percent cover of S. pulchra was relatively low, averaging 18.5% (range: 12% to 25%, with a maximum of 40%). Nevertheless, its 100% occurrence rate and the high mass of its large woody shoots were the decisive factors contributing to its large mean values. The carbon stock in its stems was 2.56 ± 0.85 t C ha⁻¹ (83.5%), while in its leaves it was 0.43 ± 0.15 t C ha⁻¹ (16.5%).

Bryophytes ranked second in terms of above-ground productivity. Their total site-weighted mean biomass was 4.08 ± 0.55 t ha⁻¹, which is 25.8% of the total above-ground biomass. The dominant species were arcto-alpine Low Arctic hygrophilic and mesohygrophilic species, among which the leafy moss Aulacomnium palustre prevailed (mean percent cover = 31%, occurrence = 100%, mean above-ground biomass = 1.01 t ha⁻¹). It was co-dominated by A. turgidum and Dicranum elongatum, and in some communities, patches of Sphagnum warnstorfii were present. The average thickness of the moss cover was 10–15 cm. Bryophytes had a high percent cover (40% to 60–70%) in almost all studied ecosystems. This, combined with their significant biomass, results in a substantial contribution to the long-term accumulation of necromass and peat. The site-weighted mean carbon accumulated by bryophytes was 2.19 ± 0.28 t C ha⁻¹.

The third-largest contributor to productivity was dwarf birch (Betula nana), with a total site-weighted mean above-ground biomass of 3.15 ± 0.42 t ha⁻¹ (19.9% of total above-ground biomass). Its mean percent cover (34%) was higher than that of Salix pulchra, yet its above-ground biomass was approximately half, mainly due to its low-growing habit. The vast majority of its carbon stock was in woody parts (88.1%), with only 11.9% in leaves. The site-weighted mean carbon stock in the above-ground organs of B. nana was 1.60 ± 0.24 t C ha⁻¹.

A significant contribution to tundra biomass was also made by the family Cyperaceae, with Eriophorum vaginatum being the most important species. In the surveyed tundra types, the occurrence of this species was 100%. In dwarf shrub–tussock cottongrass–green moss tundra (sites Chok-3 and -4), its above-ground biomass increased to 1.44 ± 0.15 t ha⁻¹, with a percent cover ranging from 1% to 40%. The main mass of cottongrass is found in the dead parts of the tussock, consisting of numerous dry leaves, sheaths, and dead roots, which were included in the necromass and litter fractions.

Table 3. Mean C stocks in above- and below-ground living biomass and necromass of the southern Low Arctic tundra near the “Chokurdakh” station (Yana–Indigirka Lowland, Northeastern Siberia).

Mean C Stocks, t C ha−1	Chok-1	Chok-2	Chok-3	Chok-4	Chok-5
In above-ground living biomass	3.62 ± 1.06	5.59 ± 1.31	6.66 ± 1.33	7.75 ± 1.54	11.93 ± 1.51
In below-ground living biomass	9.68 ± 2.69	9.34 ± 3.56	10.88 ± 4.00	10.76 ± 3.01	10.26 ± 4.61
In necromass	7.08 ± 1.61	7.64 ± 1.73	12.30 ± 1.29	12.22 ± 1.30	20.14 ± 3.81

presents the mean C stocks in above- and below-ground living biomass and necromass by tundra subtype. Stocks varied significantly among subtypes, largely due to the heterogeneous horizontal structure and pronounced fine-scale heterogeneity of the communities. The highest C stocks in above-ground biomass were observed at site Chok-5, where a deciduous shrub layer of Salix pulchra and Betula nana had a combined percent cover of 60%. Conversely, the lowest content was at site Chok-1, where the shrub cover was only 30%. Because these woody species have high C concentrations (48.3% and 51.4%, respectively), higher cover translates to higher carbon stocks. Carbon in below-ground living biomass did not differ significantly among sites, despite variations in above-ground organs. This is likely due to the greater stability of ecological conditions in the subterranean environment. Carbon in necromass showed noticeable variations between PSPs, which can also be largely explained by the different levels of shrub cover, which contributes significantly to litter and deadwood.

The site-weighted mean annual production in our studied tundra ecosystems (Figure 4) was 16.3% of the above-ground biomass. The ratio of above-ground biomass to annual production was 6.1:1, indicating the slow growth rate and low productivity of tundra plants and lichens. Photosynthetic tissues of vascular plants played the main role in the structure of production, accounting for 85.3%. The contribution of slow-growing mosses and lichens to annual above-ground production was small, at 14.7%. The total annual C production in the studied ecosystems reached 1.58 ± 0.18 t C ha⁻¹.

The ratio of above-ground to below-ground biomass is an important indicator of ecosystem structure. Our results showed that below-ground living biomass (roots, rhizomes, underground shoots) was 1.73 times greater than above-ground biomass (63.4% of total living biomass). We found that Betula nana and Salix pulchra accumulate the largest portion of their below-ground biomass in the lower parts of their shoots that are embedded in the soil. The transition to prostrate and hemiprostrate morphotypes and to rhizomatous rather than seed-based reproduction is one of the most important adaptations of tundra shrubs to extreme environmental conditions. The developed root systems of northern plants allow for the temporary storage of significant amounts of carbohydrates in the roots, which are necessary for the formation of above-ground organs when photosynthesis is limited by permafrost-related factors. This characteristic of tundra plants significantly increases the below-ground biomass of southern tundra communities and, consequently, their carbon storage: 4.39 ± 0.72 t C ha⁻¹ is accumulated in the underground shoots of Betula nana, and 2.83 ± 0.82 t C ha⁻¹ in those of Salix pulchra.

3.2. Carbon Pools and Stocks by Tundra Type

Table 4. Pearson correlation coefficients between mean C stocks in above- and below-ground living biomass and necromass, and mean C stocks in the ground and soil covers of southern Low Arctic tundra (n = 5 sites).

Correlation Coefficient *	TOC Stocks in Ground and Soil Cover
	Chok-1	Chok-2	Chok-3	Chok-4	Chok-5
TOC in above-ground living biomass	0.77	0.99	0.93	0.90	0.99
TOC in below-ground living biomass	0.86	0.86	0.92	0.89	0.89
TOC in necromass	0.73	0.73	0.87	0.89	0.88

Note: * Significance level p ≤ 0.05.

presents a correlation analysis for the five study sites, providing reliable data on the variability of organic carbon in the tundra ecosystem components. The correlation coefficient values show a very strong positive relationship between the various biomass components and carbon stocks in the ground and soil cover. For above-ground living biomass, the correlation coefficients range from 0.77 to 0.99, indicating a close relationship between C stocks in this fraction and the overall ecosystem organic carbon level. For below-ground living biomass, the correlation is also significant, ranging from 0.86 to 0.92, which highlights the importance of underground plant organs in organic carbon accumulation. For necromass, the correlation coefficients are in the range of 0.73 to 0.89, confirming the significance of this fraction in biogeochemical processes.

Figure 6 presents the final results on carbon pools and stocks in the Low Arctic southern tundras of the Yana–Indigirka Lowland. The region’s climate leads to shallow soil thawing, which limits the thickness of the seasonally thawed layer. This layer serves as the active zone for biochemical processes and is a key factor determining ecosystem organic carbon generation. Maximum C stocks were recorded in the litter and peat horizons of the different tundra types. Total C stocks varied from 257.9 t C ha⁻¹ at site Chok-1 to 511.4 t C ha⁻¹ at sites Chok-3 and -4. The litter-peat horizon accounted for 234.3 and 449.5 t C ha⁻¹, respectively. In the 0–10 cm and 0–30 cm mineral layers, 6.1 to 29.5 t C ha⁻¹ and 17.6 to 49.8 t C ha⁻¹ were accumulated, respectively. The fluctuating values for carbon pools and stocks are due to the initial C content in the organic horizon and the soil, which are unevenly distributed across the tundra sites and down the soil profile. In the southern tundra, anaerobic conditions increase C accumulation in the organic-accumulating layers. At the same time, the accumulated plant and organic material insulates the soil, limiting heat penetration into the mineral layer and reducing the thaw depth.

The distribution of C stocks is most pronounced in the litter, which is associated with the density of organic material, the degree of moss development and decomposition, plant species composition, and their respective C content. Moreover, the varying amount of stored carbon in the organic-accumulating horizons does not significantly affect the C stock of the mineral horizons. As a result, a minimal amount of carbon enters the mineral part of the soil to be stored in the upper ice complex deposits. The vast majority of the organic matter produced is preserved on the surface as a moss mat and a peat horizon. These layers are subject to degradation by abiotic environmental factors, but their mineralization rate under natural conditions remains low. The formation and persistence of permafrost ensure the preservation of carbon-containing materials until the permafrost degrades. This degradation can occur both locally and globally as a result of cyclical climate change. To understand the functioning of biological processes within the geological formations of the permafrost zone, it is important to understand their relationship with the potential destabilization of the environment.

3.3. Heterotrophic Activity of the Organic and Mineral Soil Horizons

To understand the functioning of the studied ecosystems, we conducted microbiological experiments to assess the activity of the microbial community in releasing carbon dioxide from the active layer. This clearly demonstrates their viability upon activation and with increasing temperature under laboratory conditions. This finding is applicable to natural permafrost ecosystems, as it reveals the mechanisms by which stored organic residues and their carbon serve as a substrate for microbial activity. This activity drives CO₂ emissions from degrading permafrost. This degradation is accompanied not only by biological processes but also by the physical disruption of frozen deposits due to thermokarst. Thus, carbon that has accumulated over millennia and is locked in the permafrost can become an ecosystem source of atmospheric gases, along with water vapor formed during the thawing of ground ice. This process continues until the bulk of the ground ice is lost.

Table 5. Variations in microbial activity by tundra type.

Tundra Type	Soil Horizon (cm)	Basal Respiration (BR), μg C g−1 h−1	Microbial Biomass Carbon (MBC), μg C g−1	Metabolic Quotient (qCO2), μg C mg−1 Cmic h−1	Relationship Between Microbiological Parameters and TOC Stocks
					BR	MBC	qCO2
Chok-1	O (0.0–1.0)	9.82 ± 2.02	5827.23 ± 896.71	1.69 ± 0.30	0.40	0.57	0.52
	T (1.0–20.0)	16.05 ± 2.91	10,173.98 ± 3344.22	1.65 ± 0.32
	GC⟂ (20.0–29.0)	0.48 ± 0.11	2515.04 ± 367.68	0.19 ± 0.03
Chok-2	O (0.0–2.0)	17.6 ± 0.94	23,745.00 ± 2043.27	0.74 ± 0.02	0.58	0.70	0.93
	T (2.0–17.0)	8.19 ± 0.58	70,627.77 ± 18,983.98	0.12 ± 0.02
	GC⟂ (17.0–30.0)	1.32 ± 0.36	6292.76 ± 307.76	0.21 ± 0.06
Chok-3	O (0.0–1.5)	18.2 ± 1.52	13,175.99 ± 4619.50	1.50 ± 0.57	0.73	0.45	0.97
	T (1.5–19.0)	4.79 ± 1.67	15,554.83 ± 3591.38	0.31 ± 0.07
	GC⟂ (19.0–32.0)	0.64 ± 0.04	946.81 ± 93.76	0.68 ± 0.06
Chok-4	O (0.0–3.0)	20.52 ± 2.91	22,332.51 ± 5654.38	0.94 ± 0.12	0.54	0.42	0.91
	T (3.0–12.0)	9.52 ± 1.80	16,214.92 ± 1931.95	0.58 ± 0.04
	GC⟂ (12.0–24.0)	0.20 ± 0.08	3476.42 ± 1389.86	0.06 ± 0.00
Chok-5	O (0.0–1.1)	21.86 ± 1.00	53,644.28 ± 2271.74	0.41 ± 0.03	0.46	0.72	0.60
	T (1.1–17.0)	11.99 ± 1.10	15,245.78 ± 643.65	0.79 ± 0.05
	GC⟂ (17.0–41.0)	0.44 ± 0.11	2829.54 ± 244.57	0.16 ± 0.04

presents the results for microbiological indicators. For instance, the maximum values of basal respiration were recorded in the organic (O) horizon, up to 21.86 μg C g⁻¹ h⁻¹. The minimum values were in the gleyic C (GC⟂) horizon, from 0.20 to 1.32 μg C g⁻¹ h⁻¹. Basal respiration in the mineral horizon is evidently lower due to the reduced availability of organic matter and the influence of the underlying permafrost. The peat horizon (T) is an intermediate layer between O and GC⟂, with a wide range of values from 4.79 to 16.05 μg C g⁻¹ h⁻¹, indicating heterogeneous conditions, most likely related to varying moisture levels.

The highest microbial biomass carbon was recorded in the O horizon of site Chok-5, at 53,644 μg C g⁻¹. The T horizon accumulates the most microbial biomass carbon, for example, at site Chok-2 (70,628 μg C g⁻¹). The sharp decrease in microbial biomass carbon in the gley horizon (GC⟂) of site Chok-3 to 947 μg C g⁻¹ is associated with low metabolic activity of microorganisms under permafrost conditions, a pattern generally true for all tundra types, albeit with varying values.

The values of the metabolic quotient In the O horizon varied from 0.41 (Chok-5) to 1.69 (Chok-1) μg C mg⁻¹ Cmic h⁻¹. High values were noted at sites Chok-1 and Chok-3, which may indicate physiological stress caused by low temperatures and moisture fluctuations. In the T horizon, the metabolic quotient decreased, ranging from 0.12 (Chok-2) to 0.79 (Chok-5) μg C mg⁻¹ Cmic h⁻¹, indicating more efficient carbon use by microorganisms. In the GC⟂ horizon, the lack of available carbon drives the metabolic quotient close to zero, leading to the cessation of microbiological processes.

The correlations between microbial activity Indicators and C stocks highlight the heterogeneity of microbial processes even at a local scale within a single ecosystem. This fact underscores the importance of predictive assessments when developing global and regional programs for organic carbon sequestration in the circumpolar region. Nevertheless, our studies show moderately to very strong links between the studied parameters. For the most part, correlation coefficients were moderate to high and positive, and in some cases, very high (0.93–0.97 for the metabolic quotient at sites Chok-2 and Chok-3, respectively). Ranking the coefficients for each parameter, high values were characteristic for basal respiration and microbial biomass carbon, ranging between 0.54 and 0.73. Very high values were characteristic of the metabolic quotient, from 0.91 to 0.97, which is a significant indicator of the state of the ecosystem and its functioning under permafrost conditions. These results also indicate the need for further study of microbial processes in the context of the carbon cycle and climate change adaptation.

The role of microorganisms in decomposing organic matter and forming soil in a sharply continental climate is invaluable. Figure 7A shows the general microbial respiration characteristics of the soil cover. The data reflect the stratification of the studied horizons: O (organic), T (peat), and GC⟂ (gleyic C). Basal respiration (BR) was highest in the organic horizons across all sites, ranging from 8.3 to 19.7 μg C g⁻¹ h⁻¹. This is associated with microbial activity during seasonal thawing and heat penetration into the surface layers. Deeper in the mineral soil layer, BR intensity reached a minimum threshold, from 0.3 to 0.9 μg C g⁻¹ h⁻¹. This indicates very weak microbial activity due to excessive moisture, low temperatures, and a lack of available biogenic elements. Based on our dataset, it was not possible to distinguish a specific tundra type by its BR parameter alone. First, the organic horizons are saturated with carbon but have insufficient nitrogen. Second, this lack of nitrogen slows the decomposition of plant residues, so background microbial respiration does not differ significantly among the tundra types.

Microbial biomass carbon (MBC) was a more informative indicator of biological activity (Figure 7B). The shrub-lichen-green moss tundra ecosystem (sites Chok-2 and -5) stood out. It is capable of mineralizing and transforming a sufficient amount of carbon, averaging 29,212 μg C g⁻¹ of soil, versus 8295 μg C g⁻¹ in the shrub-sphagnum-green moss tundra (Chok-1) and 12,979 μg C g⁻¹ in the lichen-green moss tundra with Eriophorum vaginatum (Chok-3 and -4).

The microbial metabolic quotient (qCO₂) of the southern tundra ranged from 0.14 to 0.21 μg C mg⁻¹ Cmic h⁻¹ (GC⟂ horizon), to 0.44–0.97 μg C mg⁻¹ Cmic h⁻¹ (T horizon), and up to 0.73–1.69 μg C mg⁻¹ Cmic h⁻¹ (O horizon) (Figure 7C). The qCO₂ values of the lower soil horizons hardly differed among the studied tundra types. In the mineral soil layer, there is low utilization of organic material by microorganisms, as it is inherently poor in available nutrients. High values of the microbial quotient were found at site Chok-1 (0.19–1.69). A higher qCO₂ suggests less efficient C use by microorganisms and may indicate physiologically stressful conditions. In contrast, lower qCO₂ values at sites Chok-2 and -5 (0.14–0.73 μg C mg⁻¹ Cmic h⁻¹) suggest that heterotrophic microorganisms in these tundra types demonstrate significant activity in the biogeochemical carbon cycle and can efficiently assimilate incoming organic material. Thus, the microbial communities of the Yana–Indigirka Lowland have the natural potential to transform organic matter and maintain the stable functioning of the permafrost ecosystem. However, this effect could be amplified with an increase in air and soil temperature.

These microbiological studies, conducted in the southern Low Arctic shrub tundra of the Yana–Indigirka Lowland—a key Arctic region and global carbon reservoir since the Late Pleistocene—show that even under low-temperature conditions, microorganisms retain their metabolic potential and exhibit significant activity with increasing temperature. Permafrost degradation caused by climate change leads to the release of previously stabilized carbon as CO₂ and CH₄, increasing the concentration of these atmospheric gases. A high potential for emissions is associated with the upper soil horizons and permafrost deposits, where a large amount of TOC is stored. Under optimal conditions, microbial activity can lead to extensive processing of this organic matter. Our data confirm the need for systematic monitoring of microbial processes in permafrost ecosystems, especially for forecasting long-term carbon cycle dynamics under thawing conditions and for developing methods to mitigate climate risks.

4. Discussion

4.1. Assessment of Above- and Below-Ground Biomass and Necromass in the Context of Southern Tundra Ecology and a Changing Climate

Climate change affects ecosystem productivity by altering environmental variables and, ultimately, the physiological functioning of plants [50]. Satellite imagery indicates an increase in the average Normalized Difference Vegetation Index (NDVI), reflecting the “greening” of the landscape [10,51,52]. From 1982 to 2008, maximum NDVI values increased by up to 15% in some high-latitude areas of the Canadian Arctic, northern Alaska, and Eurasia [53]. In the 2010s, NDVI values increased by 15% and 30% in the subarctic and near-subarctic territories of Russia, respectively [54]. An increase in biomass and production has been noted for southern eastern Taimyr and the interfluve of the Anabar and Olenyok rivers [55,56]. The authors argued that this process is driven by an increase in the photosynthetic tissues (grasses, leaves of shrubs and dwarf shrubs) and an expansion of more productive plant communities, such as grass, sedge, and cottongrass communities (meadowing) and willow thickets (shrubification). Using a regression model between NDVI and above-ground biomass [57], they found that from 1982 to 2010, vegetation biomass in the southern subzone of Eastern Siberia (D-E) increased by 23.4%. In the Yana-Kolyma floristic province, the authors established a biomass increase of 81.6 g m⁻², at a rate of 2.9 g m⁻² yr⁻¹. In Chukotka, observations of vegetation cover [58] showed that from 2000 to 2017, the treeless tundra remained quite stable, with an increase in forest-tundra and shrub tundra noted only in the northern taiga and tundra-taiga transition zones. Recently, reports have emerged about tundra ‘browning’ in the Yana–Indigirka Lowland, caused by spring floods and local drought on convex yedoma landscapes [59], as well as by wildfires [60].

For the southern Low Arctic shrub tundra of the Yana–Indigirka Lowland, a body of information has accumulated regarding shifts in the timing and length of the growing season, the expansion of shrubs and graminoids, the degradation of ice complex landscapes, and resulting fluctuations in carbon emissions and uptake [61,62]. An analysis of long-term data from the Chokurdakh meteorological station [59] revealed a shifting trend in some climatic parameters. The average monthly temperature for July increased by 2.04 °C from 1975 to 2015. In the 1960s–80s, more precipitation fell in the summer, but since 2000, it has been concentrated in the autumn-winter months. The long-term average maximum snow depth increased by 42.9 cm from 1975 to 2024 [60]. In light of these ongoing changes, it is necessary to consider the life strategy and current state of the dominant, ecosystem-structuring shrubs of the southern tundra: Salix pulchra and Betula nana. These Low Arctic plants are among the most dynamic species in the study area and play an important landscape-forming role in the southern shrub tundra.

The harsh high-latitude climate impacts all components of the tundra vegetation. The most important factors affecting plant growth are cold, low-snow winters with a harsh wind regime; a short growing season; low summer temperatures; high air humidity; nearly 24-h daylight during the polar day; and acidic, nutrient-poor soils with stagnant moisture and a shallow active layer limited by permafrost.

In the floodplain of the Beryolyokh River (a left tributary of the Indigirka) and on south-facing coastal cliffs—where temperature and wind regimes are milder and snow accumulates—Salix pulchra forms large bushes up to 2.0 m high with branched stems 5–6 cm in diameter and leaf blades up to 5.0–5.5 cm long (dry). This can be identified as a floodplain ecotype adapted to intrazonal conditions. Nearby, on the low-lying depositional plain where our research was conducted, the willows are approximately 3 to 7 times shorter (30–70 cm), corresponding to the upper limits of the snow cover. Their leaf blades are also 1.3–2.7 times smaller (2–4 cm). This represents a suppressed tundra ecotype adapted to survival on open, low-snow plains. The annual tips of their shoots are desiccated by winter winds, abraded by snow, and broken by frost, limiting growth and branching.

The influence of snow cover on shrub development is demonstrated by experiments [63] in the Alaskan Arctic tundra, where artificially deepened snowdrifts caused an increase in shrub cover and canopy height. Furthermore, a study [64] on the annual rings of Salix pulchra indicates that this willow responds positively to early summer temperatures. Thus, low winter precipitation, wind, snow abrasion, low air temperatures, and cold soils with a thin active layer greatly reduce the growth potential of this dominant tundra willow. Its biomass is several times lower than what might be possible under more favorable conditions. Based on this, an increase in heat supply during the growing season, combined with increased winter precipitation, may positively affect the productivity of this willow. This would, in turn, enhance its ecosystem-structuring, cooling, and permafrost-stabilizing functions. The above-ground shoots of this shrub live for up to 40–60 years, and the underground parts live even longer. Thus, by virtue of its high biomass and the long-term storage of carbon in its woody shoots, this species is one of the main carbon sequesters in the southern tundra of the Yana–Indigirka Lowland.

The strategy of Betula nana in the tundra we studied is more conservative. It prefers to shelter in the shade of willows and utilize microrelief depressions to avoid unfavorable conditions. It tends to minimize its growth form, thereby conserving energy and resources. Nevertheless, thanks to its 100% occurrence rate, high percent cover, and active development of underground shoots, this birch species also accumulates significant mass and is an important carbon sink. Dendrochronological analysis (1974–2018) of Betula nana stems showed a positive relationship between radial growth and summer temperature [65,66]. In the lower reaches of the Kolyma River (Kolyma Lowland), it was also found that Betula nana accumulates maximum biomass under moderately warm summer conditions [67,68].

Insufficient assimilation of mineral nutrients from cold soils, despite adequate light and high rates of photosynthesis, is one reason for the low biomass accumulation and dwarfism (nanism) of tundra plants. Under these conditions, they are forced to enhance nutrient uptake, which involves increasing the size and surface area of their root systems. Consequently, in the tundra zone, below-ground biomass is typically almost twice as large as above-ground biomass [69]. In our studies, below-ground biomass exceeded above-ground by 1.7 times, which is slightly lower than this indicator and is quite typical for southern tundra with extensive shrub cover. Studies at the “Chokurdakh” station [70] showed that the positive response of B. nana to changes in summer air temperature may be indirectly related to improved soil nutrient availability.

It has been established [71] that rising air temperatures boost the above-ground productivity of tundra ecosystems, while the below-ground component does not show a similar response. In warmer conditions, biomass allocation shifts toward above-ground parts. This may affect the ecosystem carbon cycle by changing the input and distribution of litter and altering root systems. In our studies, the majority of below-ground biomass (56.8%) consisted of the lower, rooting parts of Salix pulchra and Betula nana shoots that were embedded in the soil. Their total mean mass exceeded the root biomass by 1.4 times.

An analysis of 20 years of Net Ecosystem Exchange (NEE) data at the “Chokurdakh” station showed a trend towards an increasing annual carbon sink in the southern tundra; approximation of eddy covariance data indicated a positive trend of increasing carbon uptake. The authors attribute these changes in the carbon balance to the expansion of shrub communities driven by climate warming. They also noted a 2-week extension of the growing season, a significant factor for ecosystems and carbon exchange. R.E. Petrov et al. [61,72]. Several authors suggest that the expansion of deciduous shrubs in the Arctic may reduce permafrost thaw and thus partially compensate for its degradation [7,8,57,61]. In experiments [7,66,70], the removal of the B. nana canopy initiated the thawing of ice-rich permafrost, leading to the collapse of elevated areas and their transformation into waterlogged, concentric depressions within 6 years [66]. It was found that even a small-scale disturbance of the shrub cover leads to thermokarst, resulting in waterlogged areas with an increasing presence of graminoids (Arctagrostis latifolia (R. Br.) Griseb., Carex sp., Eriophorum angustifolium Honck.) that became a source of methane.

Based on these experimental results, it is highly probable that the restoration of Salix pulchra and Betula nana thickets in these waterlogged areas will be practically impossible. With ground subsidence and the development of water bodies and bogs, these shrubs will likely retreat to elevated microhabitats. Salix pulchra is a hygro-mesophyte, while Betula nana is more mesophilic. Although both species might adapt to some waterlogging, neither can withstand prolonged inundation. This is evidenced by the mass mortality of Salix pulchra after large-scale floods in the Alazeya River valley (Kolyma Lowland) in the early 2000s, which were linked to climate change [68]. It is also supported by the high mortality of Betula nana after a wet summer, resulting from waterlogging and thermokarst at the “Chokurdakh” station [65]. Thus, with large-scale disruption of the shrub canopy or with further warming, the expansion of water bodies and grass-sedge bogs could lead to a carbon imbalance and increased GHG emissions in the southern tundra.

Bryophytes play a significant role in the carbon pool of southern tundra ecosystems. It is believed that mosses became an important component of the tundra vegetation after the Late Pleistocene [69], and the peat deposits they have formed since then play a significant role in the GHG balance. In the ground layer of tundra communities, these organisms perform crucial ecosystem-structuring, thermoregulatory, and permafrost-protective functions. Studies [66] at the “Chokurdakh” station showed that mosses can have a noticeable effect on water, heat, and energy flows, and that changes in the moss cover can cause permafrost thawing and affect the carbon balance in tundra soils [73]. Our studies have confirmed that bryophytes in southern shrub tundra act as key dominants of the lower strata. They form a dense cover with high percent cover and biomass (up to 60–70%) and perform foundational functions, regulating not only the soil hydrothermal regime but also the establishment of many other plant species. In the green biomass of the studied ecosystems, they are dominant, exceeding the contribution of vascular plants. However, they contribute much less to annual production due to their inherently slow growth. The longevity, density, slow decomposition, and large biomass of the moss turf underscore its significant role in the long-term sequestration of carbon in the tundra zone.

Eriophorum vaginatum is an arctoboreal, dense-tussock, winter-green species that forms cottongrass communities interspersed throughout various tundra types. It is usually confined to saucer-shaped depressions with excessive moisture, where water from snowmelt and the active layer stagnates. In the studied ecosystems, it forms tussocks 25–30 cm high and 20–25 cm in diameter. Tussock tundras dominated by this species are an ancient Beringian formation [69]. In the Low Arctic, they are at their ecological optimum and are key components of tundra and tundra-bog biomes in moderately to heavily moist microhabitats. It was found [74] that a warm, wet autumn and a deep snow cover promote an increase in the green mass of cottongrass, and year-to-year productivity changes are related to air and soil temperature and thaw depth. Tussock-forming graminoids, especially Eriophorum vaginatum, also make a large contribution to the necromass and litter due to their accumulation of a large number of dead shoots. Considering that these tussocks can persist for 100 years or more [69], they are also significant carbon sequesters in the southern tundra subzone of northeastern Siberia.

The ratio of living to dead organic matter provides insights into the balance of production and decomposition processes. The largest components of the necromass are undecomposed and semi-decomposed wood of willows and birches, and partially humified remains of mosses, cottongrass, and sedge tussocks. The high proportion of above-ground necromass is explained by the shedding of massive stem parts from willows and birches and the large mass of dead above-ground material from tussock-forming sedges and cottongrass. The maximum mean necromass was recorded in those communities where, in addition to the shrubs, Eriophorum vaginatum and other dense-tussock sedges played an active role. Overall, the slow rate of decomposition, due to the limited activity of soil decomposers at low temperatures and high humidity, retards the decomposition of plant residues and slows the cycling of matter and energy.

A key indicator of plant residue decomposition is the carbon-to-nitrogen (C/N) ratio. This ratio reflects the relative proportions of carbon and nitrogen during decomposition, which correlates with the degree of organic matter transformation in the soil [25,75]. We found that the woody parts of shrubs and dwarf shrubs had the highest C/N values (Figure 3). For leaves, this ratio is determined by the lifespan of the photosynthetic apparatus (i.e., evergreen vs. deciduous). High ratios are found in the leaves of evergreen shrubs and in lichen thalli. The leaves of deciduous shrubs and the vegetative shoots of graminoids are characterized by lower, more optimal values, meaning their litter will be humified more quickly. Higher C/N ratios in evergreen leaves, coupled with their thick cuticle, slow their decomposition, increase soil acidity, and reduce nitrogen mineralization.

The slowest decomposition rates are observed for stems, branches, and perennial shoots of shrubs, evergreen leaves of dwarf shrubs, and lichen thalli. In contrast, the leaves of shrubs and the annual vegetative shoots of grasses are more susceptible to rapid mineralization, as they are an accessible substrate for microbial processing due to their higher nitrogen content. The mineralization of biogenic elements under permafrost conditions is extremely slow, which explains the high C/N ratio in the biomass. Nevertheless, the transformation of carbon and nitrogen in Arctic ecosystems is a key process influencing the productivity of plant communities and the accumulation of litter and peat.

The productivity, biomass structure, and necromass values of the studied tundra ecosystems of the Yana–Indigirka Lowland are generally consistent with figures reported in the 1990s for the southern tundras of European and Asian Russia. According to data from that period, total mean biomass in subarctic tundras ranged from 10 to 40 t ha⁻¹, necromass from 20 to 60 t ha⁻¹, and annual production from 2 to 4 t ha⁻¹ [11,76]. Our results fall within these ranges. The total above-ground production we measured is close to that of subarctic typical spotted tundras on slopes and watersheds, and to small-hummocky shrub-dwarf shrub-moss and dwarf shrub-shrub-moss-lichen southern subarctic tundra types of Russia. In terms of annual production, our southern tussock tundra ecosystems are comparable to hummocky shrub-dwarf shrub-moss and dwarf shrub-herb-moss-lichen southern tundra of other regions. The biomass structure is also characterized by similar parameters [11].

Calculations showed that the mean C content in the biomass at the “Chokurdakh” station is 0.70 ± 0.10 kg C m⁻² [77]. This is slightly higher than the 0.5 kg C m⁻² calculated [55] for the circumarctic bioclimatic subzone E [78], where the station is located. In our studies, the mean C content in the above-ground biomass was 0.78 kg C m⁻², and in the total living biomass, it was 1.99 kg C m⁻².

4.2. Influence of Permafrost on Biogenic Elements and Microbial Activity

In the southern Low Arctic shrub tundra, the litter and peat pool is the main repository of SOC. This persists despite rising temperatures and thermokarst processes, and is also due to the slow decomposition of plant residues resulting from low TN content. Some authors believe [79] that litter can decompose faster under certain dominant species, and that the limiting factor for mineralization is not nitrogen, but phosphorus. The TOC stocks in the mineral soil horizon (the upper layer of the yedoma ice complex) depend on the specific soil formation history of the region. Given that thermokarst has been active in the Yana–Indigirka Lowland for thousands of years—resulting in the physical transport of organic material into the cracks of syngenetic ice wedges and its subsequent preservation—this warrants further research on deep C pools. Undoubtedly, thermokarst and soil organic matter stocks affect the release of carbon-containing gases from permafrost. Reducing the intensity of permafrost degradation requires an increase in plant biomass and, consequently, greater peat accumulation [80]. We note that the yedoma ice complex of the Yana–Indigirka Lowland began to form during the late Neopleistocene in MIS 4 and is correlated with the upper part of the Allaikhovskaya suite [81].

Our microbiological results are consistent with a number of other studies [82]. For example, in the eastern Russian Arctic, microbial biomass carbon is high and depends mainly on moisture and biogenic elements, rather than on ground temperature. On one hand, the TN content in all tundra types is consistently low, while TOC content is high. This explains the high C/N ratio, which limits organic matter decomposition and microbial growth. On the other hand, the shrub-green moss (sites Chok-2 and -5) and shrub-lichen-green moss (sites Chok-3 and -4) tundra ecosystems are characterized by high basal respiration activity—and thus high carbon dioxide release—and a lower metabolic quotient compared to site Chok-1. A higher metabolic quotient suggests that the microbial community is under greater physiological stress. The larger this coefficient, the stronger the negative impact of environmental factors on permafrost landscapes.

Model experiments conducted at the “Chokurdakh” station [83] showed that soil warming increases the above-ground biomass of sedges but does not affect other plant community types. The application of mineral fertilizers (NPK) accelerates the growth of above-ground dwarf shrub biomass, while grasses respond with an increase in both above- and below-ground biomass. The authors conclude that the response of southern tundra plant communities to climate change is determined by thaw depth and soil nutrient availability, as well as the accumulation of organic matter in the surface soil layers.

Soils in cryogenic landscapes have lower microbial diversity compared to forest soils of temperate latitudes, as reflected in the composition and abundance of microscopic fungi [84]. Soils in temperate latitudes with a narrow C/N ratio release more greenhouse gases due to more optimal hydrothermal conditions and active mineralization. Microorganisms inhabiting permafrost deposits have low kinetic energy and limited access to nitrogen and carbon [85,86]. This leads to greater preservation of plant residues, which limits the cycling of elements between the soil and the atmosphere. The composition and density of the vegetation cover determine the input of TOC and TN into the soil. In the shrub tundra, plant litter forms a dense “cushion” of varying degrees of decomposition, with underlying horizons of felt and peat. This acts as a buffer that prevents heat from penetrating deep into the soil, which in turn slows microbial activity and limits the formation of available nitrogen. Thus, low TN content limits not only plant productivity but also microbial metabolism. In the studies of [87], methanogenesis decreases with depth due to the transition to a mineral horizon, which is less saturated with TOC, and a change in pH, which is not favorable in anaerobic conditions. Conversely, with increased permafrost thawing in Siberia, a greater release of greenhouse gases from tundra soils is expected. According to [23], data from the “Chokurdakh” station show that methane fluxes are not sensitive to soil temperature but depend on hydrological conditions. Carbon sequestration [88] in the southern tundra of the Yana–Indigirka Lowland does not increase with the length of the growing season, and the influence of increased summer temperatures does not lead to a decrease in carbon exchange between the ecosystem and the atmosphere.

Microorganisms living in permafrost are vulnerable to climate change and can affect the ecosystem in different ways. First, they must be adapted to obtaining energy from available substrates. Second, permafrost thawing will lead to a shift in biogeochemical processes, which will have serious consequences for the metabolic potential of microorganisms. Third, the permafrost microbiota reflects the local climate, vegetation, and soil properties at the time of its formation. Therefore, understanding these factors is important for predicting changes in the transformation of TOC and TN in the context of climate change [89,90,91].

The geomorphology of the landscape and the geochemical characteristics of the soil, which affect the metabolic capacity of microorganisms, are undeniably important, since the composition of the microbial community at the local level is associated with landscape elements [92,93]. In our studies, this factor was not considered, as the permanent sample plots are located on a low-lying depositional plain and have a uniform soil cover of Gleysols.

This article presents the results and discussion of the first stage of our research, providing initial assessments from detailed fieldwork. The research has so far covered only one ecosystem type, developed in a depositional lowland of the alas type; floodplain landscapes, polygonal bogs, tundra mires, and yedomas were not included. Additional research is necessary to identify seasonal fluctuations and trends in biomass and carbon content induced by climate warming. We acknowledge the limitations of our research, including the single time-point sampling. The pronounced patchiness, fine-scale heterogeneity, and stochastic distribution of plants and patch types, which are particularly evident in southern tundra, exacerbate these challenges.

The results obtained can contribute to the assessment of biological productivity parameters for similar ecosystem types in the northeastern sector of Eurasia. The data from our in situ field studies may be valuable for the parameterization and validation of mathematical models, and for compiling regional assessments of ecosystem GHG fluxes, allowing for the detection of trends in large-scale biotic changes.

Our results can be extrapolated to similar shrub–tussock tundra ecosystems in alas-type landscapes in northeastern Siberia. Based on the identified patterns of biomass structure in southern Low Arctic tundra, it is possible to use earlier, less differentiated data on the productivity of these ecosystem types to approximate organic matter stocks and carbon content, given the ratio of biomass fractions.

5. Conclusions

We quantified plant biomass and necromass, carbon stocks, and microbial activity in permafrost landscapes of the southern Low Arctic shrub–tussock tundra of northeastern Siberia.

We found that the most significant producers, accumulating the largest biomass, are deciduous shrubs, bryophytes, and graminoids. The willow Salix pulchra develops the largest mean biomass, accounting for nearly half of the total biomass of the studied tundra ecosystems. Bryophytes rank second in above-ground productivity, with the leafy moss Aulacomnium palustre showing the highest values. The third-largest contributor to biomass accumulation is the shrub Betula nana. Among graminoids, Eriophorum vaginatum accumulates the largest biomass and necromass. Bryophytes predominate in the green, assimilating portion of the biomass, while vascular plants prevail in terms of annual production. The contribution of woody organs of shrubs to above-ground biomass is high. More than half of the below-ground biomass is formed by the rooting shoots of the shrubs Salix pulchra and Betula nana. Necromass stocks are 1.8 times higher than above-ground biomass and 10.8 times higher than annual production, which indicates a significant slowdown in the microbial decomposition of plant residues and a low plant growth rate.

The main carbon reservoirs are Salix pulchra, Betula nana, and bryophytes. The taxonomic composition, life forms of dominant species, their growth habits, abundance, and rates of decomposition are the key parameters that determine the quantitative indicators of carbon sequestration.

We established that the woody parts of shrubs, evergreen leaves, and lichen thalli have the highest C/N values. Optimal C/N values were found in the vegetative shoots of graminoids and the summer-green leaves of shrubs.

We anticipate that in the event of large-scale thermokarst processes with continued climate warming, dominant deciduous shrubs will be forced to retreat. It is highly probable that these destabilizing processes will cause the expansion of hygrophilic and hydrophilic graminoids (Carex, Eriophorum, and Poaceae) on disturbed microhabitats. An increase in the proportion of grass-sedge and cottongrass bogs, alongside a reduction in shrub tundra area, could lead to an increase in GHG emissions in the southern tundra.

The carbon pool of the above-ground biomass is an order of magnitude larger than the stocks in the mineral soil. In the studied tundra ecosystems, the above-ground component is dominant in the formation of C pools and stocks; on average, this value for the ecosystem is 346.2 t C ha⁻¹, compared to 20.8 t C ha⁻¹ in the 0–10 cm mineral layer. The permafrost conditions and geomorphological features of the Yana–Indigirka Lowland promote the accumulation of organic matter on the soil surface as a living moss cover and a moss mat. At the same time, there is no significant migration of carbon into depositional landforms or mineral soil horizons. Conditions at the study plots favor the local formation of carbon stocks, regardless of their facies affiliation.

For the first time in the landscapes of the Yana–Indigirka Lowland, we have documented heterotrophic activity partitioned between biomass and soil. We established that the intensity of basal respiration and microbial biomass carbon in the litter and peat is determined by the amount of accumulated carbon. In contrast, in the mineral horizon, where TOC content is minimal, microbial processes are suppressed. Based on microbial biomass carbon and the metabolic quotient, the shrub-dwarf shrub-green moss tussock tundra (sites Chok-2 and -5) stands out, indicating more favorable ecosystem conditions compared to other tundra types (sites Chok-1, Chok-3, and Chok-4).

The results of this research can be used to populate databases on biomass stocks and carbon storage in plant communities and soils and for monitoring the dynamics of organic matter and climatically active gases in the southern tundra subzone. This observational data can also be valuable for remote sensing studies and for forecasting the dynamics of tundra ecosystems in a changing climate. The obtained results can be extrapolated to the Low Arctic shrub–tussock tundra of northeastern Siberia.