Pyrogenic Transformation and Carbon Sequestration in Forested Bog Soils of the Middle Taiga in Northeastern European Russia

Abstract

A comprehensive paleoecological study of a forested bog located in the middle taiga subzone of northeastern European Russia was carried out. According to the ¹⁴C radiocarbon dating and botanical composition analysis, the bog began forming 8200 calibrated years ago, evolving in three stages from grassy wetlands to its current state as a pine-Sphagnum peatland. Analysis revealed substantial carbon storage (81.4 kg m⁻²) within the peat deposit. Macrocharcoal particles were consistently present throughout the peat deposits, demonstrating continuous fire activity across the bog’s developing. High charcoal particle accumulation rates occurred not only during warm periods like the Holocene thermal maximum but also during colder and wetter periods. These periods include recent centuries, when high charcoal accumulation rates are likely due to increased human activity. Statistical analysis showed significant relationships between macrocharcoal content and several peat characteristics: higher charcoal levels correlated with increased soil carbon (r = 0.6), greater aromatic compounds (r = 0.8), and elevated polycyclic aromatic hydrocarbons (r = 0.7), all with p < 0.05. These findings highlight how fire has consistently shaped this ecosystem’s development and carbon storage capacity over millennia, with apparent intensification during recent centuries potentially linked to anthropogenic influences on fire regimes in the boreal zone.

1. Introduction

Fire is a widespread factor in the Earth system, playing an important role in shaping ecosystem structure and spatial and temporal distribution [1,2,3]. However, fire should be understood as a complex, multilevel, and dynamic process influenced by numerous factors, ultimately driving the formation of post-fire ecosystems [2,4]. Among the most vital components of boreal ecosystems are soils, which serve a stabilizing function by maintaining ecosystem resilience and underpinning key biogeocenotic processes.

Peat soils are of particular importance as they store vast carbon stocks [2,5,6] and act as a valuable archive of paleoecological data [7,8,9]. Projections under current climate change scenarios suggest an increase in both the frequency and intensity of fires [9,10,11]. Enhanced fire activity in the boreal zone may exacerbate carbon emissions, potentially accelerating climate change and triggering irreversible ecosystem transformations.

The interplay of natural and anthropogenic factors further complicates assessments of fire impacts on high-latitude soils. Reconstructing wildfire regimes in boreal regions can provide critical insights into broader global processes [12,13]. Moreover, understanding the effects of fire on Histosols and particularly those prone to pyrogenic alteration is essential for evaluating fire-induced changes in soil organic matter (SOM) [2,3,4].

The climatic conditions of the boreal zone limit decomposer activity, resulting in a significant fraction of soil-stored carbon remaining sequestered rather than re-entering the active carbon pool. Among boreal soils, peatlands are particularly notable for their substantial organic matter (OM) accumulation and long-term burial potential [14]. Furthermore, fires can alter ecosystems and soil organic matter (SOM), generating pyrogenic residues resistant to degradation [15], which may persist in soils for millennia [9,16,17].

Despite these observations, the scientific literature lacks comprehensive and reliable data quantifying the relationship between carbon accumulation, pyrogenic influences, and soil organic matter alterations. Addressing this knowledge gap requires a multifaceted approach, combining the analysis of macroscopic charcoal particles [9,17,18] with complementary methods. Such as taxonomic assessments of biological remains in peat deposits [8,17]. This integrated methodology enables the reconstruction of paleoecological conditions, identification of fire-induced vegetation shifts, and elucidation of long-term ecosystem responses to pyrogenic disturbances. While traditional methods [3,4,18] provide valuable insights into fire-induced alterations of soil organic matter (SOM), a more precise quantification of SOM transformations—resulting from both natural and anthropogenic factors—can be achieved by supplementing classical approaches with advanced analytical tools. These include solid-state ¹³C nuclear magnetic resonance (NMR) spectrometry [19,20,21] and analysis of polycyclic aromatic hydrocarbons (PAHs) [22,23,24]. PAHs, known for their carcinogenicity, environmental persistence, and resistance to degradation, represent a significant ecological concern due to their impact on biotic communities [25,26,27]. Their stability, mobility, and tendency to accumulate in various environmental matrices, including peat deposits [28,29], are well-documented. Moreover, PAHs serve as effective markers of both natural [30,31] and anthropogenic environmental changes [32,33,34], which makes peat soils convenient sources of information for reconstructing natural and anthropogenic impacts on boreal ecosystems.

While paleoecological research has advanced considerably, existing studies (e.g., [7,9,17]) predominantly examine open oligotrophic bogs, leaving forested bog ecosystems significant for comprehensive carbon cycle assessment, markedly understudied [20]. The aim of the work is to study the influence of wildfires on transformation and carbon storage in forested bog soils in the Holocene by assessing changes in the composition and stability of organic matter. The objectives of this study were (1) to reconstruct the dynamics of wildfires in the Holocene, (2) to assess the effect of fires on carbon sequestration, (3) to quantify fire-induced effects on long-term carbon sequestration potential. The reconstruction of historical wildfire events was accomplished through radiocarbon dating, botanical composition analysis, and microscopic charcoal particle quantification. To evaluate long-term carbon sequestration dynamics and the biogeochemical impacts of fire, we employed elemental (C and N) analysis, solid-state ¹³C-NMR spectroscopy, and PAHs analysis.

2. Materials and Methods

2.1. Area Description and Soil Sampling

The studies were carried out in the European North-East of Russia (61°41′48″ N 50°39′17″ E). A forested peatland, classified as a forest-type Histosol, was selected for detailed paleoecological analysis. The classification of the studied soils was determined using the World Reference Base for Soil Resources [35]. The study site comprises a peatland located in the northeastern part of the East European Plain, where the surrounding soils are predominantly Peaty-Podzolic Gleyic, Albic Podzols, and Gleyic Podzols [35,36]. Paleoecological analyses focused on a 32-hectare forested oligotrophic bog representing a raised oligotrophic mire, with vegetation characterized by a moss-shrub community dominated by ericaceous species (Ledum palustre L., Andromeda polifolia L., Chamaedaphne calyculata L., and Vaccinium uliginosum L.), herbaceous plants (Rubus chamaemorus L. and Eriophorum vaginatum L.), and Sphagnum mosses (S. angustifolium L., S. fuscum (Schimp.) Klinggr., and S. balticum (Russow) C.E.O.Jensen).

The studied forested bog is situated west of Syktyvkar, near the boundary between the subarctic and temperate climatic zones. According to the M. Köppen classification [37], the region falls under the “Dfc” category, characterized by cold summers and cold, snowy winters. Climatic data (until the 1990s) [38] indicate an average annual temperature range of 0 to −2 °C, with mean annual precipitation of 500–600 mm. However, recent records (1990–2020) from the nearest meteorological station (Komi CGMS—MII Syktyvkar) show a slightly higher value, with an average annual temperature of 1.9 °C and precipitation of 652 mm. During the 2020 field expedition, undisturbed peat cores were extracted from the central part of the bog using a peat corer (50 cm length and 2.5 cm diameter). Additionally, a soil profile section was established (Figure 1), and bulk density measurements were taken in triplicate at 10 cm intervals (27 samples total) using a 50 cm³ soil drill [39] to facilitate carbon stock calculations. Sampling was conducted without disrupting the structural integrity of the material. To prevent potential overestimation of reserve values, recalculation was performed after excluding the mineral fraction, as determined through ash content analysis.

2.2. Radiocarbon Dating of Peat Samples

The peat organic material in three nearby columns (at the same depths) was selected by peat corer and then radiocarbon dating carried out at the equipment provided by Shared Research Facilities of the Tomsk Scientific Center SB RAS (Tomsk, Russia) by liquid scintillation method using a spectrometer-radiometer Quantulus 1220 (Wallac, Finland). The number of sampling points was determined according to the change in the botanical composition and morphological changes. Peat organic material was subjected to pretreatment to eliminate potential contaminants. It is essential to differentiate between radiocarbon age (¹⁴C yr BP) and calibrated age (cal yr BP). Radiocarbon ages are derived from measured ¹⁴C concentrations, normalized to the atmospheric standard of 1950 [40,41]. However, these ages require calibration to account for temporal variations in atmospheric ¹⁴C. Age-depth modeling was conducted using Bacon v2.3.4 [42], with calibration performed in CALIB REV8.2 and Bacon v2.3.4, employing the IntCal20 calibration curve [43]. The Holocene period boundaries were defined according to Marcott et al. [44].

2.3. Botanical Composition of Peat Sediments

The botanical composition of peat sediments was analyzed at the Institute of Biology, Karelian Scientific Center of the Russian Academy of Sciences in accordance with GOST 28245-89 standards [45]. The percentage content of plant macrofossils was estimated using a polarizing microscope “ADF U300B” with eyepiece PL10X/22 wide field. The ratio of macrofossils of all identified peat components was reported in 5% increments. The degree of decomposition was determined in 5% increments. The identification of plant residues as belonging to a certain plant species was performed using the atlas of plant residues in peats [46]. Stratigraphic diagrams of peat composition were constructed using the software CorelDraw 11.0 (Alludo, Ottawa, ON, Canada).

2.4. Analysis of the Amount of Macroscopic Charcoal Particles and Dendrochronology

Wildfire dynamics were analyzed through charcoal macroparticle quantification following established methods [47]. Peat samples (1 cm³) were collected at 2 cm intervals, soaked in 5% sodium hypochlorite (NaOCl) for 24 h, and washed with distilled water through 125 μm mesh to isolate macroscopic charcoal particles, with this size threshold selected to optimize detection sensitivity [12]. Charcoal counts were processed using the R environment (v.4.3.1) with specialized packages including CharAnalysis [48], paleofire [49], caTools [50], mclust [51], and Matching [52]. A LOWESS smoothing function (1000-year moving window) was applied to achieve a robust signal-to-noise index (SNI) > 3.0 [53], enabling precise fire history reconstruction through CharAnalysis [48]. Moving-window charcoal peak values exceeding the 99th percentile threshold of the modeled noise distribution were identified as fire episodes.

Dendrochronological studies were employed to supplement wildfire data. Tree-ring samples for the estimation of fire scars were obtained following the methodology outlined by Madany et al. [54]. On the forested bog, a total of 30 cores from living trees and 30 cross-sections from trees were collected within a 5 km radius. The preparation of wood samples, including both cores and cross-sections, for fire dating was conducted according to the procedures described by Fritts [55] and Grissino-Mayer [56]. The width of each tree ring was measured with a precision of 0.01 mm using a LINTAB semiautomatic device under a binocular microscope with 40x magnification. The calendar year of each ring and fire scar was determined using the TSAPWin software environment with cross-dating techniques. To ensure dating accuracy, the collected data were subjected to cross-correlation analysis using the COFECHA software.

2.5. Carbon and Nitrogen Content Analysis

The total C and N content were performed in the Ecoanalytical laboratory of the Institute of Biology of Komi Science Centre of the Ural Branch of the Russian Academy of Sciences (IB FRC Komi SC UB RAS) is conformed to requirements of ISO/IEC 17025:2017 (GOST ISO/IEC 17025-2019) and recorded to the Register of Accredited CABs under number POCC RU.0001.511257. The total carbon (C) and nitrogen (N) were determined by dry combustion on an EA-1100 analyzer (Carlo Erba, Milano, Italy).

2.6. CP/MAS ¹³C NMR Spectroscopy

The molecular composition of organic matter was characterized using solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. Spectra were acquired on a Bruker Avance III 400WB spectrometer (100.53 MHz) employing cross-polarization magic angle spinning (CP/MAS) methodology with a sample rotation frequency of 12.5 kHz, 2 ms contact time, and 2 s recycle delay at the Resource Center of the Research Park “Magnetic Resonance Research Methods” of Saint-Petersburg State University, Russia. Chemical shifts were referenced to tetramethylsilane (0 ppm), using adamantane as an external standard. Prior to analysis, samples were treated with 10% hydrofluoric acid to eliminate paramagnetic iron impurities [57,58]. Spectral processing was performed quantitatively in TopSpin 3.2 (Bruker, Rheinstetten, Germany) by integrating chemical shift regions corresponding to specific functional groups: aromatic C (AR) were quantified by integrating 110–145 ppm (CAr-H) and 145–165 ppm (CAr-O,N) regions, while aliphatic components (ALs) were determined from 0 to 110 ppm, 165 to 185 ppm, and 185 to 220 ppm regions [59]. The degree of aromaticity (fa) was calculated as the sum of all aryl C components [60], and organic matter decomposition was assessed using the alkyl (0–45 ppm) to O,N-alkyl (45–110 ppm) ratio [21].

2.7. Polycyclic Aromatic Hydrocarbon Determination

Polycyclic aromatic hydrocarbons (PAHs) were extracted from organic material according to Yakovleva et al. [24] and Gabov et al. [29] using an ASE-350 accelerated solvent extraction system (Thermo Fisher Scientific, Waltham, USA) at the Ecoanalytical Laboratory of the IB FRC Komi SC UB RAS. Prior to extraction, samples were homogenized by sieving through a 0.25 mm mesh. Subsequently, a 1 g sample was placed in an extraction cell and subjected to extraction three times with a mixture of methylene chloride and acetone (1:1) at 100 °C. The extracts were concentrated using a Kuderna-Danish apparatus at a thermostat temperature of 70 °C, and the solvent was replaced with hexane. The obtained sample concentrate of 3 cm³ in volume was purified from inorganic impurities by column chromatography using aluminum oxide of Brockman activity level (II). Extraction was performed using 50 mL of a hexane/methylene chloride (4:1) solvent mixture. The resulting eluate was concentrated to 5 mL using a Kuderna-Danish apparatus at 85 °C, followed by addition of 3 mL acetonitrile and subsequent evaporation at 90 °C to remove residual hexane. PAH analysis was conducted by high-performance liquid chromatography (HPLC) using a Maestro liquid chromatograph (Interlab LLC, Moscow, Russia) equipped with a Rheodyne 7725i injector (10 μL loop), PDA 5430 diode array detector, FLD 5440 fluorescence detector, and TCC 5310 column oven. Separation was achieved on an Agilent Zorbax Eclipse PAH analytical column (5 μm, 250 × 2.1 mm) with matching guard column (5 μm, 12.5 × 2.1 mm). Separation was carried out at a flow rate of 0.2 mL/min and a temperature of 30 °C. A gradient of acetonitrile and water was used as the mobile phase (0 min–60/40, 5 min–60/40, 25 min–100/0, 50 min–100/0). Fluorescence detection was carried out using a program of excitation (Ex) and emission (Em) wavelengths: 270/330 nm from 0 to 16.1 min (naphthalene, acenaphthene, and fluorene), 250/375 nm from 16.1 to 20.4 min (phenanthrene and anthracene), 240/440 nm from 20.4 to 22.2 min (fluoranthene), 240/390 nm from 22.2 to 27.0 min (pyrene), 260/385 nm from 27.0 to 31.9 min (benzo[a]anthracene, chrysene), 290/410 nm from 31.9 to 44.3 min (benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, dibenz[a,h]anthracene, benzo[ghi]perylene), 245/460 nm from 44.3 to 50.0 min (indenopyrene). The UV detection was carried out using diode array detection at a selected wavelength of 251 nm. The method quantified 14 priority PAHs: NP—naphthalene, ACE—acenaphthene, FL—fluorene, PHE—phenanthrene, ANT—anthracene, FLA—fluoranthene, PYR—pyrene, BaA—benzo[a]anthracene, CHR—chrysene, BbF—benzo[b]fluoranthene, BkF—benzo[k]fluoranthene, BaP—benzo[a]pyrene, DahA—dibenzo[a,h]anthracene, and BghiP—benzo[g,h,i]perylene.

2.8. Statistics

Statistical analyses were performed using Excel 2010 (Microsoft Corporation, Redmond, WA, USA) and R programming language (R Foundation for Statistical Computing, Boston, MA, USA). Correlation coefficients (r-Pearson) were calculated using the STATISTICA 10.0 (Stat. Soft Inc., Tulsa, OK, USA); differences were considered significant at the significance level p < 0.05. Age-depth relationships and charcoal macroparticle data were processed using the CharAnalysis package [48] within the R environment. Additional R packages, including Readxl [61] for data import and Corrplot [62] for visualization of correlation matrices, were employed in the statistical analysis. To meet the assumptions of parametric tests, charcoal macroparticle concentration data were log-transformed (natural logarithm) to stabilize variance and approximate normal distribution.

3. Results

3.1. Age Models and Chronologies of Peat Development

Radiocarbon dating indicates that peat deposition (90 cm depth) initiated approximately 8200 cal. yr BP (

Table 1. Radiocarbon dating results (¹⁴C).

Laboratory Sample Number	Sampling Depth, cm	Age, 14C yr BP	Age, cal yr BP (1δ)	Age, cal yr BP (2δ)
IMCES—14C1976	25–35	990 ± 90	853–1042	762–1244
IMCES—14C2111	55–65	2730 ± 115	2811–3064	2566–3241
IMCES—14C2139	80–90	7350 ± 300	7915–8500	7652–9044

). During the initial development phase (the first five millennia), peat accumulation rates reached a minimum of 0.06 mm yr⁻¹. This period coincided with the colonization of waterlogged biotopes by mesophytic vegetation, dominated by sedge communities including Carex lasiocarpa Ehrh., C. rostrata Stokes, C. globularis L., and C. pauciflora Lightf., accompanied by Calla palustris L. (Figure 2).

The transitional stage marks a pronounced shift in peat botanical composition, characterized by a substantial increase in woody remains to 75% of the total organic matter. This woody fraction comprised Pinus Sylvestris L. bark (40%), Betula sp. remains (25%), and Picea sp. fragments (10%). Concurrently, herbaceous components declined to 20%, dominated by Eriophorum vaginatum and Carex rostrata, while Sphagnum mosses first appeared in the record. This compositional change coincided with an accelerated peat accumulation rate of 0.15 mm yr⁻¹—more than double the previous rate—persisting until approximately 1991 cal yr BP (Figure 3).

The third developmental stage was characterized by a distinct plant community composition, featuring a pine component (Pinus sylvestris, 5–10%) co-occurring with dominant mesotrophic sedges (collectively 40%), including Carex lasiocarpa, C. rostrata, and C. limosa L. Eriophorum vaginatum accounted for 30% of the assemblage, while Scheuchzeria palustris represented a minor component (5%). During this stage, peat accumulation rates reached 0.32 mm yr⁻¹, accompanied by the establishment of oligotrophic bog vegetation. Sphagnum mosses became the dominant group, with Sphagnum angustifolium constituting 40% of the moss layer, followed by S. magellanicum Brid. (including its form S. divinum Flatberg & K. Hassel, 10%) and S. fuscum (5%). Sphagnum species present included S. fallax H. Klinggr. and S. jensenii H. Lindb.

3.2. Macroscopic Charcoal Particles Analysis Results

Macroscopic charcoal analysis revealed distinct patterns of fire activity throughout the bog’s developmental history. During the Holocene thermal maximum, charcoal concentrations peaked at 127 particles (pc) cm⁻³ (Figure 3), with evidence for three local fire events. The subsequent Subboreal period showed increased fire activity, as indicated by higher charcoal concentrations (reached up to 137 pc cm⁻³) and three additional recorded fire events. Following this period, fire frequency moderately declined before experiencing notable reactivation during recent centuries.

The most pronounced charcoal accumulation (1084 pc cm⁻³) occurred at the acrotelm to catotelm boundary (10–15 cm depth), reflecting intense fire activity during the recent historical period. Dendrochronological dating precisely identified local fire events in 1874 and 1919 [63], accounting for this pronounced peak.

The long-term average charcoal concentration across the entire peat sequence was 55 pc cm⁻³, revealing generally low fire activity during initial bog formation. However, two periods of enhanced pyrogenic activity emerged: between 8000 and 3000 calendar years BP and during the last two centuries. This bimodal distribution suggests potential climate-mediated controls on fire regimes, with both mid-Holocene warming and possible anthropogenic influences contributing to increased fire frequency during these intervals.

3.3. Carbon and Nitrogen Analysis Results

The analysis of C and N content (

Table 2. Carbon and nitrogen concentrations and stocks of the investigated bog soils.

Depth	Horizon	C,%	N,%	C/N	C, kg m−2	N, kg m−2
0–10	T1	44.0 ± 1.5	0.95 ± 0.10	54.0	2.2	0.05
10–20	T2	43.3 ± 1.5	1.22 ± 0.13	41.4	2.2	0.06
20–30	T3	45.8 ± 1.6	1.52 ± 0.17	35.2	5.0	0.17
30–40	T4	55.2 ± 1.9	1.20 ± 0.13	53.7	6.1	0.13
40–50	T5	51.8 ± 1.8	1.50 ± 0.16	40.3	5.7	0.17
50–60	T6	53.3 ± 1.9	1.50 ± 0.16	41.5	14.9	0.42
60–70	T7	51.6 ± 1.8	1.58 ± 0.17	38.1	14.4	0.44
70–80	T8	50.0 ± 1.8	1.42 ± 0.16	41.1	14.0	0.40
80–90	T9	51.1 ± 1.8	1.23 ± 0.14	48.5	16.9	0.41

) and their distribution in pyrogenic residues (Figure 3) demonstrates significant fire-induced transformations of soil organic matter (SOM) in the bog ecosystem. Carbon content exhibits a pronounced vertical gradient, increasing from 43.3% in surface horizons to 55.2% in deeper peat layers, with minimum values occurring at the organic–mineral (OM) transition zone. Nitrogen content ranges from 0.95% to 1.58%, showing a positive correlation with horizons containing abundant pyrogenic inclusions. Total elemental stocks amount to 81.4 kg C m⁻² and 2.2 kg N m⁻², reflecting the system’s substantial carbon sequestration capacity. These patterns likely indicate that pyrogenic processes enhance carbon stabilization in deeper peat strata while simultaneously influencing nitrogen cycling dynamics.

3.4. Results of Polycyclic Aromatic Hydrocarbon Analysis

The bog sediments exhibited considerable variation in polycyclic aromatic hydrocarbon (PAH) concentrations, ranging from 294.8 to 6260.7 ng g⁻¹ (

Table 3. Polycyclic aromatic hydrocarbon concentration in peat deposits.

Depth, cm	2-Ring			3-Ring		4-Ring				5-Ring				6-Ring
	NP	ACE	FL	PHE	ANT	FLA	PYR	BaA	CHR	BbF	BkF	BaP	DahA	BghiP
0–10	116.2 ± 58.1	nd	nd	385.6 ± 84.8	5.8 ± 2.9	25.8 ± 11.8	nd	6.2 ± 2.6	13.0 ± 6.7	74.7 ± 31.4	3.4 ± 1.6	4.2 ± 2.1	11.8 ± 5.7	13.9 ± 6.1
10–20	177.4 ± 88.7	14.9 ± 6.0	19.4 ± 7.8	150.2 ± 33.1	9.4 ± 4.7	55.7 ± 25.6	52.1 ± 24.0	14.2 ± 5.9	44.6 ± 23.2	313.0 ± 81.4	10.3 ± 5.0	11.3 ± 5.6	28.6 ± 13.7	12.5 ± 5.5
20–30	261.8 ± 130.9	nd	26.2 ± 10.5	177.7 ± 39.1	11.6 ± 5.8	42.0 ± 19.3	34.2 ± 15.7	118.2 ± 30.7	913.9 ± 255.9	495.0 ± 128.7	33.3 ± 16.0	150.9 ± 36.2	21.1 ± 10.1	23.4 ± 10.3
30–40	508.8 ± 183.2	88.2 ± 19.4	36.3 ± 14.5	518.7 ± 114.1	53.8 ± 12.9	163.0 ± 61.9	63.6 ± 29.3	106.1 ± 27.6	903.0 ± 252.8	1860.6 ± 409.3	78.5 ± 37.7	1425.9 ± 256.7	288.8 ± 86.6	165.5 ± 39.7
40–50	172.5 ± 86.3	24.6 ± 9.8	13.8 ± 5.5	63.5 ± 31.7	2.1 ± 1.0	nd	nd	nd	28.5 ± 14.8	643.3 ± 167.3	6.9 ± 3.3	49.2 ± 24.6	nd	nd
50–60	228.0 ± 114.0	nd	12.6 ± 5.0	75.2 ± 37.6	1.8 ± 0.9	nd	nd	6.4 ± 2.7	100.8 ± 34.3	2112.0 ± 659.8	9.2 ± 4.4	277.8 ± 66.7	156.8 ± 47.0	nd
60–70	188.3 ± 94.1	13.5 ± 5.4	17.5 ± 7.0	40.8 ± 20.4	2.5 ± 1.3	nd	nd	nd	34.5 ± 17.9	75.3 ± 31.6	1.8 ± 0.8	7.8 ± 3.9	nd	nd
70–80	213.8 ± 106.9	17.8 ± 7.1	25.0 ± 10.0	61.9 ± 31.0	4.1 ± 2.0	nd	nd	nd	31.7 ± 16.5	52.1 ± 21.9	2.4 ± 1.2	nd	nd	nd
80–90	163.4 ± 81.7	8.9 ± 3.6	12.5 ± 5.0	34.6 ± 17.3	1.9 ± 1.0	nd	nd	nd	15.2 ± 7.9	15.0 ± 6.3	nd	1.2 ± 0.6	nd	21.5 ± 9.4

).

Variation in vertical direction revealed substantial compositional differences, with low-molecular-weight PAHs (2–3 aromatic rings) constituting 10.7–75.1% of total PAHs, while high-molecular-weight compounds (predominantly five-ring structures) accounted for 24.9–89.3%. Statistical analyses identified significant relationships between PAH composition and pyrogenic residues. A strong positive correlation emerged between charcoal macroparticle concentration and high-molecular-weight polyarenes (r = 0.79, p < 0.05). Moderately significant correlations were observed between charcoal content and specific PAHs: naphthalene (r = 0.52, p < 0.05), acenaphthene (r = 0.54, p < 0.05), benz[k]fluoranthene (r = 0.51, p < 0.05), benz[a]pyrene (r = 0.60, p < 0.05), and dibenz[a,h]anthracene (r = 0.66, p < 0.05). Notably, pyrogenic residues demonstrated particularly strong associations with benz[b]fluoranthene (r = 0.82, p < 0.05) and total PAH content (r = 0.68, p < 0.05).

3.5. CP/MAS ¹³C NMR Data

Solid-state ¹³C NMR spectroscopy revealed pronounced vertical stratification in the molecular composition of organic matter within the peat profile (

Table 4. Percentage distribution (%) of signal intensity between selected chemical shift regions (ppm) of CP/MAS ¹³C NMR spectra.

Depth, cm	Alcyl C	O-Alkyl C			Aryl C		Carboxyl C/Amide/Ester		Alkyl/ O, N-alkyl	fa
	CAlk-H	CCH3-O	CAlk-O	CO-Alk-O	CAr-H(C)	CAr-O,N	CCOOH(R)	CC=0
	0–45	45–60	60–95	95–110	110–145	145–165	165–185	185–220
5–15	35.9	7.1	26.1	6.4	13.1	4.6	5.6	1.2	0.9	17.7
20–25	21.5	6.7	40.8	10.1	9.6	4.0	5.4	1.9	0.4	13.6
30–35	28.2	8.2	20.0	5.4	24.8	7.0	5.3	1.1	0.8	31.8
40–50	20.0	8.7	21.0	5.1	29.8	8.3	5.0	2.1	0.6	38.1
55–65	37.0	7.5	21.6	5.6	14.4	5.6	6.5	1.8	1.1	19.9
65–70	42.6	7.3	18.3	5.2	14.5	4.9	6.5	0.7	1.4	19.4

fa—the degree of aromaticity.

). The degree of aromaticity (fa) exhibited a distinct depth-dependent pattern, peaking at 38.1% in middle horizons (40–50 cm depth) while remaining significantly lower (13.6–19.9%) in both surface (5–25 cm) and basal layers (55–70 cm). A strong positive correlation (r = 0.79, p < 0.05) was observed between fa values (range: 13.6–38.1%) and macroscopic charcoal particle content. This vertical distribution was mirrored in the aromatic-to-aliphatic (AR/AL) ratio, which increased from 0.2 in surface layers to 0.7 at 40–50 cm before decreasing to 0.3 in basal strata. Carboxyl group content (C_COOH(R)) demonstrated relative stability throughout the profile (5.0–6.5%). Notably, both upper and lower horizons retained higher proportions of less transformed alkyl structures compared to the more aromatic middle layers.

4. Discussion

Comprehensive paleoecological analysis of the peat bog revealed fire dynamics spanning the last 8200 calendar years and established key patterns of fire-mediated transformations in soil organic matter. The atypical pyrogenic activity during the Holocene’s coldest and wettest phase [64] likely reflects early anthropogenic expansion of this territory [65,66]. Following this period, fire frequency gradually declined before experiencing renewed intensification during recent centuries, potentially driven by combined climatic variability and increased anthropogenic pressures.

Dendrochronological analyses are consistent with these interpretations, identifying recent fire events preserved both in understory vegetation and discrete pyrogenic horizons at the acrotelm boundary (10–15 cm depth). This stratum contains peak macroscopic charcoal concentrations (1084 particles cm⁻³), with the anomalous 1874 and 1919 fire events [63] particularly evident. These elevated concentrations likely reflect intensified anthropogenic ignition sources during the historical period [20]. Statistical treatment identified the maximum charcoal value (up to 1084 particles cm⁻³) as an outlier (Dixon’s test, α = 0.05), which was subsequently replaced with the sample median following robust estimation protocols [67,68]. This winsorization approach maintains data integrity while accounting for extreme pyrogenic events.

The integrated results demonstrate that bog ecosystem evolution has been fundamentally shaped by millennial-scale interactions between climatic drivers and fire regimes, collectively governing peat accumulation rates, vegetation succession, and pyrogenic dynamics at regional scales. Macroscopic charcoal analysis revealed continuous fire presence throughout the peat stratigraphy, with particle concentrations averaging 55 particles cm⁻³ across the depositional history. Two distinct periods of enhanced fire activity emerged, namely between 8000 and 3000 calendar years BP and during the most recent two centuries, as evidenced by elevated charcoal accumulation rates. These active phases bookend an overall pattern of limited fire frequency during initial bog development stages.

The persistent stratigraphic record of charcoal particles underscores fire as a perennial ecological factor in these peatland systems. However, the observed bimodal distribution of pyrogenic residues suggests alternating periods of fire suppression and intensification, likely mediated by both climatic variability and, more recently, anthropogenic influences. This fire regime variability has consequently modulated organic matter preservation and biogeochemical cycling throughout the bog’s developmental history.

An extensive body of research underscores the strong interdependence between plant community dynamics and climatic variability [64]. The transformative role of pyrogenic processes in shaping phytocenoses has been rigorously demonstrated, particularly through studies by Molinari et al. [69] and Gorbach et al. [17]. Paleoecological reconstructions (Figure 2) suggest that the incipient stages of peatland development were characterized by persistently waterlogged conditions, which facilitated rapid organic matter accumulation.

A marked intensification of fire regimes occurred during the Holocene thermal maximum (Atlantic period) and the subsequent two millennia. This shift likely resulted from enhanced climatic continentality, greater thermal variability, and the proliferation of pyrophytic vegetation, including heathlands (Ericaceae), pine (Pinus spp.), birch (Betula spp.), and willow (Salix spp.) communities [69]. The findings suggest that elevated pyrogenic pressures likely resulted from a combination of climatic changes and anthropogenic activities rather than from climatic factors alone. This dual forcing may have contributed to the establishment of fire-adapted plant communities. The late Holocene witnessed diminished pyrogenic frequency concurrent with climatic cooling and increased humidity, fostering the proliferation of fire-resistant phytocenoses [70]. This pattern aligns with broader regional trends, wherein summer temperature regimes and moisture availability emerge as principal determinants of vegetation succession, mediating feedback loops between plant community composition and fire regime dynamics.

Hydrological drawdown during arid intervals likely enhanced peat aeration, promoting substrate desiccation and elevated fire susceptibility [7,13,17]. This process created conditions where senescent biomass experienced intensified microbial mineralization and pyrogenic alteration before hydrological stabilization could occur. While such fire-affected horizons with high carbonaceous content would typically show elevated alkyl/O-alkyl ratios, our profile analysis revealed no consistent spatial pattern in this relationship. This apparent discrepancy may indicate either that thermal alteration of organic matter was limited in these fires or that pyrogenic biomarkers subsequently underwent degradation. Collectively, these observations highlight how fires operate as fundamental drivers rather than mere disturbances in peatland ecosystems. Through millennial-scale feedback loops involving climate fluctuations, hydrological changes, and pyrogenic adaptation, fire events actively participate in shaping peatland biogeochemical cycling and structural organization, demonstrating their integral role in long-term ecosystem evolution.

Despite their relatively shallow peat layers, the studied ecosystems exhibit substantial carbon stocks that surpass those reported for dominant forest soils in the Komi Republic [71]. Previous research has established that forested bogs with thin peat deposits possess remarkable carbon sequestration capacity, potentially exceeding the storage potential of deeper peatland systems. This depends largely on the density of the material and the percentage of carbon. While our data suggest a trend of enhanced carbon storage in horizons containing pyrogenic inclusions, the statistical significance of this relationship remains unconfirmed (p > 0.05) due to limited sample size. However, complementary studies have demonstrated a significant positive correlation between pyrogenic residue content and carbon stocks [16]. Contemporary observations reveal that increasing fire frequency, extent, and intensity in boreal peatlands—driven by global climate change—are resulting in substantial carbon stock depletion. In some regions, carbon losses from peat combustion have already surpassed long-term accumulation rates [72]. These findings carry critical implications for understanding the carbon balance of northern peatlands, which represent a globally significant reservoir of soil organic matter.

The combined analytical results of PAH distributions and ¹³C NMR spectroscopy demonstrate that elevated concentrations of aromatic carbon structures—particularly complex high-molecular-weight compounds with benzene ring configurations—serve as persistent pyrogenic markers. These molecular signatures can endure for millennia, substantiating pyrogenic carbon (PyC) as a significant long-term carbon reservoir. Our findings align with established pyrogenic indicators, including a light-to-heavy PAH ratio < 1 [73], consistent with the predominance of higher-molecular-weight PAHs in charcoal-rich horizons, and BaA/(BaA + CHR) ratios exceeding 0.35 [31], further confirming the pyrogenic origin of these compounds in layers with abundant macroscopic charcoal. The current study does not establish relationships between carbonaceous residues and the PAH ratios characteristic of pyrogenic sources. These ratios include FLA/(FLA + PYR) > 0.5 [74,75], (FLA + PYR)/(PHE + CHR) > 0.5 [25,76], and ANT/(ANT + PHE) > 0.1 [77].

Fires can transform peat soils through both indirect and direct mechanisms. Indirect effects include the deposition of charcoal particles on bog surfaces, which subsequently become incorporated into the peat layer. Direct transformation occurs when peat bogs themselves burn, a process that may arise from underground smoldering under suitable conditions. Estimates of the global stock of pyrogenic carbon vary, but Reisser et al. [78] suggest that 14% of soil organic material exists in pyrogenic form, with peat soils accounting for a substantial proportion (~50%) of this pool [79]. The decomposition-resistant pyrogenic residues—primarily macroscopic charcoal particles—result from the thermal transformation of soil organic matter (SOM) during wildfires, reflecting incomplete oxidation of natural biopolymers [15,20,80].

During bog fires, the formation of aromatic carbon structures is enhanced through pyrolysis [15,16,81]. The degree of aromaticity (fa) is influenced by multiple factors, including pyrolysis temperature, feedstock composition (with lignin-rich woody material producing more aromatic pyrogenic carbon), moisture content, and oxygen availability. However, differentiating pyrogenic aromatic carbon from its non-pyrogenic counterparts in soil matrices remains analytically challenging. Our solid-phase ¹³C-NMR analyses (Table 4) demonstrate that fa values range from 13.6% to 38.1%, showing a significant positive correlation with macroscopic charcoal content (r = 0.79, p < 0.05). These findings confirm that fire promotes the formation of stable, recalcitrant carbon forms, providing new insights into how pyrogenic processes interact with ecosystem dynamics during peatland development.

5. Conclusions

This study provides a detailed reconstruction of Holocene pyrogenic activity and evaluates its impact on the formation and evolution of organic matter in peat soils from a forested bog in the middle taiga of the Komi Republic. Our analysis identified periods of enhanced fire activity that correlate not only with warm Holocene phases but also with periods of anthropogenic land use during colder, wetter climatic intervals. Pyrogenic processes significantly alter soil organic matter composition, promoting the formation of stable, aromatic-rich carbon forms. The persistence of pyrogenic carbon over millennia highlights its role in long-term carbon storage. Our results demonstrate that fires can enhance the long-term stability of soil carbon, though their net effect depends on fire frequency and severity. While this study focuses on a single site, the observed mechanisms are consistent with broader peatland carbon dynamics, emphasizing the need for further regional comparisons to refine estimates of pyrogenic carbon storage at larger scales. These findings advance our understanding of fire as a transformative agent in peat soil evolution. However, future studies incorporating multi-site comparisons and regional paleoenvironmental data will help strengthen generalizations about the role of fire in carbon cycling in the boreal region.