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Article

Pyrogenic Transformation of Soil Organic Matter in Larch Forests of the Discontinuous Permafrost Zone

by
Anjelica Kondratova
1,
Kazuto Sazawa
2,
Naoya Wada
3 and
Semyon Bryanin
1,*
1
Institute of Geology and Nature Management, Far East Branch, Russian Academy of Sciences, Blagoveshchensk 675000, Russia
2
Department of Natural and Environmental Sciences, Faculty of Science, Academic Assembly, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
3
Global Research Centre for Advanced Sustainability Science, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
*
Author to whom correspondence should be addressed.
Forests 2025, 16(8), 1309; https://doi.org/10.3390/f16081309
Submission received: 7 July 2025 / Revised: 30 July 2025 / Accepted: 8 August 2025 / Published: 11 August 2025
(This article belongs to the Section Forest Soil)
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Abstract

The increasing frequency of wildfires in larch forests across the discontinuous permafrost zone of Eastern Eurasia heightens the vulnerability of soil organic matter (SOM) under a warming climate. However, post-fire SOM thermal stability in this frequently burned forest region remain poorly understood. We assessed the long-term effects of wildfire on SOM structure and thermal stability in burned and unburned larch forests using complex analytical approaches: pyrolysis–gas chromatography/mass spectrometry (TMAH-py-GC/MS) and thermogravimetry/differential thermal analysis (TG/DTA). The focus was on the upper mineral soil horizon, where fire impacts may persist for decades. Sixteen years post-fire, total carbon content did not differ significantly between burned and control soils. Nonetheless, the molecular composition and thermal properties of SOM showed marked post-fire alterations. Burned soils exhibited higher proportions of lignin-derived compounds and reduced levels of short-chain fatty acid methyl esters. A lower degradation temperature (T50) and a higher thermal mass loss of labile fractions indicate a decrease in the thermal stability of SOM after fire. Our study shows that recurrent forest fires in larch forests of the Russian Far East decrease the thermal stability of soil organic matter, thereby increasing its vulnerability to subsequent fire degradation.

1. Introduction

The southern limit of boreal larch forests in Eastern Eurasia coincides with the south edge of discontinuous permafrost [1,2]. Research shows that soil organic matter (SOM) in permafrost-underlined soils is highly sensitive to climate warming [3]. While wildfire has historically shaped larch forest dynamics [4], their increasing frequency presents additional risks to the stability of SOM and carbon storage. Wildfires modify soil properties and vegetation, with studies suggesting that soil recovery can take about 30 years post-fire [5]. Recent research in the cryolithozone of Eastern Eurasia has examined the effects of fire on litter decomposition, soil characteristics, and vegetation recovery [5,6,7,8]. In other boreal and temperate ecosystems, the molecular composition of soil organic matter (SOM) and its thermal sensitivity are linked to fire-induced changes in soil properties, vegetation, and microbial communities [9]. However, in larch-dominated permafrost forests, SOM research has primarily focused on litter decomposition [8], leaving the post-fire molecular and thermal dynamics of SOM largely unknown. To our knowledge, this is the first study to examine long-term post-fire changes in SOM molecular composition and thermal properties in permafrost-affected larch forests of Eastern Eurasia.
Pyrolysis–gas chromatography/mass spectrometry using tetramethylammonium hydroxide (TMAH-py-GC/MS) is a powerful method for investigating changes in SOM molecular structure within complex matrices such as soil [10,11]. It is particularly effective for identifying aromatic enrichment and the formation of pyrogenic compounds in burned soils, as well as the losses in labile fractions [12,13]. Post-fire short-term shifts in fatty acid profiles—favouring short-chain compounds—have also been reported [14]. In Canadian permafrost soils, gradual inputs of fresh organic matter may restore SOM composition over decades [15].
Molecular changes in SOM directly affect its thermal properties. Thermogravimetry (TG) and differential thermal analysis (DTA) are widely employed to assess SOM thermal stability in fire-affected forest soils [16]. These methods quantify mass loss during heating, revealing the balance between labile and recalcitrant carbon pools. Comparative studies using TG-DTA and TMAH-py-GC/MS in burned deciduous and coniferous forests demonstrate increased SOM aromaticity and thermal stability [17,18], with peak combustion temperatures for polyphenolic and lignin compounds often shifting downward. In Southern Siberian forests, fires were found to deplete thermolabile SOM fractions and enhance aromaticity and T50 values [19]. However, vegetation regrowth and litter inputs may obscure long-term SOM changes, complicating detection [20].
Despite these advancements, the long-term effects of fire on the molecular composition and thermal properties of SOM—particularly beyond the ten-year mark post-disturbance—remain unclear. In the larch forests of discontinuous permafrost in Eastern Eurasia, no studies have systematically examined these long-term dynamics.
This study aims to assess the long-term effects of wildfire on the physico-chemical properties and soil carbon in the larch forests at the edge of permafrost zone south of Eastern Eurasia, where ongoing climate-derived permafrost degradation is most rapid. We hypothesise that post-fire recovery leads to increased input of labile compounds, reducing SOM aromaticity and thermal stability over time. Using TG-DTA and TMAH-py-GC/MS, we evaluate how fire influences SOM thermal stability and molecular composition. By comparing paired burned and unburned sites, our integrative approach offers new insights into post-fire carbon cycling in climate-sensitive boreal ecosystems.

2. Materials and Methods

2.1. Study Sites

This study was conducted in far eastern Russia at 53°50′ N, 127°10′ E, within a natural larch–birch forest stand (Larix gmelinii (Rupr.) Rupr. and Betula platyphylla Sukaczev) located at 569 m a.s.l., which represents the typical vegetation cover of the region. The study area consisted of two plots: a control (unburned) plot, which showed no evidence of wildfire, and a post-fire (burned) plot that was affected by a severe surface fire in 2003. Initially, this forest was a single, continuous stand, bisected by a stream that ultimately served as the fire boundary. Consequently, we attribute all observed differences in the studied soil properties to the effects of this fire event. The fire resulted in the mortality of approximately 86% of birch trees and 70% of larch trees. The study site is located at the southern limits of the discontinuous permafrost zone. The mean annual temperature is −0.7 °C, with the coldest monthly mean occurring in January (−19.3 °C) and the warmest in July (+19.1 °C). The mean annual precipitation is 527 mm. According to the World Reference Base for Soil Resources [21], soils in the region are classified as Dystric Cambisols and have developed on granite parent material.

2.2. Soil Sampling

We sampled soil in late summer, sixteen years after the fire; soil samples were collected from burned and unburned plots. Following the removal of the litter layer, samples were taken from the upper mineral horizon (0–5 cm, Ah horizon) at ten replicate points per plot. Hereafter, we refer to soils from the unburned larch forest as unburned soil and those from the fire-affected stand as burned soil.
All samples were transported to the laboratory, air-dried, homogenised, and sieved through a 0.25 mm mesh to prepare them for soil analysis. For thermogravimetric (TG), differential thermogravimetric (DTA) and pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS) analyses, subsamples from the ten replicates were thoroughly mixed to produce a single representative composite sample for each treatment (burned and unburned). While composite sampling minimises intra-plot variability, it precludes formal statistical comparisons for Py-GC/MS and TG-DTA results.

2.3. Soil Analysis

Total organic carbon (C) was analysed using the thermal oxidation method in solid samples with a TOC-L analyser (Shimadzu, Kyoto, Japan). Soil total organic nitrogen (N) content was determined using the Kjeldahl acid digestion method. Soil pH was determined in water (with a water-to-soil ratio of 2.5:1).

2.4. TMAH-py-GC/MS

The chemical structure of soil organic matter was analysed using pyrolysis–gas chromatography/mass spectrometry with tetramethylammonium hydroxide derivatisation (TMAH-py-GC/MS). Soil samples (5 mg), composited from ten replicates per plot, were pyrolysed at 590 °C for 5 s using pyrofoil capsules (pyrofoil F590, Japan Analytical Industry, Tokyo, Japan). Prior to pyrolysis, each sample was treated with 25 μL of TMAH in methanol (40 mg/mL) and 10 μL of nonadecanoic acid in acetone (0.06 mg/mL) as an internal standard (ISTD). After solvent removal under reduced pressure, the samples were tightly sealed in pyrofoil and introduced into a Curie-point pyrolyser JHP-5 (Japan Analytical Industry, Tokyo, Japan) connected to a Shimadzu GC-MS-QP2010 system equipped with a QP-2010 mass detector (Shimadzu, Kyoto, Japan).
Compound peaks in the pyrolysates were identified using the National Institute of Standards and Technology (NIST) mass spectral library, applying a minimum match threshold of 80% and data from relevant studies [17,22,23]. For each soil sample, the relative peak areas were calculated by dividing the individual peak areas by the signal of the internal standard (C20H40O2). The proportional abundance (semi-quantitative estimation) of each compound class was then calculated as a fraction of the total peak area, normalised to the internal standard.

2.5. TG-DTA

TG-DTA was used to evaluate the thermal stability of soil organic matter (SOM). Combustion characteristics of the soil were determined using a Thermogravimetry–Differential Thermal Analysis system (Thermo Plus 2, Rigaku, Tokyo, Japan). Approximately 10 mg of each sample was heated from 30 to 600 °C at a rate of 10 °C min−1 under an airflow of 200 mL min−1. Mass changes were recorded at a frequency of 1 Hz, with a precision of 0.001 mg. Alumina was used as the reference material.
The thermogravimetric data (TG) were transformed into derivative thermogravimetric data (DTG) curves to characterise the rate of mass loss as a function of temperature. Based on the TG curves, the T50 value—defined as the temperature at which 50% of SOM mass is lost—was calculated. T50 values were obtained directly from the TG curves within the temperature range of 170–600 °C, excluding the initial loss of free and unstructured water. In addition, SOM thermal stability was evaluated using the ratio of labile to stable SOM fractions.
Soil C, N, and pH were analysed at the Analytical Centre of Mineral-Geochemistry Investigation of the Institute of Geology and Nature Management, Far Eastern Branch of the Russian Academy of Sciences, Blagoveshchensk, Russia. Analyses of soil organic matter properties and structure (TG, DTA, and Py-GC/MS) were conducted at the University of Toyama, Toyama, Japan.
Statistical analyses were conducted using RStudio version 4.3.2 [24]. Initially, all data were tested for normality and homogeneity of variances. Hypothesis testing for differences and the statistical significance of calculated parameters between unburned and burned soil treatments was undertaken using the t-test for parametric data and the Wilcoxon rank-sum test for non-parametric data. Differences in soil properties were regarded as statistically significant at p < 0.05. Differences in TG, DTA, and Py-GC/MS values between sites were evaluated through descriptive comparison due to the compositing of soil samples for organic matter structural analysis.

3. Results

3.1. Soil Parameters

In the 0–5 cm soil layer, the unburned soil contained 7.37% C and 0.33% N (Table 1). Both carbon and nitrogen contents were generally higher in the burned soil (9.50% and 0.44%, respectively) compared to the unburned soil; however, these differences were not statistically significant (p > 0.05). The CNratio remained consistent between the soil samples, ranging from 21 to 22. The pH value in the burned soil was 5.91, which was 0.77 units higher than that in the unburned soil (p = 0.0014).

3.2. Molecular Composition of SOM

TMAH-pyrolysis GC/MS chromatograms of the unburned and burned soil samples are presented in Figure 1. The primary pyrolysis-derived compounds identified from the chromatograms are detailed in Table S1, with their representative molecular structures illustrated in Figure 2. The chromatograms show peaks corresponding to lignin-derived compounds (G, S, C), methyl esters of fatty acids (F) (F < 20: with a chain length less than 20 carbon atoms; F ≥ 20: with a chain length more than 20 carbon atoms), non-lignin aromatic structures (O), and nitrogen-containing heterocyclic compounds (N). The lignin peaks included derivatives of guaiacyl (G), syringyl (S), and cyanamide-related (C) structures. The predominant lignin-related peaks were linked to p-hydroxyphenol and guaiacyl units (C and G compounds, respectively). The methyl esters of fatty acids identified in the larch forest soil samples varied in carbon chain length from C6 to C28.
Nitrogen-containing heterocycles formed during TMAH thermochemolysis were primarily derivatives of indole and pyrrolidine. Normalising the peak areas to the internal standard demonstrated that the total abundance of pyrolytic products was greater in burned soil than in unburned soil, mainly due to an increase in G, S, C, F, O, and N compound groups (Figure 3a). In the unburned soil sample, the relative distribution of molecular classes was as follows: 11.0% F ≥ 20, 63.2% F < 20, 1.47% N, 4.41% O, 2.20% S, 10.3% G, and 7.35% C (Figure 3b). Compared to unburned soil, the burned soil exhibited a higher relative abundance of lignin-derived compounds and polycyclic aromatic hydrocarbons, along with a decrease in the proportion of short-chain fatty acid methyl esters (mainly F < 20).

3.3. Derivatographic Determination of SOM Content and Stability

The DTA curve was utilised to assess the character of thermal peaks (exothermic or endothermic). The oxidation of thermal degradation products yielded a pronounced exothermic peak. The TG curve was employed to ascertain the rate of thermal decomposition at a specified temperature.
On the DTA curve, both the unburned and burned soil samples showed a small endothermic peak at 61 °C (Figure 4a). Furthermore, exothermic peaks were observed in both samples within the temperature range of 250–450 °C. The height and area of these peaks were more pronounced in the burned soil compared to the unburned soil.
Analysis of the TG curves indicated that the total mass loss of the burned soil at 600 °C was greater than that of the unburned soil—24.4% and 20.7%, respectively (Figure 4b). The DTG derivative curves (TG) exhibited distinct peaks at 308 °C (Peak A) and 394 °C (Peak B) in the unburned soil (Figure 4c). In the burned soil, Peak A appeared at 301 °C, while Peak B was less pronounced. In the temperature range of 210–360 °C, mass loss in the unburned soil accounted for 53.3% of total SOM decomposition; in the range of 360–460 °C, it was 38.7%; and in the range of 460–600 °C, it was 8% (Figure 4d). In the burned soil, compared to the unburned soil, mass loss increased in the 210–360 °C range and decreased in the 360–460 °C range. The ratio of mass loss from the thermally labile SOM pool to the thermally stable pool was 1.14 for the unburned soil and 1.41 for the burned soil. The temperature at which 50% of SOM was lost (T50) was 13 °C lower in the burned soil (335 °C) than in the unburned soil (348 °C).

4. Discussion

Studies conducted across various climatic and ecological zones have shown that, in the short- and medium-term periods following wildfire events (<10 years), there is a significant decrease in soil organic matter [6,10,25,26]. This loss primarily impacts the thermally labile fraction of SOM, including compounds such as carbohydrates and proteins [27], while relatively more recalcitrant compounds, particularly aromatics, tend to increase. Previous research by González-Pérez et al. [12] showed that the peak abundance of pyrolytic products drops sharply after a fire. A decrease in labile carbon and increase in thermally stable SOM pools three years after wildfire in the forest–tundra of Western Siberia were interpreted as a general rise in the thermal stability of SOM [25].
However, long-term (over 10 years) post-fire effects are highly context-dependent. For example, 11 years after a severe wildfire in a pine forest in Spain, the content of organic carbon and nitrogen in Andosol soils was lower than in unburned soils [10]. In nearby Leptosols, similar long-term consequences of wildfire were also evident as a decrease in soil organic carbon [20]. In contrast, coniferous forests in Colorado showed no significant wildfire effects on soil C and N contents 14 years post-fire [23]. Our study, conducted 16 years post-fire in larch forests of the discontinuous permafrost zone in north Asia, found a trend toward increased total C and N contents in the 0–5 cm mineral soil layer (Table 1), potentially due to vegetation recovery and the gradual decomposition of fresh litter. Similar to other studies, we observed a relative increase in aromatic structures. However, this does not always directly reflect an increase in the thermal stability of organic matter [23].
In our study, Py-GC/MS (TMAH-py-GC/MS) analysis revealed an increase in the relative abundance of guaiacyl-derived compounds and compounds originating from non-lignin organic matter in the burned soil (Figure 3b). Guaiacyl compounds are primarily derived from the wood of gymnosperms (e.g., larch). In contrast, syringyl compounds originate from angiosperm wood (e.g., birch, aspen). Cinnamyl-derived compounds, which are characteristic of herbaceous vegetation, are known to be less thermally stable than lignin inputs from woody sources [28]. In the lignin group, guaiacyl compounds predominated over syringyls in both soil types, but the GS ratio was higher in the burned soil. These changes in the composition of recalcitrant SOM components (i.e., lignin) 16 years after fire may be associated with shifts in the plant community structure—specifically, the decline of broadleaf tree species and the expansion of grasses and shrubs, as reported in our previous publication [29]. Additionally, earlier studies have demonstrated that microbial decomposition preferentially reduces the contribution of syringyl units. Syringyl is less chemically and thermally stable than guaiacyl, which can lead to a relative accumulation of guaiacyl fragments over time [30,31]. The SG (syringyl-to-guaiacyl) and CG (cinnamyl-to-guaiacyl) ratios are widely used as indicators of lignin degradation and structural alteration [30]. In our study, the decreases in SG and CG ratios in the burned larch forest soil likely reflect a combination of vegetation change and slower microbial processing, but direct confirmation requires microbial activity data.
The lower relative abundance of short-chain fatty acid methyl esters (F < 20) in burned soils compared to unburned soils (Figure 3b) suggests potential suppression of microbial processing, as F < 20 originate from both microbial and plant sources [14]. Long-chain fatty acids methyl esters (F ≥ 20) are mainly linked to the breakdown of higher-plant waxes. In the short-term after fire, the ratio of short-to long-chain fatty acid methyl esters generally increases in burned versus unburned soils, indicating oxidative cleavage of long-chain homologues in fire-affected soils [12,14,32].
In contrast, our study found that 16 years after the fire, the short-long-chain fatty acid methyl esters ratio had decreased in the burned soil compared to the control. This pattern is likely due to microbial degradation of short-chain fatty acid methyl esters, that were initially formed by thermal cleavage in the years immediately following the fire. Simultaneously, vegetation recovery leads to the gradual accumulation of long-chain aliphatic compounds (C20–C32), which are more resistant to microbial decomposition.
The thermal stability of soil organic matter (SOM) in burned and unburned larch stands was evaluated using TG/DTA analysis. DTA curves indicate the temperatures at which thermal events occur and whether decomposition is endothermic or exothermic. An endothermic peak below 200 °C usually indicates moisture loss [33]. In our study, such a peak appeared at 61 °C (Figure 4a). Exothermic effects associated with SOM combustion occurred between 200 and 550 °C and are linked to organic matter oxidation [25,33,34]. We observed notable exothermic peaks in the range of 200–450 °C, with greater intensities in the burned soil (Figure 4a). These thermal reactions were supported by mass loss, recorded by TG analysis (Figure 4b), and this loss was greater in the burned soil than in the unburned soil, which matches the higher carbon content measured at the burned site (Table 1). In our study, mass loss between 200 and 400 °C (Figure 4c) can be attributed to the breakdown of labile and moderately stable SOM fractions [35,36]. Above 400 °C, organic matter degrades more slowly, probably due to the gradual breakdown of thermally resistant lignin compounds. Past TG studies on mineral soils under varied land uses, including forests, have shown that mass loss in the 180–450 °C range closely relates to organic carbon content [37], aligning with our results.
TG/DTA thermal analysis showed apparent differences in thermal mass loss behaviour between burned and unburned soils (Figure 4). Burned soils exhibited a higher total mass loss and more intense exothermic peaks between 200 and 450 °C, consistent with an elevated SOM content (Table 1). The first DTG peak occurred at slightly lower temperatures in burned soils (301 °C vs. 308 °C), suggesting a larger labile fraction. The broader secondary peak in unburned soils (394 °C) and higher T50 (348 °C vs. 335 °C) indicate greater SOM thermal stability in unburned sites. Similar to findings from a long-term post-fire study in a Mediterranean forest ecosystem [20], we found no clear evidence of increased SOM thermal stability in the burned soil—such as greater dominance of recalcitrant pools over labile ones, higher T50 values, or peak shifts towards higher temperatures. On the contrary, the unburned soil showed not only a peak at 308 °C but also a secondary peak at a higher temperature (394 °C), suggesting greater thermal stability of SOM in undisturbed soils. T50 is defined as the temperature at which 50% of SOM is lost [35]. A shift in T50 toward higher values is generally interpreted as an indicator of increased SOM stability. In our study, the reduced thermal stability in the burned soil compared to the unburned soil was confirmed by a lower T50: 335 °C versus 348 °C. Furthermore, the mass loss ratio of labile to stable SOM fractions, which indicates relative stability, was higher in the burned soil (1.41) than in the unburned soil (1.14).
It is essential to acknowledge several limitations that may impact the interpretation of our findings. First, using composite soil samples for Py-GC/MS and TG-DTA analyses prevented a statistical comparison of differences between burned and unburned sites. Therefore, the observed trends should be viewed as indicative rather than conclusive. Second, although we interpret specific chemical patterns (e.g., lower short-chain fatty acid methyl esters, altered S/G ratios) as indicating suppressed microbial activity, we did not directly measure microbial biomass or enzymatic functions. Consequently, these inferences should be made with caution. Future studies should include microbial biomass carbon (MBC), respiration assays, and microbial community analyses to reinforce causal interpretations.
One possible explanation for the lower thermal stability of the carbon pool in the burned site may be a decline in the proportion of microbially transformed organic matter. The TG/DTA results are supported by the Py-GC/MS data, which showed that the proportion of short-chain fatty acid methyl esters (F < 20) in the burned soil was lower relative to the unburned forest (Figure 3b). Ludwig et al. [38] reported that microbially derived carbohydrates are positively correlated with SOM thermal stability. More recent studies have also confirmed a positive relationship between microbial biomass carbon (MBC) and SOM stability, suggesting that microbially processed organic matter is more thermally resistant than previously assumed [39]. Additionally, the decline in low-molecular-weight lignin-derived compounds (<C11) in the unburned soil indicates the advanced transformation of lignin residues and their integration into the high-molecular-weight humic fraction. Organic matter losses that typically occur shortly after fire may be gradually compensated over time through vegetation recovery, litterfall, and fine root input—primarily from herbaceous vegetation [20]. An increased proportion of lignin compounds originating from herbaceous species, combined with a reduction in microbially transformed compounds due to suppressed microbial activity, likely contributes to the lower thermal stability of SOM in the burned soil.
Our results demonstrate that even 16 years after wildfire, the SOM composition and thermal behaviour remain altered, with potential long-term effects on soil carbon dynamics. These findings are relevant for models of permafrost carbon vulnerability, as post-fire SOM recovery may follow slow and nonlinear trajectories depending on the vegetation type, microbial resilience, and climatic constraints. Integrating microbial, hydrological, and vegetation data is essential for capturing feedback in post-fire permafrost landscapes. Continued monitoring of recovery processes can inform land management and climate adaptation strategies, particularly as the fire frequency increases in boreal permafrost regions.

5. Conclusions

Sixteen years after wildfire disturbance in a larch forest within the discontinuous permafrost zone, the organic carbon content in the 0–5 cm mineral soil layer had recovered to levels comparable with unburned soils. However, results from TMAH-py-GC/MS analysis indicated that molecular alterations in soil organic matter (SOM) persist for decades after a fire, including an increased proportion of lignin-derived compounds and a reduced contribution of microbially transformed material. Despite the enrichment in lignin-related compounds, SOM in fire-affected soils exhibited lower thermal stability compared to that in unburned larch forest soils.
TG/DTA analysis showed a decrease in T50 and an increase in the ratio of thermally labile to stable SOM pools in the burned soil, indicating reduced SOM recalcitrance. These findings suggest that, while the SOM quantity may recover, its composition and thermal properties remain altered, potentially reducing the resilience of SOM to future disturbances. The loss of thermal recalcitrance may lead to accelerated carbon turnover and a diminished capacity for long-term carbon retention, which, when coupled with other post-fire changes such as vegetation shifts, could contribute indirectly to increased vulnerability of boreal soils to climate-driven feedback. Future research should integrate microbial activity metrics, vegetation dynamics, and hydrological processes to fully assess SOM recovery pathways and to inform adaptive land management strategies in permafrost-affected forest regions.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/f16081309/s1, Table S1: The assignments for the peak identified in the py-TMAH-GC/MS analyses of soils. C: compounds derived from p-hydroxyphenol (cinnamyl) structures; G: compounds derived from guaiacyl structures; S: compounds derived from syringyl structures; O: other non-lignin derived compounds; N: nitrogen-containing compounds; F: fatty acid methyl esters.

Author Contributions

Methodology, A.K. and K.S.; Software, K.S.; Investigation, K.S., N.W. and S.B.; Data curation, A.K. and K.S.; Writing—original draft, A.K.; Writing—review & editing, A.K. and S.B.; Visualization, A.K.; Supervision, S.B.; Project administration, N.W. and S.B.; Funding acquisition, K.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science and Higher Education of the Russian Federation.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Forests 16 01309 g001
Figure 1. Pyrograms are total ion chromatograms of unburned and burned soil. The numbers on the peaks correspond to the pyrolysis compounds listed in Table S1. Asterisks represent fatty acids. C20H40O2: nonadecanoic acid methyl ester in acetone internal standard (ISTD).
Figure 1. Pyrograms are total ion chromatograms of unburned and burned soil. The numbers on the peaks correspond to the pyrolysis compounds listed in Table S1. Asterisks represent fatty acids. C20H40O2: nonadecanoic acid methyl ester in acetone internal standard (ISTD).
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Forests 16 01309 g002
Figure 2. Chemical structures of pyrolysate compounds. Numbers on the chemicals refer to Table S1.
Figure 2. Chemical structures of pyrolysate compounds. Numbers on the chemicals refer to Table S1.
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Forests 16 01309 g003
Figure 3. The fractions of groups for pyrolysate compounds of unburned (UB) and burned (BS) soil. The relative abundance was calculated from the peak intensity of nonadecanoic acid methyl ester. C: compounds derived from p-hydroxyphenol (cinnamyl) structures; G: compounds derived from guaiacyl structures; S: compounds derived from syringyl structures; O: other non-lignin-derived compounds; N: nitrogen-containing compounds; F < 20: methyl esters of fatty acids (F) with a chain length less than 20 carbon atoms; F ≥ 20: methyl esters of fatty acids (F) with a chain length more than 20 carbon atoms. Graph (a) shows the components as the ratio of the peak area to the standard (C20H40O2), graph (b) shows the proportion of each group of compounds from the total area of peaks normalised to the internal standard.Within the lignin group, guaiacyl compounds were more abundant than syringyl compounds in both types of soil. The SG and CG ratios were 0.21 and 0.71 in the unburned soil, and 0.19 and 0.61 in the burned soil, respectively. Furthermore, the number of lignin-related compounds with C5–C11 carbon backbones was lower in the unburned soil compared to that of the burned soil.
Figure 3. The fractions of groups for pyrolysate compounds of unburned (UB) and burned (BS) soil. The relative abundance was calculated from the peak intensity of nonadecanoic acid methyl ester. C: compounds derived from p-hydroxyphenol (cinnamyl) structures; G: compounds derived from guaiacyl structures; S: compounds derived from syringyl structures; O: other non-lignin-derived compounds; N: nitrogen-containing compounds; F < 20: methyl esters of fatty acids (F) with a chain length less than 20 carbon atoms; F ≥ 20: methyl esters of fatty acids (F) with a chain length more than 20 carbon atoms. Graph (a) shows the components as the ratio of the peak area to the standard (C20H40O2), graph (b) shows the proportion of each group of compounds from the total area of peaks normalised to the internal standard.Within the lignin group, guaiacyl compounds were more abundant than syringyl compounds in both types of soil. The SG and CG ratios were 0.21 and 0.71 in the unburned soil, and 0.19 and 0.61 in the burned soil, respectively. Furthermore, the number of lignin-related compounds with C5–C11 carbon backbones was lower in the unburned soil compared to that of the burned soil.
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Figure 4. (a) Differential thermal analysis (DTA) curves, (b) thermogravimetric (TG) curves, (c) derivative thermogravimetric analysis (DTG) curves, and (d) percentage of thermal mass loss within defined temperature ranges for unburned and burned soil samples.
Figure 4. (a) Differential thermal analysis (DTA) curves, (b) thermogravimetric (TG) curves, (c) derivative thermogravimetric analysis (DTG) curves, and (d) percentage of thermal mass loss within defined temperature ranges for unburned and burned soil samples.
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Table 1. Physico-chemical and chemical properties of unburned and burned larch forest soils at 0–5 cm depth (means ± SE, n = 10). Statistically significant differences between unburned and burned soil are indicated by asterisks: ** p < 0.01; ns = not significant.
Table 1. Physico-chemical and chemical properties of unburned and burned larch forest soils at 0–5 cm depth (means ± SE, n = 10). Statistically significant differences between unburned and burned soil are indicated by asterisks: ** p < 0.01; ns = not significant.
Soil SampleC, % (ns)N, % (ns)C/NpHH2O (**)
unburned soil7.37 ± 1.030.33 ± 0.0422.3 ± 1.275.14 ± 0.07
burned soil9.50 ± 2.180.44 ± 0.0921.2 ± 0.525.92 ± 0.12
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Kondratova, A.; Sazawa, K.; Wada, N.; Bryanin, S. Pyrogenic Transformation of Soil Organic Matter in Larch Forests of the Discontinuous Permafrost Zone. Forests 2025, 16, 1309. https://doi.org/10.3390/f16081309

AMA Style

Kondratova A, Sazawa K, Wada N, Bryanin S. Pyrogenic Transformation of Soil Organic Matter in Larch Forests of the Discontinuous Permafrost Zone. Forests. 2025; 16(8):1309. https://doi.org/10.3390/f16081309

Chicago/Turabian Style

Kondratova, Anjelica, Kazuto Sazawa, Naoya Wada, and Semyon Bryanin. 2025. "Pyrogenic Transformation of Soil Organic Matter in Larch Forests of the Discontinuous Permafrost Zone" Forests 16, no. 8: 1309. https://doi.org/10.3390/f16081309

APA Style

Kondratova, A., Sazawa, K., Wada, N., & Bryanin, S. (2025). Pyrogenic Transformation of Soil Organic Matter in Larch Forests of the Discontinuous Permafrost Zone. Forests, 16(8), 1309. https://doi.org/10.3390/f16081309

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