Root Signaling Substances Regulate Carbon Allocation Mechanism in the Plant and Soil of Peatlands under Permafrost Degradation

Abstract

As the regulator of water and nutrient changes in the active layer after permafrost degradation, root signaling substances affect the plant–soil carbon allocation mechanism under climate warming, which is a key issue in the carbon source/sink balance in permafrost regions. To explore how plant root signaling substances regulate carbon allocation in plants and soils under permafrost degradation, the changes in carbon allocation and root signaling substances in the plants and soils of peatland in different permafrost regions at the time of labeling were studied by in situ ¹³C labeling experiments. The results showed that the fixed ¹³C of Larix gemlini, Carex schumidtii, and Sphagnum leaves after photosynthesis was affected by permafrost degradation. In regions with more continuous permafrost, the trend of the L. gemlini distribution to underground ¹³C is more stable. Environmental stress had little effect on the ¹³C accumulation of Vaccinium uliginosum. Nonstructural carbohydrates, osmotic regulatory substances, hormones, and anaerobic metabolites were the main root signaling substances that regulate plant growth in the peatlands of the three permafrost regions. The allocation of carbon to the soil is more susceptible to the indirect and direct effects of climate and environmental changes, and tree roots are more susceptible to environmental changes than other plants in isolated patches of permafrost regions. The physical properties of the soil are affected by climate change, and the allocation of carbon is regulated by hormones and osmotic regulators while resisting anoxia in the sporadic regions of permafrost. Carbon allocation in discontinuous permafrost areas is mainly regulated by root substances, which are easily affected by the physical and chemical properties of the soil. In general, the community composition of peatlands in permafrost areas is highly susceptible to environmental changes in the soil, and the allocation of carbon from the plant to the soil is affected by the degradation of the permafrost.

1. Introduction

Permafrost is one of the world’s most important soil carbon reservoirs, accounting for about 60% of the Earth’s terrestrial carbon pool and more than twice the amount currently stored in the atmosphere [1,2]. In the context of climate warming, the balance between sources and carbon sinks in permafrost regions is a key issue [1]. Longer growing seasons and higher temperatures in permafrost regions are expected to drive productivity gains (increased carbon sinks), which are partially offset by changes in soil moisture, physical and chemical properties, and increased soil respiration (increased carbon sources) as a result of permafrost degradation [2,3,4,5]. Tundra and coniferous forest are the main types of permafrost regions, which are mainly distributed in the middle and high latitudes of the northern hemisphere [1]. Due to the influence of climate warming, the Arctic tundra shrub has expanded in recent years, affecting the carbon balance of the permafrost region by improving the carbon absorption of the ecosystem [6]. At the same time, the study of the carbon cycle in Canada and the Tibetan Plateau after the thawing of the permafrost shows that the ecosystem in the permafrost region is transforming from a carbon sink to a carbon source [7,8]. As the southern boundary of the Eurasian permafrost zone, Xing’an Mountains is characterized by the interphase distribution of different permafrost types (including isolated patches of a permafrost zone, sporadic permafrost zone, and discontinuous permafrost zone). It has a high forest coverage rate and is dominated by mixed coniferous forest, rich vegetation types, and a wide distribution of peatlands, and is very sensitive to climate change [9,10]. As the main distribution area for discontinuous and island permafrost, peatland is easily affected by permafrost degradation, which causes plant stress and affects productivity [11,12]. Meanwhile, carbon sequestration by plants is an important carbon cycle process in peatlands [13,14]. Plants are important sources of organic carbon in the soil as they absorb CO₂ to form photosynthetic carbon from plants and then transport it to the soil [15,16]. Changes in the composition of the vegetation community alter the carbon balance of ecosystems by affecting soil–plant interactions [6]. Therefore, the study of the plant–soil carbon allocation mechanism under permafrost degradation is a key issue regarding the carbon source/sink balance under climate change, which will contribute to the study of carbon cycles in permafrost regions.

Dynamic changes in root signaling substances can reflect the above-ground growth relationship of plants, regulate the redistribution and utilization of photosynthetic products (¹³C, soluble sugar, and starch) in different organs, and ultimately determine the growth strategy of plants and their productivity [17,18,19]. Plant organs use carbon for growth, defense, storage, and respiration [20,21]. Soluble sugar and starch are the main products of plant photosynthesis, and their content represents the formation and transformation process of carbohydrates in plants [22]. At the same time, the degradation of permafrost easily causes the plants in the peatland to be stressed. The physiological mechanism of plant growth in different permafrost areas under environmental stress can be explored through the physiological indexes of hormones, the anaerobic metabolism process, osmotic regulation substances, and the antioxidant system of plant root tissue [23,24,25]. Plants have a set of mechanisms for dealing with the damage caused by destructive substances in the body when dealing with stress. Hormones are important signaling molecules that respond first [26]. The interaction of ethylene, abscisic acid, and gibberellin is an important factor affecting the subflooding tolerance of plants [27]. The increase in ethylene content is the upstream signaling molecule that allows for most plants to cope with flood stress [26]. Abscisic acid is one of the important signal substances involved in plant stress resistance. It is involved in the transport process of photosynthates and plays a role in the regulation of stomatal opening in the mechanism of resistance to plant waterlogging [28]. A low-oxygen environment leads to the production of reactive oxygen species, and excessive reactive oxygen species can cause irreversible damage to plants [25]. Proline is one of the osmotic regulatory substances in plants. It regulates the osmotic balance and antioxidant system of cells and can improve resistance to plant stress [26]. Therefore, by studying whether plants activate the anaerobic metabolic pathway and what pathway they use to consume starch and soluble sugar, we can understand the distribution of photosynthetic products and plant growth in the mechanism of the removal of reactive oxygen by plants in peatlands under permafrost degradation.

Stable carbon isotopes are commonly used to track the plant–soil carbon distribution and transfer [29]. The use of in situ isotope labeling techniques to provide new insights into the fate of recently assimilated carbon in organs and carbon-containing compounds has been applied to crops, forest plants, and wetland plants [30,31]. Permafrost degradation plays a key role in the carbon allocation of plant–soil ecosystems. However, the mechanism of root signaling substances regulating the ¹³C allocation of plant–soil systems under permafrost degradation is rarely studied using ecological thinking. Therefore, we used stable isotope labeling in situ to study the regulation of plant root signaling substances on plant–soil system ¹³C allocation under permafrost degradation. The mechanism of the regulation of carbon allocation in the plant–soil system by root signaling substances in peatland plants (L. gemlini, V. uliginosum, C. schumidtii, and Sphagnum) in different permafrost regions (Yichun, isolated patches of the permafrost; Heihe, sporadic permafrost; Mohe, discontinuous permafrost) was analyzed. Two hypotheses were proposed: (i) the effects of root signaling substances on carbon allocation in plant and soil in peatlands in different permafrost regions change with labeling time; and (ii) with different degrees of permafrost degradation, climate change directly or indirectly affects root signaling substances through soil properties to regulate plant and soil carbon allocation.

2. Materials and Methods

2.1. Study Area

Peatland carbon allocation mechanisms were studied in the Greater and Lesser Xing’an Mountains in northeast China (Figure 1). With an altitude of 300–700 m, the annual average temperature is −2.8 °C, the frost-free period is 90–110 days, and the annual average precipitation is 746 mm, demonstrating a cold temperate continental monsoon climate. Permafrost is mainly distributed in river valleys, foothills, shady slopes, and other areas, including discontinuous permafrost, sporadic permafrost, and isolated patches of permafrost [32,33]. The peatland is widely distributed, and the main tree species include L. gemlini, Betula platyphylla, and Pinus koraiensis.

2.2. ¹³C Pulse Labeling

The pulse labeling of plants was performed on peatlands in Mohe (Figure 2a–c), Heihe (Figure 2d–f), and Yichun (Figure 2g–i), which represent different degrees of permafrost degradation (

Table 1. In-site measurement information.

Sites	Permafrost Zone	Forest Type	Active Layer Depth (m)
Yichun	Seasonally frozen soil	Shrub wetlands	1.5
Heihe	Sparsely patched permafrost	Shrub wetlands	1.2
Mohe	Predominantly continuous and island permafrost	Shrub wetlands	1

). Five grams of Na₂CO₃ were placed into a beaker before labeling, and the CO₂ concentration in the labeling room was monitored using a CO₂ monitor. The culture chamber was immediately sealed to prevent the soil from absorbing CO₂ gas in the air, and a ¹³CO₂ labeling test was started after standing for 5 min. At the beginning of labeling, a syringe was used to inject ¹³CO₂ gas with an abundance of 99.9%. The initial concentration of CO₂ in the labeling chamber was 450 μmol/mol, and the temperature was 27–28 °C. During the labeling process, the CO₂ concentration in the labeling chamber was lower than 400 μmol/mol. After absorbing 1 mol·L⁻¹ HCI with a needle and adding Na₂CO₃ from the entrance of the external catheter, ¹²CO₂ gas was released for the CO₂ concentration in the labeled gas chamber, reaching 450 μmol/mol. The marking time was 120 min. After marking, it was placed in a ventilated place for 10 min, and the sealed culture room was opened for use.

2.3. Sample Collection and Testing

The plant roots, stems, leaves, and soil of the labeled samples and control samples were collected 2, 24, 120, 216, and 360 h after the end of labeling. Three parallel samples were collected from each sample site. The sample was freeze-dried and pulverized with a ball mill. ¹³C isotope values of plant and soil samples were determined using IsoPrime-IRMS. The soil organic carbon (SOC) content was determined using the potassium dichromate oxidation method. Take 0.2 g of air-dried soil and add 10 mL 0.4 mol/L potassium dichromate sulfuric acid solution into the deboiling tube, deboil at 180 °C for 10 min, then transfer to a conical bottle after cooling, add 3–5 drops of phenanthroline indicator, and titrate with 0.2 mol/L ferric sulfate solution. The color changes from orange-yellow to blue-green to brick red, and brick red is the end point of the reaction. Total nitrogen (TN) was determined using the Kjeldahl method. Weigh 1 g air-dried soil and transfer it to the bottom of the Kjeldahl flask, add 2 g mixed catalyst, shake well and add 5 mL sulfuric acid, heat and boil with electric furnace. After boiling for 90 min, take off the Kjeldt flask, add 5 mL boric acid indicator solution into a 150 mL conical flask after cooling, open the condensate water, add 20 mL sodium hydroxide solution through the tee pipe, immediately close the distillation chamber, open the steam clip, and ensure steam distillation. Titrate with a standard solution of hydrochloric acid until the solution changes from blue-green to purple-red as the end point. Total phosphorus (TP) was determined using the perchloric acid–sulfuric acid method. Weigh 1 g of soil sample and place it in a 50 mL triangle bottle, add 48 mL concentrated H₂SO₂, shake it, and add 10 drops of 70% perchloric acid to shake well. After boiling for 60 min, wash the cooled liquid with water into a 100 mL volumetric bottle. Absorb 2–10 mL of filtrate into a 50 mL volumetric bottle, dilute it with water to 30 mL, add 2 drops of dinitrophenol indicator, and adjust the pH with dilute sodium hydroxide solution and dilute sulfuric acid solution until the solution is just slightly yellow. Add 5 mL of molybdenum–antimony color developer, with the colorimetric at 700 nm wavelength on a spectrophotometer, to read the absorption value. The soil pH and soil moisture content (SWC) were measured using a portable detector. The thickness of the active layer (ALT) was measured with a 1.5 m T-shaped metal rod. Mean annual temperature and precipitation data were obtained from Weather net (https://rp5.ru/, accessed on 25 May 2022). The contents of abscisic acid (ABA), ethylene (ETH), gibberellin (GA), peroxidase (POD), superoxide dismutase (SOD), pyruvate decarboxylase (PDC), ethanol dehydrogenase (ADH), and acetaldehyde decarboxylase (ALDH) in plant tissues were determined by enzyme-related immunoassay [34,35]. The contents of soluble sugar (SS) and starch (ST) were determined by anthrone colorimetry. The soluble sugar and starch in the sample can be separated by 80% ethanol, the starch can be decomposed into glucose by acid hydrolysis, and the glucose content can be determined by the anthrone colorimetric method [34]. Proline (Pro) was determined by ninhydrin colorimetry [34].

2.4. Statistical Analysis

To examine root signaling substances that regulate carbon allocation in the plant and soil of peatland under permafrost degradation, samples of four plant species were collected from three peatlands in three permafrost regions, and three parallel samples were collected for each plant (

Table 2. Number of replicates at the appropriate scale.

Scale of Inference	Scale at Which the Factor of Interest is Applied	Number of Replicates at the Appropriate Scale
Permafrost zones	Plant	3 permafrost zones, 3 peatlands, 4 plant species, 3 samples were collected from each species at each time

).

The δ ¹³C value and ¹³C abundance (F) were calculated as follows:

$${\mathsf{\delta }}^{13}\mathrm{C}\left(\text{‰}\right)=\frac{{\mathrm{R}}_{\mathrm{sample}}-{\mathrm{R}}_{\mathrm{PDB}}}{{\mathrm{R}}_{\mathrm{PDB}}}\times 1000,$$

(1)

$$\mathrm{F}(\%)=\frac{\left({\mathsf{\delta }}^{13}\mathrm{C}+1000\right)\times {\mathrm{R}}_{\mathrm{PDB}}}{\left({\mathsf{\delta }}^{13}\mathrm{C}+1000\right)\times {\mathrm{R}}_{\mathrm{PDB}}+1000}\times 100,$$

(2)

where R_PDB is the ¹³C/¹²C ratio of the standard substance, 0.0112372 is the Pee Dee Belemnite (PDB) (standard from the Cretaceous Pee Dee Formation, SC, USA), and R_sample is the ¹³C/¹²C ratio of the sample.

A principal component analysis (PCA) of the main physiological indicators affecting the growth of peatland plants in different permafrost areas was carried out using Origin Pro 2022. A linear mixed-effect model in a two-way analysis of variance was used to determine the difference in the main physiological indexes of different plant types in the peatlands of different frozen soil areas with a change in marking time. The effectiveness of the model was tested through a visual estimation of residuals and quantile plots. The model included the nested random factors of the sampling date within the sampling point ID as random effects. Logarithmic likelihood tests were used to determine fixed effects and possible interactions. The final model included species and permafrost types as fixed effects and sampling point IDs as nested random effects. The main effects were further explored by an expanded comparison of the mean and the p-value adjustment of Tukey’s significant difference. p < 0.05 showed a significant difference. Piecewise structural equation modeling (SEM) was used to assess the direct and indirect effects of key factors on carbon allocation in plant–soil systems in different permafrost regions. In the model, climate factors, soil’s physical properties, soil’s chemical properties, and plant root signaling substances were set as fixed effects, and plant types were set as random effects. The analysis was conducted using the “piecewiseSEM” package of R [36]. The linear mixed-effect model was fitted to the “lme” function of the “nlme” package.

3. Results

3.1. Carbon Allocation in the Plant–Soil System of Peatlands in Different Permafrost Regions

The leaf ¹³C allocation ratio of the four species in the three permafrost areas was the highest and decreased with an increase in marking time, while the stem, root, and soil increased with an increase in marking time (Figure 3). The leaf ¹³C allocation of L. gemlini was the highest in Mohe (discontinuous permafrost zone) (Figure 3i) and lowest in Yichun (isolated patches of the permafrost zone) (Figure 3a). L. gemlini had the lowest root ¹³C allocation at 120 h and the highest soil ¹³C allocation at 120 h in Yichun. L. gemlini had the highest root and soil ¹³C allocation at 120 and 360 h, respectively, in Heihe (sporadic permafrost zone). L. gemlini had the highest root and soil ¹³C allocation at 360 h in Mohe. V. uliginosum had the lowest leaf ¹³C allocation at 216 h and the highest stem ¹³C allocation at 216 h in Yichun. C. schumidtii had the highest leaf ¹³C allocation at 360 h and the highest stem ¹³C allocation at 216 h; root ¹³C allocation at 24 and 216 h was higher than at other marking times and soil allocation increased with marking time in Yichun. The ¹³C allocation in leaves of C. schumidtii was the highest at 24 h, and decreased with marking time in stems, and that in roots and soil increased with marking time in Heihe and Mohe. The ¹³C allocation of Sphagnum leaves changed little with marking time in Yichun and Heihe and decreased with marking time in Mohe. Sphagnum ¹³C allocation decreased with marking time in Yichun and Heihe and increased with marking time in Mohe.

3.2. Main Root Signaling Substances Affecting Plant Growth in Peatlands in Different Permafrost Regions

A PCA was carried out of the physiological indexes (hormones, antioxidant system, anaerobic metabolism, non-structural carbohydrates, and osmotic regulatory substances) of peatland plants in different permafrost areas. At Yichun (isolated patches of the permafrost zone), PC1 and PC2 explained 51.8% of the variance (Figure 4a), and the indexes with high contribution rates were starch, pyruvate decarboxylase (PDC), alcohol dehydrogenase (ADH), and soluble sugar. At Heihe (sporadic permafrost zone), PC1 and PC2 together explained 43.9% of the variance (Figure 4b), and the indicators with higher contribution rates were proline, PDC, aldehyde decarboxylase (ALDH), and gibberellic acid (GA). At Mohe (discontinuous permafrost zone), PC1 and PC2 explained 41.4% of the variance (Figure 4c), and the indexes with high contribution rates were starch, proline, ADH, and ALDH.

3.3. Changes in Plant Root Signaling Substances with Labeling Time in Different Permafrost Regions

The mixed effects of the main indexes affecting plant growth in different frozen soil areas were compared and analyzed (Figure 5). The starch content of V. uliginosum varied significantly with labeling time in Yichun (isolated patches of the permafrost zone) (p < 0.05). At 120 h, the starch content of V. uliginosum was the lowest and the starch content of the other three plants was the highest. The soluble sugar content of L. gemlini was higher in Yichun, and the difference with changes in labeling time was significant (p < 0.05). The PDC and ADH contents of V. uliginosum were low, and the difference was significant with the labeling time in Yichun (p < 0.05). The ADH of L. gemlini varied significantly with labeling time (p < 0.05), and the highest ADH content was found in Yichun at 120 h (Figure 5a). Proline and PDC first increased and then decreased with marking time, and ALDH decreased with marking time in Heihe (sporadic permafrost zone). The GA content of L. gemlini, V. uliginosum, and C. schumidtii significantly changed in the Heihe River with labeling time (p < 0.05) (Figure 5b). The starch content of L. gemlini, V. uliginosum, and C. schumidtii changed significantly with the labeling time (p < 0.05), and the lowest content of C. schumidtii was found in Mohe at 24 h. The proline content of V. uliginosum changed significantly with marking time (p < 0.05), and the highest content was found in Mohe (discontinuous permafrost zone) at 120 h. The ADH and ALDH of the four plants decreased with marking time, and the ALDH of L. gemlini and V. uliginosum significantly changed with marking time in Mohe (p < 0.05) (Figure 5c).

3.4. Root Signaling Substances Regulate Carbon Allocation in Plant–Soil Systems under Permafrost Degradation

Climate change has indirect effects on plant–soil carbon allocation by influencing soil’s physical properties in Yichun (isolated patches of the permafrost zone) (Figure 6a and Figure 7a). MAP and SWC affected carbon allocation in roots and stems through PDC, and SWC affected the carbon allocation of roots and soil through ADH. MAP affected the carbon allocation of leaf stems and roots through starch, and ALT affected the carbon allocation of roots and soil through SS in Yichun. Climate change had a significant impact on soil’s physical properties and indirectly affected plant–soil carbon allocation by affecting plant root signaling substances in Heihe (sporadic permafrost zone) (Figure 6b and Figure 7b). MAP affected stem and root carbon allocation through PDC, and SWC affected root carbon allocation through ALDH. MAT affected root carbon allocation through soluble sugar, and proline affected leaf and stem carbon allocation through Heihe. Root signaling substances significantly affected plant–soil carbon allocation in Mohe (discontinuous permafrost zone) through the indirect effects of climate change on soil’s physical and chemical properties (Figure 6c and Figure 7c). SOC and SWC affected root and soil carbon allocation through ADH, C:N and ALT affected leaf carbon allocation through starch. Proline affected stem and root carbon allocation, and ALDH affected root and soil carbon allocation in Mohe.

4. Discussion

4.1. ¹³C Allocation for Plant–Soil Systems in Peatlands in Permafrost Regions

The allocation of carbon to photosynthates in the plant–soil system after the photosynthesis of peatland plants in the permafrost area is the key to the influence on the size of the wetland carbon sink. A study of the seasonal carbon allocation of L. gemlini in Siberian permafrost suggests that carbon is stored primarily from the end of June until the end of the growing season [37]. In a study of grassland plant soil carbon allocation under grazing on the Qinghai–Tibet Plateau, the greater the underground carbon allocation of grassland plants, the lower the CO₂ outflow from soil organic matter, and moderate grazing had a positive impact on soil carbon sequestration [38]. The peatlands in the Greater and Lesser Xing’an Mountain permafrost areas are greatly affected by the degradation of the permafrost under climate warming, which changes the physical and chemical properties of the peatland soil and therefore affects the growth of plants [10]. L. gemlini, as the main tree species, is widely distributed in the permafrost areas of the Xing’an Mountains. Compared to the other three species, ¹³C fluctuated more with the marking time, which may be related to the carbon cycle of the plant rhizosphere [37]. The ¹³C allocation of L. gemlini leaves was the highest in the discontinuous permafrost area and the lowest in the sporadic permafrost area, indicating that the ¹³C that was fixed in leaves after photosynthesis was affected by permafrost degradation. Compared to the island permafrost area, the L. gemlini has an increasing trend of ¹³C allocated to the root and soil with marking time in the discontinuous permafrost area, indicating that the trend of L. gemlini ¹³C allocated to the underground area is more stable in the continuous permafrost area. The ¹³C of V. uliginosum root and soil increased with the marking time in the three permafrost regions, indicating that the flooding state had little effect on ¹³C accumulation. The distribution of ¹³C from C. schumidtii and Sphagnum leaves was the highest during sporadic permafrost at 360 h and the highest during discontinuous permafrost at 120 h, indicating that the distribution of ¹³C to the ground was slower during sporadic permafrost and faster during discontinuous permafrost. The ¹³C in root and soil also showed fluctuations with marking time, which was probably related to the waterlogged state of the plants.

4.2. Effects of Permafrost Degradation on Plant Root Signaling Substances

In response to the stress brought about by environmental changes, plants usually use root signaling substances (hormones, antioxidant systems, anaerobic metabolites, and osmotic regulatory substances) to regulate nutrients and water in the plant–soil system [27]. The degradation of permafrost causes changes in the growth environment of plants in peatland [17], which can result in plants being in a state of stress. The main root signaling substances regulating plant growth in the peatlands of the three permafrost areas were nonstructural carbohydrates, osmotic regulatory substances, hormones, and anaerobic metabolites, indicating that plants’ root signaling substances started regulating growth under the degradation of permafrost. L. gemlini, C. schumidtii, and Sphagnum showed a decrease in ST content, an increase in PDC content, and a decrease in ADH content at the beginning of labeling, indicating that plants consume a small amount of starch and glucose through the glycolytic and anaerobic metabolism to ensure growth in isolated patches of the permafrost regions under water flooding stress. This may explain the slower distribution of ¹³C underground. Consistent with the results of ¹³C allocation with labeling time, V. uliginosum ¹³C accumulation was less affected by flooding in sporadic permafrost regions. The root signaling substances of V. uliginosum and L. gemlini varied significantly with the labeling time, indicating that tree roots in sporadic permafrost areas were more susceptible to environmental changes than other plants. The L. gemlini that grows in the permafrost is threatened by the loss of permafrost in isolated permafrost areas, which will have an impact on the carbon cycle of permafrost ecosystems [39]. Plants in the sporadic permafrost region use hormones to regulate plant growth after waterlogging. Ethylene-mediated GA drives starch and soluble sugar catabolism in plants, and the Pro content increases at 2–120 h to remove reactive oxygen species and maintain osmotic pressure. The results indicated that the growth of plants in the sporadic permafrost area was affected by transient flooding stress. The ST content of C. schumidtii in the discontinuous permafrost area decreased from 2 to 24 h, while that of other plants decreased after 120 h, indicating that C. schumidtii first reacted to stress and the Pro content increased at 120 h to remove ROS. Sphagnum’s ADH content decreased significantly at 2–24 h to resist hypoxia and quickly adapted to environmental changes. This is consistent with the fact that C. schumidtii and Sphagnum distributed ¹³C underground more rapidly in discontinuous permafrost regions, and they showed different defensive strategies during flooding stress.

4.3. Carbon Allocation in Plant–Soil System Affects Carbon Cycle under Permafrost Degradation

Against the background of global warming, permafrost degradation appears [40,41], which will lead to changes in soil’s physical and chemical properties and affect carbon allocation in the plant–soil system [42], thus affecting carbon cycling in permafrost regions [43,44]. Permafrost degradation manifests itself in the form of thermal collapse [14] and thickening of the active layer [10]. The study of the composition and productivity of plant species after permafrost thawing in the Alaskan tundra shows that the tundra ecosystem is changing to one dominated by shrubs and mosses [45]. Permafrost degradation releases more carbon dioxide than is absorbed by primary productivity and can lead to changes in the structure of the community, which in turn affects the hydrothermal processes in the permafrost and active layers [46]. A study of plant carbon allocation and soil organic matter turnover in Canadian permafrost found that recently synthesized photosynthates drive the mineralization of older SOC under tundra vegetation [14]. Located at the southern edge of the Eurasiatic permafrost, the Xing’an Mountains have three types of permafrost and rich vegetation types. Therefore, permafrost degradation is manifested by differences in latitude and the deepening of the thickness of the active layer [46]. The thickness of the active layer is the main factor affecting the distribution and sum of photosynthetic ¹³C. The distribution of ¹³C to the ground is mainly because the discontinuous permafrost area is larger than the island permafrost area [47]. SOC, C:N, ALT, and SWC regulate the allocation of plant carbon by influencing anaerobic metabolites and ST in the discontinuous permafrost region, indicating that root signaling substances in the continuous permafrost region are easily affected by soil’s physical and chemical properties (Figure 7a), which is consistent with the study on the Qinghai–Tibet Plateau [48]. In sporadic permafrost regions, MAP, MAT, and SWC affect plant carbon allocation by influencing anaerobic metabolites and SS, and are regulated by hormones and osmoregulatory substances when resisting anoxia (Figure 7b). These results indicate that the allocation of carbon from the plant soil in the sporadic permafrost region is susceptible to climate change and transient flooding stress. MAP and SWC affect plant–soil carbon allocation by influencing anaerobic metabolites, and MAP and ALT affect carbon allocation by regulating nonstructural carbohydrates in sporadic island permafrost regions (Figure 7c). Therefore, the water content and the thickness of the active layer are the key factors affecting carbon allocation, which is consistent with the findings of the study of the Arctic carbon cycle [49]. Plant–soil carbon allocation in isolated patches of permafrost regions is more susceptible to the indirect and direct effects of many factors. The physical properties of the soil are affected by climate change in sporadic permafrost regions, and the allocation of carbon is regulated by root signaling substances. Carbon allocation in the discontinuous permafrost areas is mainly regulated by root signaling substances that are affected by soil’s physical and chemical properties under climate change. Compared to the sporadic permafrost regions, the carbon allocation in discontinuous and isolated patches of the permafrost regions is more affected by the regulation of root signaling substances. Therefore, the mechanism of the regulation of carbon allocation between plants and soils is affected by permafrost degradation.

5. Conclusions

The carbon allocation mechanism of plant–soil systems in different permafrost regions is affected by permafrost degradation. In isolated patches of permafrost regions, plants consume a small amount of starch and glucose to ensure growth through glycolysis and anaerobic metabolism during flood stress. Tree roots are more susceptible to environmental changes than other plants. MAP and SWC affect plant–soil carbon allocation by influencing anaerobic metabolites, and MAP and ALT affect carbon allocation by regulating nonstructural carbohydrates. Plants use hormones to regulate plant growth after flooding. MAP, MAT, and SWC affect plant carbon allocation by influencing anaerobic metabolites and SS, and regulate this via hormones and osmoregulatory substances when resisting anoxia in the sporadic permafrost region. SOC, C:N, ALT, and SWC regulate plant–soil carbon allocation by influencing anaerobic metabolites and ST. Root signaling substances are easily affected by the physical and chemical properties of soil in discontinuous permafrost regions.