Introduction

Terrestrial carbon pools in forests are important natural sinks for sequestering atmospheric CO2. Climate change will significantly impact on the forest carbon cycle (Frank et al. 2015). Seasonal freeze–thaw is an important ecological factor affecting forest ecosystem processes at mid- and high-latitudes and altitudes can affect the carbon cycle by influencing the structure and function of terrestrial ecosystems. These cold areas of boreal terrestrial ecosystems are critical to the global carbon cycle, where low temperatures slow the decomposition of carbon (Wei et al. 2004; Wang et al. 2007; Yang et al. 2007). Average yearly temperatures are rising across the planet, especially at mid- and high latitudes (Pithan and Mauritsen 2014), and winter temperatures are also expected to increase (Xia et al. 2014). The most significant warming is projected at mid- and high latitudes and during winter. It is critical to understand the effect of ongoing warming on terrestrial ecosystems in seasonal freeze–thaw zones where soil organic carbon (SOC) in forests is particularly sensitive to climate change (McGuire et al. 2009). SOC accumulated in forests is vulnerable to climate change and may disintegrate quickly, particularly in the litter and soil layers (Dutta et al. 2006). Litter is a critical component of forest ecosystems and is vital link in the nutrient cycle (Wu et al. 2000). The rate of decomposition and conversion of litter reflects the rate of nutrient cycling (Su et al. 2007) and is important for the forming soil organic matter and nutrient release (Wang and Huang 2001; Wen et al. 2014). Intrinsic factors refer to the physical and chemical features of the disintegrating material, and extrinsic factors to the external environment that influence decomposition and encompasses both biotic and abiotic components. The type, number, and activity of heterotrophic microorganisms and soil animal groups engaged in decomposition are biotic factors. Climate, soil, and atmospheric composition are examples of abiotic factors (Peng and Liu 2002). Freezing is an abiotic factor that leads to the physical and degree of fragmentation of litter in forest areas with a seasonal freeze–thaw cycle, and speeds up litter decomposition and affects litter organic carbon (LOC) and SOC. Over thousands of years, carbon in the soil beneath the litter has been buried deeper in the permafrost layer, resulting in a massive stock (Koven et al. 2011). This carbon is not active in the carbon cycle, but if the permafrost thaws, it might be released into the atmosphere, influencing climate change. Therefore, the issue of climate change causing changes in carbon dynamics in seasonal freeze–thaw areas must be determined.

Thinning is a well-known management technique for reducing competition and increasing individual tree and stand growth (Curtis 1971). The goal of thinning is to reduce stand density and increase the amount and quality of light penetration, allowing for more rational nutrient buildup and utilization in the understory (Pukkala et al. 2020; Pan et al. 2022). Thinning alters the microclimate, vegetation conditions, and the physical and chemical qualities of the soil, which can lead to changes in soil microorganisms (Bai et al. 2018; Zhang et al. 2022). The effect of thinning on the relationship between litter input and the rate of decomposition determines the change in carbon density of the litter layer (Wu et al. 2016). Thinning alter stand structure, litter yield, soil temperatures, moisture, and SOC (Zhou et al. 2019).

The interconnected processes by which the seasonal freeze–thaw cycle drives ecological and physiological processes and alters the soil carbon balance are poorly understood. In this study, thinning was used to investigate the response: (1) by the forest ecosystem in areas of seasonal freeze–thaw cycles under climate change; (2) to the rate of forest litter decomposition and litter organic carbon; and, (3) to SOC storage at different decomposition levels (undecomposed and semi-decomposed) and factors affecting organic carbon dynamics. The material and energy cycles between litter and soil in the forest ecosystem were investigated and the optimum thinning intensity determined that improved carbon storage capacity of the litter and soil in seasonal freeze–thaw areas.

Materials and methods

Study area

The study location was in Dongfanghong Forestry Field, Dailing Forestry Experimental Bureau, Xiaoxinganling District, Heilongjiang Province, 13.9 km southeast of Dailing District (46°50′8″–47°21′32″ N, 128°37′46″–129°17′50″ E). The site has an average slope of 10°, elevation of 504 m a.s.l., and average annual precipitation of 661 mm. The minimum (− 35 °C) and maximum temperatures (33 °C) occur in January and July, respectively, with an average annual temperature of 2.7 °C. The climate is temperate continental monsoon, with cool and rainy, humid summers, and dry, cold winters that are windy and snowy. The soil is primarily dark brown loam, with a small amount of valley meadow and swampy soil and has an average thickness of 40 cm. Mean stand age was 70 years, and densities were greater than 0.8 (Table 1).

Table 1 Information on sample plots for each thinning intensity
Full size table

Experimental procedures

One uncultivated control site (CK) and six cultivated sites with 10% (A), 15% (B), 20% (C), 25% (D), 30% (E), and 35% (F) thinning intensities were randomly set up in the Xiaoxinganling area. Three 30 × 30 m subplots were randomly set up in each sample site. Litter was collected from the undecomposed layer (fresh litter with slight color change, structural integrity, and no signs of decomposition) and a semi-decomposed layer (black color, structure destroyed, and most litter decomposed). To collect soil samples, the surface was cleaned and samples collected from the upper 10 cm layer with 20 cm length and width specifications.

Experiment boxes were established and 50 g of soil placed in each box after sampling was completed and returned to the laboratory. In each sample plot, three replicate experiment boxes were set up for each temperature cycle and litter placed on the soil surface in the boxes. To eliminate the effect of litter quality on organic carbon, 2.00, 3.00 and 4.00 g of litter were placed in the three replicate boxes and then put the replicates in a constant temperature incubator. The water content of the soil was measured.

Differences for temperature cycling were based on preliminary observation and temperature dynamic characteristics (Fig. 1). The study was carried out over four seasons, with the average temperature of each period chosen as representative. The first period was the unfrozen season, i.e., summer (July), and temperatures were 17 °C and 30 °C. The second period was the freeze–thaw season (October), and temperatures were between − 5 and 5 °C. The third season was the frozen season (January) with temperatures between − 30 and − 16 °C. The last season, the fourth, was the thaw period (March) with temperatures between 5 and − 5 °C. The two temperatures extremes alternated every 12 h for the incubation of the experiment boxes. The boxes were removed at the end of beginning, and the 1st, 3rd, 5th, 7th, 15th, and 30th temperature cycle in each season. The experiment boxes were set up separately for each temperature cycle to prevent the structure of the samples from being damaged; 1176 experiment boxes were set up in the four seasons (4 seasons × 7 thinning intensity plots × 2 types of litter × 7 temperature cycles × 3 replicates). The boxes were removed after the temperature cycles had been completed, the soil in the boxes air-dried and crushed, passed through a 0.15 mm sieve, and the various levels of litter were removed from the boxes. Table 2 shows the initial composition of the samples.

Fig. 1
figure 1

Temperature monitoring 1 year from July 2020 to 2021 before the study

Full size image
Table 2 Composition of dead litter and initial soil fractions at different decomposition
Full size table

Data calculation

Residual rate of litter mass:

$$Q\left( \% \right) = \frac{{M_{t} }}{{M_{0} }} \times 100\%$$
(1)

Mass loss of litter at each stage:

$$L\left( {\text{\%}} \right) = \frac{{M_{t - 1} - M}}{M}{ } \times 100{\text{\% }}$$
(2)

Rate of mass loss of litter at each stage (time):

$$R_{T} = \frac{{\left( {M_{t - 1} - { }M_{t} } \right)}}{\Delta T}$$
(3)

Olson decomposition index decay model for litter:

$$Y = { }e^{ - kt}$$
(4)

where M0 is the initial mass of the litter; Mt is the residual amount of litter at each sampling time; (Mt−1 − Mt) is the difference between the residual amounts of the two adjacent litter collections after drying and weighing (Note: Mt−1 is the Mt measured in the previous period); ΔT is the interval between the two adjacent sampling times; Y is the residual rate of litter, k is the litter decomposition coefficient (Olson 1963), and t is the number of decomposition time.

Analytical statistics

IBM SPSS Statistics 26 (IBM, Armonk, NY, USA) was used to analyze the data and Origin 2022 (OriginLab, Northampton, MA, USA) to plot the results. Canoco 5 (Biometris, Plant Research International, Wageningen, Netherlands) determined redundancy analysis (RDA), and non-linear regression analysis to fit the litter decomposition curves. Repeated measurement ANOVA tested the significant differences in litter mass residual rate and SOC under the influence of season, thinning intensity, litter decomposition, and temperatures (a = 0.05).

Results

Environmental factors on the decomposition of litter, LOC and SOC

According to redundancy analysis (RDA), the four environmental factors, season, time, thinning, and degree of decomposition, accounted for 99.8% of the overall variance. The first and second axes explained 84.0 and 15.8% of the total variance Season (F = 345, P = 0.002), number of temperature cycles (F = 160, P = 0.002), and degree of litter decomposition (F = 18, P = 0.002) were significant influences on the mass residual rate of litter, LOC, and SOC. Seasons had a positive effect, whereas the degree of litter decomposition and the number of temperature cycles had a negative effect. Season had a significant positive correlation with LOC, whereas the degree of litter decomposition was negative. For soil, the more temperature cycles, the higher the SOC content (Fig. 2).

Fig. 2
figure 2

Redundancy analysis (RDA) to assess the impact of environmental parameters (red arrows) on litter mass residue, litter organic carbon (LOC), and soil organic carbon (SOC; black arrows)

Full size image

The response by the forest ecosystem to environmental changes is complicated. Therefore, the interactions of various environmental factors on the variables being assessed were investigated. The effects on litter mass residual rate and LOC and SOC were determined by repeated measures analysis of variance. The four environmental factors, as well as their two-by-two interactions, had significant effects on litter mass residue and LOC (Table 3). The number of temperature cycles, season, and their interactions also had significant effects on SOC and thinning intensity, as well as their interaction with the number of temperature cycles.

Table 3 Repeated measurement ANOVA of environmental factors on residual litter mass, LOC, and SOC
Full size table

Influence of environmental factors on the decomposition of litter

The fluctuation ranges of k values for the unfrozen, freeze–thaw, frozen, and thaw seasons were 0.007–0.047, 0.003–0.007, 0.005–0.008, and 0.003–0.005, respectively (Table 4). The undecomposed and semi-decomposed litter had k values of 0.003–0.013 and 0.003–0.047, respectively. In the thaw season, litter decomposition rate was significant at a 10% thinning intensity. In the unfrozen and frozen seasons, undecomposed and semi-decomposed litter had a k maximum under 35% and 20% thinning intensities, respectively. During the freeze–thaw season, undecomposed litter degraded at the highest rate under 10% thinning intensity. In the 15–25% thinning intensity, semi-decomposable litter is easier to decompose.

Table 4 Decomposition, semi-decomposition, and 95% decomposition coefficients as a function of seasonal changes of litter at two degrees of decomposition under various thinning intensities
Full size table

The season, number of temperature cycles, degree of decomposition, and thinning intensity were all important factors impacting the mass residue rate of litter after redundancy analysis, and the specific effects of these environmental factors were investigated independently. After 30 temperature cycles, the residual rate of litter in the unfrozen season was significantly lower than that of the other three seasons for each thinning intensity (Fig. 3). The residual rate of litter in the thaw season was the highest overall, and the final residual rates of litter in the freeze–thaw and frozen seasons were not significantly different (P > 0.05). The unfrozen season’s quality residual rate of semi-decomposed litter was lower than the undecomposed litter. The mass residue rate of undecomposed litter was significantly lower than the initial level after 30 temperature cycles in the unfrozen, freeze–thaw, and frozen seasons. Only the mass residue rate after temperature cycles in 10% thinning was significantly lower than the initial value in the thaw season. However, according to SPSS pairwise comparisons, the mass loss in all other thinning intensities was not significant. For semi-decomposed litter, the litter mass residual rate after 30 temperature cycles in the unfrozen and frozen seasons was significantly lower than the initial value. Under a 20–35% thinning intensity, the litter mass residual rate after 30 temperature cycles in the freeze–thaw season was significantly lower than the initial value, and the litter mass loss under zero-10% thinning was insignificant. After 30 temperature cycles under 10% and 20% thinning intensities in the thawing season, the litter mass residual rate was significantly lower than the initial value, while the litter mass loss was insignificant under zero, 15%, and 25–35% thinning intensities. In the unfrozen season, the level of litter decomposition is an essential element impacting decomposition. At 10% thinning intensity, the difference in residual litter mass between different decomposition levels after the 5th temperature cycle was significant, and the difference in residual litter mass between different decomposition levels at the end of the 1st and 3rd temperature cycles was significant at 15% thinning intensity but not in the later temperature cycle. The starting decomposition levels of 20% and 30% had a substantial impact on the mass residue rate throughout the temperature cycle.

Fig. 3
figure 3

Analysis of environmental influences on the residual rate of litter quality using non-linear regression. Thinning intensity: A (10%), B (15%), C (20%), D (25%), E (30%), F (35%), and CK (control, 0%)

Full size image

Impact of environmental factors on LOC

The main factors affecting LOC were the degree of decomposition and season. The carbon content of litter fluctuated under every thinning intensity during the temperature cycles. However, an increase in litter carbon levels after the 30 temperature cycles in the unfrozen and freeze–thaw seasons, and a decrease in the frozen and thaw seasons was recorded (Fig. 4c). During the unfrozen season, the carbon content of undecomposed litter was higher than that of semi-decomposed litter at the end of cultivation in all stands except those with thinning intensities of 25–30% (Fig. 4a). The carbon content of semi-decomposed litter was higher than that of undecomposed litter at the end of the temperature cycles under 25–35% thinning intensity during the freeze–thaw season. During the frozen season, the carbon content of semi-decomposed litter was higher than that of undecomposed litter under 10–25% thinning intensity. During the thaw season, the carbon content of undecomposed litter was higher than that of semi-decomposed litter under 10%, 20%, 25%, and 35% thinning. The carbon content of litter increased after cultivation in the unfrozen season and decreased in the frozen and thaw seasons in controls (zero thinning). The carbon content of undecomposed litter decreased after cultivation in the freeze–thaw season and increased after cultivation in the semi-decomposed litter (Fig. 4b). After the temperature cycles, the level of carbon in undecomposed litter was highest in the thaw season > freeze–thaw season > frozen season > unfrozen season, and the C content of semi-decomposed litter changed from frozen season > thaw season > freeze–thaw season > unfrozen season to frozen season > thaw season > freeze–thaw season > unfrozen season. Compared to the controls, carbon levels of both decomposition levels under 25–30% thinning intensity in the unfrozen season was similar to the other three seasons. However, the carbon content of the unfrozen season in the other thinning intensities was significantly lower than that of the other three seasons. Under the 25–30% thinning intensity in the unfrozen season, the carbon content of litter decreased and then increased slightly after the freeze–thaw season, and the carbon content of litter decreased after 30 temperature cycles in frozen and thaw seasons.

Fig. 4
figure 4

Effect of environmental factors on LOC. Sample thinning intensity: A (10%), B (15%), C (20%), D (25%), E (30%), F (35%), CK (zero); a effect of thinning intensity on organic carbon of two litter species in different seasons; b organic carbon content of litter with different decomposition levels in different seasons and thinning intensities; c effect of the number of temperature cycles on sample plots with different nurturing thinning intensities in different seasons. Data represent mean ± standard deviation

Full size image

Effects of the environment on SOC

Season and the number of temperature cycles were the most important factors influencing SOC. SOC in the thawing season was significantly lower than that in the other three seasons (Fig. 5a). SOC in the sample plot with a 35% thinning intensity was significantly higher than that of the sample plot with 25% thinning intensity (Fig. 5b). In Fig. 5c, after 30 temperature cycles, the season with the highest soil organic carbon under undecomposed litter shifted back, with increasing thinning from zero to 20%. SOC in the frozen season was higher than the other three seasons at 20% thinning intensity; at 20–30% thinning intensity, the peak of SOC under undecomposed litter in the four seasons gradually returned to the previous season. SOC in the unfrozen season was higher than in the other three seasons under 30% thinning intensity, and in the frozen season was higher than that of the other three seasons at 35% thinning intensity. The trend of SOC in the semi-decomposed litter was the same as in the undecomposed litter in the four seasons, but the rate of change was slower.

Fig. 5
figure 5

Effect of environmental factors on SOC. Thinning intensity: A (10%), B (15%), C (20%), D (25%), E (30%), F (35%), and CK (zero%); a the effect of seasonal variation on soil organic carbon under different thinning intensities; b the effect of thinning intensity on soil organic carbon, different lowercase letters indicate significant difference; c the effect of temperature cycling on soil organic carbon under two types of litter under different thinning intensities in different seasons

Full size image

At zero thinning, the freeze–thaw season had the lowest soil carbon content of the four seasons, and the SOC was the lowest in the season before the timeline as thinning increased. SOC content in the unfrozen season was the lowest when the thinning intensity was 25%. Soil carbon in the freezing-thaw season was the lowest among the four seasons when thinning intensity was 35%. In the unfrozen season, the number of temperature cycles at 25%–35% thinning had a significant effect on SOC under undecomposed litter (P < 0.05), while the number of temperature cycles at 20%–30% thinning had a significant effect on SOC under semi-decomposed litter (P < 0.05). The SOC in the 1st, 5th, 15th, and 30th temperature cycles at 30% thinning and the 15th and 30th temperature cycles at 35% thinning were significantly lower than the initial content. At 35% thinning during the frozen season, SOC was significantly lower than the initial content (52.1 g·kg−1) in the 7th and 30th temperature cycles.

Discussion

Effect of environmental factors on the decomposition of litter

Studying the change in forest litter quality during seasonal freeze–thaw cycles has considerable ecological importance and reveals the decomposition mechanism of forest litter and ecosystem processes in these areas (Deng et al. 2010). Forests serve as a source, reservoir, and sink of carbon in the global carbon cycle, and any change in these carbon stocks impact atmospheric CO2 levels (Hopmans 2004). By altering the number of crowns in the upper canopy, thinning enhances the understory environment and indirectly stimulates the breakdown of understory litter. Olson’s decomposition coefficient k is a commonly used measure to describe the pace at which litter decomposes (Olson 1963). The decomposition rate of litter in the unfrozen season was higher than in the other three seasons (Fig. 3). As litter on the forest surface at high latitudes and high altitudes is subjected to freeze–thawing for an extended period, it is surprising that the rate of decomposition of litter in the frozen season was higher than that in the freeze–thaw and thaw seasons. Freeze-thawing influenced the physical structure of the litter and the leaching of biological elements through mechanical fragmentation (Sulkava and Huhta 2003; Withington and Sanford 2007). Moreover, semi-decomposed litter decomposes faster than undecomposed litter, implying that litter with evidence of fragmentation decomposes faster. This could be because semi-decomposed litter has a larger contact surface, or because it contains microbes or enzymes that accelerate litter decomposition that are absent in undecomposed litter (Scheu and Wolters 1991). The decomposition of litter is influenced by both inherent and external variables. Changes in forest habitat are among the elements that influence litter decomposition. Further, changes in the intensity of rainfall leaching on the litter, as well as light, temperature, and concentration of litter directly impact the decomposition rate of litter after thinning. The relationship between stand density and understory litter decomposition was noted as the extent to which the understory environment improved. The higher the thinning intensity during the unfrozen and frozen seasons in seasonal freeze–thaw areas, the faster the decomposition rate of undecomposed litter. This may be due to thinning intensity, light levels, and temperature changes. Thinning intensity of 20% is suitable for decomposition of semi-decomposed litter. In the freeze–thaw season, 20% is also the most suitable thinning intensity for decomposition of intact litter; however, 10% is the most suitable thinning intensity for decomposition of litter in the thaw season. This could be because the litter and soil are frozen after the frozen season, and a denser stand can maintain the intra-forest temperature, allowing litter decomposition to continue; however, the final decomposition degree in the thaw season was still the same. It may be concluded that, after 30 temperature cycles, the mass residue of litter in all seasons was much lower than the initial level at 10% thinning intensity. In the unfrozen season, the initial decomposition degree had the greatest impact on litter decomposition, and the number of temperature cycles required for a significant difference between litter and initial mass residue rate rose as thinning intensity increased.

Effect of environmental factors on LOC

In cold climates, mass loss and nutrient release from forest litter play a substantial role in the carbon and nutrient cycles (Wu et al. 2010). The most important abiotic factor influencing the decomposition of litter is the weather (Vitousek et al. 1994). The structure, function, dynamics, and distribution of forest vegetation in seasonal freeze–thaw zones are all affected by rising global temperatures, and these changes have an impact on forest carbon sequestration (Weng and Zhou 2006; Hui et al. 2017; Guo et al. 2021). Litter accumulation and breakdown processes are closely linked to temperature regulation in the carbon cycle (DeLucia et al. 1999; Park and Cho 2003; Yang et al. 2007). In Fig. 4b, litter acts as a carbon sink in the unfrozen and freeze–thaw seasons, fixing carbon in the environment, and as a carbon source in the frozen and thaw seasons, releasing carbon into the environment. Moreover, the degree of litter decomposition has an impact on LOC (Fig. 4a and b). The carbon content gradually increases during the unfrozen season, carbon elements are continuously enriched into the litter, and the undecomposed litter layer changes from low to high compared with the semi-decomposed layer at the end of the temperature cycles. This indicates that the litter has a carbon-fixing effect during the unfrozen season, and that the undecomposed litter has a greater carbon fixing ability. By the end of the freeze–thaw season, the litter has the greatest carbon content, and the carbon from the undecomposed litter layer has moved into the semi-decomposed layer. The carbon content of the litter gradually decreased during the frozen season, and that of the undecomposed litter was significantly lower than that of the semi-decomposed litter. At this point, the carbon content transferred to the semi-decomposed litter and was gradually released into the soil. The carbon content of the undecomposed litter in the thaw season was higher than that of the semi-decomposed litter, at which time the carbon in the semi-decomposed litter might have been released to the soil. Thinning can promote the decomposition of understory litter directly or indirectly by enhancing the diversity of understory organisms and improving the understory habitat. Microbial fixation can affect the dynamics of LOC and can result in high concentrations of LOC or even an increase in absolute levels (Gessner 2001; Li 2022). On the other hand, microbial-based decomposition is complicated, and microorganisms may transport some elements to the litter during decomposition to maintain their concentration balance, resulting in a net sequestration of elements and a slow increase or decrease in litter quality (Li et al. 2007; Nonini et al. 2021). Owing to the high density of trees, inadequate ventilation, and limited light penetration in the forest, the decomposition rate in the control and light thinning (10–15%) intervals were lower than in the medium (20–25%) and heavy (30–35%) thinning intensities. Moderate thinning ensures the variety and quantity of tall trees is maintained and also ensures the normal growth and development of the undergrowth, including herbs. Moreover, sunlight, water, and temperatures are more suitable for microbial activities, stable decomposition, transformation, and accumulation of litter. In the moderate thinning area, there are fewer trees, sufficient light, good ventilation, and the highest rate of decomposition. However, the pace of decomposition and transformation is too rapid for plants to absorb and use the nutrients effectively. This could result in nutrient loss during the rainy season. Soil nutrients, light, moisture and temperatures in the medium thinning intensity are all favorable for the accumulation of carbon. Litter is an important part of the ecosystem and examining the volume and nutritional dynamics of litter is an important part of studying the ecosystem’s material cycle and energy flow (Witkamp 1963; Maguire 1994). Some carbon in the decomposed litter is released directly into the atmosphere while the rest penetrates the soil and becomes part of the soil carbon or is released indirectly into the atmosphere through soil respiration (Wang et al. 2022; Di et al. 2022). The rate of litter decomposition and conversion indicates the pace of nutrient return and cycling (Su et al. 2007). Nutrient release during litter decomposition is controlled by biotic and abiotic processes and includes leaching-enrichment-release, enrichment-release, and direct release (Zhang 2006). Plant communities absorb nutrients from the soil and are subsequently eaten by herbivores. The nutrients are cycled back into the soil through animal excrement and the eventual death of the animals, or directly from the plants when they drop leaves. The link between plants and soil in nutrient cycling is litter, which serves as the basic carrier of nutrients. As a result, litter is crucial in preserving soil strength and encouraging the regular material cycle and nutrient balance in forest ecosystems (Lin et al. 2004).

Effect of environmental factors on SOC

Heilongjiang Province has had the strongest meteorological warming trend in China since the twentieth century, with an overall increase of 1.1 °C. As a result, the active permafrost layer has deepened, has melted or completely degraded, and the permafrost zone has shifted northward (Jin et al. 2007). The rate of SOC decomposition is affected by changes in temperature because the physical protection of organic matter is reduced and erosion increases (Lal 2005). Northern ecosystems are predicted to face the largest increase in ambient temperature as a result of climate change. Warming will increase the depth of the active permafrost layer, releasing carbon frozen in the upper layer and increasing the quantity of SOC that can generate greenhouse gas emissions (Ping et al. 1997). Forest SOC stores are affected by seasonal and temperature cycles. There is concern that rising global temperatures could lead to long-term depletion of SOC reserves (Jenkinson et al. 1991); other researchers believe that significant warming at high latitudes may make tundra and boreal forests a net source of carbon (Wang and Polglase 1995; Ping et al. 1997). Melillo et al. (2002) reported the findings of a 10 years warming experiment in mid-latitude broadleaf forests. Soil warming enhanced organic matter breakdown and CO2 release. Carbon sequestration in forest soils can slow the rate of CO2 release. However, global warming accelerates the breakdown of SOC, which releases CO2 and accelerates the warming trend. Even if climate change ceases, CO2 release from permafrost will continue for many years. Because permafrost has high thermal inertia, there will be a temporal lag between when the permafrost warms and when it thaws (Schaefer et al. 2011). Increasing the forest soil carbon pool can be achieved by management, and thinning is the most common management activity. According to Bolat (2014), thinning can raise soil temperatures and microbial biomass by reducing stand density to allow more sunlight to reach the soil. Logging at a suitable intensity boosts soil microbial biomass, improves litter decomposition, and enhances fertility (Zhang et al. 2016; Yang et al. 2017). Carbon and nitrogen contents of broadleaf forests were reduced dramatically as a result of thinning (Johnson and Curtis 2001). In Fig. 5a, it shows that low temperatures slow the release of carbon, and therefore, SOC was at a maximum in the frozen season at a moderate thinning intensity of 20%. In the unfrozen season and with a low thinning intensity (0–15%), stands have a dense canopy, low light transmission and intensity, whereas, with a high thinning intensity (25–35%), stands are sparse and have abundant light. Differences in temperatures due to light intensities were the most influential factor in affecting carbon content. Photosynthesis makes plants efficient at carbon fixation, and litter releases some carbon into the soil. Therefore, soil carbon is highest during the unfrozen season. Warming has two primary effects on soil organic carbon. First, it influences plant growth, resulting in a change in the amount of plant waste returned to the soil. Second, it changes the rate of organic carbon decomposition, resulting in a change in the amount released from the soil (Jenkinson et al. 1991). The physical breakdown of litter increases as the number of temperature cycles increases. This promotes litter decomposition and is an important component of the carbon pool of forest ecosystems. Litter decomposition plays a critical role in the formation of soil organic matter and in the forest carbon cycle (Yang et al. 2007; Chen et al. 2021). The litter layer is closely linked to the soil layer, and soil animals and microorganisms use surface litter as a food source and shelter, speeding up litter degradation through chewing and biological metabolism. Litter improves soil physicochemical structure through ecological effects, allowing for the replenishment of effective nutrients and the maintenance of fertility. Semi-decomposed litter has a less evident effect on SOC than undecomposed litter (Fig. 5c). In the unfrozen season, SOC was lower under a 25% thinning intensity than in the other three seasons. In the thaw season, it was lowest under low and high thinning intensities. Under a thinning intensity of 25–30%, the number of temperature cycles had a significant effect on SOC, especially after 30 cycles. In the context of global climate change, a reduced snowpack will increase soil frost in some areas, while in other areas, warming will reduce soil frost intensity, frequency, and duration. Freeze–thaw cycles also affect the conversion of carbon and nitrogen in soils (Matzner and Borken 2008),and understanding the mechanisms and factors underlying organic carbon dynamics in forest soils is important for identifying and enhancing carbon sinks to mitigate climate change.

Conclusion

The unfrozen season had the fastest litter decomposition rate, and the rate of semi-decomposed litter was faster than that of undecomposed litter.

Carbon sequestration of forest litter was good in the unfrozen and freeze–thaw seasons; however, the opposite was true in the frozen and thaw seasons. The effect of undecomposed litter on increasing soil organic carbon was more obvious than that of semi-decomposed litter, indicating that litter integrity affected organic carbon content as well as that of the soil.

Carbon can be sequestered by both litter and soil, and carbon moves from undecomposed litter to semi-decomposed litter and subsequently to the soil. Low temperatures slow the release of carbon from litter and soil to the environment, which allows forests to continue to function as carbon sinks.

Thinning intensity of 20–25% in seasonal freeze–thaw forest areas can effectively slow the decline of organic carbon.