Next Article in Journal
Genome-Wide Identification of CBL Gene Family and RNA-Seq Analysis Under Alkaline Stress in Poplar
Previous Article in Journal
Do Stands Self-Thin Through a Common Point? An Additional Concept for the Self-Thinning Rule
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Forests
Volume 16
Issue 2
10.3390/f16020197
forests-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Effects of Herbaceous Plant Encroachment on the Soil Carbon Pool in the Shrub Tundra of the Changbai Mountains

by
Xiaoyun Xu
1,
Yinghua Jin
1,
Jiawei Xu
1,*,
Yingjie Zhang
2 and
Jiaxing Yang
1
1
Key Laboratory of Geographical Processes and Ecological Security in Changbai Mountains, Ministry of Education, School of Geographical Sciences, Northeast Normal University, No. 5268, Renmin Street, Changchun 130024, China
2
School of Teacher Education, Hebei Normal University, Shijiazhuang 050024, China
*
Author to whom correspondence should be addressed.
Forests 2025, 16(2), 197; https://doi.org/10.3390/f16020197
Submission received: 27 December 2024 / Revised: 18 January 2025 / Accepted: 20 January 2025 / Published: 22 January 2025
(This article belongs to the Topic Plant Invasion)
Browse Figures
Versions Notes

Abstract

:
Under global warming, vegetation composition changes induced by plant encroachment have a significant impact on the carbon balance of tundra ecosystems. The encroachment of herbaceous plants into indigenous shrub communities has changed the aboveground and belowground litter carbon input and the characteristics in the shrub tundra of the Changbai Mountains. However, the impact of variations in litter characteristics and litter carbon input on the dynamics of soil organic carbon (SOC) pool concentrations and SOC stability remains ambiguous. In this study, aboveground and belowground litter and soil samples were collected for lab experiments. Our results showed that the increase in aboveground litter and belowground litter due to Deyeuxia purpurea encroachment increased the SOC concentration. Simultaneously, D. purpurea encroachment decreased the soil C/N by decreasing the components of both aboveground and belowground litter that were resistant to decomposition (C/N and lignin/N) and increased the soil mineralization ability and available N concentrations, increased the CO2 release rate, and ultimately decreased the SOC concentration. D. purpurea encroachment enhanced soil decomposition capacity by increasing the concentration of organic carbon molecular structures, such as carbohydrates, in the aboveground and belowground litter, thereby increasing the concentration of decomposable organic carbon molecular structures and active organic carbon in the soil, while simultaneously reducing the concentration of recalcitrant organic carbon. Even more, D. purpurea encroachment reduced the recalcitrant components of the aboveground and belowground litter enhanced soil mineralization capability and increased soil nitrogen concentration, which collectively increased the carbon oxidation state (COX) and decreased SOC stability. In general, global warming has led to herbaceous plant encroachment, which changes the aboveground and belowground litter carbon inputs and properties in the tundra, in turn reducing the SOC concentration and soil carbon pool stability, enhancing soil carbon emission capacity, and increasing atmospheric CO2 concentration, forming a vicious cycle.

1. Introduction

The soil organic carbon (SOC) pool, containing an estimated 1462–1548 Pg of carbon, surpasses the combined carbon content of both the vegetation and atmospheric pools within terrestrial ecosystems [1]. In alpine environments, the decomposition of organic matter is notably slower due to lower temperatures, resulting in a higher concentration of SOC compared to other ecological zones [2]. Notably, the northern coniferous forests and the Arctic tundra serve as significant carbon sinks, collectively holding nearly one-third of the total terrestrial carbon [3]. Climate change significantly influences both the concentration and stability of SOC, as well as the composition of vegetation. A critical area of investigation within the global carbon cycle pertains to how changes in vegetation, driven by climate warming, affect the SOC pool [4,5,6]. Consequently, international research on carbon dynamics has increasingly concentrated on understanding the variability and primary influences on SOC concentration and composition in cold biomes. This focus is essential for elucidating the impacts of global climate change, thereby providing foundational knowledge for predicting future carbon cycle dynamics.
Plant encroachment can lead to alterations in local vegetation types and diversity, which in turn affect the nature and volume of litter produced. This shift in litter characteristics directly influences soil carbon storage and its dynamics [7]. Litter not only represents the principal input for soil organic carbon (SOC) but also provides essential material and energy for decomposers. Consequently, any modification in litter quality and quantity impacts soil microbial communities and enzyme activities, thereby modulating the decomposition rates of both litter and SOC, which ultimately govern the flux of carbon into and out of the soil system [8]. There exists a dual perspective on the effects of increased litter input on SOC dynamics. On one hand, it is proposed that an increase in litter can elevate SOC concentrations and potentially slow the decomposition of pre-existing SOC, suggesting a stabilization effect [9,10]. Conversely, other studies indicate that augmented litter inputs might initially enhance SOC mineralization, thus reducing SOC concentrations and augmenting carbon emissions [11]. Furthermore, the quality of litter plays a pivotal role in SOC formation. Cotrufo [12] argues that litter rich in easily decomposable compounds can be rapidly assimilated by soil microbes, leading to the production of microbial by-products and metabolites that contribute to stable SOC through aggregation and mineral association. However, high-quality litter can also induce a positive priming effect, accelerating the decomposition of native soil carbon and potentially decreasing overall SOC concentrations due to increased CO2 emissions [13]. This dichotomy underscores the complex interplay between litter quality, microbial activity, and SOC stability, highlighting the need for further research to fully understand these dynamics.
Research concerning soil organic carbon (SOC) stability suggests that an increase in plant diversity leads to enhanced litter species richness, thereby providing more substrates for soil hydrolases and promoting extensive microbial proliferation. This dynamic results in elevated levels of SOC, dissolved organic carbon (DOC), easily oxidizable organic carbon (EOC), and microbial biomass carbon (MBC) while concurrently diminishing the stability of the soil carbon pool [14,15,16]. However, Zhang [17] observed that the increase in aboveground and belowground litter species diversity would significantly increase SOC stability. The chemical composition of litter also plays a crucial role in affecting SOC stability by modifying microbial biomass and substrate utilization [12]. Cotrufo [12] posits that the presence of high-quality litter components, such as proteins, amino acids, and monosaccharides, which are easily decomposed by microbes, can lead to the formation of stable mineral-bound SOC, thereby enhancing SOC stability. Conversely, other research suggests that litter rich in high-quality components might stimulate microbial activity, accelerating SOC mineralization and potentially reducing SOC stability [18,19,20,21]. This indicates a complex relationship between plant encroachment, litter characteristics, and SOC dynamics that warrants further investigation.
In the tundra zone of the Changbai Mountains, the predominant vegetation type is shrub tundra. Over the past three decades, with rising temperatures and an extended growing season, significant vegetative changes have been observed. Notably, the encroachment of Deyeuxia purpurea, a low-altitude herbaceous species, has led to a decline in native tundra shrubs like Rhododendron aureum and Vaccinium uliginosum [22]. This encroachment has resulted in the emergence of five distinct vegetation groups, including areas dominated by Rhododendron aureum (Rho), with varying degrees of D. purpurea encroachment classified as mild (Mil), moderate (Mod), severe (Sev), and complete dominance by D. purpurea (Dey) (Figure 1b). The encroachment of D. purpurea is anticipated to increase both the quantity and diversity of litter, alongside accelerating the decomposition rate of aboveground litter [23]. Despite these changes, the precise impact of D. purpurea’s encroachment on the SOC concentration and characteristics within the shrub tundra of Changbai Mountains remains unclear, necessitating further research to clarify these ecological transformations.
In this study, field samples from the Changbai Mountains tundra were evaluated to elucidate dynamic changes in the SOC concentration and components following herbaceous plant encroachment. Concentrations of various components (DOC, EOC, MBC, POC [particle organic carbon], MAOC [mineral-associated organic carbon]), 13C NMR spectra, aboveground and underground litter quality, total carbon input, lignin, cellulose, and litter components were compared under different degrees of encroachment. Soil physical and chemical properties (pH, total carbon (TC) concentration, total nitrogen (TN) concentration, total phosphorus (TP) concentration, nitrate nitrogen (NO3−) concentration, and ammonium nitrogen (NH4+) concentration) were also analyzed. The aims of this study were as follows: (a) to measure changes in litter properties and contents caused by the encroachment of D. purpurea, (b) to clarify how the encroachment of D. purpurea affects soil C emissions, and (c) to clarify how the properties of the SOC pool respond to the encroachment of D. purpurea.

2. Materials and Methods

2.1. Study Area

The tundra zone is situated at the highest area of the Changbai Mountains volcanic cone. (Soil in the tundra zone can freeze and become snow-covered in winter.) The climate is characterized by extremely cold winters, cool and brief summers, low temperatures throughout the year, and substantial precipitation. However, the seasonal distribution is unbalanced, with the highest temperatures in the summer, high humidity levels throughout the year, and more than 260 days of wind and foggy conditions annually. The average temperature during the growing season (June to September) is 6.37 °C, while the average precipitation is 153.20 mm (Figure A1) [24]. The predominant soil type in the Changbai Mountains alpine tundra zone is Entisols, which is influenced by volcanic eruption parent material comprising volcanic ejecta. Through intense physical weathering and water erosion, three subclasses are formed: primitive (stony) tundra soil, tundra soil, and scrub tundra soil. The extent of soil development is minimal, with a thickness of typically 10–20 cm, attributable to the brief soil formation period characteristic of tundra soil [24]. Simultaneously, the significant variation in water and thermal conditions across different altitudes in the Changbai Mountains results in a characteristic vertical zonation of vegetation, which includes, from highest to lowest elevation, the polar frigid alpine tundra belt, the subfrigid birch forest belt, the subfrigid coniferous forest belt, and the frigid temperate red pine broadleaf mixed forest belt [25].
The sample plot (43°59′35″ N, 128°1′18″ E, elevation 2200 m) was on the tundra within the area of D. purpurea encroachment on the western slope of the Changbai Mountains (Figure 1a). The area was 20 m × 20 m with a 5–8° slope and mostly level ground. The plot contained five representative plant communities: Mil (D. purpurea coverage was 30%), Mod (D. purpurea coverage was 50%), Sev (D. purpurea coverage was 70%), Dey (all D. purpurea), and Rho (no D. purpurea encroachment, all Rhododendron aureum) (Figure 1b). In every plant community, three 1 m × 1 m quadrats were erected (for a total of 15).

2.2. Sample Collection

The aboveground litter collection methods varied for D. purpurea, a perennial bud herb, and R. aureum, a perennial evergreen shrub. Nylon nets were used to cover the entire R. aureum plant in September 2021, and the litter within the nets, representing the R. aureum aboveground litter throughout the year, was collected in September 2022. At the end of the growing season, the aboveground portion of D. purpurea was collected in close proximity to the ground (roughly representing the aboveground litter). Using the root drilling method, fine roots (diameter less than 2 mm) were examined at the end of the growing season. The fine roots of D. purpurea and R. aureum from the current year were selected after the samples were sieved out of the sand and washed with water. Every year, there were approximately equal amounts of new and dead roots. Drying and weighing the aboveground and underground litter of D. purpurea and R. aureum enabled the quantification of the overall volume of the litter and determination of its biochemical characteristics.
In September 2022, a five-point sampling method was used to collect soil from each quadrat. Samples were brought to the laboratory and passed through a 2 mm sieve to remove visible roots and gravel. Part of the soil was stored in a refrigerator at 4 °C for the measurement of soil moisture content and MBC and DOC concentration, while the rest was air-dried for the measurement of other physical and chemical properties.

2.3. Litter Chemical Properties

Aboveground and belowground litter TN and TC concentrations were analyzed via the dry combustion method using an elemental analyzer (EA3000; Euro Vector, Milan, Italy). Aboveground and belowground litter lignin and cellulose concentrations were analyzed via the acid-washing method using a lignin analyzer (FIWE, VELP, Milan, Italy).

2.4. Soil Physico-Chemical Properties

Plant growth and soil microbial decomposition in the Changbai Mountains tundra zone are primarily concentrated between June and September [23]. Therefore, soil CO2 respiration rate was measured via the EGM-5 portable soil carbon flux measurement system (PP Systems, Haverhill, MA, USA) from June to September 2022. Soil moisture content was measured by oven-drying the soil samples to a constant weight at 105 °C. Soil pH was measured in 1/5 soil and distilled water suspensions using a pH meter (Time Power, Beijing, China). Soil TN and TC concentrations were analyzed via the dry combustion method using an elemental analyzer (EA3000; Euro Vector, Milan, Italy). SOC concentration was consistent with the TC concentration because the pre-experimental data indicated that the tundra on Changbai Mountains soil had a low inorganic carbon concentration. Soil TP concentration was analyzed via the molybdenum-antimony anti-colorimetric method using a spectrophotometer. Before colorimetry, the soil samples were subjected to a reaction with 5 mL of nitric acid and 0.8 mL of perchloric acid solution in a microwave digestion apparatus (CEM MARS6, CEM, Charlotte, NC, USA) to yield the TP digestion solution. Soil NO3− and NH4+ concentrations were analyzed via the potassium chloride (30 mL, 2 mol/L) extraction method using a continuous flow analyzer (SAN++; Skalar, Breda, Holland). DOC concentration was measured via the ultrapure water extraction method using a total organic carbon analyzer (1030S; OI, Houston, TX, USA). EOC concentration was measured via potassium permanganate oxidation using a full-wavelength microplate reader (SynergyH4, Bio Tek, South Burlington, VT, USA). MBC concentration was measured via chloroform fumigation-potassium sulfate extraction using a total organic carbon analyzer (1030S; OI, USA). MAOC and POC concentrations were determined via the sodium hexametaphosphate—wet sieve method using an elemental analyzer (EA3000; Euro Vector, Italy).

2.5. Quantification of Litter- and Soil-Derived Organic Carbon Molecular Structure

Measurements were made on aboveground and belowground litter and soil organic carbon molecular structure using a superconducting nuclear magnetic resonance spectrometer (AVANCE NEO 400 WB, Bruker, Bill, Switzerland), including seven functional groups [26] (Table A1). Calculations were then performed using the method described by Nelson [27] and Baldock [26] to yield six components: carbohydrate component, protein component, lignin component, lipid component, carbonyl component, and char component. Among them, carbohydrate and protein were considered easily decomposed organic carbon molecular structures. Lignin, lipid, carbonyl, and char were considered recalcitrant decomposed organic carbon molecular structures [26,27].
The oxidation state of carbon (COX) could be used to indicate the stability of aboveground litter, belowground litter, and soil samples. COX was inversely proportional to sample stability. COX was calculated from the normalized molecular formula of CXHYOZNW as described by Masiello [28].

2.6. Statistical Analysis

Before statistical analysis, the data were assessed for normality and homogeneity of variance, then log-transformed when necessary. Since the compositional data were calculated using a combination of model formulas and the actual conditions of the study area, some values of 0 remain untreated by log-ratio transformed. Aboveground and belowground litter carbon input, aboveground and belowground litter properties, composition of the soil carbon pool, and characteristics under various degrees of encroachment were analyzed using single-factor analysis (one-way ANOVA) and LSD testing. SPSS 2019 was used for ANOVA, and Origin 2019 was used to generate histograms. Correlation analyses were utilized to describe the relationship between aboveground and belowground litter properties, soil properties and SOC concentration, and COX by Origin 2019. Structural equation modeling (SEM) was conducted using the SPSS AU to analyze the direct and indirect consequences of total aboveground and belowground litter carbon input, aboveground and belowground litter properties, soil properties, soil organic carbon pools, and molecular structure on SOC concentration and COX. The SEM fit was evaluated using p-values > 0.05, chi-square values/degree values (χ2/df), the goodness-of-fit index (GFI), and the root mean square error of approximation (RMSEA).

3. Results

3.1. Effects of Herbaceous Plant Encroachment on Soil Carbon Input

The total litter carbon input of aboveground and belowground litter increased significantly as a result of D. purpurea encroachment in the R. aureum community (Figure 2a). The total carbon input of the aboveground litter was significantly lower (p = 0.02) than that of the belowground litter (Figure 2a). D. purpurea encroachment caused significant reductions in C/N and lignin/N (p < 0.001). On average, C/N and lignin/N were higher in belowground litter than in aboveground litter (Figure 2b,c).
In the aboveground and underground litter, D. purpurea encroachment resulted in increases in the protein and carbohydrate components and decreases in lignin, lipid, and char compounds. However, carbonyl components were not significantly affected by encroachment (Figure A2a,b). On average, the carbohydrate, protein, lipid, and char concentrations of belowground litter were higher than those of aboveground litter, and the lignin concentration of belowground litter was lower than that of aboveground litter (Figure A2a,b).
The COX in aboveground litter and belowground litter gradually increased with D. purpurea encroachment, indicating a decrease in aboveground and belowground litter stability (Figure A2c). At the same time, the COX of belowground litter was lower than that of aboveground litter (Figure A2c).

3.2. Effects of Herbaceous Encroachment on Soil Carbon Emissions

During the sampling period, the soil respiration rate in each of the different degrees of encroachment first increased and then decreased. Simultaneously, the rate of soil respiration increased as the degree of D. purpurea encroachment increased throughout the sampling period (Figure 3). As the degree of D. purpurea encroachment increased, the average soil respiration rate also increased over the course of the growing season. This might be due to the fact that August temperature and precipitation were higher than September, which was suitable for microbial activity and would accelerate soil respiration. In addition, soil respiration was accelerated due to the fact that more organic matter was imported in August as a result of strong plant growth.

3.3. Effects of Herbaceous Plant Encroachment on the Soil Organic Carbon Pool

The SOC concentration decreased significantly in response to D. purpurea encroachment (Figure 4a). There were no significant differences between estimates for the other two communities, except the SOC concentration in the Dey community was much lower than those in the other communities (p = 0.03). At the same time, COX increased significantly with different degrees of D. purpurea encroachment increase, indicating a decrease in soil stability (Figure 4b). Among them, the COX values in Dey and Sev were significantly different from each other and significantly higher than in the other communities (p < 0.001).
The active organic carbon concentrations and molecular structure in SOC were altered significantly by D. purpurea encroachment (p = 0.01, Figure 5). On average, the DOC concentration and MBC concentration exhibited an increase trend as the degree of D. purpurea encroachment increased (Figure 5a,c). The DOC and MBC concentrations in the Dey community were significantly higher than those of the other communities. The EOC concentration and MAOC concentration (the soil grain-size distribution in Figure A4) exhibited a declining trend as the degree of D. purpurea encroachment increased (Figure 5b,e). The EOC and MAOC concentrations in the Dey community were significantly lower than the Rho community, and the POC concentration exhibited an increasing and then decreasing trend as the degree of D. purpurea encroachment increased (Figure 5d). At the same time, the POC concentration in the Dey community was significantly lower than in the Rho community.
Overall, the D. purpurea encroachment increased the concentrations of carbohydrates, proteins, and char components in the SOC and decreased the lipid component concentration. No significant differences were observed in the lignin and the carbonyl components concentrations with respect to encroachment (p < 0.001, Figure 5f).

3.4. Main Influencing Factors of Soil Organic Carbon Concentration and COX

Based on Pearson correlation analysis (Figure A3), the characteristics of soil, aboveground litter, and belowground litter were significantly related to SOC concentration and COX.
Structural equation modeling (SEM) was used to determine the key factors affecting the SOC concentration and COX (Figure 6). The primary factors influencing the SOC concentration were the total aboveground and belowground litter carbon input, aboveground and belowground litter properties (C/N and lignin/N), soil properties (C/N, NO3−, and NH4+), and soil carbon emissions (CO2, Figure 6a). Among these, an increase in total aboveground and belowground litter carbon input directly promoted an increase in the SOC concentration, explaining 35% of the variance in the SOC concentration. A decrease in recalcitrant decomposed components (C/N and lignin/N) in aboveground and belowground litter directly promoted an increase in the SOC concentration, explaining 30% of the variance in the SOC concentration. The increase in total aboveground and belowground litter carbon input and decrease in refractory components in aboveground and belowground litter also reduced the SOC concentration by increasing the soil mineralization rate and soil CO2 release rate, respectively, which indirectly explained the changes in the SOC concentration by 20% and 11%.
The primary factors influencing the change in COX were the aboveground and belowground litter organic carbon molecular components (carbohydrates and proteins), aboveground and belowground litter properties (C/N and lignin/N), soil properties (TN and NH4+), soil organic carbon pool (MBC, DOC, and MAOC), and soil organic carbon components (carbohydrates and proteins) (Figure 6c). Among these, the increase in easily degradable organic carbon (carbohydrates and proteins) in aboveground and belowground litter increased the COX and reduced SOC stability via increases in active organic carbon (DOC and MBC) in soil and a decrease in recalcitrant organic carbon (MAOC), explaining 15% and 43% of the variance in SOC stability, respectively. The decrease in refractory components (C/N and lignin/N) in aboveground and belowground litter indirectly explained 3% of the variance in SOC stability by increasing the soil mineralization rate, soil N concentration, and COX and reducing SOC stability.

4. Discussion

4.1. Effects of Herbaceous Plant Encroachment on SOC Concentration

Total aboveground and belowground litter carbon input increased significantly as D. purpurea encroachment continued, consistent with the findings of other studies. This may be because D. purpurea has a large specific leaf area and a high net photosynthetic rate [29,30]. This enabled D. purpurea and R. aureum to obtain an advantage in the competition for resources, such as light and nutrients [31], and increased both aboveground and belowground biomass in D. purpurea [29]. Concurrently, as all D. purpurea were deciduous toward the end of the growing season, and a segment of R. aureum were also deciduous at this time [32], the total aboveground and belowground litter carbon input increased with the ongoing encroachment of D. purpurea. The SOC concentration in the community eventually decreased with an increase in the degree of encroachment in this study, despite the fact that the increase in total litter carbon input directly promoted an increase in the SOC concentration and explained 35% of the variance in the SOC concentration. This contradicted the results of most studies showing that the SOC concentration increases with an increase in the amount of litter. This occurred because SOC concentration resulted from a dynamic balance between total aboveground and belowground litter carbon input and soil mineralization and decomposition. [33]. This study revealed that the increase in total aboveground and belowground litter carbon input, alongside the reduction of recalcitrant components in both litter types, diminished SOC concentration by enhancing soil mineralization and CO2 release rates, thereby accounting for 20% and 11% of the variation in SOC concentration, respectively (Figure 6a). D. purpurea encroachment increased soil respiration rate dramatically, indicating that D. purpurea encroachment enhanced the soil mineralization capacity. D. purpurea encroachment increased substantial nutrient input to the soil from both aboveground and belowground litter, hence enhancing microbial activity and accelerating nutrient turnover rate. Furthermore, the increase in the concentration of easily decomposed organic carbon compounds, such as carbohydrates, in both aboveground and belowground litter resulting from D. purpurea encroachment enhanced the metabolic capability of microorganisms [34]. This led to enhanced microbial capacity to decompose and utilize soil aged carbon, as well as an enhanced capacity for soil mineralization. In addition, soil characteristics had a direct negative effect on the SOC concentration (Figure 6a). In this study, the soil C/N ratio was significantly reduced and was lower than the optimal ratio for microbial growth (25/1); additionally, the pH was significantly reduced (Table A2). The concentrations of NO3− and NH4+ increased, which promoted the growth and reproduction of microorganisms, increased the microbial decomposition ability, increased SOC consumption, and reduced the SOC concentration in the soil [34]. Therefore, in this study, SOC concentration consumed by soil mineralization generated by microbial utilization might be higher than SOC concentration input directly into the soil by aboveground and belowground litter decomposition, resulting in a significant decrease in SOC concentration in the soil with D. purpurea encroachment.

4.2. Effects of Herbaceous Plant Encroachment on SOC Stability

The ongoing D. purpurea encroachment resulted in an increase in COX in the soil, signifying a decline in soil stability, which was associated with alterations in the molecular structure of organic carbon and characteristics in both aboveground and belowground litter. D. purpurea encroachment led to a reduction in the growth and metabolic cycle of surface vegetation, altering the vegetation type. This resulted in a diminished cycle of aboveground and belowground litter input to the soil, as well as an expedited decomposition of both aboveground and belowground litter. The faster degradation of aboveground and belowground litter was accompanied by an increase in the concentration of easily decomposed organic carbon compounds, including carbohydrates and proteins, within the aboveground and belowground litter. (Figure A2a,b). This study revealed that an augmentation in the molecular structure of easily decomposed organic carbon in both aboveground and belowground litter indirectly diminished soil stability by 43% through the increase in active organic carbon concentration and the decrease in recalcitrant organic carbon concentration in the soil. (Figure 6c). This was because the increase in easily decomposed organic carbon, such as carbohydrates, in aboveground and belowground litter would provide a large number of decomposition substrates for soil hydrolases. This would increase the organic carbon sources that microorganisms can absorb and utilize [35,36], improving the efficiency of the “microbial carbon pump” [37,38], which promotes the accumulation of MBC and DOC concentration and reduces soil stability. In addition, a decrease in soil pH caused an increase in the fungal concentration, which in turn promoted the production of carbon decomposition enzymes [13,39]. In addition, it also accelerated the process by which fungi and enzymes broke down the easily decomposed organic carbon components in aboveground and belowground litter, increased active organic carbon concentration in the soil [40], accelerated the process by which microorganisms mineralize plant carbon sources, decreased the adsorption of plant residues by minerals, decreased recalcitrant organic carbon (MAOC) concentration [41], and decreased soil stability. Furthermore, an increase in the concentration of easily decomposed organic carbon molecular structures in the aboveground and belowground litter would indirectly decrease soil stability by 15% due to an increase in organic carbon molecular structures in the soil, such as carbohydrates and proteins (Figure 6c). Increased easily decomposable organic carbon components concentrations, such as carbohydrates, could provide adequate nutrition for soil microbes [42]. This could cause an increase in extracellular enzyme activity, which in turn speeds up the breakdown of plant carbon sources and soil aged carbon [43] and decreases soil stability.
In addition to the effect of the easily decomposed organic carbon molecular structure in aboveground and belowground litter on soil stability, the aboveground and belowground litter properties also had an effect on soil stability [12]. This study showed that the decrease in the recalcitrant decomposed components of aboveground and belowground litter (Lignin/N, C/N) following D. purpurea encroachment indirectly accounted for 31% of the reduction in soil stability by increasing soil TN and NH4+ concentrations (Figure 6c). On the one hand, high TN concentration in the soil alleviated nitrogen limitations on microbial activity, thereby increasing decomposition rates, enhancing microbial substrate utilization efficiency [21,44], and ultimately reducing soil stability. On the other hand, high-quality aboveground and belowground litter, with lower C/N ratios, were more efficiently utilized by microorganisms, stimulating the production of hydrolytic enzymes that accelerated the recalcitrant decomposed soil components, such as lignin [12], further reducing soil stability. Consequently, the decrease in SOC stability observed in this study was primarily attributed to an increase in the molecular structure of easily decomposed organic carbon in aboveground and belowground litter, leading to its greater input into the soil. This resulted in higher concentrations of decomposable and active organic carbon in the soil, a decline in recalcitrant organic carbon, and accelerated soil mineralization due to enhanced microbial activity stimulated by the reduction in recalcitrant components of the aboveground and belowground litter.

5. Limitations of the Study

There might be some possible limitations in this study. First, a total of 15 sample plots were only established in this study. There were limitations due to the location of the study within a national nature reserve, which restricted the number of sample plots that could be established. Second, this study focused exclusively on changes in CO2 among greenhouse gases, while changes in other greenhouse gases, such as CH4 and N2O, were not considered. In the future, we can further measure other greenhouse gases to make the study more comprehensive. Third, the Fe and Al concentrations were not determined in this study, which might have influenced the variation in MAOC concentration. However, the volcanic ash and weathering materials of the Changbai Mountains are rich in potassium, resulting in a distinct tundra ecosystem that differs from other tundras. This ecosystem exhibits relatively low acidity, with pH values generally ranging from 5 to 6. The minimum pH recorded at the sampling sites in this study was 4.19, indicating that most soils do not qualify as strongly acidic. Therefore, the Fe and Al concentrations might have little influence on the variation in MAOC concentration.

6. Conclusions

D. purpurea encroachment in the Changbai Mountains tundra zone directly increased SOC concentration within the community by enhancing aboveground and belowground litter inputs by 113.25% and 68.7%, respectively, while reducing the recalcitrant components of the litter. However, the low C/N ratio in aboveground and belowground litter caused by D. purpurea encroachment could create a favorable environment for microbial growth and activity by reducing the soil C/N ratio and accelerating microbial decomposition, leading to increased SOC concentration consumption. Simultaneously, the increase in total aboveground and belowground litter carbon inputs and easily decomposed components also indirectly reduced SOC concentration within the community by increasing soil mineralization capacity and increasing CO2 release by 71.20%. Ultimately, the D. purpurea encroachment resulted in a decrease of 41.63% in SOC concentration due to the dynamic balance between the total aboveground and belowground litter carbon inputs and soil mineralization and decomposition. Simultaneously, with the D. purpurea encroachment, there was an increase in easily decomposed organic carbon concentrations (such as carbohydrates and proteins) of more than 8% in the aboveground and belowground litter, which led to an increase in active organic carbon concentrations (such as MBC) of more than 33% in the soil and a decrease in recalcitrant organic carbon concentrations (such as MAOC) of more than 23%. At the same time, it lowered recalcitrant deposition components (such as lipids) in aboveground and belowground litter by more than 6%, accelerated soil mineralization, and decreased SOC stability by 74.90%. Therefore, D. purpurea encroachment would decrease the recalcitrant decomposed components concentrations in aboveground and belowground litter, increase easily decomposed organic carbon concentrations, and accelerate the decomposed of carbon pool in the tundra soil. We posited that this would progressively convert the tundra zone from a carbon sink to a carbon source, thereby expediting the release of nutrients and further facilitating the encroachment of herbaceous plants such as D. purpurea, which would in turn accelerate CO2 emissions.

Author Contributions

Conceptualization, X.X. and J.X.; methodology, X.X. and Y.Z.; software, X.X. and Y.Z.; validation, Y.J.; formal analysis, X.X.; investigation, X.X. and J.Y.; resources, J.X. and Y.J.; data curation, X.X. and J.Y.; writing—original draft preparation, X.X.; writing—review and editing, X.X., J.X., Y.J. and Y.Z.; visualization, X.X. and Y.Z.; supervision, J.X.; project administration, Y.J.; funding acquisition, J.X. and Y.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Science Foundation of Jilin, China, grant number 20220101151JC, the National Natural Science Foundation of China, grant number 41571078 and Science Research Project of Hebei Education Department, grant number SQ2023237.

Data Availability Statement

Data are available upon a reasonable request.

Conflicts of Interest

The author declares no conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
ANOVAAnalysis of variance
CO2Carbon dioxide
COXCarbon oxidation state
DOCDissolved organic carbon
EOCEasily oxidizable organic carbon
LSDLeast significant difference
MAOCMineral-associated organic carbon
MBCMicrobial biomass carbon
NH4+Ammonium nitrogen
NO3−Nitrate nitrogen
POCParticle organic carbon
SEMStructural equation model
SOCSoil organic carbon
TCTotal carbon
TNTotal nitrogen
TPTotal phosphorus

Appendix A

Table A1. Definition of particular regions in the 13C NMR spectra.
Table A1. Definition of particular regions in the 13C NMR spectra.
Functional GroupsChemical ShiftsCarbon Types
Alkyl0–45 ppmAlkyl carbon
N-alkyl/methoxy45–60 ppmN-alkyl methoxy carbon
O-alkyl60–95 ppmAlkoxy carbon
O2-alkyl95–110 ppmDi-alkoxy carbon
Aromatic110–145 ppmAromatic carbon
O-aromatic145–165 ppmPhenolic aromatic carbon
Carbonyl165–210 ppmCarbonyl carbon
Table A2. Physical and chemical properties of surface soil. Different lowercase letters represent significant differences in the physical and chemical properties of surface soil at different sampling points.
Table A2. Physical and chemical properties of surface soil. Different lowercase letters represent significant differences in the physical and chemical properties of surface soil at different sampling points.
Sampling SitepHTC (g/kg)TN (g/kg)C/N (%)TP (g/kg)NH4+ (mg/kg)NO3− (mg/kg)
Rho(4.28 + 0.01) a(225.87 + 10.88) a(10.47 + 0.22) bc(21.60 + 1.17) a(1.10 + 0.03) c(10.56 + 0.23) c(0.18 +0.07) b
Mil(4.27 + 0.02) a(208.59 + 7.39) a(9.27 + 0.47) c(22.65 + 1.61) a(0.98 + 0.08) c(10.76 + 0.06) c(0.06 + 0.02) b
Mod(4.23 + 0.02) ab(203.82 + 1.87) a(9.56 + 0.20) c(21.34 + 0.61) a(1.21 + 0.06) bc(11.55 + 0.15) b(0.06 + 0.02) b
Sev(4.25 + 0.04) ab(205.25 + 7.15) a(11.42 + 0.01) b(17.97 + 0.62) b(1.30 + 0.06) b(11.38 + 0.06) b(0.27 + 0.19) b
Dey(4.19 + 0.02) b(131.85 + 8.45) b(13.57 + 0.92) a(9.83 + 0.98) b(1.62 + 0.13) a(16.78 + 0.14) a(4.92 + 3.24) a
Different lowercase letters correspond to significant differences in the chemical characteristics of soil (p < 0.05).

Appendix B

Forests 16 00197 g0a1
Figure A1. Monthly variations in average temperature and precipitation in the Changbai Mountains tundra, 2022.
Figure A1. Monthly variations in average temperature and precipitation in the Changbai Mountains tundra, 2022.
Forests 16 00197 g0a1
Forests 16 00197 g0a2
Figure A2. Changes in molecular structure of organic carbon and COX in aboveground and belowground litters under different degrees of Deyeuxia purpurea encroachment. (a) Aboveground litter, (b) belowground litter, (c) COX.
Figure A2. Changes in molecular structure of organic carbon and COX in aboveground and belowground litters under different degrees of Deyeuxia purpurea encroachment. (a) Aboveground litter, (b) belowground litter, (c) COX.
Forests 16 00197 g0a2
Forests 16 00197 g0a3
Figure A3. Correlation analysis of SOC and COX with various indicators in soil, aboveground litter, and belowground litter. (a) Soil, (b) aboveground litter, (c) belowground litter. * 0.01 < p < 0.05; ** 0.001 < p < 0.01; *** p < 0.001.
Figure A3. Correlation analysis of SOC and COX with various indicators in soil, aboveground litter, and belowground litter. (a) Soil, (b) aboveground litter, (c) belowground litter. * 0.01 < p < 0.05; ** 0.001 < p < 0.01; *** p < 0.001.
Forests 16 00197 g0a3
Forests 16 00197 g0a4
Figure A4. Variation in soil grain-size distribution.
Figure A4. Variation in soil grain-size distribution.
Forests 16 00197 g0a4

References

  1. Batjes, N.H. Projected Changes in Soil Organic Carbon Stocks Upon Adoption of Recommended Soil and Water Conservation Practices in the Upper Tana River Catchment, Kenya. Land Degrad. Dev. 2014, 25, 278–287. [Google Scholar] [CrossRef]
  2. Schulze, E.D.; Freibauer, A. Environmental science—Carbon unlocked from soils. Nature 2005, 437, 205–206. [Google Scholar] [CrossRef]
  3. Pan, Y.; Chen, J.M.; Birdsey, R.; McCullough, K.; He, L.; Deng, F. Age structure and disturbance legacy of North American forests. Biogeosciences 2011, 8, 715–732. [Google Scholar] [CrossRef]
  4. Zhang, Y.; An, C.B.; Zhang, W.S.; Zheng, L.Y.; Zhang, Y.Z.; Lu, C.; Liu, L.Y. Drivers of mountain soil organic carbon stock dynamics: A review. J. Soils Sediments 2023, 23, 64–76. [Google Scholar] [CrossRef]
  5. Zhao, G.Y.; Li, S.Y.; Zhou, W.Z.; He, L.; Zou, R.; Yu, J.; Chen, Z.; Bai, X.; Zhang, J.; Sun, X. Comparative Analysis of Soil Organic Carbon and Soil Oxide Minerals across Different Climates and Forest Types. Austrian J. For. Sci. 2024, 141, 53–78. [Google Scholar]
  6. Ofiti, N.O.E.; Schmidt, M.W.; Abiven, S.; Hanson, P.J.; Iversen, C.M.; Wilson, R.M.; Kostka, J.E.; Wiesenberg, G.L.B.; Malhotra, A. Climate warming and elevated CO2 alter peatland soil carbon sources and stability. Nat. Commun. 2023, 14, 7533. [Google Scholar] [CrossRef]
  7. Xu, M.P.; Zhi, R.C.; Jian, J.N.; Feng, Y.Z.; Han, X.H.; Zhang, W. Changes in Soil Organic C Fractions and C Pool Stability Are Mediated by C-Degrading Enzymes in Litter Decomposition of Robinia pseudoacacia Plantations. Microb. Ecol. 2023, 86, 1189–1199. [Google Scholar] [CrossRef]
  8. Abiven, S.; Menasseri, S.; Chenu, C. The effects of organic inputs over time on soil aggregate stability—A literature analysis. Soil Biol. Biochem. 2009, 41, 1–12. [Google Scholar] [CrossRef]
  9. Mitchell, E.; Scheer, C.; Rowlings, D.; Cotrufo, M.F.; Conant, R.T.; Friedl, J.; Grace, P. Trade-off between ‘new’ SOC stabilisation from above-ground inputs and priming of native C as determined by soil type and residue placement. Biogeochemistry 2020, 149, 221–236. [Google Scholar] [CrossRef]
  10. Zhong, Y.; Yan, W.; Wang, R.; Shangguan, Z. Differential responses of litter decomposition to nutrient addition and soil water availability with long-term vegetation recovery. Biol. Fertil. Soils 2017, 53, 939–949. [Google Scholar] [CrossRef]
  11. Gao, J.; Feng, J.; Zhang, X.; Yu, F.H.; Xu, X.; Kuzyakov, Y. Drying-rewetting cycles alter carbon and nitrogen mineralization in litter-amended alpine wetland soil. Catena 2016, 145, 285–290. [Google Scholar] [CrossRef]
  12. Cotrufo, M.F.; Wallenstein, M.D.; Boot, C.M.; Denef, K.; Paul, E. The Microbial Efficiency-Matrix Stabilization (MEMS) framework integrates plant litter decomposition with soil organic matter stabilization: Do labile plant inputs form stable soil organic matter? Glob. Change Biol. 2013, 19, 988–995. [Google Scholar] [CrossRef] [PubMed]
  13. Tian, J.; Zong, N.; Hartley, I.P.; He, N.; Zhang, J.; Powlson, D.; Zhou, J.; Kuzyakov, Y.; Zhang, F.; Yu, G.; et al. Microbial metabolic response to winter warming stabilizes soil carbon. Glob. Change Biol. 2021, 27, 2011–2028. [Google Scholar] [CrossRef]
  14. Jin, Z.; Shah, T.; Zhang, L.; Liu, H.; Peng, S.; Nie, L. Effect of straw returning on soil organic carbon in rice-wheat rotation system: A review. Food Energy Secur. 2020, 9, e200. [Google Scholar] [CrossRef]
  15. Yan, W.; Zhong, Y.; Zhu, G.; Liu, H.; Peng, S.; Nie, L. Nutrient limitation of litter decomposition with long-term secondary succession: Evidence from controlled laboratory experiments. J. Soils Sediments 2020, 20, 1858–1868. [Google Scholar] [CrossRef]
  16. Wang, Q.; Yu, Y.; He, T.; Wang, Y. Aboveground and belowground litter have equal contributions to soil CO2 emission: An evidence from a 4-year measurement in a subtropical forest. Plant Soil 2017, 421, 7–17. [Google Scholar] [CrossRef]
  17. Zhang, W.P.; Fornara, D.; Yang, H.; Yu, R.P.; Callaway, R.M.; Li, L. Plant litter strengthens positive biodiversity-ecosystem functioning relationships over time. Trends Ecol. Evol. 2023, 38, 473–484. [Google Scholar] [CrossRef]
  18. Pérez-Harguindeguy, N.; Díaz, S.; Cornelissen, J.H.C.; Vendramini, F.; Cabido, M.; Castellanos, A. Chemistry and toughness predict leaf litter decomposition rates over a wide spectrum of functional types and taxa in central Argentina. Plant Soil 2000, 218, 21–30. [Google Scholar] [CrossRef]
  19. Shaw, M.R.; Harte, J. Control of litter decomposition in a subalpine meadow-sagebrush steppe ecotone under climate change. Ecol. Appl. 2001, 11, 1206–1223. [Google Scholar] [CrossRef]
  20. Mcculley, R.L.; Archer, S.R.; Boutton, T.W.; Hons, F.M.; Zuberer, D.A. Soil respiration and nutrient cycling in wooded communities developing in grassland. Ecology 2004, 85, 2804–2817. [Google Scholar] [CrossRef]
  21. Liao, J.D.; Boutton, T.W. Soil microbial biomass response to woody plant invasion of grassland. Soil Biol. Biochem. 2008, 40, 1207–1216. [Google Scholar] [CrossRef]
  22. Zong, S.W.; Xu, J.W.; Wu, Z.F. Investigation and Mechanism Analysis on the Invasion of Deyeuxia angustifolia to Tundra Zone in Western Slope of Changbai Mountain. J. Mt. Sci. 2013, 31, 448–455. [Google Scholar]
  23. Zhang, Y.J. Effects of Deyeuxia purpurea Expansion on the Changes in the Litter Decomposition of the Changbai Mountain Tundra and Soil Nutrients. Ph.D. Thesis, Northeast Normal University, Changchun, China, 2022. [Google Scholar]
  24. Zong, S.W.; Wu, Z.F.; Du, H.B. Study on Climate Change in Alpine Tundra of the Changbai Mountain in Growing Season in Recent 52 Years (in China). Arid Zone Res. 2013, 30, 41–49. [Google Scholar]
  25. Zong, S.W. Mechanism Research on the Vegetation Changes of the Sub-Alpine Tundra, Changbai Mountains. Ph.D. Thesis, Northeast Normal University, Changchun, China, 2014. [Google Scholar]
  26. Baldock, J.A.; Masiello, C.A.; Gélinas, Y.; Hedges, J.I. Cycling and composition of organic matter in terrestrial and marine ecosystems. Mar. Chem. 2004, 92, 39–64. [Google Scholar] [CrossRef]
  27. Nelson, P.N.; Baldock, J.A. Estimating the molecular composition of a diverse range of natural organic materials from solid-state 13C NMR and elemental analyses. Biogeochemistry 2005, 72, 1–34. [Google Scholar] [CrossRef]
  28. Masiello, C.A.; Gallagher, M.E.; Randerson, J.T.; Deco, R.M.; Chadwick, O.A. Evaluating two experimental approaches for measuring ecosystem carbon oxidation state and oxidative ratio. J. Geophys. Res. Biogeosciences 2008, 113, G03010. [Google Scholar] [CrossRef]
  29. Ehrenfeld, J.G. Effects of exotic plant invasions on soil nutrient cycling processes. Ecosystems 2003, 6, 503–523. [Google Scholar] [CrossRef]
  30. Nie, M.; Shang, L.; Liao, C.; Li, B. Changes in Primary Production and Carbon Sequestration After Plant Invasions. In Impact of Biological Invasions on Ecosystem Services; Vilà, M., Hulme, P., Eds.; Springer: Cham, Switzerland, 2017. [Google Scholar] [CrossRef]
  31. Lambers, H.; Chapin, F.S.; Pons, T.L. Plant Physiological Ecology; Springer: New York, NY, USA, 1998; 540p. [Google Scholar]
  32. Biederbeck, V.O.; Janzen, H.H.; Campbell, C.A.; Zentner, R.P. Lsbile soil organic-matter as influenced by cropping practices in an arid environmentent. Soil Biol. Biochem. 1994, 26, 1647–1656. [Google Scholar] [CrossRef]
  33. Fearnside, P.M.; Barbosa, R.I. Soil carbon changes from conversion of forest to pasture in Brazilian Amazonia. For. Ecol. Manag. 1998, 108, 147–166. [Google Scholar] [CrossRef]
  34. De Deyn, G.B.; Cornelissen JH, C.; Bardgett, R.D. Plant functional traits and soil carbon sequestration in contrasting biomes. Ecol. Lett. 2008, 11, 516–531. [Google Scholar] [CrossRef]
  35. Feng, X.; Wang, S. Plant influences on soil microbial carbon pump efficiency. Glob. Change Biol. 2023, 29, 3854–3856. [Google Scholar] [CrossRef] [PubMed]
  36. Huang, Y.; Liang, C.; Duan, X.; Chen, H.; Li, D. Variation of microbial residue contribution to soil organic carbon sequestration following land use change in a subtropical karst region. Geoderma 2019, 353, 340–346. [Google Scholar] [CrossRef]
  37. Shao, P.; Liang, C.; Lynch, L.; Xie, H.; Bao, X. Reforestation accelerates soil organic carbon accumulation: Evidence from microbial biomarkers. Soil Biol. Biochem. 2019, 131, 182–190. [Google Scholar] [CrossRef]
  38. Lauer, F.; Koesters, R.; Du Preez, C.C.; Amelung, W. Microbial residues as indicators of soil restoration in South African secondary pastures. Soil Biol. Biochem. 2011, 43, 787–794. [Google Scholar] [CrossRef]
  39. Huang, R.; Crowther, T.W.; Sui, Y.; Sun, B.; Liang, Y. High stability and metabolic capacity of bacterial community promote the rapid reduction of easily decomposing carbon in soil. Commun. Biol. 2021, 4, 1376. [Google Scholar] [CrossRef]
  40. Geethanjali, P.A.; Jayashankar, P.M. A Review on Litter Decomposition by Soil Fungal Community. IOSR J. Pharm. Biol. Sci. 2016, 11, 1–3. [Google Scholar] [CrossRef]
  41. Angst, G.; Mueller, K.E.; Castellano, M.J.; Vogel, C.; Wiesmeier, M.; Mueller, C.W. Unlocking complex soil systems as carbon sinks: Multi-pool management as the key. Nat. Commun. 2023, 14, 2967. [Google Scholar] [CrossRef]
  42. Cragg, S.M.; Friess, D.A.; Gillis, L.G.; Trevathan-Tackett, S.M.; Terrett, O.M.; Watts, J.E.; Distel, D.L.; Dupree, P. Vascular Plants Are Globally Significant Contributors to Marine Carbon Fluxes and Sinks. Annu. Rev. Mar. Sci. 2020, 12, 469–497. [Google Scholar] [CrossRef]
  43. Ding, Y.; Wang, D.; Zhao, G.; Chen, S.; Sun, T.; Sun, H.; Wu, C.; Li, Y.; Yu, Z.; Li, Y.; et al. The contribution of wetland plant litter to soil carbon pool: Decomposition rates and priming effects. Environ. Res. 2023, 224, 115575. [Google Scholar] [CrossRef]
  44. Yang, J.; Wu, F.; Wei, X.; Zhang, X.; Wu, Q.; Yue, K.; Ni, X. Global Positive Effects of Litter Inputs on Soil Nitrogen Pools and Fluxes. Ecosystems 2023, 26, 860–872. [Google Scholar] [CrossRef]
Forests 16 00197 g001
Figure 1. Overview of the study area (a) and five types of tundra vegetation created by Deyeuxia purpurea encroachment (b).
Figure 1. Overview of the study area (a) and five types of tundra vegetation created by Deyeuxia purpurea encroachment (b).
Forests 16 00197 g001
Forests 16 00197 g002
Figure 2. Histogram of the total carbon input, C/N and lignin/N of belowground and aboveground litter for various degrees of Deyeuxia purpurea encroachment. (a) Shows the total carbon input, (b) shows C/N, (c) shows lignin/N. Different lowercase letters indicate significant differences in the total carbon input, C/N, and lignin/N of aboveground litter (p < 0.05), while distinct uppercase letters indicated differences in the total carbon input, C/N, and lignin/N of belowground litter (p < 0.05). Mil (D. purpurea coverage was 30%), Mod (D. purpurea coverage was 50%), Sev (D. purpurea coverage was 70%), Dey (all D. purpurea), Rho (no D. purpurea encroachment, all Rhododendron aureum).
Figure 2. Histogram of the total carbon input, C/N and lignin/N of belowground and aboveground litter for various degrees of Deyeuxia purpurea encroachment. (a) Shows the total carbon input, (b) shows C/N, (c) shows lignin/N. Different lowercase letters indicate significant differences in the total carbon input, C/N, and lignin/N of aboveground litter (p < 0.05), while distinct uppercase letters indicated differences in the total carbon input, C/N, and lignin/N of belowground litter (p < 0.05). Mil (D. purpurea coverage was 30%), Mod (D. purpurea coverage was 50%), Sev (D. purpurea coverage was 70%), Dey (all D. purpurea), Rho (no D. purpurea encroachment, all Rhododendron aureum).
Forests 16 00197 g002
Forests 16 00197 g003
Figure 3. Soil respiration rates with respect to the extent of D. purpurea encroachment in various months. Significant differences between sites at the same sampling time are indicated by distinct lowercase letters (p < 0.05). The X-axis represents soil respiration rate from June to September, and “Average” represents the average soil respiration rate from June to September.
Figure 3. Soil respiration rates with respect to the extent of D. purpurea encroachment in various months. Significant differences between sites at the same sampling time are indicated by distinct lowercase letters (p < 0.05). The X-axis represents soil respiration rate from June to September, and “Average” represents the average soil respiration rate from June to September.
Forests 16 00197 g003
Forests 16 00197 g004
Figure 4. Effects of different degrees of Deyeuxia purpurea encroachment on SOC concentration and COX. (a) SOC concentration, (b) COX. Significant differences between sites are indicated by different lowercase letters (p < 0.05). Mil (D. purpurea coverage was 30%), Mod (D. purpurea coverage was 50%), Sev (D. purpurea coverage was 70%), Dey (all D. purpurea), Rho (no D. purpurea encroachment, all Rhododendron aureum).
Figure 4. Effects of different degrees of Deyeuxia purpurea encroachment on SOC concentration and COX. (a) SOC concentration, (b) COX. Significant differences between sites are indicated by different lowercase letters (p < 0.05). Mil (D. purpurea coverage was 30%), Mod (D. purpurea coverage was 50%), Sev (D. purpurea coverage was 70%), Dey (all D. purpurea), Rho (no D. purpurea encroachment, all Rhododendron aureum).
Forests 16 00197 g004
Forests 16 00197 g005
Figure 5. Changes in active organic carbons and molecular structure in the soil organic carbon according to the extent of Deyeuxia purpurea encroachment. Different letters denote significant differences between sites (p < 0.05). (a) DOC concentration, (b) EOC concentration, (c) MBC concentration, (d) POC concentration, (e) MAOC concentration, (f) Molecular structure.
Figure 5. Changes in active organic carbons and molecular structure in the soil organic carbon according to the extent of Deyeuxia purpurea encroachment. Different letters denote significant differences between sites (p < 0.05). (a) DOC concentration, (b) EOC concentration, (c) MBC concentration, (d) POC concentration, (e) MAOC concentration, (f) Molecular structure.
Forests 16 00197 g005
Forests 16 00197 g006
Figure 6. Structural equation model (SEM) of the total aboveground and belowground litter carbon input (TLCI), aboveground and belowground litter properties (LP), soil properties (SP), and soil carbon emission (SCE) with SOC concentration (a). Standardized effects of driving factors on SOC concentration (b). SEM of aboveground and belowground litter components (LC), aboveground and belowground litter properties (LP), soil properties (SP), soil components (SC), and soil organic carbon pool (SOCP) with COX (c). Standardized effects of determinants of COX (d). Solid red lines and solid blue lines represent significantly positive and negative relationships, respectively. * 0.01 < p < 0.05; ** 0.001 < p < 0.01; *** p < 0.001.
Figure 6. Structural equation model (SEM) of the total aboveground and belowground litter carbon input (TLCI), aboveground and belowground litter properties (LP), soil properties (SP), and soil carbon emission (SCE) with SOC concentration (a). Standardized effects of driving factors on SOC concentration (b). SEM of aboveground and belowground litter components (LC), aboveground and belowground litter properties (LP), soil properties (SP), soil components (SC), and soil organic carbon pool (SOCP) with COX (c). Standardized effects of determinants of COX (d). Solid red lines and solid blue lines represent significantly positive and negative relationships, respectively. * 0.01 < p < 0.05; ** 0.001 < p < 0.01; *** p < 0.001.
Forests 16 00197 g006
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Xu, X.; Jin, Y.; Xu, J.; Zhang, Y.; Yang, J. Effects of Herbaceous Plant Encroachment on the Soil Carbon Pool in the Shrub Tundra of the Changbai Mountains. Forests 2025, 16, 197. https://doi.org/10.3390/f16020197

AMA Style

Xu X, Jin Y, Xu J, Zhang Y, Yang J. Effects of Herbaceous Plant Encroachment on the Soil Carbon Pool in the Shrub Tundra of the Changbai Mountains. Forests. 2025; 16(2):197. https://doi.org/10.3390/f16020197

Chicago/Turabian Style

Xu, Xiaoyun, Yinghua Jin, Jiawei Xu, Yingjie Zhang, and Jiaxing Yang. 2025. "Effects of Herbaceous Plant Encroachment on the Soil Carbon Pool in the Shrub Tundra of the Changbai Mountains" Forests 16, no. 2: 197. https://doi.org/10.3390/f16020197

APA Style

Xu, X., Jin, Y., Xu, J., Zhang, Y., & Yang, J. (2025). Effects of Herbaceous Plant Encroachment on the Soil Carbon Pool in the Shrub Tundra of the Changbai Mountains. Forests, 16(2), 197. https://doi.org/10.3390/f16020197

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop