The Hidden Role of Forest Tree Species in Driving Soil Organic Carbon Dynamics

Abstract

The role of soil organic carbon (SOC) dynamics in forest carbon (C) balance has been widely recognized. The processes that mediate the relationships between forest tree species composition and the formation, turnover, and stabilization of SOC are not sufficiently understood. This paper aimed to compile the state of knowledge on the involvement of tree species composition in the regulation of SOC dynamics through litter quality, root properties, root exudates, microbial-mediated processes, and soil mineral interactions. A greater emphasis is given to the role of the SOC pool subdivision into active (labile) and passive (non-labile) fractions. These fractions turn over at a significantly different rate and have also been proven to be considerably different in terms of long-term stability. The properties of the trees and soil in the rhizosphere influence the rate of short-term and chemical conversion of plant material into the persistent or passive fraction of soil C through the mediating process of microorganisms. Evidence confirmed that the functional interactions between the mix of tree species increase the rate of SOC stabilization through an increase in the rate of active to passive fraction transition. This synthesis presents a trait-based approach for considering and addressing the dynamics of SOC in the environment.

1. Introduction

The soil organic carbon (SOC) pool can be a major contributor to the carbon (C) budget of a forest ecosystem, surpassing the amount of C in the above-ground biomass pool [1]. The forest soil is a major C pool because this soil retains decomposed organic material that originates from litter fall and root turnover [2,3,4]. This SOC pool exerts a major influence on ecosystem C stocks and can determine whether a forest functions as a C sink or a C source [5].

SOC also plays a crucial role in regulating C cycle processes in forested ecosystems [6,7]. One of the largest C fluxes in forest ecosystems is soil respiration, driven by organic matter decomposition and root and microbial metabolic activities [8,9,10]. Additionally, SOC supports the long-term maintenance of forests and their role in sequestering C from the atmosphere [11]. While aboveground biomass is erratic in terms of its retention due to aspects such as fire, wind, and harvest, SOC is often stored in forests for many years and even centuries. This is precisely what is needed to ensure that forests remain vital in sequestering C [12].

Tree species are important factors influencing the formation of SOC, since they significantly tend to control the input of organic matter into the soil [13,14]. Variations among tree species in litter production rates, litter chemistry, root biomass, and root exudation can regulate both the quantity of carbon inputs to the soil and the rate of carbon decomposition. In particular, species producing litter rich in lignin and low in nutrient concentrations tend to slow decomposition rates due to reduced litter quality and limited microbial accessibility [15,16,17]. Furthermore, tree species can affect soil microbial populations and biogeochemical cycling, which are essential for maintaining soil C [18]. The microorganisms present in tree roots, specifically the mycorrhizal fungi in the soil beneath them, vary among different tree types [19]. They can influence C cycling in the soil, determining whether the carbon will be respired or enter the soil’s C pools [20]. Moreover, tree species also influence soil physical and chemical properties, which in turn regulate the stability of SOC [21]. Tree species, therefore, can be considered an important, although less recognized, factor in the stability of SOC in forests [22].

SOC can be considered a heterogeneous reservoir, consisting of different fractions that can be distinguished based on their chemical compositions, turnover rates, and storage [23]. The soil stabilization factors influence the long-term durability of SOC in soil [24]. Physical protection in soil aggregates can prevent microbial interactions with organic matter [25,26,27], while chemical protection is achieved through the interactions between organic matter and soil minerals [28]. Biologically mediated processes, for instance, microbial degradation of plant organic matter into stable organic compounds, also influence the formation of SOC [29]. The relative importance of these stabilization processes varies depending on the soil type, climate, and vegetation [30,31]. In forest soils with high clay content or abundant reactive mineral surfaces, C stabilization is enhanced through organo-mineral associations, promoting the formation and persistence of mineral-associated organic carbon (MAOC) [28]. Knowledge of the processes involved in the formation and protection of SOC fractions is essential for predicting SOC reactions [32]. Despite intensive research, significant uncertainties persist in understanding SOC processes in forest ecosystems [12,33,34]. While numerous studies have reported differences in C storage among tree species, these differences are often context-dependent and influenced by environmental and edaphic conditions. The mechanistic basis underlying such differences and their linkage to species-specific functional traits remains insufficiently synthesized. A critical knowledge gap exists in the limited integration of tree species traits with SOC stabilization pathways. Rather than a lack of evidence that tree species influence SOC, the main limitation lies in explaining how trait-driven processes regulate SOC formation and persistence. Additionally, the sensitivity of different SOC fractions and processes involved in their stabilization to changes associated with tree species in the forest remains unclear. Furthermore, the variability in SOC fractionation and depth and the lack of a standard for presenting the results make it difficult to synthesize the information obtained.

The review study focused on the short- and long-term dynamics of SOC in the rhizosphere, highlighting gaps in understanding SOC sequestration and the sensitivity of its fractions to different tree species, and integrating current knowledge of SOC, specifically regarding processes within the rhizosphere, where tree species have a ‘hidden’ influence.

2. Tree Species as Drivers of C Inputs to Soil

2.1. Litter Production

Litterfall constitutes an important component of net primary production, as a portion of the biomass produced through photosynthesis is ultimately returned to the soil. Through this process, C and nutrients are transferred from vegetation to the soil system [35,36,37]. Several factors directly impact litter production among tree species. One of these factors is leaf area index (LAI) [38]. Chen and Black [39] defined this parameter as the total one-sided leaf area per unit of terrestrial surface area and indicated that the forest LAI represents half of the total leaf area per unit horizontal ground area. This parameter is crucial for determining canopy structures [40,41]. Therefore, forest tree species with a higher LAI have a significant impact on C dynamics and can sequestrate and store more C in the soil [42]. The biomass turnover rate can also vary among tree species [43]. Therefore, a higher biomass turnover rate results in higher litter input and surface C content [44]. Furthermore, nutrient resorption efficiency, which varies among tree species, can impact the C cycle and C content of the litter [45,46]. The C content of the litter of tree species with higher nutrient resorption efficiency is more than that of the lower ones [47]. Evergreen and deciduous species exhibit different litter compositions and patterns, varying in both quantity and seasonality [48,49,50]. These differences alter microbial activity [51], the rate of decomposition [52], and the timing of C addition to pools of organic matter in the soil [53]. The mixed litter fall in the forest can also be considered a significant parameter for improving soil fertility and enhancing the SOC pool. In this regard, Polyakova and Billor [54] noted that mixed pine and deciduous litter decomposed more rapidly and exhibited higher nutrient concentrations and lower C:N ratios than pure pine litter, thereby enhancing nutrient circulation and SOC dynamics, benefiting ecosystem functioning in pine forests.

2.2. Litter Quality

The lignin and polyphenols, as well as the C:N ratio in litter, are factors that significantly contribute to litter quality, which varies according to tree species [55,56,57,58]. For example, deciduous species typically produce litter with lower lignin and polyphenol concentrations and more favorable C:N ratios compared with conifers, resulting in faster decomposition and greater microbial activity [59,60]. These chemical variations in the litter result in different forms of litter-derived C in the soil [61]. Cotrufo et al. [62] reported that elevated atmospheric CO₂ (600 ppm) altered litter quality by decreasing N content and increasing lignin concentrations and C:N and lignin: N ratios, which slowed decomposition and is expected to enhance soil C storage. Furthermore, Thirunavukkarasu [63] highlighted that microbial community composition and lignin chemical composition, rather than lignin content alone, regulate early-stage decomposition and contribute to soil organic matter formation, highlighting the mechanistic interplay between microbial activity and litter chemistry in soil C dynamics.

Decomposable litter supports microbial life in the soil and acts as a source for the short-term C pool in the soil C budget [64]. Fanin and Bertrand [65] demonstrated that litter quality significantly influences C mineralization rates more than soil type and microbial community composition. The resistant components of the litter would persist in the soil for a long time and act as an indirect source for the stable C pool in the soil [66,67]. Consequently, the species-specific chemistry of litter is essential in determining the rates of decomposition, C mineralization, and stabilization.

2.3. Root System Characteristics

The architecture of the root system varies significantly among tree species, which in turn affects the levels of C deposited in the soil profile [68,69]. Tree species with a deep-rooted vegetation system can deposit C into the sub-soil layers, resulting in increased organic matter deposits in the soil at a slower rate [70]. Species with shallow and fine root systems deposit C primarily in the upper soil layers, where fine root biomass is often concentrated (e.g., 10–20 cm depth) and strongly associated with higher SOC levels in the 0–30 cm soil profile; in these layers, high bacterial activity and species-specific differences in root decomposition regulated by root chemical traits such as the C:N ratio and soil depth (e.g., Alnus glutinosa, Pinus sylvestris, and Picea orientalis) further influence C turnover and sequestration [71,72,73]. Jackson et al. [74] also indicated that a substantial fraction of the C sequestered by photosynthesis in forests is allocated to the development of fine roots (diameter < 2 mm). Fine root turnover (FRT) is another key contributor to belowground organic C inputs. Those plant species with higher rates of fine root production (FRP) and turnover contribute significantly to organic C inputs into the soil through root death [75]. In addition, certain root morphological properties, such as specific root length and root diameter, mediate decomposability, colonization, and soil mineral interactions, thereby controlling the turnover of root organic C [76,77,78].

2.4. Mycorrhizal Associations (AM vs. ECM Species)

Plant species are characterized by associations with types of mycorrhizae, mostly arbuscular mycorrhizal (AM) or ectomycorrhizal (ECM) fungi, depending on the species [20,79]. According to Smith and Read [80] AM fungi penetrate root cortical cells, with their colonization rates influenced by the volume of available root cortex. Conversely, ECM fungi expand externally, creating a hyphal network that envelops the root epidermal cells. As mentioned above, tree species may associate with different mycorrhizal types (e.g., AM or ECM). In general, AM tree species primarily utilize inorganic nutrient availability and allocate limited C resources for fungal associations, and require less C compared to host trees [81,82]. These fungi enable tree species to allocate more C to the root rather than the fungal tissue [83].

ECM tree species allocate more C resources underground through organic nutrients from enzymatic degradation, impacting soil C cycling and stabilization. This ECM-driven approach leads to slower litter decay and increased organic matter accumulation due to competition with free-living decomposers [84,85]. By contrast, associations with soil microbes tend to be related to enhanced soil nutrient cycling [86] and microbial turnover, which can be associated with soil C cycling and stabilization [87]. Overall, the ECM leads to suppressing litter decay and helps form a more stable soil organic matter (SOM); therefore, it results in an increased soil C pool [88,89]. Therefore, these fungi can increase soil C:N in the SOM [90]. In contrast, AM with higher rates of nutrient cycling [91] and microbial activity results in higher C cycling within the system [92] but lower C retention in the soil. In this regard, Yin et al. [90], indicated that AM-treated trees promoted significantly more rhizosphere priming and enhanced soil C breakdown compared to ECM-treated trees. Thus, it can be reported that ECM-associated trees probably enhance soil carbon retention more effectively than AM-associated trees, due to reduced microbial enzyme expenditure on carbon breakdown. Soudzilovskaia et al. [93] also highlighted that AM fungi predominantly influence the C pool in living plant biomass, while ECM fungi primarily impact the soil carbon stock directly.

According to Liu et al. [94] ECM-treated soils contained younger C in the upper layer of the soil but older C in the deeper layer of the soil compared to AM-treated soils. As expected, Carteron et al. [95] reported that deciduous forest ECM stands contained more soil C (up to 20 cm depth) compared to adjacent AM-dominated stands. However, in the deeper horizons of the ECM, the organic matter was decomposed more slowly and of lower quality.

3. Control of SOC Fractions by Tree Species

Fractionation serves as an effective method for monitoring alterations in various labile, recalcitrant, and total soil C stocks. Consequently, it helps local authorities determine appropriate management strategies tailored to specific soil types, taking into account both economic productivity and soil quality [96]. The Species of the tree also affects not only the total stock of SOC but also its distribution based on various functional C components or fractions [97]. The different functional SOC components vary according to their availability to microorganisms [98], soil residence times [99], and their role in C storage [23]. The properties of various tree species govern their role in the formation, conversion, and stability of the C fraction [5].

SOC can be categorized into labile (active), intermediate, and passive (stable) fractions based on turnover time and stabilizing mechanisms [100]. The labile or active C (AC) pool portion of SOC is defined by its hot water-soluble C (HWSC), or dissolved organic C (DOC), permanganate-oxidizable C (POXC), and microbial biomass C (MBC) [97,101,102].

The bridge of AC and PC often represents the intermediate SOC pool (IC); therefore, this fraction can be considered a transitional C fraction with both labile and stable characteristics [103,104]. Thus, particulate organic C (POC) can be regarded as the IC fraction. The passive SOC (PC) pool, which facilitates long-term soil C sequestration, primarily consists of mineral-associated organic C (MAOC), stabilized by robust interactions with soil minerals such as clays and metal oxides (Figure 1) [105,106,107]. The stabilization of SOC results from microbial degradation of the labile C pool, followed by its transfer to mineral-associated fractions.

3.1. Labile SOC Pools

3.1.1. Dissolved Organic Carbon (DOC)

DOC results from the leaching of soluble components from litter and roots as exudates and provides energy for microbial life in the soil, and helps the C and nutrient cycle in the forest [108,109,110,111]. Tan et al. [112] highlighted the essential role of forests to sequester C by tree species and soil in the form of DOC, which can be conveyed via streams. This fraction of the SOC can be mixed by minerals not only in the soil, but also in water, and can be transported to the deep ocean or can be changed to CO₂ by microbial activity in water and affect the global C cycle and climate change [113,114]. Tree species differ in their volumetric production of DOC, depending on the solubility of their litter and the activity of their roots [115]. According to Uselman et al. [116] root litter serves as a significant DOC in forests. Presumably, it plays a crucial role in the accumulation of SOM and the stabilization of C during the development of forest ecosystems. Wölfelschneider et al. [117] also indicated that forests, especially mangrove forests, significantly facilitate the generation and export of DOC from soil by leaching and runoff to aquatic ecosystems. As indicated above, DOC plays a crucial role in the vertical transport of C within the soil profile [118]. This SOC fraction also serves as an essential interface between plant inputs and microbiological processing, and the microbial activity, along with plant species, that significantly contributes to DOC creation [119,120]. The interactions between DOC and soil minerals also control the further processing of this C as mineralized, immobilized, or stabilized C [121]. In this line, Kalbitz et al. [122] reported that the stability of DOC by mineral association is a crucial mechanism for long-term soil C sequestration; yet, its effectiveness is markedly affected by hydrological variability and biological processes in natural soils. Fröberg et al. [123] reported that the soil under spruce and pine had a higher input of DOC to mineral soil compared to birch. Hansson et al. [124] mentioned that the variations in DOC flux were mainly ascribed to reduced forest floor C reserves in birch. Likewise, DOC inputs to the upper and mineral forest soil were determined to be greatest under Norway spruce and beech [125]. The composition of the predominant forest plant species significantly influences the quality of DOC and its decomposition due to variations in litter chemistry and interspecies interactions that impact C cycling in forests [126].

3.1.2. Permanganate-Oxidizable Carbon (POXC)

POXC serves as a prevalent indicator of the labile organic C reservoir in soils and can be rapidly oxidized by potassium permanganate [127]. POXC is closely associated with fresh plant material, microbial biomass, and a labile fraction of SOC, hence influencing the rapid turnover of organic C [100,128,129]. POXC’s fast reaction to environmental alterations renders it a prevalent indicator for assessing C dynamics, soil health, and soil quality indicator [130], and vulnerability to shifts in forest vegetation [5]. Forest tree species significantly influence POXC through differing litter inputs, litter quality, and root C contributions. Pine litter contains a higher C:N ratio and persists in decomposition [131]. Tree species that generate litter with a low C:N ratio, minimal lignin content, and elevated nutrient concentrations are likely to sustain a substantial POXC pool. Oliveira et al. [132] stated that under loblolly pine plantations, soil POXC diminished with increasing soil depth and has a positive correlation with SOC and nitrogen; nevertheless, its responsiveness to silvicultural methods is constrained and scale dependent. Tree species that generate litter functioning as a resistive input, like pine trees, are expected to diminish the turnover rate of the POXC pool [131]. Plant roots provide an important source of soil AC by rapidly transferring fresh C to symbiotic microorganisms, especially bacteria and mycorrhizal fungi, which can lead to the increased dynamics of POXC [133]. In addition to direct C intake, forest tree species indirectly influence POXC via regulating soil physicochemical qualities [134] and biological activity [135]. Tree species influence soil pH [136], water content [137], and the ratio of fungal to bacterial populations [138], which subsequently regulates microbial breakdown of organic waste and the creation of POXC. Consequently, alterations in forest species composition due to species replacement, invasion, and succession can lead to significant changes in POXC levels, thereby affecting the potential for soil C to be converted into a stable soil fraction [97].

3.1.3. Microbial Biomass Carbon (MBC)

MBC is the labile portion of SOC [139]. As reported by Das et al. [140] MBC in forest ecosystems is markedly affected by tree species, temperature, precipitation, soil depth, and the quality of organic matter, exhibiting a positive association with SOC and soil basal respiration. Tree species and their age significantly impact MBC as they control rhizosphere environments through root exudation, microbial C and N dynamics, litter, and nutrient release [141]. Tree species with high-quality litter and active rhizosphere environments tend to facilitate more microbial growth and bacterial diversity [142]. Putra et al. [143] revealed that agroforestry systems augment soil MBC via increased canopy cover and litter inputs relative to annual cropping and degraded grasslands. Microbial associations play a crucial role in the processing of the global C cycle [144]. Turnover of microorganisms, followed by necromass production, is a significant source of stable C in soils [145]. Camenzind et al. [146] also illustrated that microbial mortality pathways significantly influence the content and longevity of microbial necromass, thereby affecting SOC sequestration, and emphasizing that both microbial proliferation and mortality are crucial to the soil microbial C pump.

Plants with attributes of high microbial productivity or efficiency in using C in soils can increase the contribution of microbial residues to SOC [147]. Therefore, species-specific rhizosphere microenvironments can control microbial biomass [148,149] and indirectly control SOC in soils. Jiang et al. [150] found that tree plantations influence soil microbial biomass, and the conversion of grassland to plantation increased MBC. In the significance of tree species on MBC content Wu et al. [151] also mentioned that the mix of tree species plays a vital role in increasing MBC in the forest compared to a pure stand. In this regard, Duan et al. [18] also emphasized that the increased tree species diversity promoted microbial C use efficiency via modifying soil microbial biomass and, therefore, boosting soil C storage or MBC in subtropical forests. Similarly, an increase in soil MBC content by enhancing tree species diversity in subtropical secondary forests was reported by Yuan et al. [152].

3.2. The Intermediate SOC Pool (IC)

Particulate Organic Carbon (POC)

POC is a fraction of SOC that is mechanically defined by its particle size (commonly 53–250 µm) [153] derived mainly from plant residues that are not fully decomposed and contain high amounts of fungal materials and lignin [154]. As a result, there is an intimate relationship between POC and the amount of litter [155,156]. Tree species that generate high-quality and large amounts of organic litter materials, particularly those of high complexity, contribute significantly to the development of POC, especially in surface soil [157]. On the other hand, forest management and forest conversion can decrease this fraction of SOC [158,159]. Differences in chemical composition and decomposition rates among various species also affect both the accumulation and stability of POC. Specifically, POC stability was higher beneath species with higher root N concentrations and lower amounts of acid-insoluble compounds, and lower beneath species with higher tissue calcium (Ca) content. In contrast, MAOC (see Section Mineral-Associated Organic C (MAOC)) appeared less sensitive to species identity over the timescale studied. These results indicate that tree species regulate particulate and mineral-associated C pools through tissue composition, root traits, and decomposition patterns, with POC being more responsive to species-specific traits, while MAOC remains relatively stable [160]. Even if POC is quite susceptible to physical disturbance and microbial degradation, some species provide greater physical protection against disturbance through aggregation, enhancing their long-term stability [161]. Tong et al. [162] reported that afforestation, particularly with Robinia pseudoacacia L. (robinia) and Populus tomentosa Carrière (poplar), increased the physical protection of POC by enhancing intra-microaggregate formation and improving soil structure. These species resulted in greater overall soil SOC accumulation than Caragana korshinskii Kom. (caragana) and Hippophae rhamnoides L. (buckthorn), which showed smaller increases and more limited effects across the soil profile. Angst et al. [160] also emphasized the essential role of tree species in regulating soil C stabilization by controlling the decomposition and physical protection of POC. Consequently, changes in the diversity of species can significantly influence the contribution of POC to total SOC.

3.3. The Passive SOC Pool (PC)

Mineral-Associated Organic C (MAOC)

MAOC is a form of organic C in the soil that forms and is stabilized due to the association of organic matter with minerals like clay and oxides of Fe and Al [163]. The formation of MOAC can be affected by climate conditions, as reported by Li et al. [164] SOC stock increases in the form of MOAC content in the environment with less rainfall. Organo-mineral materials in this condition act as a barrier to C degradation by microorganisms; hence, C stabilization occurs through MAOC [165,166]. MAOC is considered a stable form of C in the soil [167]. Furthermore, MAOC mainly comprises microbial products with elevated nitrogen content, which remains in the soil due to chemical bonding with calcium, magnesium, and manganese oxides, in addition to physical protection afforded by macroaggregates [168,169]. Moreover, the MOAC content in the soil depends on several parameters, including polyvalent cations, acidification, and nutrient availability, which affect microbial biomass content. Thus, the leaching of calcium and magnesium through soil acidification and reducing nutrient availability and microbial biomass leads to a decrease in MOAC content in the soil [166,170,171,172].

Forest tree species largely indirectly control the formation process of MAOC, primarily through the regulation of both the quantity and chemistry of C inputs to the soil [173]. In this regard, Chen et al. [174] reported that in subtropical forests characterized by various tree species, MAOC was 78%–86% of SOC content, establishing it as the predominant SOC pool and a crucial regulator of microbial biomass and enzyme activities. The microbial symbiosis of the ECM with forest tree roots also impacts the formation of more persistent MAOC [94]. Differences among forest tree species in root and leaf litter chemistry, as well as exudation, influence microbial processing, a key factor in the formation of MAOC [175,176,177]. Dai et al. [178] stated that the addition of plant litter of London plane tree (Platanus × acerifolia) enhanced MAOC by 0.90 to 2.09 times, with leaf litter exhibiting the most significant effect and branch litter the least. The creation of MAOC occurs through two pathways, referred to as ex vivo and in vivo. The ex vivo process involves the mineral adsorption of low-molecular-weight DOC resulting from exoenzyme depolymerization or plant litter exudate, in the absence of microbial transformation. Conversely, the in vivo route necessitates the microbial digestion of plant litter, resulting in the production of microbial secretions or necromass [166,178,179,180]. Consequently, numerous recent and previous studies have also emphasized the critical and vital role of plant species in the formation of MAOC and, therefore, SOC stabilization [181,182,183,184]. The effects of different tree species or functional groups on soil organic carbon (SOC) and its fractions, the impacted soil components, and the relevant references are summarized in

Table 1. Effects of tree species on soil organic carbon (SOC) fractions in forest ecosystems.

Tree Species/Functional Group	SOC and Its Fraction Content	Impacted Components	References
Mixed forests	MAOC ↑, and POC ↑ (relative to younger forests or monocultures)	forest age ↑, MAOC/SOC ↓, but soil depth ↑, MAOC ↑ (MAOC ↑ in deeper soils).	[173]
Pinus tabuliformis	microbial necromass C (MNC)/MAOC ↓ (11.9%) (relative to Quercus aliena at 20–40 cm depth)	total phosphorus, soil moisture, and texture, and cellulose content in litter.	[185]
Quercus aliena	MNC/MAOC ↑ (21.2%) (relative to Pinus tabuliformis at 20–40 cm depth)
Primary mixed broadleaved–Korean pine forest → secondary broadleaved forest	free POC 20.3% ↓ and aggregate-occluded POC 57.2% ↑ (relative to primary forest before conversion)	litter and superior fine root quality ↑, C/N ratio ↓, SOC, and microbial biomass content ↑	[159]
Primary mixed broadleaved–Korean pine forest → coniferous plantation	Free POC 49.1% ↑, while aggregate-occluded POC 42.4% ↓ and MAOC 9.0% ↓ (relative to primary forest before conversion)	litter and fine root quantity and quality ↓, SOC ↓, microbial biomass ↓, and microbial residue C ↓
Populus tomentosa (deciduous broadleaf)	SOC ↑; MAOC ↑; MAOC/SOC ↑ (relative to Pinus tabuliformis in the same urban green space)	Fungal-dominated microbial communities ↑, C/P ↓, hydrolytic enzyme activities ↑, SOC accumulation, and MAOC ↑	[186]
Pinus tabuliformis (evergreen conifer)	SOC ↓ or NC; MAOC/SOC ↑ (site-dependent) (relative to Populus tomentosa in the same urban green space)	oxidative enzyme activity ↑, organic matter oxidation ↑, and SOC ↓
Quercus wutaishanica	SOC:NC; POC:NC; MAOC:NC (relative to the same forest before N addition)	microbial communities: NC to N addition, SOC fraction dynamics: NC	[184]
Betula platyphylla	SOC:NC; POC:NC; MAOC ↑ (relative to the same forest before N addition and relative to Quercus wutaishanica)	Microbial residues (amino sugars) ↑, microbial biomass, and enzymes: NC
mixed-species plantations (five-species mixtures)	SOC ↑, MAOC ↑ (76.55%) (relative to reference secondary forest and relative to monoculture and two-species mixture plantations)	Diverse necromass inputs, organo-mineral interactions ↑, available N ↑, bacterial + fungal necromass ↑	[187]
Pure coniferous plantation → coniferous–broadleaf mixed forest	POC ↑ (80.2%–169.8%) (relative to pure coniferous plantation before conversion)	physically protected by macroaggregates and Fe oxides ↑	[188]
	MAOC ↑ (41.1%–137.3%) (relative to pure coniferous plantation before conversion)	microaggregate formation ↑ and long-term SOC stabilization ↑
Quaking aspen (Populus tremuloides)	Stable SOC ↑ (relative to conifers in the same site)	Root–microbe–soil (rhizosphere) interactions	[189]
Conifers	Labile SOC ↑ (relative to Populus tremuloides in the same site)	Root detritus inputs
Conifers	SOC stock 44% ↑ (relative to broadleaved species)	litter C ↑	[190]
Broadleaved	SOC 44% ↓ (relative to conifers)	litter C ↑
AM-associated	SOC stock 39% ↓ (relative to ECM-associated species)	litter decomposition ↑
ECM-associated	SOC stock 39% ↑ (relative to AM-associated species)	litter decomposition ↓
Mixed broadleaved forests (3–5 species)	MAOC ↑, SOC pools ↑ (relative to monospecific forests)	Tree species diversity and fine root biomass and turnover ↑	[191]
Juniperus excelsa (Juniper), Cedrus libani (Cedar), Abies cilicica (Fir)	Total SOC ↑, PC ↑, C Management Index (CMI) ↑ (mostly under Juniper, Cedar, Fir relative to oak and shrubby land)	Species-specific rhizosphere effects	[97]
Quercus L. (Oak)	Labile SOC ↑ (relative to Juniper, Cedar, Fir, and shrubby land)
Pinus brutia	SOC 22% ↑ AC ↑, PC ↑, SOC lability ↓ (relative to Quercus coccifera)	Species-specific rhizosphere effects	[5]
Quercus coccifera	AC ↑, PC ↑, SOC lability NC (relative to deforested shrubby land)
Deforested shrubby land	SOC ↓-AC ↓, PC ↓, SOC lability ↑ (relative to both Pinus brutia and Quercus coccifera forests)	SOC ↓, PC:AC ↓
Deciduous broadleaved	Stable SOC stock ↑ (relative to pre-afforestation or shrubby land)	Consistent SOC ↑ after afforestation; strong performance on grasslands	[192]
Sempervirent broadleaved	SOC stock ↓ (young trees) (relative to stand age)	Initial SOC ↓ following afforestation
	SOC stock ↑ (mature trees) (relative to stand age)	SOC ↑ (~20 years after afforestation)
Sempervirent conifer	SOC stocks ↓ (low rate) (relative to pre-afforestation or baseline soil)	Lowest SOC stock change among species groups

Note. Increase = ↑, decrease = ↓, NC = no detectable change, conversion = →.

.

4. Tree Species Effects on Soil Carbon Stabilization Pathways

Tree species affect the mechanisms by which C is stabilized in soils. The regulation of biological, chemical, and physical processes by tree species influences these phenomena [160]. Tree species-specific materials interact with microorganisms and minerals in soils to determine the efficiency of the stabilization process [193]. An understanding of the role of various forest tree species in enhancing or inhibiting the progression of stabilization processes is essential and relevant, especially in forest management [194,195].

4.1. Mineral-Associated C Formation

As indicated above in the MAOC section, tree species influence the formation of mineral-bound C through the quality and quantity of organic material entering the soil [160]. These organic materials as exudates and microbial metabolic by-products, resulting from plant matter, will precipitate on especially clay mineral surfaces to create C-stable associations [196]. However, tree exudates are not the same in C stabilization in this regard Li et al. [197] reported that robust organic ligands (e.g., oxalic acid) swiftly facilitate the mobilization of MAOC through direct mineral dissolution and desorption. In contrast, simple sugars (e.g., glucose) encourage a more gradual, microbially mediated MAOC mineralization. Additionally, reductants (e.g., catechol) can initiate both processes, suggesting that the longevity of MAOC is co-regulated by mineral reactivity and biological contributions at the root-soil interface. Although, according to recent studies, it is to be considered that soil C stabilization embodies an intrinsic trade-off wherein root exudates and mineral composition together facilitate the development of MAOC but concurrently augment C losses via microbial respiration [198]. Tree species with high microbial turnover will also lead to an increase in mineral-reactive organic material [159,199]. Variations in litter chemistry and rhizosphere processes between species also influence organo-mineral interaction. Organic matter with suitable chemical properties tends to bind to the surfaces of clay and metal oxides [200]. Therefore, forest species diversity influences the soil’s ability to sequester and stabilize C through mineral associations [28].

4.2. Soil Physical Protection and Aggregation

Tree species influence soil structure through their effects on root growth [201], fungal networks [202], and organic matter inputs [203], all which impact soil aggregation [204]. Soil aggregates physically protect organic matter from microbial decomposition due to limited accessibility [1]. Plant species with dense rooting systems or extensive mycorrhizal networks can commonly promote the formation of aggregates [205]. Moreover, root development enhances soil structure by improving porosity, which supports water circulation, nutrient cycling, and protects SOC, while soil compaction undermines these benefits and reduces soil quality [206].

Aggregation patterns in soil vary significantly, depending on species-specific influence on SOC distribution within soil aggregates [207]. Stable soil aggregates can store C for an extended period, primarily when associated with fine mineral particles [1]. In this line, Ozlu and Arriaga [208] reported that the stabilization of SOC is mainly influenced by silt dominance and mineralogical composition, rather than clay concentration. Similarly, Li et al. [209] also emphasized the planting of mixed-species plantings to increase SOC sequestration by promoting soil aggregation, especially macroaggregates, and improving soil nutrient cycling in afforestation systems. Earlier research by Rodríguez et al. [210] reported enhanced SOC storage associated with increased root-derived macroaggregate formation for restoring SOC stocks in deforested Amazonian soils. Thus, different tree species indirectly control physical protection mechanisms, leading to long-term C stabilization and storage [211].

Table 2. Tree species effects on soil organic carbon (SOC) stabilization pathways in forest ecosystems.

Tree Species/Functional Group	SOC Stabilization	Affected Components	Location	References
Secondary forest (Inceptisols)	SOC stabilization ↑ (relative to pasture)	Sorption of organic C to clay minerals ↑	Ecuador	[212]
Secondary forest (Andisols)	SOC stabilization ↓ (relative to pasture)	Formation of metal–humus complexes and allophane
Early succession (Lespedeza bicolor)	SOC stabilization ↓ (relative to late succession, Quercus liaotungensis)	Macroaggregate formation ↑; C transfer ↑ (rapid)	China	[213]
Late succession (Quercus liaotungensis)	SOC stabilization ↑ (relative to early succession, Lespedeza bicolor)	C occlusion in silt and clay within aggregates (root–microbe-mediated)
Leucaena leucocephala (most effective afforested species, 20 years)	SOC stabilization ↑ (relative to other afforested species)	Physical protection in heavy fraction (<0.25 mm) and moderate biochemical recalcitrant C	China	[214]
Primary natural broadleaf → secondary and plantation forests	SOC stabilization ↓ (relative to primary natural broadleaf forests)	Macroaggregate-associated SOC ↓ + tree biomass (litter and root) ↓ and Fe/Al oxide concentration ↓	China	[215]
Norway spruce	SOC stabilization ↓ (relative to mixed forest)	Litterfall accumulation in the forest floor	Czech Republic	[216]
European beech	SOC stabilization ↑ (moderate) (relative to Norway spruce)	Root-derived C inputs to mineral soil
Mixed forest (Norway spruce + European beech)	SOC stabilization ↑ (relative to monocultures of Norway spruce and European beech)	Litterfall and root turnover complementarity (root-driven stabilization)
Temperate forest	SOC stabilization ↓ (high microbial activity reduces MAOC)	Litter (high-quality) ↑ + microbial growth ↑, SOC decomposition rate ↑	United States	[217]
Pinus brutia	SOC stabilization ↑ (relative to non-forest soils in the region)	Protection of SOC by micro-aggregate (<2 mm)	Türkiye	[1]
Quercus coccifera	SOC stabilization ↑ (relative to non-forest soils in the region)	High protection of SOC by fine-aggregates (0.25–0.05 mm)
Pinus tabulaeformis and Forsythia suspensa	SOC stabilization ↑ (relative to P. tabulaeformis monoculture)	Bacterial necromass-driven MAOC ↑	China	[218]
Pinus tabulaeformis and Quercus wutaishanica	SOC stabilization ↑ (relative to P. tabulaeformis monoculture)	Fungal necromass-driven POC ↑
Forest (mature)	SOC stabilization ↓ (relative to grassland)	Microbial activity ↑; Ca-mediated stabilization ↓	Russia	[219]
Grassland (climax)	SOC stabilization ↑ (relative to forest)	Organo-mineral interactions ↑; Ca-bound humic acids ↑
Cunninghamia lanceolata plantations	SOC stabilization: NC (relative to untreated control)	Microbial–enzyme interactions in litter and soil; smoke effects on C cycling	China	[220]
Shorea robusta (no/low disturbance (ND/LD))	SOC stabilization ↑ (relative to HD forest)	litterfall and root-derived C inputs ↑ and soil nutrients ↑	India	[221]
Shorea robusta (moderate disturbance (MD))	SOC stabilization ↑ (relative to HD forest)	biomass ↑ > other disturbance regimes
Shorea robusta (highly disturbed (HD))	SOC stabilization ↓ (relative to ND/LD and MD forest)	soil nutrients ↓ and erosion risk ↑
Populus euphratica (native) (1), Eucalyptus camaldulensis (introduced) (2), Prosopis juliflora (introduced) (3), Tamarix ramosissima (native) (4), open area (no vegetation) (5)	SOC stabilization ↑ (High to very low) in1 > 2 > 3 > 4 > 5	SOM ↑, SOC sequestration↑, bulk density ↓ (in 5 ↑) SOC sequestration in 1 (9.08 t ha−1) > 2 (8.37 t ha−1) > 3 (5.20 t ha−1) > 4 (2.93 t ha−1) > 5 (1.33 t ha−1)	Iran	[222]
Alnus glutinosa (1), Carpinus betulus- Acer velutinum (2), Populus deltoides (3), Cupressus sempervirens, var. horizontalis (4), Degraded land (5)	SOC stabilization ↑ (High to very low) in 1 > 2 > 3 > 4 > 5	Litter quality ↑, soil N, P, K, SOC ↑, microbial biomass ↑, enzyme activity ↑ In contrast to non-native species and poor degraded land, Alnus glutinosa showed the highest SOC stabilization↑ due to N fixation ↑ and litter decomposition rate ↑, and soil recovery after 25 years	Iran	[223]

Note. Increase = ↑, decrease = ↓, NC = no detectable change, conversion = →, > = greater than.

presents an overview of how tree species or functional groups influence SOC stabilization, highlighting the affected components, study locations, and cited references.

5. Species-Specific Rhizosphere Processes and Priming Effects

The rhizosphere is an interface characterized by intense interaction between the root organs of plants, microorganisms, and organic matter in soils [224,225]. Tree species differ in their ability to regulate rhizosphere interactions via root exudation [226], microorganisms [227], and nutrient exchange [228]. These interactions have a substantial effect on SOC through priming interactions [229,230].

Priming can either stimulate or hinder the decay of existing SOC, depending on species and microbial responses [231]. It thus behooves us to understand species-specific rhizosphere interactions to better predict C feedback to the ecosystem.

5.1. Variability of Root Exudate Quantity and Chemical Composition Among Species

Root exudates are sugars, organic acids, and amino acids of varying complexity, depending on the species, which affects the quantity of labile C released in the soil, thereby influencing the amount of easily accessible C provided to soil microorganisms [232,233]. Similarly, Yan et al. [234] by meta-analysis of 104 publications, indicated that root exudate chemistry and environmental context jointly influence priming effects on SOC decomposition, highlighting the general importance of root-derived inputs in soil C cycling. Bolan et al. [235] likewise, observed that carboxylates from rhizosheath, derived from plant roots and soil microorganisms, enhance the turnover of SOC by facilitating nutrient uptake and increasing microbial activity, hence inducing priming effects that govern C decomposition processes. However, according to recent research, the mixing of tree species primarily alters the chemical composition of root exudates rather than their quantity, with these changes reflecting interactions among species and varying by stand age, thereby influencing belowground C dynamics [226]. Exudate chemical differences also control microbial nutrient demand and metabolism [236]. Depending on their release, unique release method, and content, root exudates consist of diffusates (low-molecular-weight organic molecules that passively permeate the rhizosphere), secretions (which the plant meticulously regulates, encompassing chemicals that facilitate signaling, nutrient absorption, and stress resilience), and excretions (including mucus, which facilitates root penetration) [237,238,239]. Exudates are also classified by molecular weight: low-molecular-weight exudates include amino acids, sugars, organic acids, and secondary metabolites, whereas high-molecular-weight exudates are predominantly composed of proteins and polysaccharides [237,238]. The presence of higher proportions of low-molecular-weight compounds stimulates faster microbial uptake [240], while the production of more complex compounds may delay microbial activity. Bacteria preferentially use compounds derived from root exudates rather than from native SOM under the condition that C and N are abundant [241].

Tree species show significant variation in the rate and composition of root exudates released in the rhizosphere [242,243]. According to Neumann et al. [244] about half of the C assimilated by soil-grown plants is preserved as root tissue, and half of this C in root is related to the composition of root products, including root exudates, border cells, and root debris. Parameters, including plant species, cultivar, age, soil characteristics, and stress levels (like drought and salinity stress), affect the volume of these exudates [245]. Consistent with these findings Brunn et al. [246] revealed that in drought conditions, Fagus sylvatica and Picea abies maintained root exudation despite a nearly 50% decrease in C assimilation, allocating 1.0% and 2.5% of net assimilated C, respectively, with drought enhancing C allocation to exudation by two- to three-fold. In this line, Li et al. [247] reported reduced root C, not N exudation in Picea crassifolia, resulting in a lower C:N ratio of root exudates under drought stress conditions. Furthermore, Jing et al. [248] found that upon the addition of N and variation in soil properties, various plant species exhibited distinct responses to root exudation. The Pinus tabulaeformis, a tree in the forest canopy, produced the highest amount of exudate, followed by the Rosa xanthina, a shrub in the understory, and the Carex lancifolia, a grass in the understory. Carbohydrates constituted the primary component of trees and shrub exudates, whereas fatty acids were predominant in grass exudates. The leaf and root exudate metabolomes of four subtropical forest trees, including Cinnamomum camphora, Cyclobalanopsis glauca, Daphniphyllum oldhamii, and Schima superba, were determined by Weinhold et al. [242]. They reported that Flavonoids predominate in leaf metabolites. At the same time, carboxylic acids and prenol lipids were more prevalent in root exudates, which suggested that changes in root exudate composition reflect quality differences rather than merely an increase in total C input to the rhizosphere resulting from diversity. In addition, as discussed above, by promoting microbial activity and bacterial community composition, C-rich root exudates, such as carboxylic acids and sugars, have essential roles in C dynamics [249]. Therefore, the role of species exudation rates as critical components contributes significantly to variability in rhizosphere effects and helps explain why plant species differ in their impacts on soil C dynamics.

5.2. Microbial Community Responses in the Rhizosphere

The root inputs specific to a species tend to have a significant impact on the population and composition of microorganisms in the rhizosphere [250]. This occurs depending on the plant’s diversity, which regulates the dominant functional groups of microorganisms, such as bacteria and fungi, with varied decomposition methods [251]. These microbiological changes affect the overall activity of microorganisms and the efficiency of C processing and the C cycle [144]. Plant diversity helps to promote microbial communities in the rhizosphere, thereby enhancing C sequestration [252]. Tree species known to encourage the presence of microorganisms can play a role in optimizing the turnover of C in the rhizosphere, with some species allocating more plant-derived C to microbial communities and thereby enhancing microbial activity [253]. Regarding this point, Lv et al. [254] indicated that enhancing tree species variety augments soil C dynamics by elevating MBC, improving microbial community and activity, respiration, and expediting C turnover through root-derived labile C. As a result, accelerated C cycling transpires in the rhizosphere, influencing the equilibrium of C stabilization and decomposition in forest soils. On this matter, Li et al. [255] revealed that the integration of broad-leaved trees with Pinus massoniana and Pinus elliottii enhanced the quality and mineralization rates of rhizosphere SOC by decreasing the ratios of aromatic to aliphatic compounds and increasing copiotrophic bacterial taxa, particularly Proteobacteria and Bacteroidetes. Regarding this issue, a prior study reported the significant role of mycorrhizal associations with tree species, such as Larix spp. and Suillus grevillei, which function as crucial reservoirs for plant-derived C. As mentioned above, the differences in mycorrhizal growth forms and hyphal lifespan affect the quantity, distribution, and sequestration of C in forest ecosystems [256]. Furthermore, additional evidence has indicated that in organic soil, Betula pendula, relative to Pinus sylvestris and Picea abies, demonstrated elevated MBC, alongside heightened fungal and Gram-positive bacterial indicators but in mineral soil, all tree species similarly stimulated microbial activity and C mineralization, with no notable differences among species, indicating that vegetation presence rather than species identity drives microbial responses [257]. Similarly, changes in the microbial communities in the rhizospheres and rhizoplanes of two forest tree species, including Larix eurolepis and Picea sitchensis, predominantly altered differences in soil C compounds, including sugars, carboxylic acids, and amino acids, reported by Grayston and Campbell [258]. Notably, Jia et al. [259] emphasized the different origins of soil C with different microbial life under different tree species and reported that ECM-Pinus massoniana resulted in increased accumulation of plant and microbial-derived C. In contrast, AM-Castanopsis eyrie exhibited diminished microbial necromass stability and a reduced contribution of microorganisms to rhizosphere C in the Gutianshan subtropical forest of China. Along similar lines, Sinha et al. [260] indicated significant potential of forest tree Aegle marmelos with the highest MBC and turnover rate, for restoring soil C functioning in damaged mining soils of Dhanbad, India. Overall, accumulating evidence from multiple studies supports the importance of tree species in shaping microbial communities as an essential parameter in soil C dynamics [261,262,263,264,265].

5.3. Enzyme Activities and SOC Decomposition

Various species of trees regulate the enzymatic process in the breakdown of organic matter [266]. The amount of C from litter and roots influenced the enzymatic activity in the decomposition of cellulose, lignin, and organic matter [267]. However, according to Xu et al. [268] the activity and variability of enzymes did not significantly influence long-term C sequestration in the forest zone. They reported that polyphenol oxidase activity in the forest soil increased with recovery time, while urease and catalase activity decreased. Saccharase showed a cubic response, but despite these enzyme changes, SOC stability was mainly unaffected, suggesting that enzyme dynamics have a minimal impact on long-term C storage in the Loess Plateau of China.

Rhizosphere microorganisms can also change the soil’s enzymatic content for the decomposition of organic matter in the forest [269]. Regarding this issue Lin et al. [270] stated that the rhizospheric microorganisms in Kashmir forests demonstrate considerable enzymatic activity that facilitates the decomposition of conifer litter, hence reducing litter accumulation and fostering the natural regeneration of Picea smithiana and Abies pindrow.

Various species of trees found in a forest significantly impact soil enzyme function, as different species result in varying levels and qualities of organic matter, which is added to the soil [271]. Although greater organic inputs can generally stimulate microbial activity through increased substrate availability, tree species also differ in litter chemistry and root inputs, which can influence enzyme activity patterns. For instance, deciduous or evergreen species, or mixed species, create different enzymatic activities in the soil. As reported by Wang et al. [272], mixed-species plantations of Quercus variabilis and Platycladus orientalis enhance enzymatic activity and overall soil quality in comparison to monocultures. In this context, Hu et al. [273] also stated that composite leaf litter from Cunninghamia lanceolata, Liquidambar formosana, and Alnus cremastogyne species enhanced soil microbial biomass and enzymatic activity (urease, invertase, and dehydrogenase), thereby promoting SOC transformation. Conversely, the monospecific litter of Chinese fir (C. lamceolata), characterized by a high C:N ratio, enhances polyphenol oxidase activity and prolongs the decomposition process. The significant role of Species mixing in Chinese fir plantations in enhancing enzymatic activities and SOC has also been reported by Guo et al. [274]. Furthermore, Fu et al. [275] explained that increased tree species diversity, enhanced acid phosphatase and urease activities, decreased the relative abundance of ECM fungi, and increased saprotrophic fungi, augmenting SOC dynamics and total N, nutrient availability in subtropical evergreen forests. As indicated above, there are interconnections in the activities of enzyme functions and SOC decomposition for different species of trees. In the decomposition of organic litter, the balance between hydrolases and oxidases plays an important role. According to Xu et al. [276], in the decomposition of litter in Robinia pseudoacacia plantations, the labile SOC fractions and stability were influenced by modifying the activity of microbial carbon-degrading enzymes. As the labile SOC fractions increase, hydrolase activity decreases while oxidase activity increases. Litter quantity, lignin concentration, soil moisture, and oxidase activity favorably affect SOC fractions, but cellulose and soil pH exert negative influences, indicating that higher soil pH and greater cellulose availability are associated with lower SOC accumulation and stability, reflecting shifts in the balance between hydrolase and oxidase activities during decomposition.

5.4. Positive and Negative Priming Effects

Forest tree species exert their effects on the rhizosphere priming effect and SOC through variations in their root systems, primarily in terms of root properties and C deposition in the soil [277]. As mentioned above, tree species differ in terms of the quantity and quality of root exudates that are released. This affects microbial activity in the rhizosphere [278]. Labile root exudates, such as organic acids and sugars, accelerate microbial growth and the production of enzymes, thereby causing positive priming and the rapid degradation of existing SOC [234]. There are also variations in litter quantities and qualities among different species of trees, and these significantly influence microbe and SOC dynamics [279]. Although there are many complex results about lignin decomposition, in general, the litter with low lignin content and low C:N ratios decomposes at a higher rate, increasing the requirements for microbial growth and thereby inducing SOC mineralization [56,280]. Litter with high lignin and phenolic compounds, found among some conifer species, decomposes at a slower rate [281], reflecting the high metabolic cost of oxidative enzyme production, and restricts microbe development, thus resulting in negative or weak priming and higher SOC stabilization [282]. Moreover, root-derived C inputs are one of the critical factors that govern microbial dynamics in the rhizosphere and significantly have a priming effect and control SOC dynamics [283]. Cheng et al. [284] indicated that the priming effect generated by root-derived C inputs varies from a reduction in SOC breakdown by 50% to an enhancement of 380%.

Arbuscular mycorrhizal associations are another important association where different kinds of tree species can regulate priming effects. In general, AM fungus-related tree species would favor saprotrophic microbial activity, leading to increased positive priming effects. In the case of ECM-related tree species, there would be competition between free microbes and nutrients for nutrients in soil, thereby increasing the preservation of SOC in soil, leading to negative or weak priming effects [90,285].

According to de Graaff et al. [286] surface soils contain elevated levels of total and labile C that may be due to the priming of SOC by root exudates, which is markedly affected by soil depth. Conversely, deeper soils exhibit diminished priming effects due to lower C availability. These differences may be related to the different root systems of plant species. For species with shallow-rooting habits, there is priming of the surface soil, while for species with deep-rooting habits, there is priming of the subsoil. Furthermore, microbial activity, which contributes to the positive priming effect of SOC, is also observed in surface soil under Picea schrenkiana forest in the Tianshan Mountains [287].

6. Influence of Tree Species Diversity and Forest Mixtures on SOC Dynamics

Tree species diversity influences SOC turnover due to variations in the amount, quality, and location of organic matter inputs into the soil [288]. A forest ecosystem combines different organic matter inputs, root systems, and microbial activities of various tree species and contributes to complex SOC turnover compared to a single-species system [13].

Knowledge of diversity effects is essential for projecting SOC response to diversity in mixed forests.

6.1. Complementarity in Litter and Root Inputs in Mixed Forests

Complementarity in litter inputs, as well as root inputs, is a significant factor that leads to improved soil function in mixed forests compared to monocultures [289]. Since different species of trees produce litter with varying chemical properties, such as C:N ratios, lignin content, or nutrient concentration, when this litter is mixed, a mutual interaction occurs during the decomposition process, leading to the quicker release of nutrients or improved decomposition of the organic matter [290]. As reported by Ding et al. [291] mixed plantations of Pinus massoniana with certain broadleaved species, particularly deep-rooted species such as Bretschneidera sinensis and Camellia oleifera, enhanced the water conservation capability of the litter–soil system more effectively than monoculture plants. The extent of this improvement was significantly influenced by species complementarity, particularly in terms of root features and litter attributes, underscoring the importance of deliberate species selection in plantation management.

In the belowground, the complementarity of root inputs further regulates forest C cycling by enhancing the diversity of C inputs to the soil [292]. Usually, the different species coexisting in a forest differ in their root dimensions, root depth distribution, root turnover, or root exudate composition, or in the mixing of tree species, allowing C to be deposited at various levels of the soil [293,294]. Species with less deep roots mainly deposit labile C to the topsoil, while those with deeper roots deposit C to the subsoil via fine root turnover and exudation. This reflects differences in vertical C distribution reported in the literature rather than distinct or exclusive C fractions. These processes may enhance C stabilization, as lower C mineralization rates in subsoil layers have been reported in previous studies [223,295]. In this regard, Ding et al. [296] observed that mixed forest stands (coniferous and broadleaf) improved soil microbial functionality, interaction networks, and ecosystem multifunctionality in comparison to monocultures, particularly in surface soils. Nonetheless, these beneficial mixing effects were diminished or negated with greater soil depth. Germon et al. [297] reported that the combination of Acacia mangium and Eucalyptus grandis significantly improved fine-root biomass, root morphological efficiency, and deep-soil exploration, resulting in elevated C inputs throughout the soil profile.

In combination with litter and root contributions, mixed forests may enhance C sequestration and contribute to greater aboveground C storage compared with monoculture systems [298].

6.2. Effects of Tree Species Diversity on Microbial Diversity and Function

A key factor influencing the dynamics of SOC in mixed forests is microbial diversity [299]. A growing body of evidence further supports this view [300,301,302]. A greater number of tree species is often associated with a greater number of taxa, as well as a higher functional diversity of microbes, which increases the range of available microbial functions [303,304]. Beyond this, microorganisms with various functions can facilitate the thorough decomposition of organic C compounds, as well as interact with one another to control the rate of C cycling [305]. An enhanced variety of tree species is associated with increased microbial diversity due to the varied C sources. Varied inputs of organic matter promote differentiation among microbial communities, thereby altering the functional composition of microorganisms [306].

Beyond these findings, increased functional diversity of microorganisms can change the stability of SOC through the effect of microbial C use efficiency and the ratio of C mineralization versus incorporation into stable organic form [307]. From a broader perspective, further research has shown that higher functional diversity of microbes often leads to the generation of diverse enzymes and biochemicals, reducing the recalcitrance of organic C and facilitating its conversion into microbes and microbial necromass [305,308]. It should be noted that microbial necromass is considered an essential and stable part of SOC, and its formation in mixed forests largely depends on the composition and functional traits of microbes [309].

Crucially, recent advances have revealed that changes in microbial community composition in mixed forests, such as the shift from dominance by bacteria, fungi, or mycorrhizae, affect SOC significantly in mixed forests [302]. Greater fungal and mycorrhizal diversity is considered to result in reduced C turnover or improved C stabilization through interactions with soil minerals or the formation of soil aggregates [310]. Similarly, Ortiz et al. [311] found that afforestation and the growth of woody vegetation in Mediterranean alpine grasslands modify soil aggregation, microbial community composition, and SOC dynamics, promoting macroaggregation while diminishing microaggregate stability. This results in a reallocation of SOC and N, as well as a transition to fungal-dominated microbial communities, signifying that SOM in high-elevation ecosystems is susceptible to persistent environmental and climatic changes. Zhang et al. [204] demonstrated that mixed forests enhance bacterial and fungal diversity in soil aggregates, particularly those smaller than 0.25 mm, in contrast to pure Castanopsis hystrix plantings. The regulation of these effects is primarily influenced by soil NH₄⁺-N, pH, and organic C, suggesting that mixing tree species is an effective strategy for improving soil microbial structure and fertility in artificial forests. In addition, Anthony et al. [312], after examination of 238 forest plots, emphasized the critical role of diverse fungal communities in the C dynamic compared to bacterial diversity in European forests. Taken together, the current body of knowledge implies that microbial diversity in mixed forests affects SOC somewhat indirectly via the efficiency of C processing or C persistence rather than the addition of C inputs in mixed forests [313,314,315].

6.3. SOC Dynamics in Mixed-Species vs. Monoculture Forests

Although the effects of mixed-species forests on microbial populations, litter, and SOC dynamics are well noted, there are other differences between mixed and monocultured forests regarding SOC dynamics. Mixed forests often exhibit higher stand productivity, improved resource use efficiency, and greater structural complexity, which may contribute to increased organic matter inputs and consequently higher SOC levels [316,317]. Additionally, variations in forest canopies may have effects on the soil microclimate, thereby affecting the rate of SOC turnover [318].

Furthermore, previous research broadly agrees that tree species composition also impacts SOC cycles in various ways, depending on differing patterns of C allocation and rooting depths [229]. In the case of mixed tree species forests, non-overlapping rooting patterns result in C accumulation in diverse depths of the soil profile, which in turn leads to an increase in the proportion of SOC that is stabilized in the deeper layers [319,320]. Monoculture forests, on the other hand, often show less vertical complementarity in rooting patterns, which may result in a comparatively greater proportion of SOC inputs concentrated in surface layers, depending on species characteristics [321].

Evidence from the literature consistently shows that physical protection of the SOC is another difference between mixed-species and monoculture stands. Increased structural heterogeneity can stimulate improvements in soil structure, leading to the development of soil aggregates and enhanced physical protection of organic C within mixed species stands, compared to monoculture stands [22,209].

6.4. Temporal Stability of SOC in Species-Rich Forests

Temporal stability of SOC is the ability of forest soils to sustain relatively constant levels of C over space and time, despite changes and variations [322]. In the context of species-diversified mixed forests, the temporal stability of SOC is of fundamental importance because of the significant role of these forests in long-term C sequestration and climate change mitigation. Compared to monoculture forests, mixed forests exhibit higher functional plant trait diversity, which influences the patterns of C entry, turnover, and storage. Further support for this pattern has been reported by Li et al. [323], who indicated that although thinning initially (after four years) marked a decrease in SOC dynamics, microbial biomass, and enzyme activity in mixed forests of Quercus aliena var. acuteserrata and Pinus tabuliformis, but commenced recovery after twelve years, approaching the levels observed in unthinned stands. These findings highlighted the transient disturbance of soil C processes caused by thinning and the resilience of forest soils, underscoring the importance of recovery periods in sustainable forest management. Notably, Yu et al. [324] stated that mixed-species broadleaved forests (older than 30 years), rather than coniferous stands, enhance productivity and C sequestration, thereby guiding sustainable forest management in subtropical regions of China.

Biodiversity in mixed stands supports SOC stability owing to complementarity in leaf litter and root addition. The leaf litter from different species of trees exhibits varying chemical properties, resulting in differing decomposition rates that do not facilitate rapid C loss in the soil [325]. Notably, recent findings suggest that incorporating oak into pine stands may significantly augment decomposition and maintain elevated soil C reserves, hence enhancing overall soil multifunctionality [326]. However, differences in rooting habits result in increased additions to the soil C pool at various soil depths. Complementarity in these C addition mechanisms enhances the stability of the soil C pool, making it less sensitive to environmental fluctuations [160,327]. Non-forest ecosystems, such as wetlands, also demonstrate depth-dependent persistence of SOC under climatic and anthropogenic pressures [328].

Additionally, interactions between plant species diversity, soil microbes, and microclimates are other factors that influence the stability of SOC over time [21]. Diverse plant species promote diverse and functional microbial communities that are involved in converting SOC into more stable forms [329]. Additionally, globally, diverse forests are known to reduce soil temperature and water variability due to variations in canopy species [330]. This ensures that microbial activity and decomposition are constant, and as such, variability in SOC is maintained [18].

The higher temporal stability of SOC in species-rich mixed forests also has significant implications for forest management and climate adaptation. Species-rich forests are generally less vulnerable to disturbance by drought, pests, and climate extremes, making it less likely that they suffer sudden losses of SOC [331,332,333]. Osuri et al. [334] emphasized the role of temporal stability of SOC in species-rich forests and reported that Species-rich forests, due to functional complementarity among tree species, exhibit greater stability and drought resistance in C sequestration compared to monodominant teak (Tectona grandis) and Eucalyptus (Eucalyptus spp.) plantations, that rendering diverse forests more dependable for long-term climate change mitigation. Therefore, promoting mixed forests and protecting natural species-rich forests may contribute to higher temporal stability of soil C. Furthermore, as climate variability increases, it is essential to maintain species-rich forests.

7. Conclusions

This review highlighted that tree species are identified as major controls of SOC dynamics processes via their influence on soil C inputs, soil C fractions, and soil C stabilization mechanisms. Variations in litter production, litter qualities, root system properties, root excretion, or mycorrhizal types in tree species can considerably affect the production, turnover, or generation of active and passive C fractions. Soil C inputs influenced by tree species can regulate microbial processes, decomposition, and interactions between soil minerals and soil organic matter, thereby controlling the balance between labile and stable soil carbon fractions and the overall SOC pool in forests.

In addition, the diversity of tree species is shown to be a crucial driver of soil C stability through the promotion of litter and root contributions, as well as the support of a range of microbiological activities. Mixed-species forests alter the nature of the rhizosphere and the related priming effects. This leads to a reduced dominance of the more rapid-turnover C fractions and, hence, an increasing likelihood of C persistence within the soil pools. With these perceptions, this study underlined tree species composition and diversity as major drivers of long-term soil C sequestration.