The Role of Bedrock Geochemistry and Climate in Soil Organic Matter Stability in Subtropical Karst Forests of Southwest China

Abstract

The stability of soil organic matter (SOM) plays a critical role in soil carbon (C) dynamics under global warming. However, the factors influencing SOM stability, particularly the significance of bedrock geochemistry and its hierarchical relationship with climate and soil properties, remain poorly understood. To address this gap, we conducted a study along a large climatic gradient (Δtemperature > 9 °C) in the subtropical karst forests of southwest China, quantifying SOM stability using thermal analysis and investigating the contributions of bedrock geochemistry, climate, and soil properties. Our results showed that SOM stability was positively correlated with mineral-associated organic C (MAOC) rather than particulate organic C. Hierarchical partitioning analysis further demonstrated that bedrock geochemistry was the predominant contributor to SOM stability variance, accounting for 23.7%. Following this, soil minerals contributed to 21.1%–22.6% of the variance, the mean annual temperature to 20.3%, and microbial biomass C to 17.2%. In particular, bedrock geochemistry—specifically the presence of calcium-rich bedrock—was found to enhance SOM stability by promoting the accumulation of exchangeable calcium and calcium carbonate in soils. Additionally, high temperature improved SOM stability by increasing the content and proportion of MAOC and soil pH. These results highlight the fundamental role of bedrock geochemistry in controlling SOM stability and emphasize the importance of considering hierarchical relationships among bedrock–soil–climate interactions for evaluating soil C dynamics.

1. Introduction

Soil organic matter (SOM) plays a crucial role in climate change mitigation [1] and soil multifunctionality [2]. Hence, protecting and replenishing depleted soil carbon (C) are vital strategies for achieving climate goals and maintaining soil functionality [1]. The stability of SOM, which refers to its resistance to microbial decomposition and environmental changes, is a critical determinant of soil C persistence [3]. Recent studies emphasize that SOM stability is primarily governed by microbial accessibility rather than intrinsic chemical composition [4,5]. Physicochemical stabilization, such as the binding of soil organic C (SOC) with minerals and its occlusion within aggregates, plays a key role in constraining microbial access to SOM [6]. While the factors influencing SOM stability have been investigated [7], understanding the relationships among them and their interactions remains limited. Therefore, it is crucial to explore the influence of various factors on SOM stability by adopting a hierarchical framework, which can contribute to better evaluating soil C dynamics [8].

Climate is often considered one of the main factors controlling SOM stability, but the relationship between SOM stability and climate is complex and context-dependent [9]. Specifically, temperature and precipitation can influence SOM stability through their effects on microbial physiology [10], soil properties, SOM molecular structure [11], and the proportion of plant-derived C [12]. Emerging evidence indicates that soil C stabilization capacities are also mediated by physicochemical protection [13]. In fact, soil physicochemical properties play a crucial role in restricting microbial access to SOM [6]. Since climate modulates long-term mineral weathering, the interaction between the climate and soil minerals should be considered when studying SOM stability [14]. However, most studies examining the factors affecting SOM stability have primarily focused on climate and soil minerals [15,16], while the significance of underlying bedrock geochemistry remains inadequately understood.

Bedrock is an essential component of the Earth’s surface and can influence soil properties and microbial communities [17,18], thereby controlling SOM stability [19]. Additionally, the chemical composition of bedrock can influence plant growth and composition by regulating the release of mineral nutrients [20] and the water-holding capacity of the regolith [21], which consequently impacts C inputs and SOC fractions. For instance, recent evidence has shown that bedrock types influence soil C fractions [22,23], which may create differences in SOM stability. Furthermore, soil mineral composition and its mineral phases are dependent on bedrock geochemistry. Moreover, weathering is mainly driven by climate and bedrock geochemistry. Therefore, linking bedrock geochemistry with soil minerals and climate would provide insights into SOM stability, especially under the global warming scenario.

Karst regions cover approximately 15% of the surface of ice-free continents globally [24]. The karst region in southwest China covers more than 0.54 million km², being one of the world’s largest carbonate rock regions where the bedrock is continuously exposed on the surface [25]. The carbonate rocks in southwest China primarily include limestone, dolomite, and a mixture of both, and they consist of highly soluble components [26]. Carbonate rocks with higher calcium (Ca) content exhibit rapid weathering and high release of active minerals. Although variations in SOC content between carbonate rock types (limestone and dolomite) have been demonstrated [27], the geochemical properties of carbonate rocks regulating SOM stability remain unexplored. Furthermore, the large elevation differences in southwest China create substantial temperature variation [28], which provides a natural climatic gradient for exploring the influence of climate on SOM stability.

In this study, we quantified SOM stability using thermal analyses—differential scanning calorimetry (DSC) and thermogravimetry (TG)—of forest soils (0–15 cm) across a large climatic gradient in the subtropical karst region. Thermal analysis is increasingly being adopted to characterize SOM stability due to its high accuracy and quantitative nature [16,28,29]. The objectives of this study were (1) to investigate the effect of different factors such as bedrock geochemistry, soil minerals, and climate on SOM stability and (2) to establish hierarchical relationships between SOM stability and bedrock geochemistry, soil minerals, and climate. We hypothesized that: (1) Ca-rich bedrock would increase SOM stability by enhancing physicochemical protection through the accumulation of Ca in soils, and (2) SOM stability is regulated by both bedrock-driven bottom-up effects and climate-driven top-down effects.

2. Materials and Methods

2.1. Site Description

The study was conducted in the karst region of southwest China (22°42′–27°53′ N, 104°82′–108°37′ E), characterized by limestone, dolomite, and their mixtures as the predominant lithology. The region has shallow soils, typically less than 30 cm in hillsides, with varying elevations ranging from 142 m to 2264 m. The mean annual temperature (MAT) ranges from 12.6 to 21.8 °C, and the mean annual precipitation (MAP) ranges from 1013 to 1607 mm yr⁻¹ across the study sites. The soil is classified as Eutric Renzzic Skeletic Leptosol (Siltic, Humic) according to the FAO classification [30].

2.2. Experimental Design and Sample Collection

The field investigation and sampling were conducted during the early fall of 2018. Secondary forests were selected from seven counties along the climatic gradient in the karst regions (Figure 1). A total of 21 plots were sampled, with three replicates in each county. Criteria were established to ensure comparability among sampling sites, considering factors such as parent material, stand age, and slope. The lithologies of all the selected forests consisted of carbonate rocks. These forests had been preserved for approximately 60 ± 5 years, and they all had slopes facing either south or southeast.

In each county, we established three 30 m × 30 m plots. The rationale for selecting this particular plot size was to minimize the influence of karst habitat heterogeneity on our sampling results. Typically, the distance between any two sampling plots within the same county was less than 5 km. However, in certain cases, this distance may exceed 5 km in order to fulfill the specified sample site criteria. Within each plot, we randomly collected 20 mineral soil cores from a depth of 15 cm using a soil auger and then composited these samples. The visible roots and stones were removed, and the soils were sieved with a 2 mm mesh. The soils were then divided into three portions. One portion was immediately stored in iceboxes and shipped to the laboratory for microbial biomass analysis. Another portion was air-dried and used to separate different SOC fractions. The remaining portion was used for soil chemical property analysis. Minimally weathered rocks were collected from three intact outcropped bedrocks in each plot.

2.3. SOM Stability

SOM stability was characterized using thermal analysis with a simultaneous thermal analyzer (STA 8000; PerkinElmer, Waltham, MA, USA), where the medium was Ultra-Zero air (21% O₂ and CO₂ free) [16]. To remove sample moisture, the soil samples were heated from 25 to 105 °C at a rate of 10 °C min⁻¹ and maintained at 105 °C for 15 min. The samples were then heated to 800 °C at a rate of 10 °C min⁻¹ to facilitate the simultaneous detection of mass loss (TG) and energy release (DSC). We used two indices to characterize SOM stability based on thermal analysis: TG-T₅₀ and DSC-T₅₀. Specifically, TG-T₅₀ refers to the temperature corresponding to half of the mass lost in the TG analysis, whereas DSC-T₅₀ indicates the temperature corresponding to half of the energy released in the DSC analysis. During thermal analysis, the active or unstable C factions are oxidized first with increasing temperature. If the soil contains larger unstable C fraction, the mass loss and energy release would be greater at lower temperatures, resulting in lower TG-T₅₀ and DSC-T₅₀ [31]. Thus, a higher TG-T₅₀ or DSC-T₅₀ indicates higher SOM stability [16].

Owing to the close correlation between DSC-T₅₀ and TG-T₅₀ (Figure 2b), we further calculated the average Z-score of DSC-T₅₀ and TG-T50 to reflect the integral stability of SOM (Equation (1)) [16] in the subsequent analyses:

$$SOM stability=(\frac{TG-T{G}_{mean}}{T{G}_{SD}}+\frac{DSC-DS{C}_{mean}}{DS{C}_{SD}})/2,$$

(1)

where $TG$ and $DSC$ denote TG-T₅₀ (°C) and DSC-T₅₀ (°C), respectively; $T{G}_{mean}$ and $DS{C}_{mean}$ represent the mean of TG-T₅₀ and DSC-T₅₀ across the sampling sites, respectively; $T{G}_{SD}$ and $DS{C}_{SD}$ represent the standard deviations of TG-T₅₀ and DSC-T₅₀ across sampling sites, respectively.

2.4. Soil Properties

The SOM was fractioned into particulate organic matter (POM, >53 μm) and mineral-associated organic matter (MAOM, < 53 μm) via size fractionation [9]. Briefly, 10 g of air-dried soil was dispersed in 30 mL of 5 g L⁻¹ sodium hexametaphosphate solution and shaken for 4 h in a reciprocating shaker. The dispersed soil suspension was passed through a 53 μm mesh after rinsing five times with distilled water. All materials remaining on the mesh were classified as POM, whereas the portion that passed through was regarded as MAOM. Thereafter, both fractions were dried at 55 °C to a constant weight. The organic C content in bulk soil, POM, and MAOM fractions was measured using the dichromate redox colorimetric method [32]. Particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) were calculated based on the organic carbon content in POM and MAOM and their ratios, respectively.

The fresh soil stored at 4 °C was used for microbial biomass carbon (MBC) analyses using the chloroform fumigation–extraction method [33]. The pH of the soil was measured using a pH meter (FE20K; Mettler-Toledo, Greifensee, Switzerland) [34]. Soil exchangeable Ca²⁺ and Mg²⁺ were displaced via compulsive exchange in ammonium acetate (1 mol L⁻¹) and were subsequently analyzed using an inductively coupled plasma emission spectrometer (5110 ICP-OES; Agilent, Santa Clara, CA, USA). Soil amorphous and free Fe-oxide contents were determined following acid oxalate extraction and the citrate–bicarbonate–dithionite method, respectively, and analyzed using the inductively coupled plasma emission spectrometer. The content of CaCO₃ in soils was determined through acid–base titration. Briefly, after dissolving CaCO₃ with 0.5 mol L^–1 HCl, the excess acid was back-titrated with 0.2 mol L^–1 NaOH to calculate the equivalent CaCO₃ [35]. The total Ca, Mg, Fe, and Al contents were analyzed with an X-ray fluorescence spectrometer (Axios mAX; Malvern Panalytical B.V., Almelo, The Netherlands), and the final values (%) were represented as CaO, MgO, Fe₂O₃, and Al₂O₃, respectively.

2.5. Bedrock Geochemistry

The weathered layer on the rock surfaces was removed with a saw. After washing with 5% H₂O₂ to remove the remaining organic matter from the rock surface, the rocks were heated to 105 °C to decompose H₂O₂. The rock samples were crushed and powdered to a particle size of 75 μm using a grinding machine. Ultimately, the contents of bedrock elements, including Si, Ca, and Mg, were analyzed with an X-ray fluorescence spectrometer (Axios mAX), and the final values (%) were represented as SiO₂, CaO, and MgO, respectively.

2.6. Statistical Analyses

The data were reviewed for normal distribution and variance homogeneity, and a natural log transformation was performed when required to satisfy the criteria. First, we used ordinary least-squares regressions to explore the relationships between SOM stability and the two indices of thermal analysis (TG-T₅₀ and DSC-T₅₀) and SOC fractions (bulk SOC, MAOC, and POC). Second, we used ordinary least-squares regressions to explore the relationships between SOM stability and climate, bedrock geochemistry, and soil properties. The above-mentioned statistical analyses were conducted using SPSS 26 (SPSS Inc., Chicago, IL, USA), and the figures were plotted using OriginPro 2021 (OriginLab, Hampton, MA, USA). Third, we used hierarchical partitioning analysis with the “rdacca.hp” package of R to evaluate the relative importance of explanatory variables on SOM stability [36]. This approach is advantageous because the overall significance of the explanatory variables can be estimated by combining the unique and shared contributions among predictors.

3. Results

Both TG-T₅₀ and DSC-T₅₀ in the soils of subtropical karst forests showed an increasing trend from northwest to southwest (Figure 2a). Accordingly, SOM stability also increased from northwest to southwest (Figure 2a). There was a significant positive correlation between TG-T₅₀ and DSC-T₅₀ (p < 0.001), and both indices were strongly and positively correlated with SOM stability (p < 0.001, Figure 2b). Across the sites, SOM stability showed a positive correlation with MAOC content (p < 0.05), which represents a stable fraction of SOC, while no correlation was observed with POC content (p > 0.05) (Figure 3a–c).

Regression analysis revealed positive correlation between SOM stability and climatic factors (MAT and MAP), bedrock geochemistry (CaO/MgO), soil minerals (Ca²⁺, CaCO₃, and CaO/MgO), and soil properties (pH and MBC) (p < 0.05, Figure 4). Notably, the relationships of TG-T₅₀ and DSC-T₅₀ with the above explanatory variables were similar to that with SOM stability (Figure A1).

Hierarchical partitioning analysis showed that bedrock geochemistry (bedrock CaO/MgO) accounted for the largest portion of the variance in SOM stability (23.7%), followed by soil minerals (soil CaO/MgO and CaCO₃, 22.6% and 21.1%, respectively), MAT (20.3%), and MBC (17.2%) (Figure 5). Pearson correlation analysis further proved that soil mineral content (Ca²⁺, CaCO₃, and CaO/MgO) and the protected SOC fraction (MAOC) were positively correlated with bedrock geochemistry (CaO/MgO) (Figure A2). Additionally, MAOC content was positively correlated with MAT and MAP (Figure A2). The results comprehensively demonstrated that the geochemical composition of carbonate rocks affected SOM stability by influencing the content of soil Ca²⁺ and CaCO₃. Meanwhile, climate impacts SOM stability by affecting SOC fraction and soil pH (Figure 6).

4. Discussion

Our results showed that SOM stability was positively correlated with MAOC content rather than POC content (Figure 3). Considering the mechanisms of persistence, MAOC and POC represent fundamentally different components of SOC [37]. Specifically, MAOC is considered to be a stable SOM fraction that can persist for decades to centuries due to protection from decomposition through association with soil minerals. Conversely, POC is considered a labile fraction, and the mean residence time varies from years to decades and is primarily protected by occlusion in large aggregates, if at all [37]. Thus, thermal stability can reasonably characterize SOM stability in the subtropical karst forests. Furthermore, our results revealed that bedrock geochemistry impacted SOM stability through its effect on soil minerals, whereas climate shaped SOM stability by influencing SOC fractions and soil pH (Figure 6). This aligns with our hypothesis that bedrock affects SOM stability via bottom-up effects, while climate influences it through top-down effects.

The bedrock CaO/MgO exerts the strongest influence on the variance of SOM stability in the karst region (Figure 5), signifying a greater role of bedrock geochemistry than previously assumed. Higher levels of bedrock CaO/MgO enhanced SOM stability by promoting the accumulation of Ca²⁺ and CaCO₃ in soils (Figure 6), thus pointing to the strong interplay between bedrock geochemistry, soil minerals, and SOM stability in karst regions. The positive correlation between soil Ca and bedrock CaO/MgO can be explained by bedrock weathering processes. The weathering state of bedrock, influenced by its geochemistry, plays a key role in determining soil mineral composition [38]. Ca reacts more readily with CO₂-containing water compared to Mg, leading to a faster weathering rate of bedrock rich in Ca [39]. The highly soluble nature and rapid weathering rate of Ca-rich bedrock could be beneficial for the accumulation of soil Ca. Specifically, karst regions are characterized by shallow, discontinuous soils but well-developed porosity, promoting root penetration into crevices within the bedrock surface for nutrient and water uptake [21]. On one hand, the increase in root penetration and root exudation within these crevices accelerates bedrock weathering and subsequently Ca release into soils. On the other hand, the Ca derived from bedrock and absorbed by plants is reincorporated into the soil through increased detrital inputs. Additionally, soil microorganisms could expedite rock weathering through organic acid and extracellular enzymes [40] and by penetrating their hyphae into tiny bedrock fissures [41], accelerating Ca-rich bedrock weathering and elemental release. Consequently, the Ca-rich bedrock facilitates the build-up of Ca within the soil. This, in turn, improves the stability of SOM through physicochemical protection mechanisms. Increasing studies have emphasized the role of soil minerals, including Ca, Fe/Al oxides, and clay, in the physicochemical protection of SOM [13,14,42]. In calcareous or alkaline soils, the physicochemical protection mediated by Ca would overshadow the function of Fe/Al oxides [43]. Consistent with this notion, our results demonstrate a positive association between SOM stability and soil Ca (Ca²⁺, CaCO₃, and CaO/MgO) (Figure 4 and Figure A3). Ca contributes to SOM stabilization primarily through cation bridging involving Ca²⁺ and CaCO₃. Abundant soil Ca²⁺ promotes SOM stabilization through outer- and inner-sphere cation bridging [43]. The positive correlation between Ca²⁺ and MAOC (Figure A4) indicates that Ca can bind to SOM in Ca-rich soils. Moreover, the dissolution–precipitation processes of CaCO₃ result in the formation of calcite crystals that fill the porous structure of aggregates, enhancing the stability of soil aggregates and occluded SOC [44]. Additionally, CaCO₃ plays a significant role in the direct sorption and inclusion of SOC, further increasing SOM stability [43]. The positive correlation between SOM stability and CaCO₃ content in the soils (Figure 4) provides vital evidence supporting our inference. Together, Ca²⁺ and CaCO₃ in karst soils, originating from the Ca-rich bedrock, contribute to cation bridge formation, aggregate stabilization, and direct sorption and inclusion, thereby stabilizing SOM.

In addition to the bottom-up effects exerted by bedrock geochemistry on SOM stability, climate, particularly MAT, demonstrated top-down influences (Figure 5 and Figure 6). The effect of MAT on SOM stability outweighs that of MAP due to greater temperature variation (Δtemperature > 9 °C) compared to precipitation variation (MAP > 1000 mm and Δprecipitation < 600 mm) in the humid karst region. Consequently, vegetation biomass, soil minerals, microbial activity, and C fractions may be more sensitive to temperature variations, resulting in MAT exerting a greater influence on SOM stability. The increased stability of SOM with MAT can be explained by two mechanisms. Firstly, climate warming increased the content and proportion of stable or protected C fractions, thereby enhancing SOM stability. High temperature promotes microbial biomass turnover, leading to an increased contribution of microbial necromass to SOC [45]. A higher proportion of microbial necromass in the soil C pool indicates the protection of a greater amount of C, as it readily binds to soil minerals [9,46]. In Ca-rich karst soils, Ca²⁺ plays a crucial role in stabilizing microbial necromass through inner- and outer-sphere cation bridging [23]. The positive correlation between MAOC and MAT provides important supporting evidence for our inference (Figure A5b). Additionally, warming generally increases soil microbial activity and preferentially promotes the decomposition of active or unprotected soil C fractions [47,48], as observed in our study where the proportion of unprotected soil C (POC to SOC ratio) decreased with increasing MAT (Figure A5c). These processes result in the decomposition of unprotected C fractions, leading to the formation of more microbial necromass that binds to soil minerals [49]. The positive correlation between the ratio of MAOC to SOC and MAT supports our inference (Figure A5d). Secondly, climate warming can increase SOM stability by raising soil pH, which is considered an important indicator of the mechanisms controlling SOM stabilization [43,50]. In Ca-rich soils, the stabilization of SOM mediated by Ca²⁺ bridging or Ca-mediated aggregation can potentially increase with soil pH [43]. The positive influence of higher temperatures on soil pH (Figure 6) can be ascribed to a pair of primary factors. Higher temperatures in humid regions (MAP > 1000 mm) increase evapotranspiration and reduce the loss of base cations, leading to increased soil pH [51]. Additionally, increased temperatures may promote the dissolution of CaCO₃ into Ca²⁺ through a process that consumes protons (H⁺), thereby increasing soil pH. Consequently, as temperature increases, soil pH increases, further enhancing SOM stability regulated by soil pH.

Besides the established significance of bedrock geochemistry and climate, the role of microbial biomass in influencing SOM stability should not be overlooked. Our results demonstrated that MBC was positively correlated with SOM stability (Figure 4h and Figure 5). Typically, soil microorganisms consume easily decomposable C fractions [52]. Thus, a higher microbial biomass corresponds to a reduction in these decomposable organic matter reserves, effectively leading to an increased proportion of stable C fractions. Simultaneously, upon the death of these microorganisms, they contribute to the transformation of C compounds into more stable forms via microbial assimilation [52]. The products of microbial biosynthesis, also referred to as necromass, are notably stable due to their affinity for binding with soil minerals [53]. Overall, the findings emphasize the unneglectable role of microbial biomass in shaping the formation and preservation of stable SOM in subtropical karst forests.

5. Conclusions

Bedrock geochemistry emerges as the predominant driver of variance in SOM stability in karst forests. The presence of Ca-rich bedrock enhances SOM stability by facilitating the accumulation of exchangeable Ca and calcium carbonate in soils. Concurrently, climate factors, most notably increasing temperature, contribute to SOM stability by augmenting the proportion of stable soil C fractions and increasing soil pH. Furthermore, elevated MBC reinforces SOM stability through microbial catabolic and anabolic processes. These results suggest that karst forest soils have significant potential for C stabilization, particularly in Ca-rich bedrock areas and regions experiencing higher temperatures. The findings advance our understanding of the factors influencing SOM stability, highlighting the combined effects of bottom-up factors driven by bedrock geochemistry and top-down factors driven by climate.