Soil black carbon decline following deforestation and farming in karst rocky desertification Southwest Guangxi, China

Black carbon (BC) is produced by the incomplete combustion of biomass and fossil fuels. After deposition in soils, BC forms a relatively stable carbon pool that contributes to long-term carbon sequestration. Understand the effects of land use on soil BC is critical for accurately interpreting the role of BC in the carbon cycle of karst areas. This study investigated the distribution characteristics of organic carbon (OC), BC, char, soot, and soil physicochemical properties within the 0–40 cm soil profiles of secondary forest, shrub, farmland, and wasteland in a rocky desertification area of Guangxi Province, a typical karst region in Southwest China. A combination of soot/char separation and δ¹³C_BC isotope analysis was used to identify the sources of BC in these soils. The result show that average soil BC content was highest in secondary forests (7.20 g/kg), followed by shrub (5.85 g/kg), farmland (3.64 g/kg), and wasteland (3.38 g/kg). BC showed a positive correlation with OC, char, soot, soil nitrogen, phosphorus, and potassium, and a negative correlation with bulk density. Among BC components, char accounted for a greater proportion than soot, and the heavy fraction of OC exhibited a higher BC concentration than the light fraction. Overall, the BC content was primarily derived from C₃ plant combustion, vehicle exhaust emissions, and fossil fuel use in agricultural production activities. Vegetation restoration improved soil BC levels and promoted OC sequestration, while thereby enhancing soil carbon stability.

Highlights

• Land uses significantly influenced soil black carbon(BC) distribution, with a decreasing trend was observed after conversion from forest land to farmland.

• Anthropogenic agricultural activities were the main factors affecting soil black carbon distribution in this area.

• Vegetation restoration significantly contributed to BC accumulation by increasing the content of char fraction.

1 Introduction

Organic carbon (OC) is a highly complex and heterogeneous mixture of readily decomposable reactive OC and refractory inert OC (Ramesh et al., 2019). Inert OC has a slow decomposition rate, a long turnover time, and relative stability, plays an essential role in the long-term sequestration of soil OC (SOC). Black carbon (BC), an important component of the inert SOC pool (Lian and Xing, 2017), is generally defined as a solid organic material rich in carbon that results from the incomplete combustion of carbonaceous fuels, such as fossil fuels and biomass. BC comprises a complex mixture of slightly charred biomass compounds and highly condensed refractory materials, including soot, char, and charcoal, and is widely distributed in carriers such as soil and atmosphere. BC has a highly condensed aromatized structure (Yamashita et al., 2022) and exhibits high oxidation resistance, Further, it is not easily degraded by microorganisms (Lian and Xing, 2017), and it contributes to long-term soil carbon storage (Coppola et al., 2022).

BC plays a crucial role in the global carbon cycle (Zhan et al., 2015). Global annual BC production is estimated at 62–294 Tg, of which 80%–90% is deposited directly into soils (Gao et al., 2023). BC is a critical component of climate change mitigation strategies (Sharma and Mishra, 2022) and is considered a vital component of the “missing carbon” in Earth’s carbon balance (Lian and Xing, 2017; Gerke, 2019), as it enhances carbon sequestration (Coppola et al., 2022), reduces greenhouse gas emissions (Harmsen et al., 2020), and improves soil fertility (Hossain et al., 2020). Studies on BC have been conducted in a number of ecosystems, including forests (Haukenes et al., 2023; Robson et al., 2025), agricultural fields (Wang et al., 2020), wetlands (Wang et al., 2022), and urban areas (Ji et al., 2017), and these studies have shown that BC can effectively increase stable soil carbon stocks and contribute substantially to mitigating the greenhouse effect.

Land use is one of the most significant and direct human interventions in soil systems, influences the sequestration of SOC by regulating carbon input, decomposition, and turnover (Zhang C. et al., 2022). In karst rocky desertification regions, where conflicts between human needs and land capacity are particularly acute, rational land use serves as a key strategy for ecological restoration. It also plays a foundational role in supporting sustainable local economic development and enhancing ecosystem carbon sequestration capacity. The peak-cluster depression area in southwestern Guangxi, a typical karst landscape, exemplifies one of the most ecologically vulnerable and severely desertified regions in Guangxi. It is a major focus for ecological reconstruction and vegetation rehabilitation (Yuan, 2015). Despite advancements in the early 21st century, traditional slash-and-burn agricultural practices remain prevalent in these areas, exerting considerable influence on regional carbon cycling (Hu et al., 2023; Mu et al., 2024). These activities may affect the storage and distribution of soil BC. Over the past two decades, large-scale ecological restoration efforts—including natural enclosure, artificial reconstruction, and assisted regeneration—have substantially increased vegetation cover and altered plant community composition in Southwest Guangxi (Yang et al., 2024). These changes are expected to impact both the storage capacity and spatial distribution patterns of soil BC and its chemical forms across the region. Adopting appropriate land-use practices can improve land-use efficiency and contribute to integrated rocky desertification control. Land-use conversions from forest to agricultural land (cropland and grassland) are especially relevant, as they are known to influence soil carbon stocks (Nobuhisa et al., 2020). Different land-use types under traditional agricultural systems may result in varied distributions and sources of BC in the soil.

Southwest Guangxi, China, is a region with diverse land use patterns. This region features extensive karst crest depressions, severe rocky desertification, and a fragile ecological environment (Jiang et al., 2016). When vegetation destruction occurs in this environment, the shallow soil layers and subsurface seepage cause carbon to be easily lost through erosion or dissolution, leading to a sharp decline in carbon sink function and increased difficulty in carbon sequestration. However, vegetation restoration can, to some extent, enhance carbon sink capacity (Jiang et al., 2016). Since the launch of the Grain-for-Green project (Xian et al., 2020) and the implementation of ecological restoration engineering initiatives, vegetation cover in parts of the region has gradually increased over the past 22 years. Previous studies in the area have examined soil microbial activity and community composition (Liu et al., 2023), soil physicochemical properties (Lyu et al., 2022), the influence of vegetation on the water cycle (Kang et al., 2020), changes in vegetation cover (Mo et al., 2023), soil enzyme activities under different land uses (Gong et al., 2022), and carbon sequestration in rocky desertification management (Qiu et al., 2022). SOC and carbon accumulation in the karst forests of the Guizhou Autonomous Region have also been investigated (Zhang L. et al., 2022). However, few studies have focused on the distributionng and storage ofing soil BC, char, and soot in Southwest Guangxi. Although some studies have explored BC in typical soils, uncertainties remain regarding BC dynamics in karst rocky desertification areas with varying land use types. Based on these objectives and the known effects of land-use change and traditional agricultural practices on soil carbon dynamics, we hypothesized that: (A) The conversion of forests to agricultural land may reduce the content of soil black carbon fractions. (B) Traditional agricultural production activities may be the main source of black carbon in the soil. To establish a scientific foundation for evaluating soil quality, this study examines the impact of human activity on soil BC accumulation during ecological restoration. It investigates how different land-use patterns influence soil BC in the rocky desertification areas of Southwest Guangxi—an issue of significant practical importance.

The objectives of this study were to: (1) Investigate the vertical distribution characteristics and potential sources of soil BC, and the correlation between soil BC and SOC under different land use patterns; (2) Elucidate the factors influencing the relationship between BC and OC in the rocky desertification region of Southwest Guangxi. In short, this study aimed to provide fundamental data on the soil carbon pool of karst ecosystems in Southwest Guangxi, and to offer a theoretical basis for rational soil use and environmental protection in rocky desertification regions.

2 Materials and methods

2.1 Study area

The study area was located in Guohua Town, Pingguo City, Guangxi Province (107^°22^′40^″–107^°25^′30^″E, 23^°22^′30^″–23^°24^′00^″N), a typical karst region with an elevation ranging from 110 to 570 m. The average annual temperature is 19.1 °C–22.0 °C, and the annual precipitation is approximately 1,500 mm. Rainfall is highly seasonal: ∼70% of the annual precipitation occurs between May and August, and the remaining 30% falls from September and April.

Soil is predominantly brown limestone, shallow, and often with exposed rock. Vegetation cover is sparse, and rocky desertification is severe. The dominant vegetation types include secondary forest, shrub, and cultivated species. The main tree species in the secondary forests are Zenia insignis, Melia azedarach, Apodytes dimidiata, and Choerospondias axillaris. The main shrub species included Alchornea trewioides, Cipadessa cinerascens, and Vitex negundo (Lyu et al., 2018).

Land use patterns were classified into four categories: wasteland, farmland, shrub, and secondary forest (Figure 1). Soil samples were collected from these four land use types. The slope gradients of the four plots are 17°, 15°, 13°, and 13°, with all sites located at mid-slope positions.

Figure 1

Figure 1. Location map of the study area, with different land use types associated with vegetation restoration.

The vegetation coverage was 85, 73, 26, and 44%, respectively. Cultivated land, primarily planted with maize, soybeans, and pitaya, has been in use for at least 110 years. Shrub and secondary forest were estimated to be approximately ∼39 and 80 years old, respectively, while the wasteland area had been abandoned from cultivation for about ∼17 years.

Our research site is typical of rocky desertification in Southwest Guangxi, openly accessible, and no permits were required for field sampling. Long-term research in this area has established strong relationships with the local residents.

2.2 Soil sampling

The four sampling sites representing the different land use types (wasteland, farmland, shrub, and secondary forest) were selected based on vegetation restoration levels. In November 2020, three replicate plots (50 × 50 m each) were established per site (12 plots total). Soil samples were collected using a 5 cm-diameter auger at four depth intervals: 0–10 cm, 10–20 cm, 20–30 cm, and 30–40 cm. Samples from the same layer were composited into one representative sample. Anthropogenic activities, such as fire and fertilization, were recorded. Samples were placed in sterile, self-sealing bags, transported in ice containers, and stored at 4 °C until analysis. After air-drying, the samples were sieved through a <2 mm mesh and stored in sealed containers for further analysis.

2.3 Sample analysis

2.3.1 BC measurement

BC was measured using a refined method based on Lim and Cachier (1996). The six-step pretreatment procedure included: (1) 3 g of fine dry soil (ө < 2 mm) was weighed into a glass container; (2) 15 mL of 3 mol·L-1 HCl was slowly added to remove carbonates, and the sample was allowed to react for 24 h; (3) 15 mL of 10 mol·L-1 HF: 1 mol·L-1 HCl was added to remove silicates, followed by a 24 h reaction; (4) 15 mL of 10 mol·L-1 HCl was added again to remove any CaF₂, and the sample was left for 24 h; (5) 15 mL of 0.1 mol·L −1 K₂Cr₂O₇: 2 mol·L-1 H₂SO₄ was added to oxidize OC, and the sample was incubated at 55 °C ± 1 °C for 65 h; (6) the remaining residue (BC) was centrifuged, dried, and analyzed using elemental analysis-isotope ratio mass spectrometry (FLASH EA–DELTA V) to determine BC content and δ¹³C (PDB).

Following Zhan et al. (2013), soil temperature was gradually increased (140, 280, 480, and 580 °C) to separate four OC fractions: OC1, OC2, OC3, and OC4. A gas mixture (2% O₂/98% He) was introduced, and temperatures were increased to 580, 740, and 840 °C to obtain three elemental carbon fractions: EC1, EC2, and EC3. The boundary between OC and BC was defined at this step. Particulate organic carbon (POC) was defined as the overlapping fraction. Total OC was defined as OC1 + OC2 + OC3 + OC4 + POC, while total BC was EC1 + EC2 + EC3 – POC. Char was defined as EC–POC, and soot was defined as EC2 + EC3, following Han et al. (2009).

2.3.2 LFBC and HFBC measurement

An improved method based on Janzen et al. (1992) was used. Light-fraction organic matter was separated from the BC using dichromate oxidation combined with hydrofluoric/hydrochloric acid treatment. The supernatant was vacuum-filtered through a 0.45 μm membrane. The residue was washed with 0.01 mol·L⁻¹ CaCl₂ (100 mL) solution and distilled water (200 mL) to yield the light-fraction black carbon (LFBC). The precipitate was mixed with 50 mL of distilled water, shaken for 0.5 h at 200 r·min⁻¹, and centrifuged at 4,000 r·min⁻¹ for 20 min. The supernatant was discarded and the washing process was repeated three times. Remaining residue was repeatedly washed with 95% ethanol until colorless to obtain heavy-fraction black carbon (HFBC).

2.3.3 Soil physicochemical properties

Soil total organic carbon (TOC) and organic carbon (OC) were measured using the high-temperature external heat potassium dichromate oxidation (Zheng et al., 2023). Total nitrogen (TN) were determined using a Vario ELIII elemental analyzer (Vario EL III; Elementar Analysensysteme GmbH, Langenselbold, Germany). Total phosphorus (TP) was determined using concentrated sulfuric acid-perchloric acid digestion (Olsen and Sommers, 1982) followed by the molybdenum-antimony colorimetric method using an Agilent 8453 UV-Vis spectrophotometer (Agilent Technologies, Santa Clara, CA, USA). Total potassium (TK) was analyzed by flame photometry after sulfuric acid-perchloric acid digestion. Available nitrogen (AN) was measured using the alkaline digestion diffusion method. Available phosphorus (AP) was determined using the 0.50 mol/L NaHCO₃ extraction-molybdenum-antimony anti-colorimetric method, and available potassium (AK) using the 1 mol/L CH₃COONH₄ extraction-flame photometry method. Fresh weight was measured using the ring knife method; samples were later oven-dried in the lab to calculate soil water content and bulk density (BD). These soil property determination methods were obtained from Bao (2000).

2.4 Statistical analytics

2.4.1 Data organization and preliminary analysis

Raw datasets were compiled and pre-processed in Microsoft Excel 2019 (Microsoft Corp.). Preprocessing included data cleaning, outlier detection, normalization, and the calculation of descriptive statistics (mean ± SD, CV%).

2.4.2 Statistical hypothesis testing

Analyses were conducted using SPSS Statistics 19.0. Data normality was assessed using the Shapiro-Wilk tests were used to verify normality (α = 0.05). One-way ANOVA (Function: aov, Variance homogeneity verified via Levene’s test (car: levene Test, α = 0.05), with Tukey’s HSD post-hoc test was applied for multi-group comparisons (Function: Tukey HSD or HSD. test (agricolae), Confidence level: 95%), while the Kruskal-Wallis test was used for non-normally distributed data (Function: kruskal.test, Post-hoc Dunn’s test (dunn.test package) if applicable). Pearson and Spearman correlation matrices were generated (Pearson: cor.test (method = “pearson”), Spearman: cor.test (method = “spearman”), Significance level: α = 0.05), with false discovery rate (FDR) correction applied for multiple comparisons.

2.4.3 Advanced analytics and visualization

Advanced statistical modeling and data visualization were performed using Origin 2019, including multivariate regression modeling, redundancy analysis (RDA) to elucidate how response variables are driven by explanatory variables, and customizable scientific plotting with ISO-compliant formatting.

3 Results

3.1 Variations in physical and chemical properties in soils of different land uses

The physical and chemical properties of soils under different land uses are presented in Figure 2. The TN (Figure 2a) values of secondary forest profiles (1.96–4.20 g·kg⁻¹) were significantly higher than those of shrub (1.18–2.50 g·kg⁻¹), farmland (0.99–2.18 g·kg⁻¹), and wasteland (0.97–1.82 g·kg⁻¹) in each soil layer. The TP (Figure 2b), TK (Figure 2c), AN (Figure 2d), AP (Figure 2e), AK (Figure 2f), and TOC (Figure 2g) values in secondary forest profiles were also significantly higher than those in shrub, farmland, and wasteland across all soil layers. The BD (Figure 2h) values of secondary forest (1.07–1.38 g·cm⁻³), shrub profiles (1.18–1.48 g·cm⁻³), and wasteland profiles (1.25–1.50 g·cm⁻³) were lower than those of farmland profiles (1.25–1.59 g·cm⁻³). TN, TP, TK, AN, AP, AK, and TOC in all four land use types decrease with depth, whereas BD increased. All indicators showed significant differences among different layers within each land use type.

Figure 2

Figure 2. Differences in soil physicochemical properties under different land use types. Note: TN: Total Nitrogen (a),TP: Total Phosphorus (b), TK: Total Potassium (c), AN: Available Nitrogen (d), AP: Available Phosphorus (e), AK: Available Potassium (f), TOC: Total Organic Carbon (g), BD: Bulk Density (h). Lower case letters indicate significant differences (P < 0.05) between different land use types in the same layer, while upper case letters indicate significant differences (P < 0.05) between different land use types in different layers.

3.2 Differential responses of SOC and BC to varied land use types

The OC, BC, char, and HFBC contents decreased with increasing depth across all four land use patterns (Figure 3). Soot content also decreased with depth in secondary forest, farmland, and wasteland, while LFBC content declined with depth in secondary forest and shrub. The δ¹³C_BC and BC/TOC ratios increased with depth. The OC (3.14–42.78 g·kg⁻¹), BC (1.95–9.70 g·kg⁻¹), and HFBC (1.12–6.09 g·kg⁻¹) values were highest in secondary forest across all depths. No significant differences in OC content between farmland and wasteland in the 20–30 cm layer, or among shrub, farmland, and wasteland in the 30–40 cm layer. In the 0–20 cm layer, δ¹³C_BC differed significantly among land use types, with the highest values observed in wasteland, followed by farmland, shrub, and secondary forest (P < 0.05). In the 30–40 cm layer, δ¹³C_BC in secondary forest was significantly lower than in other land use types.

Figure 3

Figure 3. Differences in the contents of OC, BC, char, soot, and the ratio of BC/TOC in soils under different land use types. Note: OC: Organic Carbon, BC: Black Carbon, Char: Charcoal, Soot: Soot, LFBC: Light Fraction Black Carbon, HFBC: Heavy Fraction Black Carbon, BC/TOC: Black Carbon to Total Organic Carbon Ratio, δ¹³C_BC: δ¹³C of Black Carbon. Lower case letters indicate significant differences (P < 0.05) between different layers in the same land use type, while upper case letters indicate significant differences (P < 0.05) between different land use types in the same layer.

Char content ranged from 1.04 to 5.50 g·kg⁻¹ across soil depths. Secondary forest had higher char values than shrub, farmland, and wasteland in the 0–10 cm and 20–40 cm layers, while shrub had the highest char content in the 10–20 cm layer. Soot content ranged from 0.58 to 4.21 g·kg⁻¹. Secondary forest had significantly higher soot values than other land use types in the 0–10, 10–20, and 30–40 cm layers, while shrub had the highest soot values in the 20–30 cm layer.

LFBC content ranged from 0.65 to 4.26 g·kg⁻¹. Shrub exhibited the highest LFBC values in the 0–10 cm layer, while secondary forest had the highest LFBC values from 10 to 40 cm. BC/TOC values in wasteland were significantly higher than in secondary forest, shrub, and farmland in the 0–30 cm layers. Overall, OC, BC, char, soot, LFBC, and HFBC concentrations were significantly higher in secondary forest and shrub compared to farmland and wasteland across the 0–40 cm soil profile.

3.3 Effects of various land use patterns on the proportion of soil BC composition

The proportions of soil BC components under different land use types are shown in Table 1. The BC/OC ratios in the 0–40 cm layer were 22.67%–78.62% in secondary forest, 27.02%–67.16% in shrub, 33.29%–52.47% in farmland, and 45.16%–62.10% in wasteland. BC/OC ratios increased with depth, reaching their highest values in the 30–40 cm layer, with secondary forests having significantly higher values than the other land use types.Char/BC ratios showed irregular patterns across depths. Shrub had higher char/BC values than other land uses in the 0–10, 10–20, and 30–40 cm layers, but lower values in the 20–30 cm layer. Char/soot followed a pattern similar to char/BC. Soot/BC values varied unpredictably among depths and land use types: farmland had the highest soot/BC in the 0–10 cm layer, wasteland in the 10–20 cm layer, and secondary forest in the 30–40 cm layer.

Table 1

Table 1. Percentage of BC under different land use patterns.

The LFBC/BC ratios of shrub were significantly higher than those of secondary forest, wasteland, and farmland in the 0–10 and 20–30 cm layers. Wasteland showed the highest LFBC/BC values in the 10–20 and 30–40 cm layers. Farmland had the highest HFBC/BC values in the 0–10, 10–20, and 20–30 cm layers, while shrub had the highest values in the 30–40 cm layer. In general, HFBC/BC values were significantly higher than LFBC/BC values across land use types, indicating a greater contribution of heavy-fraction black carbon to total soil BC.

3.4 Correlation between soil BC and soil physicochemical properties

The correlations between soil BC fractions and soil physicochemical properties in the 0–40 cm layer are shown in Figure 4. OC, TN, TP, TK, AN, AP, AK, and SOC were all highly significantly (and positively) correlated with BC, char, soot, LFBC, and HFBC (P < 0.001). OC and SOC were significantly positively correlated with LFBC/BC (P < 0.05), while TN, TP, TK, AN, AP, AK, and BD showed no correlation with LFBC/BC or HFBC/BC. OC and SOC were significantly negatively correlated with HFBC/BC (P < 0.05). BD was highly significantly and negatively correlated with BC, char, soot, LFBC, and HFBC (P < 0.001), but was significantly and positively correlated with δ¹³C_BC (P < 0.001). OC, TN, TP, TK, AN, AP, AK, and SOC were also negatively correlated with BC/OC (P < 0.001). These results indicate a strong correlation between the content of BC components and soil physicochemical properties.

Figure 4

Figure 4. Correlation between soil black carbon and soil physico-chemical properties. Note: OC: Organic Carbon, TN: Total Nitrogen, TP: Total Phosphorus, TK: Total Potassium, AN: Available Nitrogen, AP: Available Phosphorus, AK: Available Potassium, BD: Bulk Density, SOC: Soil Organic Carbon, BC: Black Carbon, δ¹³C_BC:δ¹³C of Black Carbon, Char: Charcoal, Soot: Soot, LFBC: Light Fraction Black Carbon, HFBC: Heavy Fraction Black Carbon, LFBC/BC: Light Fraction Black Carbon to Total Black Carbon Ratio, HFBC/BC: Heavy Fraction Black Carbon to Total Black Carbon Ratio, BC/OC: Black Carbon to Organic Carbon Ratio.

The first (RD1) and second (RD2) standard axes of the soil BC fraction accounted for 69.38% and 7.91% of the variation in the soil BC fraction, respectively (Figure 5). Correlation analysis revealed that BC was significantly and positively associated with OC content, char, soot, LFBC, TK, AP, and HFBC. There were highly significant positive correlations between BC and both soil δ¹³C_BC and HFBC/BC (P < 0.05). OC had highly significant negative correlations with BC stock, BD, and LFBC/BC, and positively correlated with LFBC and HFBC content. This analysis also showed that OC and BC component distribution were inversely related. Additionally, the correlation between LFBC and BC was weaker than that between HFBC and BC, indicating a stronger association of heavy-fraction BC with OC compared to light-fraction BC.

Figure 5

Figure 5. Redundancy analysis of soil black carbon and soil physico-chemical properties. Note: OC: Organic Carbon, TK: Total Potassium, AP: Available Phosphorus, BD: Bulk Density, BC: Black Carbon, δ¹³C_BC:δ¹³C of Black Carbon, Char: Charcoal, Soot: Soot, LFBC: Light Fraction Black Carbon, HFBC: Heavy Fraction Black Carbon, LFBC/BC: Light Fraction Black Carbon to Total Black Carbon Ratio, HFBC/BC: Heavy Fraction Black Carbon to Total Black Carbon Ratio, BC/OC: Black Carbon to Organic Carbon Ratio.

4 Discussion

4.1 Characteristics and distribution of soil BC in karst rocky desertification areas

BC, as a marker of human activities, is widely present in soils affected by anthropogenic disturbances (Koç et al., 2020). It serves as a robust proxy for such activity and exhibits distinct spatial and vertical distribution patterns in karst rocky desertification regions. The average BC content in soils from rock-desertified areas in Southwest Guangxi ranged from 3.38 to 7.20 g·kg⁻¹. This is comparable to the natural soil BC content of (2 ± 1)–(6 ± 3) g·kg⁻¹ observed by Karthik et al. (2023) in South India; higher than the BC content of 2.99–4.36 g·kg⁻¹ in woodland soils observed by Jiang et al. (2014); but lower than the values reported for Changbai Mountain (6.39–16.55 g·kg⁻¹) by Sun et al. (2016) and for northeastern forest areas (6.64–17.63 g·kg⁻¹) by Sun et al. (2018). Fire-prone boreal forests, such as those on Changbai Mountain, typically exhibit higher BC content due to frequent biomass burning.

The relatively low BC content in the study area can be attributed to its unique environmental conditions and the impacts of land use change induced by human activities in rocky desertification regions. Soil formation in these areas is slow, the soil layer is shallow, leaching is intense, and the absence of weathered parent material contributes to erosion and irreversible damage (Guo et al., 2023), all of which exacerbate BC depletion. Similar declines in BC have been reported in Mediterranean karst areas (e.g., Spain) and the Brazilian Cerrado, where erosion and limited biomass input restrict BC accumulation (Silva et al., 2017). In addition, rocky desertification leads to significant vegetation loss, reducing combustible biomass (Zheng et al., 2024). Severe soil erosion further facilitates BC migration from the topsoil via runoff, while the breakdown of soil structure accelerates microbial mineralization and BC degradation (Wang et al., 2019). Anthropogenic activities also limit BC deposition and retention in soils (Zhao et al., 2020).

Land use is a key factor controlling SOC fractions (Zhang C. et al., 2022), and in this study, land use patterns had a significant effect on both the content of BC and its proportion relative to TOC. The mean BC content in the 0–40 cm layer varied across land use types: it was 7.20 g·kg⁻¹ in secondary forest, 5.85 g·kg⁻¹ in shrub, 3.64 g·kg⁻¹ in farmland, and 3.38 g·kg⁻¹ in wasteland. These results indicate that long-term vegetation restoration has enhanced soil BC accumulation. The highest BC concentrations were generally found in surface layers, decreasing with depth—consistent with the findings of Karthik et al. (2022) and Sun et al. (2016).

BC/TOC ratios varied by land use type. In the 0–10 cm layer, the mean BC content was highest in the wasteland (32.24%), followed by farmland (21.44%), shrub (20.73%), and secondary forest (18.06%). Significant differences in the proportion of BC/TOC were also observed across soil layers within the same land use type. The rocky desertification area in Southwest Guangxi has a long history of human farming activity, resulting in severe land degradation and ecological decline due to prolonged human–land conflict (Guo et al., 2023). However, the implementation of national ecological restoration policies—such as the Grain-for-Green project—has led to substantial ecological recovery (Guo et al., 2023). With vegetation regrowth, SOC has increased in secondary forest and shrub areas, contributing to higher BC levels in these zones.

Current agricultural practices in the region, including long-term monoculture and chemical fertilization, have caused topsoil loosening and exposure, undermining soil aggregate stability. Although applying farmyard manure could enhance BC sequestration in soils (Marques et al., 2015), heavy rainfall has caused notable carbon losses. This has been further exacerbated by production methods focused on material yield, which overlook soil conservation. In wasteland areas, soil erosion due to surface exposure has further reduced SOC levels. In this study, the contents of OC, BC, char, soot, LFBC, and HFBC were lower in farmland and wasteland soils at all depths (0–40 cm), likely due to tillage, fertilization, and organic matter loss that disrupted soil structure and limited organic matter return. In contrast, BC content was significantly higher in secondary forests and shrubs, indicating greater organic carbon inputs. Moreover, past anthropogenic activities such as fire in secondary forest and shrub areas likely contributed to increased BC accumulation in these soils.

4.2 The correlation between soil BC and OC in karst rocky desertification regions

In this study, a significant positive correlation was observed between soil BC and OC, indicating that BC plays an important role in SOC fixation. These findings are consistent with results from Indian soils and European loess soils (Zhan et al., 2013; Sun et al., 2018; Zheng et al., 2024; Nath et al., 2018), suggesting that increased BC content in secondary forest and shrub areas contributes to SOC accumulation. On the one hand, this accumulation can be attributed to the unique chemical and biological inertness of BC; on the other hand, BC is capable of adsorbing and stabilizing organic matter and clay minerals. Furthermore, both BC and OC showed surface enrichment patterns, with concentrations decreasing significantly with soil depth, consistent with the findings of Sun et al. (2016).

The ratio of soil BC to OC increased proportionally with soil depth across all four land use types. One possible explanation is that, in secondary forest and shrub, a substantial amount of litter is deposited annually onto the surface soil, promoting OC accumulation. In this context, OC may act as a natural “diluent” of BC, particularly in the absence of intensive human disturbance. Although farmland surfaces experience frequent disturbances, adding large quantities of organic matter results in reduced input of fresh carbon. In contrast, lower soil layers tend to have lower microbial activity and limited permeability, reducing the degradation rate of BC (Nath et al., 2018). In addition, BC is characterized by high chemical inertness and thermal stability (Koç et al., 2020), which enhance its persistence in soil. As a result, BC is selectively enriched over time during soil formation processes (López-Martín et al., 2018).

In this study, the proportion of BC in heavy fractions was significantly greater than in light fractions, which may be associated with the unique geological background of karst rocky desertification regions. However, it contradicted the results reported by Llorente et al. (2010), who reported that BC was primarily present in the light fraction of calcareous soils. Marques et al. (2015) similarly observed that BC in Brazilian soils was mostly found in the light fraction due to the inert nature of HFBC and its incorporation within the protected carbon pool. It has also been reported that the proportion of highly aromatic carbon increases with soil depth (Ukalska-Jaruga et al., 2019), suggesting that BC content declines more slowly than TOC with increasing depth and thus contributes positively to long-term soil carbon accumulation. Under similar climatic conditions, soil-forming parent material, and topography, aboveground litter serves as the primary source of SOC input. Adding carbon substrates to the system directly influences BC content (Li et al., 2023). The amount of decaying twigs and fallen leaves entering the soil is largely determined by the type and density of aboveground vegetation. As land use patterns influence the quantity, type, and quality of litter (Maes et al., 2019), the level of BC input into the soil also varies accordingly (Zhang and Zhang, 2006). Since secondary forests produce a higher volume of litter, the corresponding black carbon content in their soils is relatively higher.

4.3 Influence of land use types on the proportional composition and origin of soil BC

The proportions of BC to OC in this study ranged from 22.67% to 78.62% for secondary forest, 27.02%–67.16% for shrub, 33.29%–52.47% for farmland, and 45.16%–62.10% for wasteland (Table 1). These values are higher than those reported in previous studies, such as 8%–26% in red and yellow soils by Zhang and Zhang (2006) and 11.3%–53.2% in agricultural soils in the northern Zhejiang Plain by Dai et al. (2009). This discrepancy may be attributed to the low OC content commonly found in rocky desertification areas. Additionally, in the surface soil, the highest BC/OC ratio was observed in wasteland (45.16%), followed by farmland (33.29%), shrub (27.02%), and secondary forest (22.67%). These results are consistent with those of Zhang and Zhang (2006), who found that OC content was highest in dryland soils, followed by tea gardens, secondary forests, and native woodlands—a trend that may be due to decreasing OC with increasing cultivation time. Moreover, the high stability of BC and the presence of incompletely burned biochar in organic fertilizers contribute to elevated BC/OC ratios in soils with agricultural inputs, suggesting that farming practices influence BC accumulation.

BC in this study was significantly positively correlated with both char and soot content. Char/BC ratios ranged from 51.09% to 81.01%, while soot/BC ratios ranged from 18.99% to 49.09%. These findings indicate that char and soot are major components of soil BC in the rocky desertification regions of Southwest Guangxi, aligning with the findings of Zhan et al. (2013). The BC/TOC ratio ranged from 18.06% to 54.41% (Figure 3), showing substantial variation and highlighting the significance of BC as a component of SOC. These ratios were much higher than those reported by Zhang and Zhang (2006) (8.3%–25.6%). The decomposition rate of native OC in the soil is influenced by BC input (Li et al., 2013), and the elevated BC/TOC observed in this study may result from either high BC input or an inherently low OC content. Different land use types showed distinct impacts on BC/TOC, with the highest ratio in the secondary forest (54.39%), followed by wasteland (44.83%), shrub (43.87%), and farmland (33.22%), indicating that BC accumulation is also influenced by human activity.

This study found that soil bulk density was negatively correlated with BC. The soil bulk density decreased in the order of wasteland > farmland > shrub > secondary forest. The permeability of the surface soil was affected by artificial tillage; therefore, its bulk density was low. However, the bulk density of wastelands is large because of human disturbance and a lack of effective management measures. Secondary forest and shrub areas provide more favorable environmental conditions, allowing stable, natural systems to develop and resulting in lower soil bulk density. This phenomenon might also explain the differences in BC/TOC, consistent with the results of Zhan et al. (2015) and Edmondson et al. (2015).

BC is mainly generated from two sources: natural emissions, primarily resulting from volcanic eruptions and fires in grasslands or forests, and anthropogenic emissions, primarily resulting from residential and commercial activities, energy production, industrial processes, motor vehicle exhaust, coal combustion, and agricultural straw burning. When analyzing the sources of BC in soil, many scholars (Zhan et al., 2015; López-Martín et al., 2016; Wang et al., 2014) have used the char/soot method. Generally, char is formed by the incomplete combustion of biomass, and soot is predominantly produced by fossil fuel combustion. Therefore, BC source analysis using char/soot has a substantial reference value. Zhan et al. (2015) reported a char/soot ratio of less than 2.0, suggesting that the primary sources of BC are herbaceous plants and fossil fuel combustion, such as motor vehicle exhaust emissions. Conversely, a ratio greater than 2.0 indicates that woody plant combustion is the main contributor to BC. This study indicated that the char/soot ratio was below 2.0 across all layers (Table 1), except 4.38 for the 10–20 cm depth in shrub, 3.73 for the 30–40 cm depth in shrub, and 2.34 for the 20–30 cm depth in wasteland, indicating that herbaceous plants and fossil fuel combustion are the primary sources of soil BC within the study area. This was mainly related to the succession sequence of herbs, shrubs, and trees during the process of vegetation restoration. Another contributing factor might be that the study area is close to roads, dust, and vehicle exhaust emissions from road traffic. Stable carbon isotope values from −13.66‰ to −28.33‰, along with the high proportion of HFBC in the soil BC suggest that the BC is mainly from the burning of C3 plants (Liu and Zhang, 2010), as well as motor vehicle exhaust emissions and fossil fuel combustion (Koç et al., 2020).

The isotopic composition of BC from C3 and C4 plants is relatively fixed. Das et al. (2010) showed that the δ¹³C values of BC were approximately from −24. 6‰ to −26.1‰ for C3 plants and from −12.3‰ to −13.8‰ for C4 plants after being burned. Due to the extensive combustion of fossil fuels, the BC particles exhibited a relative dilution of δ¹³C, leading to a reduction in atmospheric CO₂ δ¹³C values (Wang et al., 2018). The δ¹³C value of BC of carbon-contained materials on the soil surface layer also decreased. The BC content of secondary forest soils was significantly higher than that of other land use types, and the ¹³C abundance of soil BC was the lowest, indicating a great depletion of ¹³C in BC. In secondary forests, the many years of burning biomass materials and road motor vehicle exhaust containing large amounts of particulate matter, such as BC, resulted in a high soil BC content. The δ¹³BC values were lower than those in other land use modalities, indicating that secondary forests and shrubs reduced the diffusion of BC from motor vehicle emissions to surrounding areas. Road traffic dust may also significantly contribute to soil BC in the study area because the amount of BC accumulated in road dust is primarily derived from fossil fuel combustion emissions from motor vehicles (Han et al., 2009). This suggests that vehicular emission pollution has become a potential source of air pollution with the development of tourism and the increased number of motor vehicles in the rocky desertification areas of Southwest Guangxi.

The study has some limitations. First, the variation characteristics of soil dissolved black carbon and the protective effect of soil aggregates on black carbon were not deeply investigated in this study. Additionally, we did not analyze the distribution characteristics of soil organic carbon and black carbon in the deep layer (below 40 cm), thereby overlooking the possibility of the vertical migration of black carbon. These aspects warrant further investigation in future studies.

5 Conclusion

This study is the first to investigate the distribution patterns of soil black carbon in secondary forests, shrub, farmland, and wasteland within the rocky desertification area of Southwest Guangxi. The examination of black carbon (BC), char, soot contents, and the BC/TOC across these four land use types revealed certain distinct differences. Overall, BC was primarily enriched in the topsoil and showed a significant positive correlation with organic carbon. The mean BC content was highest in secondary forest, followed by shrub, with farmland and wasteland being relatively lower.

1. Land use type significantly influenced the distribution of soil BC, exhibiting a decreasing trend after the conversion of forestland to farmland. The contribution of BC to soil organic carbon increased proportionally with soil depth, and higher BC content facilitated the accumulation of soil organic carbon.

2. Both the LFBC and HFBC in the rocky desertification area of Southwest Guangxi contained certain proportions of BC, with BC being predominantly distributed in the HFBC. Significant positive correlations were identified between soil BC and char, soot, LFBC, and HFBC, indicating that char and soot are important components of BC.

3. This study suggests that soil BC in the rocky desertification area of Southwest Guangxi is considerably influenced by human activities, primarily derived from vehicle emissions, fossil fuel combustion, and the combustion of C3 plant biomass. Agricultural activities are identified as the main source of BC in cultivated farmland soils within the region. The presence of BC contributes to enhancing the carbon sequestration capacity of secondary forest soils in the rocky desertification area of Southwest Guangxi.

Data availability statement

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/supplementary material.

Author contributions

DZ: Conceptualization, Data curation, Funding acquisition, Investigation, Visualization, Writing – original draft, Writing – review and editing. QT: Conceptualization, Data curation, Funding acquisition, Writing – original draft. KH: Conceptualization, Writing – original draft. YuS: Formal Analysis, Supervision, Writing – original draft. YiS: Formal Analysis, Writing – original draft. GX: Conceptualization, Data curation, Funding acquisition, Writing – review and editing. HS: Investigation, Writing – original draft. YZ: Supervision, Writing – original draft.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This work was supported by the National Natural Science Foundation of China (42267007, 32460311); Natural Science Foundation of Guangxi (2020GXNSFBA297048); Basic Research Fund of Guangxi Academy of Sciences (CQZ-E-1912); Guangxi Key Science and Technology Innovation Base on Karst Dynamics (KDL & Guangxi 202004); Fund of Guangxi Key Laboratory of Plant Conservation and Restoration Ecology in Karst Terrain (22-035-26); Basic Research Fund of Guangxi Institute of Botany (25007).

Acknowledgements

We would like to thank Jianchun Liu, Guixia Cheng, Qianqian Yu, Lei Tian, Qilei Cheng, and Cuihong Li for testing the server. We also extend our thanks to Editage (www.editage.cn) for English language editing.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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Footnotes

^{Abbreviations:}BC, Black carbon; OC, Organic carbon; SOC, Soil organic carbon; POC, Particulate organic carbon; LFBC, Light-fraction black carbon; HFBC, Heavy-fraction black carbon; TOC, Total organic carbon; TN, Total nitrogen; TP, Total phosphorus; TK, Total potassium; AN, Available nitrogen; AP, Available phosphorus; AK, Available potassium; BDSoil bulk density.

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