Evaluation of forest conversion effects on soil erosion and soil organic carbon using 137Cs and 210Pbex tracers in the low mountain–hill region of Southwest China

Estimating both soil erosion and sedimentation in southern China’s hilly regions is essential for planning soil-conservation measures; however, the patterns of soil erosion and their influence on soil organic carbon (SOC) in different forest types remain insufficiently understood. Using a dual-tracer approach with ¹³⁷Cs and ²¹⁰Pb_ex, this study assessed soil erosion dynamics across three forest types --mixed broadleaf forest (MBF), Masson pine plantation (MPP), and Eucalyptus plantation (EUP) --in the low mountainous hills of northern Guangxi, southwest China. Both radionuclides showed exponential declines in activity concentrations with depth, with pronounced surface enrichment (peak values in the 0–5 cm layer) in all forest types. EUP exhibited significantly lower clay content and higher sand proportions compared with MPP and MBF, and SOC content was consistently lowest in EUP across all slope gradients. Mean ¹³⁷Cs and ²¹⁰Pb_ex inventories followed the order MBF > MPP > EUP, while slope position followed lower-slope > mid-slope > upper-slope. Soil erosion rates ranged from -466.90 to 2159.78 tkm^-2a^-1 for ¹³⁷Cs and ²¹⁰Pb_ex and -398.83 to 3023.27 tkm-²a^-1 for ²¹⁰Pb_ex, with erosion intensity increasing from lower to upper slopes and being highest in the Eucalyptus plantation. Significant positive correlations (p<0.01) were observed between both radionuclides and SOC and clay content. Overall, the conversion of mixed forests to economic plantations intensified soil erosion, particularly in EUP. Targeted management practices in EUP are essential for improving soil conservation and promoting sustainable forest management. The close agreement between ¹³⁷Cs and ²¹⁰Pb_ex-derived erosion estimates highlights the potential of ²¹⁰Pb_ex as an alternative or complementary tracer for monitoring soil erosion in this region.

1 Introduction

Soil erosion, a global environmental challenge, accelerates land degradation, reduces agricultural productivity, and poses a significant threat to sustainable socio-economic development. In the red soil hilly regions of southern China, severe soil erosion persists despite high vegetation coverage, due to fragmented terrain, intense and concentrated rainfall, and high population density (Wang et al., 2020). Soil erosion drives the redistribution of soil and associated soil organic carbon (SOC) across landscapes, with profound consequences for biogeochemical cycling and terrestrial carbon sequestration (Van Oost and Six, 2023). Elevated erosion rates reduce soil depth, disrupt hydrological properties, degrade productivity, and impair carbon sequestration capacity and nutrient retention (Zhang et al., 2020). To address soil erosion in these areas, large-scale plantations of timber forests—such as Masson pine and Eucalyptus—have been widely established (Huang et al., 2019).

Reforestation improves surface stability, enhances rainfall infiltration, and promotes the accumulation of soil organic matter, thereby improving physicochemical properties such as bulk density, aggregate stability, and SOC content (Zhang et al., 2020; Mongil-Manso et al., 2022). Fast-growing plantations play an important role in controlling erosion, restoring soil fertility, reversing ecological degradation, and strengthening ecosystem resilience (Sun et al., 2018; Aburto et al., 2021). However, large-scale monoculture plantations—particularly pure Eucalyptus and Masson pine—while economically beneficial, can also create ecological problems, including high susceptibility to soil erosion (Zhang et al., 2015; Ma et al., 2016). Differences in silvicultural practices among timber forests may further lead to divergent erosion patterns. Currently, limited research addresses the comparative soil erosion characteristics of different timber forests in this region and the impact of erosion on SOC content.

¹³⁷Cs and ²¹⁰Pb_ex share similar physicochemical properties: both are deposited via rainfall, bind strongly and irreversibly to soil particles, and are minimally affected by leaching or plant uptake. Their spatial redistribution is primarily governed by physical processes such as erosion and deposition (Shi et al., 2012; Guo et al., 2022). Dual-tracer techniques using ¹³⁷Cs and ²¹⁰Pb_ex are widely applied to quantify soil erosion and sedimentation rates (Putyrskaya et al., 2020). These tracers have been used to investigate SOC migration in forested hillslopes (Teramage et al., 2013) and to assess erosion–sedimentation processes in diverse landscapes, such as southern Italy (Porto et al., 2009), the hilly region of central Sichuan (Guo et al., 2022), and uncultivated slopes in the Sichuan Basin and Three Gorges area (Shi et al., 2012; Zhou et al., 2025). They have also been applied to forested slopes, Nepalese sub-basins, grassland sandy areas in northern China, and sandy margins in Mongolia (Wakiyama et al., 2010; Hu and Zhang, 2019; Yuan et al., 2021; Guo et al., 2022). However, the effects of changes in silvicultural practices on soil erosion in major timber forests of southern China’s red soil hilly regions, using ¹³⁷Cs and ²¹⁰Pb_ex tracers, remain unexplored.

This study examines soil erosion characteristics in three dominant forest types—mixed broadleaf forest (MBF), Masson pine plantation (MPP), and Eucalyptus plantation (EUP)—in the low-mountain hilly region of northern Guangxi, southwest China, using ¹³⁷Cs and ²¹⁰Pb_ex dual-tracer techniques (Figure 1). Field sampling and laboratory analyses were conducted to determine the vertical distributions of ¹³⁷Cs, ²¹⁰Pb_ex, and SOC. Soil erosion rates were estimated using established models to evaluate differences among forest types and slope positions. Correlation analyses between radionuclides, SOC, and soil texture were used to assess their interrelationships with slope gradient, forest type, and soil depth. The objectives were to: (1) assess the impact of converting mixed forests to monoculture plantations on soil erosion in the red soil hilly areas of southwest China; and (2) determine the influence and drivers of soil erosion on SOC in major timber forests. The findings provide insights into the role of forest type in shaping erosion dynamics, offering a scientific basis for improved land management, erosion mitigation, and sustainable forestry in the region.

Figure 1

Figure 1. Impacts of different plantations on soil erosion and soil organic carbon.

2 Materials and methods

2.1 Study area

The study was conducted in Huangmian Forest Farm, located at the junction of Luzhai County and Yongfu County, Guangxi, China (Figure 2); 109°43′46″–109°58′18″E, 24°37′25″–24°52′11″N. The terrain is highly undulating, with a maximum elevation of approximately 895.91 m, and is characterized predominantly by low-mountain and hilly landscapes. The region has a subtropical monsoon climate, with a mean annual temperature of 19 °C, annual precipitation of 1750 mm, and mean annual evaporation of 1426 mm. The climate is moderate overall, with hot, rainy summers and distinct seasonal variations. The dominant soils are mountain red soil and mountain yellow soil. The selected slope has a 50-year reclamation history, complex geomorphology, and was stratified into three slope positions: upper slope (16°–22°), middle slope (7°–15°), and lower slope (2°–6°).

Figure 2

Figure 2. Location of the study area.

Historically, the area was covered by subtropical primary forest, but intensive timber harvesting led to extensive deforestation. At present, the landscape is dominated by reforested non-native plantations, mainly Eucalyptus and Masson pine, both established through conversion from mixed broadleaf forest. Understory coverage rates were approximately 90% in MBF, 70% in MPP, and 20% in EUP. Both MPP and EUP were approximately 5 years old at the time of sampling. Based on field surveys conducted on 5 May 2014, three forest types—MBF, MPP, and EUP—were selected as experimental sites. Within each forest type, three slope positions (upper, middle, lower) were systematically delineated along topographic transects. In each slope position, three standardized plots (20 m × 20 m) were established, totaling nine plots per forest type. The MBF plots supported a diverse understory vegetation community, including maple (Acer spp.), moso bamboo (Phyllostachys edulis), shrubs, and herbaceous plants.

2.2 Soil sampling

For each forest type, nine full-depth soil profiles (40 cm) and 54 bulk cores were collected for analysis of ¹³⁷Cs, ²¹⁰Pb_ex, and particle size distribution. A total of 189 soil samples were obtained using a 5 cm inner-diameter auger from the three forest types. Bulk cores were stratified by slope position (upper, middle, lower), with each core sampled to a depth of 30 cm at 5 cm intervals (six layers per core).

A reference site (RES) for determining background ¹³⁷Cs and ²¹⁰Pb_ex inventories was established in an undisturbed secondary forest adjacent to the study area. This site is flat, has been free from deforestation or human disturbance since the 1950s, and has experienced negligible erosion over the past century. Twelve full-depth profiles (40 cm) and 30 bulk core samples were collected at the reference site, with sampling points spaced >2 m apart. Cores were sectioned at 5 cm intervals to a depth of 30 cm.

2.3 Sample analyses

All soil samples were air-dried, weighed, and sieved through a 2 mm mesh after removal of roots and debris. Approximately 300 g of each sample was packed into standardized cassettes for ¹³⁷Cs and ²¹⁰Pb_ex activity determination using a high-purity germanium gamma spectrometer (ORTEC, United States of America) at the Nuclear Physics Laboratory, Lanzhou University, China. ¹³⁷Cs activity was measured at the 661.6 keV emission peak, with a counting time of ≥80,000 s, achieving a precision of ±5% at the 95% confidence level. ²¹⁰Pb_ex activity was calculated by subtracting ²²⁶Ra-supported ²¹⁰Pb (determined via its daughter nuclide ²¹⁴Pb at 351.9 keV) from total ²¹⁰Pb (46.5 keV).

Soil bulk density was determined from undisturbed cores collected using the ring knife method (5 cm height). The SOC content was analyzed using a total organic carbon analyzer (Shimadzu TOC-5000A, Japan). Particle size distribution (clay: <0.002 mm; silt: 0.002–0.02 mm; sand: 0.02–2 mm) was measured using a laser diffraction particle size analyzer (Mastersizer 2000, Malvern, United Kingdom). Results were expressed as mass percentages following the International Soil Texture Classification System.

2.4 Estimation of erosion and deposition rates

Measured ¹³⁷Cs and ²¹⁰Pb_ex activity (Bq·kg^-1) were converted to area-based inventories (Bq·m^-2) using soil bulk density and layer thickness. For bulk cores and full profiles, inventories were calculated as (Equations 1, 2):

$\text{Bulk\hspace{0.17em}core\hspace{0.17em}sample}:\text{CPI}\left(\text{PPI}\right)={\displaystyle \sum _{i=1}^n}{10}^3\times {\mathrm{C}}_{\mathrm{i}}\times {\mathrm{D}}_{\mathrm{i}}\times \text{Hi}(1)$

$\text{Full\hspace{0.17em}profile\hspace{0.17em}sample}:\text{CPI}\left(\text{PPI}\right)={\mathrm{C}}_{\mathrm{i}}\times \mathrm{W}/\mathrm{S}(2)$

Where CPI (¹³⁷Cs inventory) or PPI (²¹⁰Pb_ex inventory) is the total activity of ¹³⁷Cs and ²¹⁰Pb_ex at the sample site (Bq·m^-2); i is the layer number, n is the number of layers sampled, C_i is the mass activity of ¹³⁷Cs (or ²¹⁰Pb_ex) in layer i (Bq·kg^-1), 10³ is the unit adjustment factor, D_i is the soil bulk density of layer i, H_i is the sample thickness of layer i (m), W is the weight of the sieved sample (kg); and S is the corer cross-sectional area (m²).

Because the forest types in this study are non-cultivated land, ¹³⁷Cs-based erosion rates were estimated using the non-cultivated soil erosion model (Lowrance et al., 1988) (Equation 3):

${\mathrm{M}}_{\mathrm{c}}=\left[\left({\mathrm{A}}_0-\mathrm{X}\right)\times {10}^3\right]/\left(\mathrm{N}-1954\right)\times \text{Cs}(3)$

Where M_c is the ¹³⁷Cs-derived soil erosion rate (t·km^-2·a⁻¹); A₀ is the local ¹³⁷Cs reference inventory (Bq·m^-2); X is the ¹³⁷Cs total activity of the soil profile in N years (Bq·m^-2); N is the sampling year; Cs is the mean ¹³⁷Cs mass activity of eroded soil (Bq·kg^-1).

For ²¹⁰Pb_ex, the steady-state non-cultivated model (Sun et al., 2013) was applied (Equation 4):

${\mathrm{M}}_{\mathrm{p}}=-\mathrm{H}\times \ln \left(\mathrm{\lambda }+1-\frac{\mathrm{\lambda }{\mathrm{A}}_0}{\mathrm{A}}\right)\times {10}^2(4)$

Where M_p is the ²¹⁰Pb_ex-derived soil erosion rate (t·km^-2·a⁻¹); H is the relaxation mass depth (kg∙m^-2); λ is the ²¹⁰Pb_ex decay constant (0.031a⁻¹); $A_0$ is the ²¹⁰Pb_ex reference inventory (Bq·m^-2); and A is the ²¹⁰Pb_ex total area activity at the sampling site (Bq·m^-2).

Soil organic carbon storage was calculated by following formula (Xu et al., 2016) (Equation 5):

${\mathrm{S}}_{\text{SOC}}=\sum {\mathrm{S}}_{\mathrm{i}}{\mathrm{B}}_{\mathrm{i}}{\mathrm{F}}_{\mathrm{i}}\times {10}^{-2}(5)$

Where S_SOC is organic carbon storage (kg·m^-2); S_i is SOC content in layer i (g·kg^-1); B_i is bulk density (g·cm^-3); and F_i is soil layer thickness (cm).

3 Results

3.1 ¹³⁷Cs and ²¹⁰Pb_ex inventories and depth distributions at the reference sites

The vertical distributions of ¹³⁷Cs and ²¹⁰Pb_ex activities at the reference sites are shown in Figure 3. Forty-two samples from undisturbed sites yielded regional background inventories of 1565.90 Bq·m^–2 for ¹³⁷Cs and 15,153.06 Bq·m^–2 for ²¹⁰Pb_ex. Both radionuclides exhibited peak activities within the upper 10 cm of the soil profile, followed by an exponential decline with depth. Below 25 cm, activities were negligible (<5% of surface values), confirming the absence of significant soil disturbance. The depth profiles—characterized by surface enrichment and minimal subsurface migration—support the validity of these measurements as baseline reference inventories for erosion assessments.

Figure 3

Figure 3. Distribution of ¹³⁷Cs (a) and ²¹⁰Pb_ex (b) at the reference plots.

3.2 ¹³⁷Cs, ²¹⁰Pb_ex and SOC depth distributions across plots

The vertical distributions of ¹³⁷Cs, ²¹⁰Pb_ex, and SOC across forest types and slope positions are shown in Figure 4. Mean ¹³⁷Cs activities in the 0–30 cm layer were 1.42, 2.00, and 2.54 Bq·kg^–1 at upper slopes; 2.15, 3.49, and 3.21 Bq·kg^–1 at middle slopes; and 2.65, 3.78, and 3.66 Bq·kg^–1 at lower slopes for EUP, MPP, and MBF, respectively (Figures 4a–c). Corresponding mean ²¹⁰Pb_ex activities were 16.80, 21.02, and 29.82 Bq·kg^–1 at upper slopes; 25.99, 37.18, and 35.80 Bq·kg^–1 at middle slopes; and 29.18, 45.17, and 37.05 Bq·kg^–1 at lower slopes (Figures 4d–f). In all cases, radionuclide activities declined exponentially with depth and increased downslope. MBF had the highest ¹³⁷Cs and ²¹⁰Pb_ex activities on upper slopes, MPP dominated at middle and lower slopes, and EUP consistently showed the lowest activities across all slope positions.

Figure 4

Figure 4. Depth distributions of ¹³⁷Cs (a–c), ²¹⁰Pb_ex 1, (d–f) and SOC (g–i) each plot. Different capital letters indicate significant differences among forest types within the same soil layer (p < 0.05); different lowercase letters indicate significant differences among soil layers within the same forest type (p < 0.05).

Relative to MPP and MBF, ¹³⁷Cs activities in the 0–5 cm layer of EUP were lower by 39.72% and 48.43% at upper slopes, 48.52% and 40.41% at middle slopes, and 40.86% and 32.01% at lower slopes (p < 0.05), respectively. Similarly, ²¹⁰Pb_ex activities in EUP declined by 28.93% and 43.01% at upper slopes, 38.68% and 28.14% at middle slopes, and 50.53% and 24.58% at lower slopes (p < 0.05), respectively. The SOC depth profiles are presented in Figures 4g–i. Mean SOC contents in the 0–30 cm layer were 27.73, 46.35, and 51.90 g kg^–1 at upper slopes; 32.79, 50.12, and 56.26 g kg^–1 at middle slopes; and 40.40, 55.96, and 60.14 g kg^–1 at lower slopes for EUP, MPP, and MBF, respectively. SOC increased downslope for all forest types. Across all slope positions, SOC in EUP was significantly lower than in MPP and MBF (p < 0.05), whereas MPP and MBF differences were not significant. Within each slope position, SOC declined with increasing depth.

3.3 Particle size distribution characteristics

Vertical and slope-dependent variations in soil texture are shown in Figure 5. Clay content (%) declined with depth in all forest types (Figures 5a–c), decreasing from 20.34%, 26.87%, and 30.64% in the 0–5 cm layer to 6.69%, 15.70%, and 18.43% in the 25–30 cm layer for EUP, MPP, and MBF, respectively. In contrast, sand content (%) increased with depth (Figures 5g–i), with increases of 11.38%, 10.72%, and 12.22% from 0–5 cm to 25–30 cm at upper slopes for the three forest types. Silt content showed minimal depth variation at upper and middle slopes (Figures 5d–f), averaging 56.30%, 57.53%, and 55.09% at upper slopes; 50.77%, 55.97%, and 52.53% at middle slopes; and 59.18%, 51.77%, and 48.16% at lower slopes for EUP, MPP, and MBF, respectively. MPP had significantly higher silt content at upper and middle slopes than EUP and MBF (p < 0.05). At lower slopes, MBF silt content increased with depth, while EUP exhibited a relatively high silt proportion (p < 0.05).

Figure 5

Figure 5. Particle size distribution characteristics showing Clay (a–c), Silt (d–f), and Sand (g–i) contents for the three forest types.

Mean clay contents for EUP, MPP, and MBF were 13.37%, 21.02%, and 24.42% at upper slopes; 15.36%, 22.92%, and 24.58% at middle slopes; and 21.03%, 27.10%, and 26.16% at lower slopes, respectively. At upper and middle slopes, MBF had the highest clay content and EUP the lowest; at lower slopes, MPP clay content exceeded both MBF and EUP significantly (p < 0.05). Across forest types, clay content decreased significantly from upper to lower slopes. Sand content in EUP was significantly higher than in MPP and MBF at upper and middle slopes (p < 0.05), whereas MBF showed the highest sand content at lower slopes. EUP had the highest sand percentage at middle slopes but the lowest at lower slopes.

3.4 Soil erosion rates characteristics

Variations in ¹³⁷Cs and ²¹⁰Pb_ex inventories by slope position and forest type are summarized in Table 1. The ¹³⁷Cs inventories ranged from 847.75 to 1456.51 Bq·m^-2 in EUP, 939.11–1844.48 Bq·m^-2 in MPP, and 1054.68–1920.92 Bq·m^-2 in MBF (Figure 6a). Inventories generally increased downslope (upper < middle < lower slopes), with the highest mean ¹³⁷Cs inventory in MBF (1491.84 Bq·m^–2), followed by MPP (1443.35 Bq·m^–2) and EUP (1134.95 Bq·m^–2).

Table 1

Table 1. Soil erosion characteristics of different forest types.

Figure 6

Figure 6. Relationships between S_SOC,¹³⁷Cs (a), SSOC and ²¹⁰Pb_ex (b) under different forest types.

For ²¹⁰Pb_ex, inventories ranged from 6712.71 to 12,547.21 Bq·m^–2 in EUP, 9105.07–18,692.03 Bq·m^–2 in MPP, and 12,845.96–16,857.53 Bq·m^–2 in MBF (Figure 6a). EUP and MPP both showed downslope enrichment (lower > middle > upper), whereas MBF followed a middle > lower > upper trend. Mean ²¹⁰Pb_ex inventories were highest in MBF (15,183.42 Bq·m^–2), followed by MPP (14,822.64 Bq·m^–2) and EUP (10,117.21 Bq·m^–2). These results confirm significantly lower radionuclide inventories in EUP compared to the other forest types (p < 0.05), indicating relatively greater erosion in EUP.

Erosion rate derived from ¹³⁷Cs decreased significantly with decreasing slope gradient (p < 0.05), with a similar trend for ²¹⁰Pb_ex in EUP and MPP (p < 0.05). In MBF, ²¹⁰Pb_ex-derived erosion followed an upper > lower > middle slope trend (p < 0.05). Mean erosion rates ranked as follows: EUP (¹³⁷Cs: 1,143.48 t km^-2·a⁻¹; ²¹⁰Pb_ex: 1,474.84 t km^-2·a⁻¹) > MPP (¹³⁷Cs:450.82 t km^-2·a⁻¹; ²¹⁰Pb_ex:250.25 t km^-2·a⁻¹) > MBF (¹³⁷Cs: 294.20 t km^-2·a⁻¹; ²¹⁰Pb_ex:29.35 t km^-2·a⁻¹). Maximum erosion occurred at upper slopes of EUP, exceeding MPP and MBF by 1.27–8.04 times.

Based on the soil erosion classification and grading Standard (Ministry of Water Resources of the People’s Republic of China, 2007) and the reference inventories determined in this study, ¹³⁷Cs-derived erosion intensity was classified as mild for EUP at upper and middle slopes, for MPP at upper slopes, and for MBF at upper slopes. Erosion rates for EUP at lower slopes and MBF at middle slopes (<500 t km^-2·a⁻¹) were within the allowable soil loss threshold for the red soil hilly regions of southern China. For ²¹⁰Pb_ex, moderate erosion occurred at EUP upper slopes, mild erosion at EUP middle/lower slopes and MPP upper slopes, and slight erosion at MBF upper slopes. Specifically, ²¹⁰Pb_ex results indicated that soil erosion intensity in EUP reached moderate levels at upper slopes, while mild erosion occurred at both middle and lower slopes of EUP and at upper slopes of MPP. MBF exhibited slight erosion at upper slopes. In contrast, soil deposition prevailed at middle and lower slopes of both MPP and MBF.

3.5 Relationship between soil erosion and soil organic carbon storage (S_SOC)

The linear relationship and correlation among S_SOC, ¹³⁷Cs and ²¹⁰Pb_ex activities are presented in Figure 6. Significant positive correlations were found between S_SOC and both radionuclides across all forest types (p < 0.01), indicating that higher S_SOC content generally coincides with higher radionuclide activities. ¹³⁷Cs explained 89%, 79%, 90%, and 93% of S_SOC variability in EUP, MPP, MBF, and reference sites, respectively. Corresponding values for ²¹⁰Pb_ex were 83%, 76%, 91%, and 89%. Overall, ¹³⁷Cs showed a stronger correlation with S_SOC than ²¹⁰Pb_ex. The S_SOC variation trends in EUP, MBF, and the reference sites closely matched changes in ¹³⁷Cs and ²¹⁰Pb_ex activities, whereas MPP exhibited a slower rate of change.

Regression intercepts between S_SOC and radionuclide activities differed significantly among forest types, reflecting variations in depth distributions of ¹³⁷Cs and ²¹⁰Pb_ex within their soil profiles. Correlation analyses suggest that S_SOC shares similar physical transport characteristics with both radionuclides, supporting the use of ¹³⁷Cs and ²¹⁰Pb_ex for quantitative evaluation of S_SOC spatial and temporal patterns under erosion processes. These findings demonstrate the feasibility of applying ¹³⁷Cs and ²¹⁰Pb_ex techniques to assess SOC redistribution in plantation lands of the low mountain–hill regions.

3.6 Relationship between soil erosion and grain size composition

Linear regression analysis revealed significant positive correlations between ¹³⁷Cs/²¹⁰Pb_ex content and clay content across all forest types (p < 0.01, Figure 7). Specifically, ¹³⁷Cs accounted for 89%, 87%, 92%, and 88% of clay variability in EUP, MPP, MBF, and reference sites, respectively, while ²¹⁰Pb_ex explained 85%, 83%, 91%, and 93%. Significant negative correlations between both radionuclides and silt content were observed only in MBF (p < 0.05), with no significant relationships in EUP, MPP, or reference sites.

Figure 7

Figure 7. Relationship between contents of ¹³⁷Cs and grain size distribution (a–c), and between contents of ²¹⁰Pb_ex and grain size distribution (d–f).

Both radionuclides were significantly negatively correlated with sand content (p < 0.01). ¹³⁷Cs explained 53%, 74%, 33%, and 87% of sand variability in EUP, MPP, MBF, and reference sites, respectively, whereas ²¹⁰Pb_ex accounted for 47%, 69%, 42%, and 92%. Across forest types, the strength of correlation was highest for clay particles, followed by sand, with silt showing the weakest and least consistent relationships.

3.7 Relationship between ¹³⁷Cs and ²¹⁰Pb_ex erosion rates

As shown in Figure 8, ¹³⁷Cs and ²¹⁰Pb_ex inventories were significantly positively correlated across all forest types (p < 0.01), confirming that both tracers can independently capture erosion and deposition processes over different temporal scales. EUP exhibited higher erosion rates than MPP and MBF, indicating intensified soil loss following conversion of native MBF to Eucalyptus monocultures. The overall agreement between the two tracers underscores the potential of ²¹⁰Pb_ex as a complementary or alternative method to ¹³⁷Cs for regional soil erosion monitoring.

Figure 8

Figure 8. Correlation between ¹³⁷Cs and ²¹⁰Pb_ex erosion rates.

3.8 Correlation analysis between ¹³⁷Cs, ²¹⁰Pb_ex, and soil physicochemical properties

Correlation results are presented in Figure 9. In all forest types—EUP (Figure 9a), MPP (Figure 9b), and MBF (Figure 9c)— ¹³⁷Cs, ²¹⁰Pb_ex, and SOC were significantly positively correlated (p < 0.01, p < 0.05). M_c was significantly negatively correlated with ¹³⁷Cs, ²¹⁰Pb_ex, and SOC (p < 0.01), while M_p also showed significant negative correlations with ¹³⁷Cs, ²¹⁰Pb_ex, and SOC (p < 0.01, p < 0.05), and a positive correlation with M_c (p < 0.01). Clay content was positively correlated with ¹³⁷Cs and SOC (p < 0.01), weakly correlated with ²¹⁰Pb_ex, and negatively correlated with M_c and M_p. For MPP and MBF, silt content was negatively correlated with ¹³⁷Cs, ²¹⁰Pb_ex, SOC, and clay (p < 0.01, p < 0.05), and was positively correlated with M_c and M_p (p < 0.01, p < 0.05). Bulk density showed no significant correlations with ¹³⁷Cs, ²¹⁰Pb_ex, SOC, M_c, M_p, clay, silt, or sand in any forest type. In EUP, silt content showed no significant correlations with other variables, while sand content was negatively correlated with silt, clay, ¹³⁷Cs, and SOC (p < 0.01, p < 0.05), but not with ²¹⁰Pb_ex, M_c, or M_p. In MPP, sand content exhibited no significant correlations with any variables. In MBF, sand content was negatively correlated with silt and M_c (p < 0.01), weakly negatively correlated with M_p (p < 0.05), and positively correlated with ¹³⁷Cs, SOC, clay, and ²¹⁰Pb_ex (p < 0.01, p < 0.05).

Figure 9

Figure 9. Correlations between ¹³⁷Cs, ²¹⁰Pb_ex and soil physico-chemical properties for (a) Eucalyptus plantation (EUP), (b) Masson pine plantation (MPP), and (c) Mixed broadleaf forest (MBF).

3.9 The effects of slope position, forest type and soil depth on ¹³⁷Cs and ²¹⁰Pb_ex

Multi-factor ANOVA (Table 2) revealed significant effects of slope position, forest type, soil depth, and their interactions on ¹³⁷Cs and ²¹⁰Pb_ex inventories (p < 0.01). Coefficients of determination (R²) exceeded 0.90, indicating that >90% of the variability in radionuclide distributions could be explained by these factors and their interactions. Individually, slope position, forest type, and soil depth each significantly influenced ¹³⁷Cs and ²¹⁰Pb_ex (p < 0.01). Interaction terms—slope position × forest type, slope position × soil depth, and forest type × soil depth—also significantly affected both radionuclides (p < 0.01). The three-way interaction was significant for ²¹⁰Pb_ex (p < 0.01) but not for ¹³⁷Cs (p > 0.05).

Table 2

Table 2. Significance tests for the effects of slope position, forest type, soil depth, and their interactions on¹³⁷Cs and²¹⁰Pb_ex contents.

4 Discussion

4.1 Soil erosion following the conversion of mixed forests to monoculture plantations

The determination of reference inventories is fundamental for applying the ¹³⁷Cs and ²¹⁰Pb_ex dual-tracer technique in soil erosion studies (Ritchie and McCarty, 2003; Correchel et al., 2006). By comparing measured radionuclide inventories with established regional reference values, erosion and deposition processes can be quantitatively assessed. In southern China’s red soil regions, reported ¹³⁷Cs reference inventories include 1746.64 Bq·m^–2 in the Jiangxi red loam area, 1823.94 Bq·m^–2 in the southern red soil hilly area, and 968.19 Bq·m^–2 in Jiangxi watershed hillslopes (Lu et al., 2016; Ye et al., 2022). For ²¹⁰Pb_ex, reference values range from 9437.19 Bq·m^–2 in the Najia sub-basin to 48,841.13 Bq·m^–2 in the Honghu sub-basin of Jiangxi (Zhang et al., 2006; Zheng et al., 2007; Liu, 2022). In this study, reference sites adjacent to the experimental forests yielded ¹³⁷Cs inventories of 1565.90 Bq·m^–2, closely matching comparable regional values, whereas the ²¹⁰Pb_ex inventory (15,153.06 Bq·m^–2) showed greater variation, likely due to localized geochemical and depositional conditions. The proximity of the reference sites to the experimental plots ensures the reliability of these values for comparative analyses among the three forest types.

Both ¹³⁷Cs and ²¹⁰Pb_ex exhibited surface enrichment (0–10 cm) with exponential decay down the profile, consistent with global patterns (Walling et al., 2003; Zhang et al., 2006; Meliho et al., 2019). Previous studies have reported high erosion rates in monoculture plantations: ¹³⁷Cs-derived erosion rate of 1382–2,308 t km^–2·a⁻¹ in Eucalyptus forests (Ren et al., 1999), 2,700–6000 t km^–2·a⁻¹ in Masson pine understories (Li et al., 2012), and ²¹⁰Pb_ex-derived rates averaging 1400 t km^–2·a⁻¹ in Eucalyptus woodlands (Liu, 2022). Duan (2019) combined ¹³⁷Cs and ²¹⁰Pb_ex to quantify erosion rates of 74.28 t km^–2·a⁻¹ in primary forests and 545.13 t km^–2·a⁻¹ in secondary forests. In this study, erosion rate in northern Guangxi ranged from −466.90 to 2,159.78 t km^–2·a⁻¹ (¹³⁷Cs) and −398.83 to 3023.27 t km^–2·a⁻¹ (²¹⁰Pb_ex), with intensities ranked as: EUP > MPP > MBF. Except for moderate ²¹⁰Pb_ex-derived erosion at EUP upper slopes (p < 0.05), all sites experienced mild or negligible erosion, with deposition prevailing at the lower slopes of MPP and MBF. These results indicate that converting MBF into economic forests (MPP and EUP) has aggravated soil erosion. Intensive management practices in EUP—such as understory clearance, soil loosening, and frequent weeding—disrupt root networks, weaken soil structure, and intensify erosion (Shu et al., 2023). In contrast, MBF effectively enhances soil resistance to erosion by improving soil aggregate stability (Zheng et al., 2023). To balance ecological and economic objectives, plantations should adopt species with high litter production (Zhu et al., 2019), maintain root densities above 4 kg m^–3 (Li et al., 2015), and extend harvest cycles to minimize soil disturbance (Sun et al., 2023).

4.2 Correlations between soil erosion, SOC, and grain-size composition

Soil organic matter plays a key role in forming soil aggregates, improving physicochemical properties (aeration, permeability, water retention), and providing essential nutrients (Francis et al., 1999; Tahmoures et al., 2022). In this study, SOC content ranked MBF > MPP > EUP, reflecting differences in vegetation composition, litter accumulation, and soil properties (Nie et al., 2019; Teng et al., 2020). Compared with MPP and MBF, EUP has a simplified canopy structure and microenvironment that limits litter accumulation (Tang et al., 2007). The loss of surface organic matter in EUP increases runoff, accelerates SOC loss, and reduces vegetation cover—effects exacerbated by intensive harvesting, which limits erosion control (Schuller et al., 2013). In contrast, MBF’s structural complexity and abundant litter enhance infiltration and protect against SOC loss. For all three forest types, clay content decreased with depth (MBF > MPP > EUP), while sand content increased (EUP > MBF > MPP). Silt showed no significant trends. Severe clay loss in EUP loosens soil structure and increases erosion vulnerability, whereas elevated sand content accelerates coarsening and reduces cohesion (Liu et al., 2020). Minimal ground cover in EUP exposes soil to raindrop impact, dislodging clay and fine particles along with SOC. Canopy gaps and discontinuous cover exacerbate this process; thus, afforestation should aim to minimize such gaps (Porto et al., 2009; Altieri et al., 2018). The SOC typically peaks at slopes of 15°–25°, declining on steeper slopes (Zhang et al., 2020).

In this study, ¹³⁷Cs, ²¹⁰Pb_ex, and SOC were significantly positively correlated (p < 0.01), reflecting shared transport dynamics during erosion that influence their spatial distribution (Li et al., 2006; Meliho et al., 2019; Duan, 2019; Liu et al., 2020). These findings are consistent with reports of strong SOC–¹³⁷Cs relationships (Ritchie et al., 2007; Federico et al., 2020) and significant ²¹⁰Pb_ex–SOC associations (Tang et al., 2021). Notably, ²¹⁰Pb_ex often exhibits stronger SOC correlations in forested slopes (Teramage et al., 2013), likely due to dual replenishment mechanisms: SOC via litter decomposition and ²¹⁰Pb_ex via atmospheric ²²²Rn decay. Clay particles have a high affinity for ¹³⁷Cs adsorption; for example, 35% clay content can retain 84% of ¹³⁷Cs (Lomenick and Tamura, 1965). Forest soils with high bulk density and sand content are more erosion-prone, while organic-rich soils resist erosion (Tang et al., 2007). In this study, both radionuclides were positively correlated with clay and negatively with sand (p < 0.01), patterns consistent with global observations (Walling et al., 2003; Duan, 2019). Across forest types, except for the non-significant ²¹⁰Pb_ex–clay relationship in MBF, ¹³⁷Cs, ²¹⁰Pb_ex, SOC, and clay were strongly positively correlated (p < 0.01), confirming clay and SOC as major regulators of radionuclide distribution.

The SOC depletion accelerates soil structural degradation, nutrient loss, and reduced water retention, all of which exacerbate erosion (Richter et al., 1999). The integration of ¹³⁷Cs and ²¹⁰Pb_ex tracers links erosion processes with SOC and soil texture, providing a robust approach for evaluating plantation soil sustainability. Land use, slope gradient, and soil texture are key drivers of erosion, with steep slopes (>15°) and ridge positions experiencing the highest rates (Zhang et al., 2003; Meliho et al., 2019; Teng et al., 2019). MBF is particularly effective at controlling erosion on slopes of 16°–25°, consistent with this study’s finding of erosion intensity gradients: upper > middle > lower slopes, as also noted by Lu et al. (2016). Soil from upper slopes is often deposited downslope (Zhang et al., 2023), explaining the observed erosion severity at elevated positions.

To reduce soil and nutrient losses in plantations, management should incorporate optimized harvesting practices—such as contour-aligned logging and retention of harvest residues—to mitigate slope- and rainfall-driven erosion (Schuller et al., 2013). The ¹³⁷Cs-²¹⁰Pb_ex dual-tracer method proved effective for quantifying erosion under different land uses and management regimes in northern Guangxi’s low hills, revealing significant effects on SOC, bulk density, and particle size distribution. These results highlight the urgent need for improved soil conservation measures in EUP, where erosion-driven environmental degradation is most severe.

4.3 The shortcomings of this article

While this study provides valuable insights, several methodological limitations remain. First, based on the available data, we were unable to quantitatively separate the specific contribution of forest type conversion to changes in soil carbon pool storage and carbon loss through erosion. Second, although a strong consistency was observed between the medium-term ¹³⁷Cs and longer-term ²¹⁰Pb_ex erosion estimates, the approach does not incorporate short-lived radionuclide tracers. This omission limits the ability to resolve erosion events occurring over days to months. Future research should therefore focus on two key areas: (1) Refining temporal resolution: Incorporating short-lived tracers such as Beryllium-7 (⁷Be) is necessary to capture event-scale erosion dynamics and to directly link high-intensity rainfall or specific silvicultural operations to immediate soil loss—temporal patterns that cannot be resolved within the multi-decadal integration windows of ¹³⁷Cs and ²¹⁰Pb_ex. (2) Partitioning carbon fluxes: Advanced tracing techniques, such as ¹³C stable isotope analysis, are required to distinguish between the two main pathways driving SOC depletion in EUP: physical removal via lateral transport and enhanced biological mineralization via vertical fluxes. Such methods would allow more precise attribution of the mechanisms underlying soil carbon pool instability.

5 Conclusion

1. The conversion of native MBF to EUP has markedly intensified soil erosion in the region, following a clear pattern in severity: EUP shows the greatest erosion, followed by MPP, and then MBF. The average ²¹⁰Pb_ex-derived erosion rate in EUP reached 1,474.84 t km^-2·a⁻¹, indicating a highly selective erosion process. The strong correlations (p < 0.01) between radionuclide inventories, SOC, and clay content demonstrate that erosion preferentially removes the finest and most chemically active soil components, particularly clay and clay-associated SOC. This is consistent with the observed soil properties in EUP, which exhibited the lowest SOC and clay contents and the highest sand fraction across all slope positions. Such selective erosion accelerates soil coarsening and results in substantial depletion of essential soil resources, undermining long-term soil fertility and carbon sequestration capacity in this subtropical landscape.

2. Soil erosion intensity varied significantly with topography, increasing progressively from lower to upper slope positions. Upper slopes experienced the greatest erosion, followed by middle slopes, and then lower slopes. The upper slopes of EUP were the most erosion-prone areas, exhibiting both the highest erosion rates and the lowest soil conservation capacity. This pronounced instability arises from the combined effects of soil degradation caused by forest type conversion—particularly the loss of fine particles and SOC—and insufficient ground cover (20% understory in EUP compared with 90% in MBF).

3. The strong agreement between the medium-term ¹³⁷Cs and long-term ²¹⁰Pb_ex erosion estimates (p < 0.01) confirms that the dual-tracer method is a reliable approach for quantifying soil redistribution in non-cultivated red soil hillslopes of northern Guangxi. The consistency between the two tracers also indicates that the severe erosion observed in EUP reflects a recent and rapid acceleration of soil loss associated with intensive management practices, thereby reinforcing the study’s conclusions regarding land-use impacts.

4. In summary, this study provides quantitative evidence of the long-term ecological costs associated with the shift to intensive monoculture plantations. The finding that EUP substantially increases selective soil erosion reveals a key mechanism driving soil structural degradation. Effective mitigation will require immediate management interventions, including minimizing mechanical soil disturbance and promoting understory vegetation and litter retention to enhance soil protection. The validated dual-radionuclide framework presented in this research offers a strong foundation for developing sustainable plantation management strategies in southern China.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.

Author contributions

JC: Data curation, Visualization, Writing – original draft, Writing – review and editing. YC: Data curation, Visualization, Writing – review and editing. YuS: Conceptualization, Funding acquisition, Writing – review and editing. QT: Formal Analysis, Writing – review and editing. JL: Validation, Writing – review and editing. YS: Investigation, Writing – review and editing. DZ: Investigation, Writing – review and editing. YiL: Resources, Writing – review and editing. KH: Investigation, Writing – review and editing. HS: Software, Writing – review and editing. GX: Writing – original draft, Writing – review and editing.

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); Basic Research Fund of Guangxi Academy of Sciences (CQZ-E−1912); Guangxi Key Science and Technology Innovation Base on Karst Dynamics (KDL & Guangxi 202004); 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 providing assistance with the testing of the experimental samples. The authors would like to express their gratitude to EditSprings (https://www.editsprings.com) for the expert linguistic services provided.

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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