Assessment and modelling of soil organic carbon stock under different land-use types in the Fandoghlou region, Ardabil, Iran

Introduction
Land-use change is widely recognized as one of the most significant anthropogenic drivers influencing soil organic carbon (SOC) dynamics. The conversion of natural ecosystems, particularly forests and grasslands, into agricultural land generally reduces organic matter inputs, disrupts soil organic matter turnover, and accelerates soil erosion, ultimately leading to declines in organic carbon (OC) content and carbon stocks. However, the magnitude of these changes is strongly influenced by land-use type, climatic conditions, soil characteristics, and land management practices. Over recent decades, extensive conversion of the forest ecosystems in the Fandoghlou region into scrublands, rangelands, and rainfed agricultural lands has resulted in substantial degradation of natural ecosystems and soil resources. Despite these land-use transitions, quantitative information regarding their impacts on soil organic carbon stock in this region remains limited. Therefore, the present study aimed to evaluate the effects of different land-use types on soil organic carbon concentration and soil organic carbon stocks across forest, scrubland, rangeland, and rainfed agricultural ecosystems in the Fandoghlou region.
Materials and Methods
This study was conducted in the Fandoghlou region, located in the southeastern part of Namin County, Ardabil Province, Iran. Four representative land-use types, namely forest, scrubland, rangeland, and rainfed agricultural land, were selected to represent the dominant ecosystems in the study area. Within each land-use type, three parallel transects, each 100 m in length and spaced 100 m apart, were established. Soil samples were collected from the beginning, midpoint, and end of each transect at two sampling depths: surface soil (0–20 cm) and subsurface soil (20–40 cm). Consequently, nine surface and nine subsurface soil samples were obtained from each land-use type. In total, 72 soil samples (4 land-use types × 2 soil depths × 9 replicates) were collected across the study area. Each sample was placed in an individual polyethylene bag and transported to the Central Laboratory of the University of Mohaghegh Ardabili for physicochemical analyses. The measured soil properties included soil pH, electrical conductivity (EC), soil texture, bulk density (BD), and soil organic carbon (OC) concentration. Soil organic carbon stock (SOCstock) was subsequently calculated and expressed as Mg C ha⁻¹ (equivalent to ton C ha⁻¹). Differences in SOCstock and OC between the surface and subsurface soil layers were evaluated using a paired t-test. The interactive effects of land-use type and soil depth on SOCstock and OC were assessed using two-way analysis of variance (ANOVA). When significant differences were detected, treatment means were compared using Tukey's test (Tukey HSD) at the 5% significance level (p<0.05). In addition, simple linear regression analysis was performed to examine the relationship between OC and SOCstock across the study area.
Results and Discussion
The paired t-test revealed that both OC and SOCstock were significantly higher in the surface soil layer (0–20 cm) than in the subsurface layer (20–40 cm) across all land-use types, including forest, scrubland, rangeland, and rainfed agricultural land (p<0.001). The forest ecosystem exhibited the greatest vertical decline in OC, with OC decreasing from 3.24% in the surface layer to 2.05% in the subsurface layer. Similarly, SOCstock declined from 89.86 to 65.73 Mg C ha⁻¹ with increasing soil depth. Two-way ANOVA indicated that land-use type, soil depth, and their interaction all had significant effects on both OC and SOCstock (p<0.05). The interaction means further demonstrated that the highest OC concentration was recorded in the surface layer of the forest ecosystem (3.24%), which was significantly greater than those of all other land-use types. Likewise, the highest SOCstock was observed in the surface forest soil, reaching 86.89 Mg C ha⁻¹. Simple linear regression analysis revealed a strong and significant positive relationship between OC concentration and SOCstock (p<0.001), indicating that increases in OC concentration were accompanied by proportional increases in SOCstock across the study area.
Conclusion
The findings of the present study demonstrated that land-use type plays a critical role in regulating soil OC concentration and SOCstock in the Fandoghlou region. In agreement with the study objectives, forest ecosystems exhibited the highest OC concentration and SOCstock, followed by scrubland, whereas rainfed agricultural lands contained the lowest values. Furthermore, SOCstock was consistently greater in the surface soil (0–20 cm) than in the subsurface layer (20–40 cm) across all land-use types, highlighting the concentration of carbon within the upper soil horizons and the greater susceptibility of these layers to land-use change. The significant interaction between land-use type and soil depth further indicated that the vertical distribution of OC varied among ecosystems, reflecting differences in vegetation cover, organic residue inputs, and the intensity of land management and anthropogenic disturbance. These findings underscore the critical role of vegetation and management practices in controlling SOCstock and maintaining soil quality. Overall, this study emphasizes the importance of conserving the forest and scrubland ecosystems of the Fandoghlou region as the most effective natural reservoirs of soil carbon. Continued conversion of these ecosystems to rainfed agricultural land or degraded rangelands is likely to substantially reduce the soil's carbon stock capacity and may accelerate soil degradation. The results provide a scientific basis for sustainable land-use planning, soil conservation strategies, and ecosystem service assessments aimed at enhancing long-term carbon storage and ecosystem resilience in the region.