Introduction and Objective: Forest soil is the most important carbon reservoir in terrestrial ecosystems, and the stability of its carbon depends on the combination of soil physical and chemical properties as well as the tree species composition. In plantation stands, whose structure and function are influenced by species selection and human interventions, carbon deposition patterns may differ from those of natural forests. Different tree species create distinct mechanisms in soil carbon dynamics through variations in litter, root quality, nutrient cycling, and eco-physiological processes. Therefore, identifying these differences is important for species-based management and enhancing carbon stability. The present study aimed to analyze the relative contribution of physical and chemical soil properties to soil carbon storage in three plantation stands of Caucasian alder (Alnus subcordata C.A.Mey), Chestnut-leaved oak (Quercus castaneifolia C.A.Mey), and Italian cypress (Cupressus sempervirens L.) in the Darabkola educational–research forest, Mazandaran Province, northern Iran.
Materials and Methods: In each stand, ten 20 × 20 m plots were randomly selected using a systematic–random sampling design. Soil samples were collected from a depth of 0–20 cm. Physical properties (moisture content, texture, and bulk density) and chemical properties (pH, electrical conductivity, organic carbon, nitrogen, phosphorus, potassium, calcium, nitrate, and ammonium) were measured. Soil carbon sequestration was calculated based on soil organic carbon percentage, bulk density, and sampling depth. Data were analyzed using ANOVA, principal component analysis (PCA), and variance partitioning through the rdacca.hp package in R software.
Results: Significant differences were observed among the soil properties across plantation stands. Soil moisture was significantly higher in A. subcordata stands (mean 42.47%) compared with the other two species, while bulk density was greatest in C. sempervirens stands (1.87 g cm⁻³). Among chemical properties, soils under C. sempervirens exhibited higher concentrations of phosphorus (10.42 mg kg⁻¹), potassium (608.7 mg kg⁻¹), and calcium, whereas A. subcordata soils had the highest total nitrogen and ammonium levels. Soil organic carbon (SOC) content ranged from 3.47% to 3.88% without significant differences among stands; however, soil carbon sequestration varied from 57.07 to 72.27 t ha⁻¹. PCA results revealed that the first four components explained 73.88% of total variance. The first component (PC1), strongly correlated with phosphorus, potassium, EC, and SOC, represented soil fertility and carbon stability, while the second component (PC2) showed strong associations with soil moisture, nitrogen, and ammonium, highlighting the role of biological processes in carbon dynamics. Variance partitioning analysis indicated that in A. subcordata stands, chemical variables overwhelmingly dominated, explaining about 95% of the variation in soil carbon sequestration. In C. sempervirens stands, chemical variables remained more influential (80%), though physical properties also contributed notably. Conversely, in Q. castaneifolia stands, physical variables accounted for a slightly higher share (56%) compared with chemical variables, suggesting different controlling mechanisms among species.
Conclusion: The findings of this study confirm that properties such as ammonium, soil organic carbon, electrical conductivity, and the percentages of sand and silt play key roles in enhancing soil carbon stability and storage. The results indicate that the type of plantation species can influence soil carbon stability and storage through changes in soil physical and chemical properties. Accordingly, different species affect the carbon cycle through distinct mechanisms. Therefore, selecting appropriate species combinations and implementing intelligent soil management in plantation stands can be an effective strategy to increase soil carbon sequestration capacity and mitigate the impacts of climate change in the Hyrcanian forests.