Aim
Forest ecosystems are among the most critical terrestrial carbon reservoirs, playing a fundamental role in regulating biogeochemical cycles, particularly carbon and nitrogen cycling. They cover approximately 30% of the Earth's land surface and provide a wide range of ecosystem services including timber production, biodiversity conservation, climate regulation, water purification, and soil protection. After oceans, forests are the largest carbon sinks on Earth, storing more than 70% of total global soil organic carbon. However, over the past decades, rapid population growth and increasing food demand have led to widespread land-use change, with forests being converted to agricultural lands, pastures, and orchards. According to recent studies, approximately one-third of the global land surface has undergone land-use change over the past six decades, making it one of the most significant global environmental challenges. These conversions typically involve removal of tree cover, reduced organic matter inputs, tillage operations, irrigation, and chemical fertilizer application, all of which can significantly alter soil physical, chemical, and biological properties. In most cases, such processes result in a decline in soil organic carbon (SOC), reduced microbial activity, increased bulk density, decreased porosity, and ultimately deterioration of soil quality and ecosystem functionality. In recent years, reforestation with various tree species has been considered as a promising strategy for restoring degraded lands and recovering ecological functions. However, the effects of different forest plantation types on soil properties, particularly in comparison with natural forest stands, remain poorly understood. Differences in litter quality and quantity, chemical composition of residues, lignin content, and root exudates can differentially influence decomposition pathways, carbon stabilization, nutrient cycling, and soil microbial community structure. Therefore, the present study was conducted with the following objectives: (1) to evaluate the effects of different forest cover types (natural forest, poplar plantation, and alder plantation) on selected physical, chemical, and biological soil properties, (2) to investigate the influence of soil depth (0–20, 20–40, and 40–60 cm) on these properties, and (3) to explore the relationships between SOC and key soil quality indicators through regression analysis. The central hypothesis was that forest cover type would significantly affect SOC content and consequently other soil properties, with effects most pronounced in surface layers where organic matter accumulation and biological activity are greatest.
Materials and Methods
This study was conducted in December 2022 at the Poplar Research Station in western Gilan Province, Iran. The study area is characterized by a humid climate with an average annual precipitation of 1,252.12 mm and a mean annual temperature of 17.5°C. The three land cover types selected were: (1) natural lowland forest, primarily composed of *Alnus subcordata* and *Pinus taeda*; (2) poplar plantation (*Populus euramericana* and *Populus deltoides*); and (3) alder plantation (*Alnus subcordata*). These land cover types share similar climatic conditions, topography, and soil texture (loam and silty loam), differing only in vegetation cover. Field observations indicate that approximately 50 years ago, parts of the natural forests were deforested, and subsequently some degraded lands were restored through afforestation with alder and poplar. A factorial experiment was conducted based on a randomized complete block design with three replications. In each land cover, soil sampling was performed from three depths: 0–20, 20–40, and 40–60 cm, resulting in 27 composite samples. Soil properties measured included: aggregate stability, bulk density, porosity, dispersible clay, SOC, total nitrogen, pH, CEC, T-GRSP, MBC, Cmin, and qCO₂. Statistical analysis was performed using SAS 9.4 with two-way ANOVA and Duncan's test.
Results
The interaction effect of land cover and depth was significant (P ≤ 0.01) for all measured properties, indicating that soil responses to different vegetation covers were depth-dependent. The poplar plantation exhibited the highest SOC; at 0–20 cm, SOC was 13.61% and 6.55% higher than in natural forest and alder plantation, respectively. This was attributed to the broadleaf nature of poplar species producing more readily decomposable litter compared to the needle-leaf species in the natural forest. Total nitrogen followed a similar pattern, with the highest value (3.23 g kg⁻¹) in the poplar surface layer. CEC and T-GRSP also showed the highest values in the poplar surface layer, with increases of 18.25% and 20.34% for CEC, and 8.75% and 4.96% for T-GRSP compared to other stands. Soil pH ranged from 5.35 (poplar, surface) to 4.73 (natural forest, 40–60 cm), reflecting vegetation influence on soil acidity. For physical properties, the lowest bulk density (1.14 g cm⁻³) and highest porosity (54.22%) were in the poplar surface layer, while dispersible clay was lowest there (8.78%), confirming SOC's role in structural stability. For biological properties, the highest Cmin and MBC were in the poplar surface layer, while qCO₂ was lowest there (28.89 μg C g⁻¹ MBC day⁻¹), indicating higher microbial metabolic efficiency where carbon resources were more abundant and of higher quality. Regression analysis revealed strong positive correlations between SOC and CEC (R² = 0.87), T-GRSP (R² = 0.97), MWD (R² = 0.92), and MBC (R² = 0.95), and a strong negative correlation with DC (R² = 0.73).
Conclusion
This study demonstrates that forest cover type plays a decisive role in determining soil quality through its influence on organic inputs. The poplar plantation, with broadleaf species producing readily decomposable litter, exhibited the highest SOC, which in turn led to improved physical, chemical, and biological properties. In contrast, the natural forest with needle-leaf species showed relatively lower SOC and associated soil quality indicators, although it maintained good aggregate stability due to undisturbed structure. All properties showed significant depth-dependent variations, with surface layers consistently exhibiting better quality due to greater organic matter inputs and biological activity. The strong correlations between SOC and key indicators underscore its fundamental role in soil health and ecosystem functionality. These findings suggest that reforestation with suitable broadleaf species such as poplar can be an effective strategy for restoring degraded lands and enhancing soil quality, though species selection should consider local conditions and management objectives. Future research should address long-term dynamics, mixed-species plantations, soil fauna contributions, and economic trade-offs between different reforestation options.