Document Type : Research Paper
Authors
1 Ph.D. student, Department of reclamation of Arid and Mountainous Region, Faculty of Natural Resources, University of Tehran, Iran
2 Professor of the Department of reclamation of Arid and Mountainous Region, Faculty of Natural Resources, University of Tehran, Iran
3 Associate Professor, Department of reclamation of Arid and Mountainous Region, Faculty of Natural Resources, University of Tehran, Iran
4 Associate Professor, Faculty of Natural Resources and Environment, Ferdowsi University, Mashhad, Iran
Abstract
Introduction
Mountainous regions, as the most sensitive and vital ecosystems, play a crucial role in providing fresh water, regulating climate, and preserving biodiversity. However, these areas are vulnerable to soil erosion, natural resource degradation, and climate change. Sustainable water and soil management in mountainous regions is particularly important, as soil quality is directly linked to essential ecosystem services. Natural factors such as slope and aspect influence the distribution of organic matter, soil aggregate stability, and water infiltration, while human activities like uncontrolled pasture exploitation, overgrazing, vegetation destruction, and land use changes contribute to soil degradation, reduced infiltration, and severe erosion. Management practices, including watershed structures and planting native species, can enhance soil water and nutrient retention, improving its physical and chemical properties. Paired watersheds serve as natural laboratories for monitoring and evaluating changes in soil, water, and ecosystem quality under the influence of human activities and climatic conditions. This study aims to investigate the impact of topographic variables, specifically slope aspect and the topographic wetness index (TWI), on the effectiveness of restoration measures in enhancing the physical and chemical properties of soil.
Materials and methods
This study was conducted in the paired watershed of Gonbad, located in Hamedan province, Iran. The area comprises two sub-watersheds: one subjected to watershed management operations (treated sub-watershed) and the other without such activities (control sub-watershed). To assess the effect of topography on soil properties, soil sampling was performed at various points in both the control and treated areas, based on two key topographic factors: slope aspect and TWI. For slope aspect analysis, three points were selected on north-facing slopes and three on south-facing slopes within each sub-basin. The TWI was calculated using the relevant equation, and each sub-basin was divided into three zones with varying moisture conditions (low, medium, and high TWI). Soil samples were collected at the end of the growing season from a depth of 0-15 cm while maintaining the soil structure. With three replicates, a total of 36 sampling points were established. Soil physical and chemical properties, including permeability, texture, porosity, aggregate stability, organic matter, pH, electrical conductivity, water holding capacity, and surface cover components, were measured. Restoration measures in the treated sub-watershed included biological measures (seeding of drought-resistant species such as Astragalus gossypinus and Bromus tomentellus) and managerial measures (complete grazing exclusion). To analyze the effects of restoration measures, slope aspect, and TWI on soil hydrological properties, statistical methods including analysis of variance (ANOVA) based on a nested design, and Pearson correlation were employed using SAS and R software.
Results and discussion
The findings demonstrated that restoration measures and topographic variations significantly improved soil properties. Comparison of the treated and control watersheds using the t-test revealed that the treated watershed exhibited lower bulk density (1.18±0.01 vs. 1.31±0.02 g/cm³), reduced bare soil percentage (13.06±1.38% vs. 32.5±1.61%), and higher steady-state infiltration rate (28.44±1.92 vs. 19.78±0.82 mm/h) (P<0.05). Additionally, soil porosity (51.13±0.73% vs. 41.66±1.14%) and aggregate stability (1.96±0.52 vs. 1.52±0.39 mm) were significantly greater in the treated watershed (P<0.05). Organic matter content was also higher in the treated watershed (2.15±0.62% vs. 1.5±0.38%) (P<0.05), indicating the positive influence of restoration on soil quality and erosion control. Slope aspect significantly affected certain soil properties. The t-test showed that north-facing slopes had greater aggregate stability (2.14±0.33 vs. 1.35±0.29 mm), higher organic matter (2.13±0.64% vs. 1.32±0.38%), and denser vegetation cover (60.39±3.18% vs. 48.17±3.2%) compared to south-facing slopes (P<0.05). These differences are linked to improved moisture conditions on north-facing slopes due to reduced solar radiation and denser vegetation, enhancing organic matter retention and erosion resistance. Analysis of TWI classes using ANOVA indicated that the TWI3 class had the highest organic matter (1.45±0.61%) and aggregate stability (1.96±0.52 mm), though these differences were not statistically significant (P>0.05). However, soil texture varied significantly with TWI: clay increased from 42.75±3.12% (TWI1) to 46.12±3.05% (TWI3), silt from 28.43±2.76% to 31.66±2.78%, and sand decreased from 28.82±5.53% to 22.22±5.68% (P<0.05), suggesting finer particle deposition in higher TWI zones (lower slopes). Principal component analysis (PCA) showed that TWI and vegetation were correlated with organic matter, aggregate stability, and infiltration (P<0.05), while bare soil and bulk density were associated with southern slopes and low TWI. Northern slopes and high TWI classes showed the greatest improvement in organic matter and vegetation. These findings confirm the key role of slope orientation, TWI, and biological interventions in improving soil and vegetation quality in semi-arid regions.
Conclusion
The present research showed that restoration measures generally lead to improved soil quality, but the effectiveness of these measures is significantly affected by topographic characteristics, especially slope aspect and topographic wetness index (TWI). North-facing slopes and points with high TWI showed the greatest improvement in soil parameters, especially organic matter and soil aggregate stability. The findings of this research show the importance of considering topographic characteristics in planning and implementing restoration measures, and it is suggested that in future studies, more focus should be on investigating the factors affecting the reduction of some parameters and the long-term effects of restoration measures.
Keywords
)