Environmental Erosion Research Journal

Extended abstract
1- Introduction
Soil erosion is one of the most important problems in the arid and semi-arid areas in Iran and the knowledge about its content has an important role on the management practices of soil conservation. Topography as one of the soil-making factors affects the physical properties of the soil through the aspect and the degree of the slope. The aspect of the slope changes the soil moisture, temperature, and the activity of the soil micro-organisms, causing changes in the soil organic matter at different slope degrees, and as a result, creating different soil qualities. Different slope degrees through transportation, the clay particles, and even the organic matter down to the low lands as well as the change in the soil bulk density may change the quality of the soil. In order to maximize the utilization of agricultural lands, to minimize the environmental degradation, and to prevent soil erosion, investigating the soil quality is essential. The aim of this study was to evaluate the amount of soil erosion and quality in different degrees of the southern and northern slopes in the catchment of Torogh dam located in the southwestern Mashhad.
2- Methodology
To evaluate the soil erodibility index and quality, 40 soil samples were collected from 0-30 cm depth at the southern and northern slopes including less than 10%, 10-20%, 20-30% and more than 30% slope degrees in the catchment of Torogh dam located in the southwestern Mashhad. 5 samples were taken from each position and some soil properties, including the particle size distribution, the percentage of the soil organic matter, the mean weight diameter of aggregates, the water dispersible clay and silt contents, and the soil structural stability index (SI), were measured. The soil erodibility index was calculated based on five characteristics including sand, silt with fine sand, organic matter, soil structure class, and the infiltration rate class of the soil profile. In addition, the soil quality index, i.e. Integral Quality Index (IQI), was calculated according to the data set. To calculate the soil quality, the score of each property were determined based on the scoring functions and their weights calculated by dividing the communality value of each property to the total communality of all the properties in the JMP4 software. Then, the soil quality index (i.e. IQI) was calculated through multiplying the score and weight of the studied properties and their sum.
3- Results
The results showed that the mean of the organic matter in the North aspect was significantly more than the South slop due to the coarse soil texture and the scarce cover crop in the South aspect. The weight diameter of the aggregates (MWD) in the northern aspect was significantly lower than the southern one because of the higher contents of the silt, fine sand, and the water dispersible clay and silt. Therefore, the low instability of the soil structure in the northern aspect was compared to the southern slope. Also, the MWD had a significant difference between the diverse degrees of the southern slope. In addition, the soil erodibility index in the northern aspect of the slope was significantly greater than that of the southern aspect due to more contents of the water dispersible clay and silt in the northern aspect compared to the southern one. The lowest value of the soil erodibility index was obtained at less than 10% of the North aspect that had a significant difference in relation to the other slope degrees. The difference of the soil erodibility index was not significant in different slope degrees of the South aspect. In addition, the quantitative value of the soil quality index in the northern slope was significantly more than that of the southern slope. Because of the optimal values of most soil properties in the North aspect, therefore, the soil quality in this aspect was higher than that of the South. There was no significant difference between the slope degrees in the South slope, but in the North aspect, the difference between various slope degrees of 20-30% was significant (P < 0.05) and the soil quality value was the least compared to the others.
4- Discussion & Conclusions
In the studied area, the annual precipitation is low, so that the plant residue and the organic matter are low, and as a result of this, the impact of the organic matter on soil erodibility was low. Therefore, other soil properties, i.e. the contents of silt, sand, clay, water dispersible clay and silt have important roles in controlling the soil erosion. In this research, the effect of the slope aspect on the soil quality was more than the respective degree, so that the soil erosion value in the North aspect was greater than that of the South one, and diverse slope degrees had no significant effects on the soil quality. With regard to the low content of the organic matter in this area, it is suggested that the soil management practices be performed to increase the soil organic matter, to decrease the soil erodibility, and to improve the soil quality.