Investigating the influence of parent material and topography on the soil-water retention curve and soil aggregate stability indicators (Case study: parts of Khorasan Razavi Province)

Introduction: Knowing the different factors on the characteristics of soil-water retention curve and soil aggregate stability can play an important role in better understanding the environmental potentials and managing natural areas. The soil-water retention curve shows the changes of moisture in different soil suctions, and the stability of soil aggregate is also an accurate indicator for evaluating soil quality in planning the optimal use of natural and agricultural areas. In most of the research conducted, these two characteristics have been investigated in agricultural soils. In the present study, considering the effect of the soil-water retention curve and aggregate stability indicators on the state of natural vegetation, as well as the effect on permeability, runoff emergence, and soil erosion, an attempt is made to investigate some of the factors affecting the two aforementioned characteristics. Parent materials and the topographic condition of the land are factors that can have a significant impact on the soil-water retention curve and soil aggregate stability. The mineralogical composition, texture and structure of the parent rock has a direct effect on the ability of the soil to retain and absorb water, permeability and characteristics of the retention curve. The topography conditions also plays an important role in the distribution and maintenance of soil moisture due to its influence on hydrological processes and microclimate. In addition, topography affects the processes of soil formation and the type of vegetation, which in turn affect the soil water retention characteristics and stability of soil aggregates.
Material and methods: The studied area is located in Razavi Khorasan province and the cities of Mashhad, Torbat Heydarieh, Sarkhs and Chenaran. In this research, the soil characteristics of seven types of marl, limestone (Cenozoic and Mesozoic), ophiolite, shale, granite and sandstone were selected from the relatively pure rocks of Razavi Khorasan province, Using 1:100,000 geological maps from the Geological Organization of Iran. Topographic wetness index was used to determine the factor of topographic characteristics, and this index was evaluated in two classes, positive and negative. The topographic wetness index describes the tendency of water to accumulate in any point of the sub-basin and the tendency of gravity forces to transport water downstream. The topographic wetness index is obtained from the combination of the area of the upstream basin and the slope, and it determines the effect of topography on the amount of saturation levels for the production of runoff. Based on this and according to the examination of seven types of stones, there are a total of 14 different treatments, which, including three replications for each treatment, a total of 42 samples were taken from a depth of 0-20 cm using a cylinder sampler and used to prepare a soil-water retention curve, determination of stability indicators of soil aggregates and measurement of physical and chemical properties. A pressure plate device was used to prepare the soil-water retention curve and the amount of moisture was determined in different matric suctions, including saturation and suctions of 10, 30, 50, 100, 300, 500, 1000 and 1500 kilopascals. Several indices have been used to evaluate the stability of soil aggregates, among these indices, we can refer to the weighted average of the diameter of soil aggregates in dry and wet sieving conditions, the stability index of soil aggregates and the percentage of degradation of soil aggregates.
Results and Discussion: According to the results of soil-water retention curve, the highest moisture retention is related to medium texture (Loam) and the lowest is related to light textures (Sandy loam). Changes in the slope of the curve in medium soils are gradual, but in light soils it is sudden due to the presence of large pores. Also, the results showed that shale has the highest percentage of moisture in different suctions, such that at a suction of 10 kPa, they retain more than 50% moisture. After that, marl, Jurassic limestone, Paleogene limestone show the highest amount of moisture percentage and sandstone, with only 20% moisture retention at 10 kPa suction, show the lowest amount of moisture percentage in different suctions.
The amount of saturated moisture in the soils of shale and marl, respectively, is the highest and is more than 50% in both of them. After that, the Jurassic limestone soils of Chenaran and Paleogene limestone of Sarakhs and ophiolite have the highest amount of saturated moisture.
Granite and sandstone soils generally had the lowest amount of saturated moisture with less than 30%. Also, soils containing more silt, in terms of retention curve characteristics, have higher saturated moisture and agricultural capacity, which can be seen in marl and shale soils.
Ophiolite and sandstone soils have more weight average diameter of soil aggregates than other types of rocks, with more than one millimeter. Which indicates that the percentage of silt in the soil has an inverse relationship with the average weighted diameter of the soil aggregate. It is obvious that the average weighted diameter of soil aggregates in dry state is higher than in wet state, but in the jurassic limestone soils of Chanaran, the destruction of soil aggregates was more than 80%, which indicates that soil aggregates of this rock type are more dispersed than in other soils. The percentage of clay and silt more than 60% in the Paleogene Sarakhs limestone, Jurassic Chenaran limestone, marl and shale has caused the stability of the soil aggregates in the soils of these rocks to reach less than 10%.
Conclusion: Differences in rock type can cause significant differences in the characteristics of the soil-water retention curve and aggregate stability indicators Soils formed from them. Also a significant correlation was observed between the characteristics of the soil-water retention curve and soil aggregate stability indicators in different types of rocks. On the other hand investigating the type of limestone in the second and third geological periods showed that there is no significant difference in the investigated indicators with age change. Among the chemical properties of the soil, organic matter and among the physical properties of the soil, the texture plays a large role in the retention curve and stability indicators of soil aggregate. Finally there was no significant difference between positive and negative topographic wetness index in most of the studied variables of retention curve characteristics and soil aggregate stability indicators, which indicates the lack of influence of this factor on the studied indicators.