Effects of Geomorphology of Alluvial Fans on the Physical and Chemical Changes of the soil of Alluvial Fan in the Southeast of Shah Gheib’s Salt Dome, Larestan

ABSTRACT
Alluvial fans are ideal places for investigating the effects of geomorphological processes and landforms on soil properties. This study aimed to investigate the effects of geomorphological forms and processes on physical and chemical properties of soil in an alluvial fan located in the southeast of Shah Ghaib salt diaper. The study area alluvial fan is composed of three surfaces including very old, old and young. The phosphorous, potassium, absorbable sodium, calcium carbonate, saturation moisture of soil, water’s holding capacity in soil, soil texture, and organic carbon parameters were measured in 72 soil samples on the alluvial fan surfaces.The results showed that the sodium, potassium, EC, calcium carbonate, phosphorous, and holding capacity of water, silt, and clay parameters were higher in the interfluves than the channels in inactive (very old and old) surfaces. The organic carbon, pH, saturation moisture of soil and percentage of sand is higher in the channels. Also, the sodium, potassium, EC were more in the young compared to the old and very old surfaces. The PH, phosphorous, and saturation moisture of soil were higher in the old surface than in the young and very old ones. The amount of calcium carbonate, water’s holding capacity in soil and percentage of silt were highest in the very old fan surface. The correlation coefficient between the measured parameters of the soil indicated that there are strong positive relationships between the clay and the sodium, electrical conductivity, calcium carbonate, and potassium. In general, the results show that geomorphological processes such as entrenchment in channels and weathering on the surface of the interfluves have controlled on the values of the physicochemical parameters of the soil in the study area alluvial fan
 
Extended Abstract
Introduction
As the thin layer of the earth's surface, soil changes continuously over time. This change occurs due to the effects of various factors. Geomorphological forms and processes can affect the physicochemical properties of soil. Alluvial fans are one of the most widespread landforms in Iran. Alluvial fans developed around salt diapirs of Zagros mountains are ideal places for investigating the effects of geomorphological landforms and processes on soil properties. The movement of diapir salts on the apex of alluvial fans can change the location of channel and sedimentation and, hence, the physicochemical properties of soil. This study aimed to investigate the effects of geomorphological forms and processes on soil's physical and chemical properties in an alluvial fan developed in the southeast of Shah Ghaib salt diapir in Larestan, Fars, Iran. The mentioned alluvial fan comprises three surfaces as very old, old, and young. The phosphorous, potassium, absorbable sodium, calcium carbonate, soil's saturation moisture, water's holding capacity, soil texture, and organic carbon parameters of 72 soil samples on the alluvial fan surfaces were measured.
 
Methodology
 In order to achieve the aim of this study, the borders of the studied alluvial fan surfaces were delineated by Google Earth images and field observation, and then the areas of three very old, old, and young surfaces were determined. A field survey was conducted in December 2021 to study the morphology and processes of the study area alluvial fan. Also, to analyze the physical and chemical properties of the studied alluvial fan, 9 plots in the apex, mid-fan, and toe of the very old, old, and young surfaces were selected. Seventy-two soil samples (24 samples for each surface) were acquired from locations (apex, mid-fan, and toe) and various landforms (channels, interfluves, bars, and swales) of the studied alluvial fan. The soil samples were then transferred to a laboratory. Ec, pH, phosphorous (P), potassium (K), absorbable sodium (Na), calcium carbonate (Caco), soil’s saturation moisture (Sp), water’s holding capacity in soil (WHC), soil texture (Texture) and total organic carbon (OCT) tests were conducted. The mean physical and chemical properties of soil on different surfaces (very old, old, and young) and in different landforms on inactive (channels and interfluves) and active (bars and swales) surfaces and also on different locations (apex, mid-fan, and toe) were calculated, and the results were compared. Later, the Pearson correlation coefficient was used to investigate the relationship between soil properties. Finally, the relationship between soil properties and fan morphology was analyzed.
 
Results and Discussion
A comparison of data in the landforms of inactive (very old and old) surfaces indicated that the sodium, potassium, EC, lime, phosphorous, and water holding in soil, silt, and clay parameters were higher in the interfluves than the channels. The higher amounts of the mentioned factors in the interfluves can be attributed to the more excellent weathering and soil formation and the lower topographic gradients of the interfluves compared to the channels. In channels, incision and erosion processes are predominant, reducing parameters such as potassium, sodium, and water’s holding capacity in soil, clay, and silt. Results show that organic carbon is higher in channels than interfluves, possibly due to the higher vegetation density and canopy. Despite the richer soil of the interfluves compared to the channels of old and very old surfaces, the channels are more conducive for vegetation due to their shading and greater moisture, which could increase the organic carbon of soils in the channels. Data indicate that the amount of soil sodium on the new surface is much higher than on old and very old surfaces. This can be due to salt formations (Shah Ghaib salt diapir) in the drainage basin of the alluvial fan. Meanwhile, floods and runoffs in the basin only feed the young surface of the alluvial fan, resulting in the soil salinity.
On the other hand, the alluvial fan's old and very old surfaces have been abandoned and are not affected by the floods. The alluvial fan's old and very old surfaces are washed by rainfalls and runoffs each year, reducing their salinity. Results have demonstrated that the sodium, pH, limestone, organic carbon, and clay parameters are greater in the apex of the fan, while the phosphorous, silt, and gravel are greater in the toe of the alluvial fan. The correlation coefficient between the measured parameters of the soil indicated positive and significant relationships between clay and sodium, electrical conductivity, calcium carbonate, and potassium. Results also show sodium positively and significantly correlated with potassium and Ec. The Ec shows the soil's ability to conduct electrical currents. The most important effect left by electrical conductivity on soil fertility is that electrical conductivity is an indicator of access to nutrients in soil. Electrical conductivity is often low in sandy soils with low organic carbon. In soils with high clay content, electrical conductivity is high. The clay, Ec, sodium, and potassium are high in interfluves but low in channels.
 
Conclusion
The studied alluvial fan is located southeast of Shah Ghaib diapir in Fars Province. Under the influence of salt movement towards the downslope, the stream flow in the basin outlet or the fan apex has been repeatedly relocated during the past hundreds or thousands of years, and hence, parts of alluvial fan (very old or old surfaces), have been subsequently abandoned, resulting in changing geomorphological forms and processes during previous periods. The morphological survey of the studied alluvial fan indicated that old and very old surfaces had been incised by erosive channels and had rough surfaces. In contrast, the young surface of the fan with swale and bar landforms was almost smooth. The interfluves of old and very old surfaces are affected by weathering and soil formation because much time has passed since their last flooding and sedimentation activities. At the same time, channels have been subjected to incision and erosion. Findings showed that the sodium, potassium, EC, lime, phosphorous, water holding in soil, silt, and clay factors were greater in the interfluves than in the channels. The higher amounts of the mentioned elements can be due to greater weathering and soil formation and the lower topographic slopes of the interfluves than the channels. The higher amount of organic carbon in the channels than the interfluves can be attributed to the vegetation density and canopy of the channels compared to the interfluves. Data also suggested a higher amount of soil sodium on the young surface compared to the old and old surfaces, which may be due to salt formations (Shah Ghaib salt diapir) in the drainage basin of the young surface of the fan. In general, the active processes on the alluvial surfaces, the difference in the relative age of various surfaces, and the lithology of the upstream drainage basin are among the most important factors controlling the physicochemical properties of the studied alluvial fan’s soil.
 
Funding
There is no funding support.
 
Authors’ Contribution
All of the authors approved the content of the manuscript and agreed on all aspects of the work.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
We are grateful to all the scientific consultants of this paper.