Evaluating the effect of geomorphology in the physicochemical properties of soils of alluvial fan surface (Case study: alluvial fan of northwest of Amiriyeh, Damghan)

Introduction
Soil is one of the most important agents of production and profoundly influences the human life. Nowadays, soil erosion is one of the main environmental problems that is regarded as threat to natural resources, agriculture, and environment (Rahman et al, 2009). Therefore, it is very important to evaluate the physicochemical characteristics of soils in order to prevent its erosion. Alluvial fans are suitable locations for settlements (Waters and Field, 1986; Maghsoudi et al, 2014), the groundwater recharge (Bull, 1977; Houston, 2002; Blainey and Pelletier, 2008), the exploitation of aggregates (Fookes et al, 2007; Bahrami et al, 2015), soil formation (Norton et al, 2007; Bahrami and Ghahraman, 2019), agriculture (Field, 1992; Rahaman, 2016) and other human activities. Development of alluvial fans are affected by different factors such as tectonic activity, climate, base level change, geological and morphometric properties of catchments (Beaumont, 1972; Waters and Field, 1986; Blair and Mcpherson, 1998; Crosa et al, 2004; Arzani, 2005; Bahrami, 2013, Arzani and Jones, 2018; Goswami, 2018; Özpolat et al, 2022; Peng e al, 2024; Ghahraman and Nagy, 2024). Alluvial fans as one of the most important depositional landforms of arid and semi-arid regions have considerable diversity in terms of evolution and morphometry. Every alluvial fan may be composed of surfaces with different ages such as young, old and relict surfaces. Difference in relative age of fan surfaces can result in the variation of geomorphological processes and morphometry, and hence in the variation of soil properties and development. The aim of this research is to evaluate quantitative properties of soil in different surfaces, positions, and landforms of alluvial fan located in the northwest of Amiriyeh, Damghan. The study area is located in the southern part of Alborz structural zone, and northern part of Central Iran structural zone. Damghan Playa is located in the southern part of the study area.
 
Materials and Methods
First, the borders of relict, old and young surfaces of studied alluvial fan were identified based on weathering rate, fan surface morphology, drainage pattern, color/tone on satellite images (Field, 1994; Bahrami and Bahrami, 2011) and field works. Owing to the highest concentration of vegetation root and organic carbon in the top 30 cm of soils, in this research, 24 soil samples from depths of 0–30 cm were gathered from alluvial fan surface. Samples were collected from positions (apex, toe), surfaces (relict, old and young), and landforms of young surface (bar and swale), as well as landforms of relict and old surface (interfluve and channel). The soil sampling squares at toes and apexes of different surfaces of fan were selected randomly.
The selective sampling method was used to gather soil samples from landforms (interfluve and channel on 
the relict and old surfaces, and bar & swale on the young surface). From each surface (old, relict and young), 8 soil samples were gathered (4 from apex and 4 from toe). On each position (apex and toe) of the old and relict surfaces, two soil samples were gathered from interfluves and two from channels.
Also, on each position of the young surface, two soil samples were gathered from swale and two from bar landforms. The soil samples were transformed to the laboratory and then clay%, silt%, sand%, organic carbon, Ph, and soil hydraulic conductivity (K) were measured. The soil texture was calculated by the hydrometer method (Kroetsch and Wang, 2008). The soil organic carbon (OC) was measured by Walkley-Black titration method (Walkley and Black, 1934). The hydraulic conductivity (K) of soil samples was calculated based on the Saxton et al. (1986) method:
Eq. 1:
       
 
where K is unsaturated hydraulic conductivity (m/ s) and θ is moisture content (m3/m3) as indicated by the following equation 2:
Eq. 2:
   
To compare the means of soil variables in alluvial fan positions (apex and toe), landforms of old and relict surfaces (interfluves and channels), and landforms of young surface (bars and swales), the independent sample t-tests were calculated. To compare the means of soil parameters in alluvial fan surfaces (relict, old and young) the ANOVA test was used.
 
Results and Discussion
The studied alluvial fan is composed of three surfaces including relict, old and young surfaces, each having distinct geomorphological features. The relict and old surfaces are characterized by dendritic drainage pattern. Nevertheless, channels on the old surface have lower depth compared to channels on the relict surface, implying lower erosion and relative age of old surface than relict one. Drainage pattern on the young surface is braided. The young surface of alluvial fan is characterized by relatively flat morphology, whereas the relict and old ones have crenulated and entrenched morphology. Field observation revealed that the young surface deposits lack desert varnish and weathering marks, whereas deposits on the relict and old surfaces are exposed to weathering. Results show that sand% on the studied alluvial fan surface varies from 36% (sample 12 on the toe of relict surface) to 96% (sample 11 on the toe of relict surface). The maximum value of Ph (9.27) corresponds to sample 8 on the toe of young surface, and the lowest Ph (7) belongs to sample 17 on the apex of relict surface. The value of soil organic carbon is lower than 1% in all samples, ranging from 0.07% (sample 17 on the apex of old surface) to 0.74% (sample 20 on the apex of relict surface). The value of soil hydraulic conductivity (K) varies from 1.22 (sample 15 on the apex of relict surface) to 13.71 cm/h (sample 11 on the toe of relict surface). Data show that soil texture is coarser in fan apex compared to its toe. This is due to decrease in slope gradient and also to flow diversion, and hence decrease in flow velocity that cause the coarse sediments to deposit in apex and the fine sediments to deposit in the toe of alluvial fan. In spite of the coarser texture soils of apex, the mean values of soil hydraulic conductivity do not have meaningful difference at apex and toe of alluvial fan. Although previous studies have suggested that soil organic carbon in depositional landforms is often higher compared to erosional landforms in upstream areas (Vanden Bygaart et al, 2015; Xiao et al, 2015), results of this study revealed that soil organic carbon is lower at fan toe than fan apex. The lower value of organic carbon in alluvial fan toe can be attributed to the higher moisture and hence increased microbial activity, facilitating soil organic carbon decomposition and consequently its loss (Mohseni et al, 2019). Results demonstrate that channels formed on the relict and old surfaces have remarkably coarser soils compared to interfluves. The soil texture is also coarser in swales than bars of young surface. The mean value of Ph is higher at toe than apex implying that soils of fan toe is more alkaline than its apex. The more alkaline soils of alluvial fan toe can be attributed to the finer textured soils and hence their lower leching. The higher value of soil organic carbon in channels compared to the interfluves of old and relict surfaces can be attributed to the denser vegetation of channels. Results of ANOVA test show that the means of soil hydraulic conductivity do not have significant differences in different surfaces (relict, old, and young). The values of soil hydraulic conductivity at the apex and toe of alluvial fan are similar and do not show considerable difference. Evaluation of t-test to compare means of hydraulic conductivity revealed that the means of this parameter in bars and swales of young surface have 
significant differences, implying the micro-landforms of bars and swales have fundamental impact in the variation of hydraulic conductivity. Based on t-test values, means of soil hydraulic conductivity also have significant differences in the interfluves and channels of relict and old surfaces, showing that the micro-landforms of interfluves and channels have also had strong control in the variation of soil hydraulic conductivity. Evaluation of the relation between parameters (Pearson's correlation coefficient) reveals that there is meaningful negative correlation between Ph and elevation. The soil organic carbon is strongly positively correlated with elevation. The soil hydraulic conductivity has a meaningful positive correlation with sand%, whereas it has meaningful negative correlations with clay% and silt%.
 
Conclusion
The study area alluvial fan is located in the in the northwest of Amiriyeh, and is composed of three surfaces of relict, old and young, where morphology and geomorphological processes are different on each surface. Different geomorphological processes and landforms in different surfaces and also in different positions (apex and toe) have resulted in the spatial variation of physicochemical characteristics of soils of alluvial fan. Results imply that soil hydraulic conductivity is lower in the old surface compared to the relict and young surfaces. The lower soil hydraulic conductivity of old surface can be associated with relative stability, weathering and increasing clay% of soils of old surface, resulting in reducing soils permeability and hydraulic conductivity. The values of soil texture and hydraulic conductivity in landforms (interfluves compared to channels, and bars compared to swales) have meaningful statistical differences. This implies the mentioned landforms have fundamental effect in the variation of soil hydraulic conductivity of alluvial fan. Regarding the fact that improved understanding of physicochemical properties of soils has important role in the management and conservation of soil and vegetation, it is suggested that planners and managers consider differences in quantitate properties of soils in different landforms, positions, and surfaces of alluvial fans.