Impact of Geomorphic Hillslope Components on Soil Erodibility and Soil Quality (Case Study: Hashtian Catchment, Urmia Lake)

Controlling soil erosion and maintaining soil quality is essential for agricultural and environmental sustainability. Degradation of soil quality is caused by a combination of issues including land use change, desertification, soil erosion, chemical pollution, and total loss of soil fertility. Among these factors, soil erosion is considered an important factor in reducing soil quality and degradation. Slopes and their different components of them are one of the important and effective parameters in soil erosion, especially in mountainous areas. Different components of the slope (summit, shoulder, back slope, foot slope, and toe slope) directly and indirectly affect soil erodibility and consequently the soil quality and the development of slopes. Therefore, the purpose of this study is to investigate the effect of geomorphic hillslope components on erodibility and soil quality in the Hashtian catchment.

Hashtian catchment is located in West Azarbaijan province, Urmia city. Hashtian catchment area is 21222 hectares, maximum and minimum heights are 2759 and 1639 meters, respectively. There are three types of land use including agricultural lands (irrigated farming, rainfed farming), rangeland, and garden in Hashtian catchment.In this study, 53 soil samples were gathered from the main hillslope components (summit, shoulder, back slope, foot slope, toe slope), and the physical and chemical properties of the samples including sodium (Na), potassium (K), phosphorus (P), organic carbon (OM), electrical conductivity (EC), pH, calcium (Ca), saturated percentage (SP), bulk density (BD), particle density (PD) and soil texture were determined. Then the analysis of variance was used for investigating the relationship between the measured parameters and the geomorphic hillslope components. Also, the soil quality index was estimated using the following equation:SQI=(∑_(i=1)^n▒〖W_i S_i 〗)/n*10Where Wi represents the weights of the soil indicators, Si denotes the scoring function ranks of each soil property, indexed as i, and n reflects the number of variables selected for the MDS. Soil erodibility is also calculated from the Wischmeier equation as following formula:K = ((2.1 * 10 ^ (-4) M ^ (1.14) (12-a)) + (3.25 (b-2)) + (2.5 (c-3) / 759)M = (V.F.Sand + sand) * (100-Ac)Where K is the soil erodibility, Ac is clay, a is organic matter, b is soil structure and c is the permeability code, which is obtained based on soil characteristics. Finally, the soil quality and soil erodibility index in the geomorphic hillslope components were compared using analysis of variance, and the relationship between different slopes was analyzed by Post-hoc test and Tukey method.

The results showed that only the potassium parameter had a significant relationship with the hillslope components. Some soil properties such as saturated soil percent, potassium, and silt, are affected by the slope of the area. It is always expected that the back slope will have high erosion, but the high amount of clay in the back slope indicates the transfer of particles and nutrients from the shoulder to the back slope. On the other hand, due to the high amount of clay and sufficient amount of organic matter and phosphorus, the back slope has an acceptable soil quality. This has also affected the soil erodibility in this area. The summit has better soil quality due to the presence of suitable clay and organic matter as well as more sodium and phosphorus. The foot slope has better soil quality due to the presence of high phosphorus and potassium and also the amount of suitable organic matter.The results of using PCA to determine the MDS showed that four components were calculated to have an eigenvalue >1 and can thereby be used in the MDS. The cumulative variance was 80.56%. The highest PCs loadings were considered as a condition for selecting the final MDS comprised of Na, Clay, P, PD, and SP. The indicators were transformed into a dimensionless combinable score (0–1). In this study, a 'more is better' scoring function was applied to P and SP, and a 'less is better' scoring function was employed for Na and PD. The sand content followed an 'optimal range' function. The final results revealed that Na, Clay, PD, P, and SP had the highest to lowest weights, respectively.SQI = 0.29 S Na% + 0. 27 S Clay% + 0.27 S PD% + 0.27 S P % + 0.16 S SP%The results showed that different hillslope components do not affect soil quality and the mean values of soil quality at the summit are better than other hillslope components.Finally, different hillslope components do not affect the degree of soil erodibility and have no significant relationship with the results of soil erodibility so these results are not similar to the results of Ayoubi et al. (2014) who examined the soil quality of the slopes of Chaharmahal and Bakhtiari province. However, the results are similar to the results of Nosrati (2017) and Nosrati et al. (2015) in terms of the soil erodibility in the shoulder is higher than the other hillslope components. Also, the mean values of soil erodibility in the hillslope components are not significantly related.

The potassium parameter has a significant relationship with the hillslope components, which means that different hillslope components affect this parameter and there is no significant relationship between other parameters and hillslope components. There is no significant relationship between the geomorphic hillslope components and soil erodibility. Also, the summit and foot slope has better quality than other hillslope components due to the decay of vegetation and its effect on carbon dioxide and carbonic acid production. As a result, it increases the ability to absorb organic and nutrient substances, especially phosphorus in the soil. Accumulation of selected minimum data sets on soil quality at the foot slope and the summit has also increased soil quality. The steep slope in the shoulder has caused the washing of soil organic and nutrient materials and lowered the pH, and as a result, reduced the ability to absorb nutrients and increase erodibility.Keywords: Soil quality, Soil erosion, Geomorphic hillslope components, Hashtian Catchment.