Effect of Slope Positions on Physicochemical Properties of Soils Located on a Toposequence in Deilaman Area of Guilan Province

Topography is one of the most important factors of soil formation and evolution. Soil properties vary spatially and are influenced by some environmental factors such as landscape features, including topography, slope aspect and position, elevation, climate, parent material and vegetation. Variations in landscape features can influence many phenomena and ecological processes including soil nutrients and water interactions. This factor affects soil properties by changing the altitude, steepness and slope direction of lands. In spite of the importance of understanding the variability of soils for better management, few studies have been done to assess the quality of soils located on a toposequence and most of these studies include just pedological properties. The aim of this study was to investigate physical and chemical properties of soils located on different slope positions and different depths of a toposequence in Deilaman area of Gilan province, that located in north of Iran.
The lands on toposequence that were same in climate, parent material, vegetation and time factors but topographical factor was different, were divided into five sections including steep peak, shoulder slope, back slope, foot slope and toe slope. In order to topsoil sampling, transverse sections of this toposequence were divided into three parts lengthways, each forming one replicate or block. 10*10 square was selected and after removing a layer of undecomposed organic residues such as leaf litter, three depths of 0 to 20, 20 to 40 and 40 to60 cm soil samples were collected. physical and chemical characteristics such as soil texture, bulk density, aggregate stability, percent of organic matter, cation exchange capacity, available phosphorous and total nitrogen were measured.
The results showed that, because of high organic matter content and fine textured soils on the lower slope positions including foot slope and toe slope, aggregate stability, cation exchange capacity, available phosphorous and total nitrogen were maximum in these positions, whereas, bulk density had a reverse trend and was higher in the upper slope positions than the lower slope positions. The high content of organic carbon, phosphorus and total nitrogen in the soil of foot and toe slope positions, can be attributed to soil erosion and transferred from top of the slope and their accumulation in these situations. The results also revealed that, with increasing depth, aggregate stability, organic carbon content, cation exchange capacity, available phosphorous and total nitrogen content of soils decreased, whereas, clay content and bulk density had a reverse trend and increased with increasing the depth. Reducing the amount of organic carbon with increasing depth was because of the remains of plants and roots in the surface horizons and the presence of more organic carbon. Since phosphorus and nitrogen in the soils are highly dependent on organic matter, Thus, changes in these indicators are mainly obeys from this materials.
In general, it became appears from this study, that the topography factor had important effect on studied soil properties. The changes observed in the quality of soils located on different slope positions can be attributed to the differences of the soil in erosion rate and moisture content and different sediment receptions in different positions of toposequence as affected by the amount and distribution of rainfall. Considering the effect of the position of the landscape on the physical and chemical properties of soil, recommended analysis of the landscape is better to be done in the sustainable land management and also for soil and water conservation programs. Because of the different management practices in different parts of landscape is difficult and perhaps impossible, in order to maintain soil, conservation management must be done based on soil quality in areas with maximum damage and minimum quality.