Soil evolution affected by climate and topography in Sardooeyeh-Bam Region(south of Kerman)

Climate and topography are two important soil-forming factors that affect the genesis, evolution, and classification of soils. Topography may influence soil evolution through attributes such as the direction, shape, and percentage of the slope, the amount of precipitation, penetration, and runoff, the rate of erosion, as well as the difference in the drainage conditions. On the other hand, organic carbon, the amount and type of clay, soil color, calcium carbonate, base saturation, leaching depth, and solubility of salts in the soil are closely related to the climate. Hence, this study aimed to investigate the effect of climate and topography on changing physicochemical, mineralogical, and micromorphological properties of soil.

This research focused on two regions including, Sardooeyeh and Bam. The Sardooeyeh region, with the soil moisture-temperature regime of xeric-mesic, is about 3500m above sea level. However, the mean elevation in the Bam region, with the aridic-thermic regime, is 900m. Eight representative pedons on the extrusive igneous parent material with an intermediate composition were selected, sampled, and described. Physicochemical, mineralogical, and micromorphological analyses performed on the soil samples, and the soils were classified using Soil Taxonomy and World Reference Base for Soil Resources (WRB). Soil pH, particle size distribution, electrical conductivity, equivalent calcium carbonate, cation exchangeable capacity, and gypsum were measured, then eight samples were selected for clay mineralogy investigations. A Bruker Dh8 Advance X-ray diffractometer at 30 mA and 40 kW was used to analyze the samples. Scanning electron microscopy was performed. The undisturbed soil samples from horizons were selected for micromorphological observations.

The results revealed the increase in the clay percentage, cation-exchange capacity, organic carbon, and equivalent calcium carbonate in the elevations with the xeric regime. On the other hand, an increase in the amount of electrical conductivity and gypsum percentage in the aridic regime was determined. Thus, changes in soil properties caused different soils, including a range of Mollisols and Alfisols (xeric regime) to Aridisols (aridic regime) in the Soil Taxonomy and the Chernozems, Luvisols, and Calcisols (xeric regime) to Solonchaks, Gypsisols and Cambisols (aridic regime) in the WRB system to be classified. One of the strengths of the WRB system compared to the Soil Taxonomy is the classification of buried soils and the use of the "Raptic" qualifier to indicate the lithologic discontinuity within the pedon. The results of clay mineralogy elucidate the presence of illite, palygorskite, chlorite, smectite, kaolinite, vermiculite, smectite-vermiculite interstratified minerals, and quartz. In the xeric regime, the illite mineral decreased, and vermiculite and smectite increased. However, the amount of illite increased in the aridic climate. Furthermore, the simultaneous presence of palygorskite and smectite-vermiculite interstratified minerals can be the result of the presence of polygenetic soils in this region. Calcite infilling, nodule, and calcite internal hypo-coating, with clay coatings, clay bridges, and link clay capping were among the dominant micromorphological pedofeatures observed in the xeric moisture. On the other hand, the simultaneous presence of calcite and clay coatings and infillings together with gypsum infillings and the interlocked gypsum plates prove the role of paleoclimate in soil formation.

Results of this study showed that the current climate, the paleoclimate prevailing in the region, and topography have key roles on the intensity of weathering, the amount of clay minerals, and the type of pedofeatures in the region. Besides, due to the lack of carbonate in the parent material, it seems that the increase of weathering in the xeric regime part of the area significantly influenced on the alteration of plagioclase mineral which in turn, increased secondary calcium carbonate.