Study of relationship between soil diversity index and soil-landscape evolution in Juneqan plain, Chaharmahal-Va-Bakhtiyari Province

 Addressing the concept of soil diversity over landscape as a set of different land units with different spatial distribution, shape, and arrangement that are affected by natural phenomena and human activities is essential for optimal use, proper management and conservation of this valuable resource. Soil diversity is a criterion for quantifying soil variability that deals with changes in soil properties or classes and understanding of the structure of these changes in the area. Soils evolve continuously under the interactive effects of propulsion and backward pathways, factors, processes, and endogenous and exogenous conditions. In other words, the development of soils is a function of divergent pedogenic pathways (increasing soil evolution followed by increasing soil diversity) and converging (increasing soil evolution and subsequently increasing soil uniformity). In the present study, we investigate the relationships between soil-landscape evolution in a hierarchical sequence of different soil classification and geomorphic levels using diversity indices in some parts of Juneqan plain, Chaharmahal va Bakhtiari province, as an example of semi-arid regions.

The study region with an area of nearby 16000 hectares is located in the Juneqan plain, Charmahal va Bakhtiari Province, Iran, between the coordinates 50° 33ʹ and 50°44ʹE longitude, and 32° 5' and 32°16ʹN latitude. Based on the US Soil Taxonomy (Soil Survey Staff, 2014), the study area has a Mesic soil temperature regime and the soil moisture regime is mainly Xeric and partially Aquic over the study area. A total of 102 soil profiles were dug, described and classified up to the great group level according to US Soil Taxonomy system and soil samples were collected from various genetic horizons. Mountain, hill, piedmont and low lands were the main detected landscapes in the studied area. In order to study the soil evolution the spatial structure of landscape changes, pedodiversity indices were calculated at different taxonomy hierarchical levels (from order to great group in Soil Taxonomy classification) and geomorphic hierarchical levels (landscape, landform, lithology and geomorphic surface) using appropriate indices such as the Shannon diversity index, richness index, Margalef Index, Menhinick Index and O’Neill index.

 The soils in the studied area were classified in three main soil orders including Entisols, Inceptisols and Mollisols. The results demonstrated that soil evolution in the studied area was mainly influenced by topography, parent material and locally the underground water level. In the higher lands (like mountain and hills), the lowest evolution was observed whereas, more evolved soils were observed in lower lands with more stable conditions. The results also indicated that all of the pedodiversity indices showed nearby a similar trend and increased under the decrease of the taxonomy and geomorphic hierarchy levels. So that, the minimum diversity was related to order and landscape levels and the maximum diversity was observed in soil great group and geomorphic surfaces levels. Besides, there was a positive linear relationship between species richness index and area of landform units. In other words, as the area of landform units increases, the diversity and consequently the richness index increases. The results also show a positive and nonlinear relationship between number of observations and Shannon entropy index and species richness index.

The obtained result showed that soil evolution and its properties is affected by some soil formation factors including parent material and topography. In conclusion, it seems, diversity indices are powerful tools in the demonstration of quantitatively soil diversity and provide useful information for soil mapping and optimum soil management purposes. In this study, different soil differentiation indices were calculated and reported for the level of classification hierarchy as well as geomorphic hierarchy. The results showed that by decreasing the level of hierarchy, the dispersion indices increased. This upward trend indicates that the soil evolution is divergent in this region and that as the soil evolves, its dispersal increases. The results also showed that by increasing the area of earthquake surfaces, both species richness index and Shannon entropy index increased. Also, a positive and non-linear relationship was observed between both Shannon entropy indices and species richness indices. Increasing soil richness and dispersion index in geomorphic hierarchy and soil classification as well as increasing richness and Shannon indexes with increasing extent of earthquake surfaces indicate that the soils studied are a nonlinear dynamic system.