پراکنش پوسته های زیستی خاک در سطوح تحول سنی یک مخروط افکنه نیمه‌خشک

Introduction

Studying the relationship between the living world, geomorphic processes, and landforms, developed a new branch of Earth's surface process that called Biogeomorphology. There is a significant correlation between morphological changes and soil physicochemical and biological properties. Desert ecosystems comprise about one-third of the Earth's land surface, and a collection of geomorphological elements, such as the structure of landforms, alluvial fans, and Playa's sand dunes. A common feature of arid and semi-arid environments, there is sporadic vegetation. In these areas, below vascular plants scattered and empty space between them creates a fertile environment for the emergence of nonvascular plants, which the so-called refers to biological soil crusts (BSCs). They have low water requirements and high tolerance to temperature and sunlight and therefore are able to continue their life under conditions that limit the growth of vascular plants. Although biological soil crusts are very widespread in arid regions, just covers more than 40 percent of the Earth's surface. Biological soil crusts are communities of tiny organisms consists of cyanobacteria, green algae, lichens, mosses and others associated closely with particles of surface soil, forming a cohesive thin horizontal layer. The structure of biological soil crust makes binding soil particles and have a significant effect on the stability of the soil and resistance role to erosion. n addition, BSC maintains soil moisture, reduces the pH amount and increase the availability of nutrients and providing nitrogen, which leads to vegetation cover. Soil-geomorphic relationship in arid areas forms BSC regarding geomorphology evolution. In this study, we examined the compatibility of biological soil crusts under soil physicochemical characteristics along alluvial fan with different geomorphic structures.

Materials & Methods

The research was applied in an arid alluvial fan located in Khorasan Razavi province eastern north Iran at Binaloud hillslopes. The study areas includes Quaternary formations and alluvial deposits with contained formations of hard limestone, sandstone, and quartzite. A relation between the dynamic morphology landforms with dynamic chronology (temporal evolution) was considered based on Davis evolutionary approach. The records by imagery data and field observations identified three different relative ages of geomorphology on the debris of alluvial fan including old, middle, and new formations from apex to base of fan. For biological soil crust, cover percentage, biomass, and amount of chlorophyll a and b were analyzed. Soil samples were measured in terms of physicochemical properties such as soil moisture, soil particle size, EC, pH, percent calcium carbonate, N, P, K, Na, Ca, Mg, and organic carbon.

Results and Discussion

The findings showed that BSCs distributed on debris alluvial fan involve a complex assembly regarding two or three combinations of cyanobacteria, green algae, lichen, and moss. Results indicated that geomorphology factors govern biological soil crusts dynamics significantly from apex point of alluvial fan towards base surfaces, which beck to deposition and soil properties. Deposits of apex and top points showed generally more coarse texture and poorer sorting and roundness, in front an increasing trend is in deposits from top towards base points, so we found fine deposits on base surfaces. This sorting and roundness status led to better water drainage ratio from the highlands to the lowlands and thus increasing the mosses and lichens in BSCs. In other words, from old structures to the new structures, the distribution pattern of biological soil crusts in primary sequences is related to cyanobacteria and green algae and cyanolichen and in the final sequence is regarded to lichens and mosses.
Soil texture indicates the relative frequency of sand, silt, and clay that could be changed by soil moisture conditions. Thus, results showed that soil texture could be effected on species composition and distribution of BSCs. The soil texture in this study generally lights to moderate and belongs to the class loam. There is more silt in the soil, thus increased coverage of BSCs.
With the development of BSCs (from primary sequences such as cyanobacteria to the final sequence, such as mosses and lichens), reduced the amount of calcium carbonate and soil pH.
New structures have more organic carbon compared to other structures and there is inverse relationship between organic carbon and soil pH, also increasing trend on carbon fixation in the presence of lichens and mosses. The soil nitrogen content in new structures with mosses and lichens almost 1.5 times against the soil old structures. This finding confirmed the results of multiple studies of the biological soil crust regarding soil nitrogen increasing even up to 200%.
Results showed that in the middle structures, amount of magnesium, calcium, sodium, and potassium significantly increased compared to other structures, because these elements are attached to the external surface of the cell wall lichens. When lichens are dry and wet, these elements of the wall of lichen washed into the soil, and because of the positive charge by the colloids of clay which have a negative charge are absorbed, thus increasing the amount of them in these soils.

Conclusions

In this study was investigated the distribution of biological soil crusts in connection with the physicochemical properties of soil and geomorphological features in the dynamic morphology of the alluvial fan in the arid region. In the old structures and heights, with soil textures lighter and above pH levels of the soil, placed dominated by the primary sequences of cyanobacteria and green algae. While increasing the amount of silt and loamy soil makes new structures retain more moisture and thus is associated with the development of mosses and lichens in the next sequence. Different types or levels of different geomorphic causing change the distribution and species distribution of biological soil crusts. Lithology and sediment level showed a strong role in determining the structure of biological soil crusts.