Investigating the Soil Evolution and Microbial Respiration along a Semi-Arid ‎Alluvial Fan in Northeastern Iran

Soils and landforms have a strong relationship and archive evidence of climatic and environmental changes. Alluvial fans are one of the most important landforms in arid and semi-arid regions of Iran. Climate changes in the Quaternary, especially in the late Pleistocene, had a significant effect on the evolutions of alluvial fans in arid and semi-arid regions. Alternate of sedimentation and soil formation in alluvial are the consequences of periodic climate change. Organisms are one of the main factors of soil formation. Biological crusts are part of organisms that are abundant in dry lands and especially in alluvial fans; however, their role in soil formation has been less studied. Biological soil crusts by providing the suitable biological activity, effect on trapping of aeoilian materials and hydrological processes affect the soil formation processes. The chemical properties of the soil affect the catabolic capacity of the soil and it is very different among the different layers of the soil. However, few studies have addressed the effect of processes on soil microbial respiration during change and evolution and pedogenic state. The objectives of this research were to 1) investigate the evolution of soils along the gradient from upstream to downstream of the alluvial fan and 2) investigate the changes in microbial respiration in different layers of soil and the factors affecting it.

The studied area is an alluvial fan in Razavi Khorasan province, in the southern slopes of the Binaloud mountain range. The climate of the region is semi-arid and the soil moisture and temperature regimes are Aridic border on Xeric and mesic, respectively. Three soil profile in the upper, middle, and base part of the alluvial fan were described. Bulk and undisturbed soil samples were collected from various soil horizons for subsequent physical, chemical, and micromorphological analyses. In addition, the microbial soil respiration was measured in all horizons. The soils were classified according to Soil Taxonomy and World Reference Base ‎methods. ‎

Sequences of sedimentation and soil formation were observed in the soil profiles. Vesicular (V), argillic (Bt), argillic-calcic (Btk), calcic (BCk) and cambic (Bw) horizons were the diagnostic soil horizons of the studied soils. Soil profiles of the middle and base were Xeric Calciargids in the subgroup category of Soil Taxonomy; while soil profile of the apex soil was Xeric Haplocambids. In the profiles, a thin vesicular horizon (V) was formed under the desert pavement. Below the vesicular horizon, evidence of clay illuviation, pedogenic carbonate nodules, and calcium oxalates in roots were observed in thin sections. This evidence shows the role of biological crusts in the formation of these features. In the lower horizons of the profiles, pedogenic carbonate nodules, carbonates pendants and clay coatings were observed. It seems that the upper soil (vesicular and underlying Bt horizons) were developed in the more humid periods of the Holocene, and biological crusts also played a key role in the processes of calcification and clay illuviation. The argillic horizons in the lower layers were formed during the stable periods of the late Pleistocene. The irregular microbial respiration mainly indicated difference in microbial activities labile organic matter content. The argillic horizons had the lowest microbial respiration, due to decomposition of organic materials during soil formation. In contrast, soil respiration was the highest in surface and calcic horizons. It seems that preservation of organic materials by carbonate complication. However, it is suggested to investigate the carbon fractions in relation to microbial biomass in the studied horizons.

In this area, biological crusts and vegetation affected the formation of soil in the aeolian sediments of the Vk and AVk horizons and played a significant role in creating the Bt horizon in profiles 2 and 3. The study of landform profiles showed the formation of calcic and argillic horizons in the past climate, while the Bt horizon of the upper layers was formed in the current Holocene period. This form of the argillic horizon is slightly different from the soils of the Iranian region because these horizons have not been reported so far. It has been proven that there were humid periods in the Holocene, and it needs more studies at present. The study of soil microbial respiration in landform horizons showed that argillic horizons decreased the amount of microbial respiration, while it increased in classical horizons.