Introduction: Biological soil crusts are a community of cyanobacteria, fungi, lichens, and mosses and play key roles in arid and semi-arid regions including carbon and nitrogen accumulation, soil fertility, dust capture, soil conservation and stability. Recent studies show that the formation of biological soil crusts on mobile dunes areas is extremely difficult due to low vegetation cover, strong sand flow activity, and soil surface instability. If sand activity is controlled by the wind and the soil surface is stabilized by cover establishment and/or mechanical factors, the biological and physical crusts gradually form on the soil surface. These soil crusts play an important role in establishment and evolution of sand dunes. Lichen biological soil crusts and physical crusts are widespread on early Holocene sand dunes in the north of Golestan province. The aim of this study was to investigate the effect of lichen biological soil crusts on surface soil properties and consequently evolution and stability of sand dunes.
Materials and methods: Sand dunes around the Alagol wetland located in the north of Golestan province were selected for this study. Lichen biological soil crusts species were collected after extensive filed study and transferred to the laboratory for identification. Three lichen species including Diploschistes diacapsis (Ach.) Lumbsch, Gyalolechia fulgens (Sw.) Søchting, Frödén & ArupFulgensia fulgens (Sw.) and Squamarina lentigera (Weber) were identified. After identification, soil samples were taken from two depths beneath lichen biological soil crusts and physical crusts. Then, soil samples were transferred to laboratory and some of physical, chemical and biological soil properties including total organic carbon, carbohydrate, calcium sulfate, equivalent calcium carbonate, mean aggregate diameter, particle size, pH and electrical conductivity were measured using standard methods. In addition, soil samples were taken from two meter depths (parent material) for particle size analysis. In this research, a completely randomized factorial design with three replications was used to analyze the data. The means were compared using LSD method at p < 0.05. SAS software was utilized to analyze the data, and the figures were drawn using Excel software and GRADISTAT (Version 8.0).
 
Results and discussion: The results showed that presence of lichen biological soil crusts and physical crust changes the surface soil properties. Soils derived from aeolian deposits beneath of various lichen biological soil crusts had higher total organic carbon, carbohydrate and mean aggregate diameter in comparison with physical crusts. D. diacapsis and S. lentigera species had improved physical, chemical and biological soil properties compared to G. fulgens species. The effect of biological soil crusts on soil properties decreased with increasing soil depth. There was a high correlation between mean aggregate diameter with total organic carbon and carbohydrate. Electrical conductivity strongly decreased in soils cover with D. diacapsis and S. lentigera species compared to physical crusts and G. fulgens species in two depths. Calcium sulfate was higher in soil covered by G. fulgens in comparison with D. diacapsis and S. lentigera species. The content of fine sand and coarse silt in soils covered by lichen biological soil crusts was higher than physical crusts. Whereas, the content of fine silt and clay content covered by physical crusts were higher or similar to lichen biological soil crusts. Parent material had greater particle size distribution in comparison with surface soils covered with biological soil crusts and physical crusts. These results show that soil crusts are formed in soils with finer particle size. Biological soil crusts can increase fine particle content with two mechanisms including capture dust and weathering soils. The secretion of extracellular polysaccharides (carbohydrate) and rough surface morphology of biological soil crusts may help to capture dust (fine sand and coarse silt). Soil pH and calcium carbonate were lower in soils covered with lichen biological soil crusts compared to physical crusts.
Conclusion: The presence of biological crusts on soils derived from early Holocene aeolian deposits apparently plays an important role in improving soil properties. Well-developed lichen biological soil crusts including D. diacapsis and S. lentigera species may significantly lead to establishment and stability of soils derived from early Holocene aeolian deposits. It seems that G. fulgens species grow better on coarse-grained soils with higher calcium sulfate content. Lichen biological crusts increase the mean aggregate diameter and consequently increase the stability of the soil surface.  Lichen biological soil crusts have an important role in dust capture and resulted in increased fertility of soils derived from early Holocene aeolian deposits.