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Published online February 10, 2022
https://doi.org/10.5141/jee.21.00082

Journal of Ecology and Environment (2022) 46:04

Biological soil crusts impress vegetation patches and fertile islands over an arid pediment, Iran

Adel Sepehr1* , Asma Hosseini1, Kamal Naseri2 and Atoosa Gholamhosseinian1

1Department of Desert and Arid Zones Management, Ferdowsi University of Mashhad, Mashhad 0098513, Iran
2Department of Rangelands and Watershed Management, Ferdowsi University of Mashhad, Mashhad 0098513, Iran

Correspondence to:Adel Sepehr
E-mail adelsepehr@um.ac.ir

The authors of this article dedicate the present manuscript to Asma whose name also means heavens. She left us to meet God in the Heavens.

Received: October 27, 2021; Revised: December 6, 2021; Accepted: December 7, 2021

This article is licensed under a Creative Commons Attribution (CC BY) 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/ The publisher of this article is The Ecological Society of Korea in collaboration with The Korean Society of Limnology

Abstract

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Background: Plant vegetation appears in heterogeneous and patchy forms in arid and semi-arid regions. In these regions, underneath the plant patches and the empty spaces between them are covered by biological soil crusts (moss, lichen, cyanobacteria, and fungi). Biological soil crusts lead to the formation and development of fertile islands in between vegetation patches via nitrogen and carbon fixation and the permeation of runoff water and nutrients in the soil.
Results: The present study has investigated the association of biological soil crusts, the development of fertile islands, and the formation of plant patches in part of the Takht-e Soltan protected area, located in Khorasan Razavi Province, Iran. Three sites were randomly selected as the working units and differentiated based on their geomorphological characteristics to the alluvial fan, hillslope, and fluvial terrace landforms. Two-step systematic random sampling was conducted along a 100-meter transect using a 5 m2 plot at a 0–5 cm depth in three repetitions. Fifteen samplings were carried out at each site with a total of 45 samples taken. The results showed that the difference in altitude has a significant relationship with species diversity and decreases with decreasing altitude. Results have revealed that the moisture content of the site, with biocrust has had a considerable increase compared to the other sites, helping to form vegetation patterns and fertile islands.
Conclusions: The findings indicated that biological crusts had impacted the allocation of soil parameters. They affect the formation of plant patches by increasing the soil’s organic carbon, nitrogen, moisture and nutrient content provide a suitable space for plant growth by increasing the soil fertility in the inter-patch space.

Keywords: biological soil crusts, fertile islands, soil physicochemical parameters, vegetation patches

Introduction

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The ecosystems of arid and semi-arid regions worldwide are known for their sparse vegetation and scarce biological resources (Toranjzar et al. 2009). Plants are non-homogeneously distributed in these regions and centralized in areas where water and nutrients are concentrated. Nutrients are scant in these regions and these thinly dispersed resources are not distributed evenly and continuously. Therefore, a phenomenon known as resource redistribution causes patchiness in vegetation (Ludwig et al. 2007). Plant patches are richer in terms of nutrients and more suitable conditions in terms of soil properties such as permeability, nutrient accumulation, and stability, thus called “fertile patches”. They act as an oasis or shelter during environmental stresses and have therefore been named “fertile islands” (Bolling and Walker 2002; Schlesinger et al. 1990). These islands have a major impact on changes in desert ecosystems through redistribution and dispersion of food resources under vegetation canopy and their accumulation in the root zone (Walker et al. 2001). Also, the presence of these islands leads to an increase in the amount of some soil parameters such as nitrogen (Bonanomi et al. 2008), phosphorus and other nutrients in the space under the canopy, which has a positive effect on plant growth (Bolling and Walker 2002; Walker et al. 2001; Zhao et al. 2010). There are empty vegetation-free spaces between the spots where resources such as water and soluble nutrients move freely between the spaces. This movement occurs along the downward slope when water is the actuator or along the wind direction when the wind is the actuator. Thus, islands are richer in nutrients (Thompson et al. 2005). In many desert ecosystems, voids between vascular plants are covered with biological crusts (Bowker et al. 2008; Bowker et al. 2010).

Biological crusts comprise a broad range of microorganisms including cyanobacteria, algae, lichen, moss, and fungi. These organisms live a few millimeters beneath, or on the soil surface in the free spaces between vascular plants and are referred to as autotrophs (Belnap 2003; Belnap and Büdel 2016). Biocrust can lead to optimal plant growth (Condon and Pyke 2018a). The positive effect of cyanobacteria on the germination and growth of wheat and rice seeds has been reported (Muñoz-Rojas et al. 2018). Furthermore, the use of cyanobacteria on seeds can also have beneficial effects on germination (O’Callaghan and van Sinderen 2016). The chemicals’ produced by cyanobacteria have essential roles in regulating metabolism, plant growth, development, and improving germination (Hashtroudi et al. 2013). Considering the importance, role and function of soil biological crusts in the development of fertile islands and plant establishment, the aim of this study was to investigate the effect of biological crusts on the development of fertile islands and vegetation formation in various landforms in arid and semi-arid regions. The relationship between the development of fertile islands through biological soil crusts and the development and establishment of vegetation can be used as an efficient solution to prevent desertification.

Materials and Methods

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Studied area

This research has been conducted in the Takht-e Soltan area, located in Khorasan Razavi Province, northeastern Iran, with an arid and semi-arid climate. Takht-e Soltan is located in the south-west of Sarakhs plain with a geographical longitude of 36° 16’ to 61° 58’ 20” E and latitude of 60° 59’ to 36° 15’ N (Fig. 1). The average annual rainfall in this region is 202 mm, which is the highest and lowest rainfall in February and July, respectively. The yearly minimum temperature is 1°C. and the highest temperature is 28.4°C in July. The average annual temperature is 27°C. The studied region is located in a topographic and geomorphic gradient of an erosional pediment. In terms of topography, the region consists of alluvial fans, hillslopes, and an erosional pediment. The particle size distribution in the soil shows sandy loam textures.

Figure 1. Study sites: Above image, general area of Takht-e Soltan. (a) Alluvial fan site, (b) hill slope sites, and (c) alluvial terrace sites.

Rocks are mostly sandstone, conglomerate and shale. This area is geologically and geomorphologically related to the Quaternary and Cenozoic periods.

Sampling and laboratory analysis

Sampling

Soil sampling was carried out in autumn 2018 via plot and transect method and as a systematic random sampling using a 100-meter transect and five 5 m2 plots with 20-m distancing, and 1 m2 subplots. Sampling was performed at three sites (fluvial terrace, hillslope, and alluvial fan) and 0–5 cm depth from the soil surface. In each site, two transects and five plots (in each transect) were sampled in three repetitions.

Laboratory analysis

Soil samples were transferred to the laboratory to investigate and measure chemical and physical parameters. Soil electrical conductivity in a soil extract with a ratio of 1:5 by an EC (electrical conductivity) meter (Rayment and Lyons 2011), the relative percentage of soil particles (hydrometer 52H) (Bouyoucos 1962), calcium carbonate (Nelson and Sommers 1983), organic carbon (Mingorance et al. 2007), power of hydrogen with a 1:2.5 extract using a pH-meter (Olsen 1954), total nitrogen (Bremner and Mulvaney 1983), Soil aggregate stability (Lax et al. 1994), bulk density via the paraffin coated clod method, phosphorus (Olsen 1954), potassium and sodium (Knudsen et al. 1983), and the moisture content via the weight method.

Scanning electron microscopy imaging was performed in the central laboratory of Ferdowsi University of Mashhad. This microscope enables the examination and analysis of chemical, composition, surface, and internal substructure properties in micronic and nanometric dimensions. The samples are thoroughly dried and covered with a thin layer of gold or gold alloy before taking images with the device. EDS spectrometry enables the quantitative and qualitative examination of the samples by measuring the energy of the X-rays emitted from the sample (Goldstein et al. 2018).

Investigating the biodiversity of biological soil crusts and vegetation

The identification of lichens was carried out via morphological examination using a stereomicroscope and using valid identification keys (McCune et al. 2012; Rosentreter et al. 2007; Temina and Nevo 2009; Zedda 2000). To measure the percentage of vegetation, 5 m2 plots and 1 m2 subplots were used along the 100 m transect and the plant species at each site were collected and transferred to the laboratory of the Faculty of Natural Resources and Environment for identification. The biological crusts and plants were identified to investigate the biodiversity of the species, in contrast, the species richness function, the Shannon index, and the Simpson index were used to evaluate species biodiversity. ‘Simpson’s Diversity Index (λ)’: This index shows the probability that two individuals, randomly selected from the same place or at the same time, belong to two separate species. In ecology, it is mostly used to determine the level of biodiversity in a habitat and is as follows (relation 1) (Simpson 1949). Calculation: In this regard, 1–D: Simpson diversity index (variation range 0–1), ni: number of people of each species at each sampling, N: total number of sample people, and S: number of species in the sample

1−D=1−∑ i=1s ni(n−1) N(N−1).

Shannon–Wiener index (H’): The issues of logarithmic series are easily solved with this index and it is the most prevalent method for measuring diversity (Shannon 1948). The value of this index increases with the increase of the number of species in the community and could theoretically reach very high numbers (Hayat et al. 2010). Calculation: In this regard, H’: Shannon–Wiener diversity index, S: number of species in the sample, and Pi: ratio of the number of species, i: to the total number of species

H'=−∑ i=1s(Pi)(log2Pi).

Statistical analysis

The analysis of data was performed using SPSS V26 statistical software (IBM Co., Armonk, NY, USA). The Kolmogorov–Smirnov test was used to investigate the normality of parameters, and data correlation was calculated with respect to Pearson’s correlation value. The existence of a correlation between environmental factors, the soil’s physicochemical properties, the presence of biological crusts, and vegetation density was investigated. In the case of a significant correlation between the data, the data were stated as being positive or negative. In the case of a positive correlation, it is deducted that considering the data in each site, the measured parameters are dependent on each other. Then, the significant difference of the groups was investigated via analysis of variance and Duncan’s multiple-range post-hoc method at a statistical level of 5%.

Results

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Biological soil crusts and vegetation in the three landforms studied

The total percentage of the biological soil crusts in the alluvial fan site (Total mean [%] = 52.50%) is relatively average and its predominant includes lichens (Table 1, Fig. 2). Biological crusts were not observed in the other two landforms.

Table 1 . Classification of lichens and cyanolichens of the studied site.

Crust typeCategoryOrderFamilySpecies
Lichens with cyanobacterial photobiontLecanoromycetesLecanoralesCollemataaeCollema
ChaetothyriomycetidaeVerrucarialesVerrucariaceEndocaon
Lichens with photobiont algae greenLecanoromycetesPsoraceaePsora
LecanoralesRamalinaceeToninia
CandelarialesCandelariaceCandelaella

Figure 2. Soil biocrusts photographed during a field visit to the area. picture (a) shows a view of lichens and cyan lichens. picture (b) and (c) show close-ups of cyanolichens endocarpon sp. and collema sp, respectively, taken by a stereomicroscope; picture (d) and (e) are close-ups of the lichens of Psora sp. and Candelariella sp, taken by a stereomicroscope.

In Table 2, the species richness index for the alluvial fan site is listed by height classes. Examination of the values of Simpson species diversity indices (Hill diversity index 2) in the table shows that significantly 770 m has a maximum, 752 m has a minimum and 766, 762, and 759 m are the intermediate values of the species diversity index. In general, it can be concluded that the diversity of species has decreased with decreasing altitude.

Table 2 . Biological soil crust species richness and species diversity by Shannon and Simpson's indices in alluvial fan site (by height class).

SiteHeightp-valueTest F
770 meters766 meters762 meters759 meters752 meters
Species richness index0.707 ± 3.500a0.00 ± 3.000a2.000 ± 1.440a2.000 ± 0.000a2.00 ± 0.000a0.2332.000
Shannon diversity index0.188 ± 5.710a0.007± 6.280a3.640 ± 3.109a2.770 ± 1.092a3.16 ± 0.193a0.1952.281
Simpson diversity index0.005 ± 0.017a0.00007 ± 0.009a,b0.008 ± 0.000a,b0.007 ± 0.000a,b0.006 ± 0.001b0.0445.583

Values are presented as mean ± standard deviation.

Within given parameters, different superscripts indicate a significant among sites, respectively, at p < 0.05.

The vegetation of the hillslope site was relatively moderate (total mean = 6.5%) and its predominant vegetation was Polygonum patulum and Peganum harmala. Moreover, the total vegetation percentage of the fluvial terrace was relatively low (total mean = 2.2%) and the predominant vegetation was Alhagi camelorum, Peganum harmala, and Acanthophyllum glandulosum (Table 3). The results of Simpson species diversity index (Hill diversity index 2) (Table 4) show that the difference in altitude has a significant relationship with species diversity and decreases with decreasing altitude. There was no significant difference between the Shannon–Wiener species diversity index and number of species index in altitude groups. The total vegetation percentage of the alluvial fan site was relatively high (total mean = 16.5%), (Fig. 3) its predominant vegetation consisted of various Artemisia species and Poa bulbosa, and the soil texture was loam.

Table 3 . The following table shows the plant species, the dominant species and the percentage of vegetation cover at different sites.

SiteSpecies vegetationDominant speciesMean total vegetation cover
Alluvial fanLondesia erianthaArtemisia scoparia16.5%
Poa bulbosaA. kopetdayhensis
Artemisia scopariaPoa bulbosa
A. kopetdayhensis
Gagea lutea
Amygdalus lycioides
Centaurea virgate
Peganum harmala
HillslopeAmygdalus lycioidesPolygonum patulum6.5%
Polygonum patulumPeganum harmala
Peganum harmala
Alhagi camelorum
Terrace alluvialA. kopetdayhensisAlhagi camelorum2.2%
A. scopariaPeganum harmala
Alhagi camelorumAcanthophyllum glandulosum
Peganum harmala
L. eriantha
Stenophylla carex
Acanthophyllum glandulosum
Fumaria officinalis
Euphorbia helioscopia
Alyssum minus

Table 4 . Plant species richness (total per site), species diversity (Shannon and Simpson' indices) in three studied sites.

SiteAlluvial fanHill slopTerrace alluvial fansp-valueTest F
Species richness index4.200 ± 0.477a1.800 ± 0.477b3.600 ± 2.302a,b0.0444.105
Shannon diversity index11.469 ± 1.800a3.426 ± 1.108b10.117 ± 8.720a,b0.0374.374
Simpson diversity index0.109 ± 0.082a0.230 ± 0.160a0.265 ± 0.411a0.6180.501

Values are presented as mean ± standard deviation.

Within a given parameters, different superscripts indicate a significant among sites, respectively, at p < 0.05.
Figure 3. The image shows the role of biological soil crusts in the development of fertile islands and vegetation patches.

The results of Duncan’s test, Shannon–Wiener species diversity indices, and the species richness index of the vegetation (Table 4) show that significantly speaking, species biodiversity is of the maximum value for the alluvial fan site, intermediate value for the fluvial terrace site, and minimum value for the hillslope site. The results indicate no significant difference regarding Simpson’s diversity index.

Effect of biological crusts on soil fertility and plant patch formation

The results obtained from the analysis of variance, Duncan’s multiple-range method, and Pearson’s multiple correlation test, presented separately for each site, are as follows:

According to Duncan’s comparison of means (Table 5), the vegetation of the alluvial fan site was better and more, compared to the other studied sites, and regarding the soil’s physicochemical properties (Table 5), the mentioned site has a significantly (p < 0.05) higher organic carbon, organic matter, and nitrogen content, as compared to the other sites, but ranks lower regarding the pH, EC, apparent density, and sand content. Furthermore, the results of the soil samples confirmed a significantly (p < 0.05) higher level of calcium carbonate, potassium, phosphorus, and silt content. Investigating Pearson’s correlation (coefficient of correlation = R) in this site indicated that the percentage of crust cover has the highest level of correlation with the absorbable phosphorus at a 95% level (R = 0.948, p = 0.05) and the highest level of positive correlation with the vegetation percentage at a 99% level (R = 0.996, p = 0.01). The highest level of vegetation density, with a significant difference, is observed in this site.

Table 5 . Analysis of variance of physicochemical and biological soil properties (n = 45).

Factors soil physicochemical propertiesFp-valueAlluvial fanHillslopeAlluvial terrace
pH*12.1270.0018.162 ± 0.263b8.480 ± 0.072a8.716 ± 0.146a
EC* (μS/cm)4.9170.028171.440 ± 78.670b273.168 ± 15.503a,b562.606 ± 345.305a
Soil bulk density* (g/cm3)18.904<0.0011.306 ± 0.080b1.658 ± 0.133b2.564 ± 0.557a
Soil water content * (%)4.5710.033145.190 ± 1.600a134.120 ± 9.250b136.010 ± 5.180c
Organic matter* (%)13.1890.0012.156 ± 0.986a0.714 ± 0.093b0.388 ± 0.162c
Organic carbon* (%)13.3010.0011.238 ± 0.564a0.408 ± 0.055b0.222 ± 0.091b
TKN* (%)142.742<0.0010.066 ± 0.001a0.035 ± 0.004b0.0263 ± 0.005c
P* (mg/kg)23.692<0.0010.001 ± 0.000a0.001 ± 0.000b0.000 ± 0.000c
K* (mg/kg)28.324<0.0010.002 ± 0.000a0.002 ± 0.000b0.001 ± 0.000c
Na (mg/kg)1.3220.3030.001 ± 0.003a0.001 ± 0.000a0.001 ± 0.001a
CaCO3* (%)7.0240.01040.700 ± 12.500a21.670 ± 3.630b37.151 ± 7.020a
Clay (%)0.8660.44526.400 ± 4.560a22.400 ± 8.050a22.400 ± 2.610a
Sand* (%)7.0650.00936.800 ± 5.210b52.000 ± 7.350a61.200 ± 15.530a
Silt* (%)5.2050.02436.800 ± 7.150a25.600 ± 8.880a,b16.400 ± 13.070b
Aggregate stability* (%)23.798<0.00128.652 ± 27.010c92.302 ± 11.360b131.684 ± 29.070a
Total vegetation cover* (%)19.082<0.00116.500 ± 5.950a6.500 ± 2.280b2.160 ± 1.360b

Values are presented as mean ± standard deviation.

Analyzed with the two-way ANOVA for Duncan post-hoc test.

*Represent significant at 95% confidence interval or p < 0.05.

In the hillslope site, almost all the soil’s properties are at an intermediate level in between the values obtained for the two other sites. The results showed significantly higher levels (p < 0.05) of nitrogen, potassium, phosphorus, organic carbon, and organic matter content compared to the fluvial site. Also, the results obtained from the soil samples of this site showed significantly (p < 0.05) lower levels of calcium carbonate content compared to the fluvial terrace. According to Pearson’s correlation, the results indicate that the vegetation percentage of this site has a positive correlation (R = 0.948, p = 0.05) with the total nitrogen content. The fluvial terrace was almost devoid of vegetation. The region’s soil has significantly (p < 0.05) lower nitrogen, phosphorus, potassium, organic carbon, and organic matter content compared to the other studied sites. This site also has a significantly (p < 0.05) lower EC, as compared to the other sites. The results obtained from Pearson’s correlation analysis show that the vegetation percentage of the studied site has a positive correlation (p < 0.05) with the total nitrogen, organic matter, organic carbon, and absorbable phosphorus content. Figure 4 shows an electron microscope image of the presence of biological crusts at the alluvial fan site. As can be seen in the image, the presence of these crusts causes the soil particles to coalesce, Figure 4b shows the polysaccharides produced by these crusts that hold the soil particles together.

Figure 4. The prepared SEM (scanning electron microscope) images show the stability of the aggregates. (a) Shows the red arrows, the viscous polysaccharides that bind the aggregates together. (b) Shows the fungal hyphae and they protect the soil aggregates like a net.

Discussion

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Comparison of physical and chemical parameters of soil in the presence and absence of biological crusts in three landforms

In this study, many soil parameters such as N, OC, P, K, and CaCO3 showed a significant difference between the three landforms. The alluvial fan site, which has a cover of biological crusts and a high percentage of higher vegetation, and the amount of nitrogen, potassium carbon and calcium carbonate in it is significantly higher than the other two sites. Based on the results obtained, biological crusts are able to increase soil nitrogen by up to 200% (DeFalco 1995). lichens and cyanobacteria fixate the atmospheric nitrogen and increase the soil nitrogen content up to 70% and make it available to vascular plants, mosses, fungi, and other microbial soil communities (Gholamhosseinian et al. 2020; Harper and Belnap 2001) Therefore, they can have a significant impact on plant establishment and increase vegetation. Organic materials have a major role in determining the characteristics of the soil by affecting its physicochemical properties (Whalen and Chang 2002). The color change in Figure 5 of the two sections indicates that the site with biological crust has a darker color Figure 5a and is also covered with Amaranthaceae which is a nitrogenphilous plant. These observations express the fact that crusts increase nitrogen entrance to the soil through nitrogen fixation.

Figure 5. Section (a) has a biological crust, and the presence of a nitrophilous plant (Amaranthaceae) indicates that the soil has a higher nitrogen content. Section (b) has no biological crust and its soil has less nitrogen.

Biological crusts play an important role in the producing of organic carbon and the provision of a suitable substrate for plant growth by increasing soil carbon through the deposition of atmospheric carbon (Beymer and Klopatek 1991; Danin and Ganor 1991; Hassanzadeh et al. 2018). Cyanobacteria with polysaccharide secretions play an effective role in increasing carbon content (Mager and Thomas 2011) and decomposition of organic matter as well as increasing soil particle retention (Danin and Ganor 1991; Miralles et al. 2013). They can also convert carbon dioxide to calcium carbonate, so more calcium carbonate in the soil can be attributed to the higher amount of activity of this microorganism (Wierzchos et al. 2012). Gholamhosseinian et al. (2020) and Kheirfam (2020) also showed that the amount of organic carbon had increased significantly in the presence of moss and lichen crusts. The decrease in pH can be attributed to the increase in microbial respiration at the alluvial fan site (Chamizo et al. 2012). Potassium also adheres to the wall of lichens and is absorbed when moistened in the soil by negatively charged clays, which may explain the high amount of potassium on the site with lichen cover (Gholamhosseinian et al. 2020; Hassanzadeh et al. 2018).

The impact of biological soil crusts on fertile islands development

The highest variety of biological crusts was observed at an altitude of 770 m compared to other altitudes. At higher altitudes, due to better physicochemical soil conditions, the diversity of crusts decreased with decreasing altitude. The results also show that at this height, vegetation has decreased with decreasing diversity of crusts. Soil texture in alluvial fans is loamy and according to research, biological crusts have the highest growth rate in soils with medium texture (Kleiner and Harper 1972). In the terraced alluvial terrace and the hillslopes, the sand has increased more and is accompanied by a decrease in vegetation. The percentage of clay in the presence of biological crusts at the alluvial fan site was higher than the other two sites. This significant increase in the percentage of clay in the biocrusts compared with the bare soil can be attributed to dust capture in the biocrusts of the soil surface (Gholamhosseinian et al. 2021a).

The erosive effects at the alluvial fan site, which has more biological crust and vegetation cover, were much less than the other two sites. It absorbs the lichens of the elements and then introduces them into the soil, thus reducing the loss of elements due to leaching (Williams et al. 2013). Biological crusts are stabilized by polysaccharide secretion that binds soil particles (Fig. 4) (Zhang et al. 2016). Absorption of nutrients by polysaccharides is the mechanism by which cyanobacteria provide nutrients to plants (Belnap and Gardner 1993), and leads to the formation of Fertility Islands adjacent to the biological crusts (Fig. 4). Aggregate stability was significantly higher at the biological crusts site, due to the production of polysaccharides by cyanobacteria crusts that hold the particles together.

The results of the present study have revealed that the moisture content of the site with crust has had a significant increase compared to the other sites which could be due to the roughness caused by the biological crusts since they prolong the water retention time to a great extent and therefore increase the chance of permeability and runoff reduction (Belnap 2006). During precipitation, the polysaccharide plates present in cyanobacteria and algae are capable of rapidly absorbing water several times their weight (Belnap and Gardner 1993). The observations of Campbell (1980) and Verrecchia et al. (1995), showed that all of the biological crust absorb water in various degrees and increase the moisture of the soil, particularly cyanobacteria that absorb water over 10 times their volume and 8–12 times their weight. The results of the present study revealed that the stability of soil aggregates was significantly higher in the alluvial fan site, as compared to the other studied sites. The biological soil crusts play a major role in the stability of soil aggregates. Fungal hyphae, cyanobacterial filaments, and viscous lichen polysaccharides create a consistent network for the soil layers and stabilize the soil aggregates (Belnap et al. 2001; Chamizo et al. 2012; Maestre et al. 2011) and this stability protects soil aggregates against wind and water erosion (Eldridge and Greene 1994). As observed in Figure 4, algal biofilms and lichen are able to preserve soil particles, especially cyanobacteria which stabilize soil aggregates. Sepehr et al. (2019) also achieved similar results by investigating the effect of biological crusts on the stability and strength of aggregates.

The increased apparent density in soils lacking lichen cover is due to reduced biological activities and the low organic matter content of these soils. Therefore, the soil’s reduced respiration, an index of the soil’s biological activity, could be counted as one of the reasons influencing the increase of the soil’s apparent density in the region (Niu et al. 2017). These results are in line with the studies of Jimenez et al. (2009), stating that the biological soil crusts have no type of competition with vascular plants, and this lack of competition leads to the availability of nutrients to the plants, thus promoting their establishment. Moreover, Tongway et al. (1989), observed a positive correlation between the vegetation percentage and the biological soil crusts and stated that by retaining the soil’s moisture and nutrients and promoting fertility, these crusts are able to support the growth and establishment of plants. One of the most important spatial control factors of vegetation in natural ecosystems, other than the presence of biological soil crusts, is the microtopography of the studied site (Gholamhosseinian et al. 2021b). Microtopography directly impacts environmental factors such as altitude increase and temperature reduction and has an indirect effect on the formation of soil, plant communities, and their production (Thompson et al. 2005). The results of the present study showed that microtopography is the cause of vegetation patchiness in the fluvial terrace and hillslope sites.

Conclusions

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The destruction of vegetation and the increase of coverless land is one of the signs of desertification. The biological soil crusts prevent the loss of nutrients by retaining the moisture obtained from runoff water, and increase soil fertility through the fixation of nitrogen and carbon, thus providing suitable conditions for the preservation and establishment of vegetation. They also develop fertile islands among plant patches, therefore reducing the distance between these patches which leads to a more uniform distribution in comparison to ecosystems that lack biological soil crusts. These fertile islands act as biological fertilizers and make nutrients available to plants, ultimately enhancing their growth. They also provide a suitable opportunity for the growth of other species, both annual and perennial, which leads to more diverse vegetation of the ecosystems. Furthermore, increased vegetation leads to the diversity of animal species and provides suitable conditions for herbivores.

Authors’ contributions

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AS collected the data, acquisition research grant, researched design, writing, review, and edited. AH collected the data, writing pre-original drafts. AGH analyzed and collected data, reviewed the paper, helped to write, and interpreted the results. KN collected the data. The authors read and approved the final manuscript.

References

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