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Soil crusts influence many soil parameters that affect how water moves into and through the soil, and therefore, critically influence water availability, erosion processes, nutrient fluxes, and vegetation distribution patterns in semiarid ecosystems. Soil crusts are quite sensitive to disturbance, and their alteration can lead to modification of the local hydrological regime, thus affecting general functioning of the ecosystem. The aim of this study was to analyze the influence of different types of soil crusts, physical, and biological in different developmental stages, as well as the impact of their disturbance, on infiltration. This was assessed by means of rainfall simulations conducted in two semiarid ecosystems in southeast Spain characterized by different lithologies, topographies, and soil crust distributions. Two consecutive rainfall simulation experiments (50 mm h¯¹ rainfall intensity), the first on dry soil and the second on wet soil, were carried out in microplots (0.25 m²) containing the most representative soil crust types at each site, each crust type subjected to three disturbance treatments: (a) undisturbed, (b) trampling, and (c) removal. Infiltration in the crusts was higher on coarse-than on fine-textured soils and almost two times greater on dry than on wet soil. Biological soil crusts (BSC) showed higher infiltration rates than physical soil crusts (PSC). Within BSC, infiltration increased as cyanobacterial biomass increased and was the highest in moss crusts. However, latesuccessional crustose and squamulose lichen crusts showed very low infiltration rates. Trampling reduced infiltration rates, especially when soil was wet, whereas crust removal enhanced infiltration. But this increase in infiltration after removing the crust decreased over time as the soil sealed again due to raindrop impact, making runoff rates in the scraped microplots approach those registered in the respective undisturbed crust types. Our results demonstrate that water redistribution in semiarid ecosystems strongly depends on the type of crusts that occupy the interplant spaces and the characteristics of the soils which they overly, as well as the antecedent moisture conditions of the soil. Disturbance of these crust patches results in increased runoff and erosion, which has important consequences on general ecosystem functioning.

Background Extracellular polymeric matrix (EPM) is a complex component of the organo-mineral assemblages created by biological soil crusts (BSCs). Mainly of polysaccharide origin, it embeds soil and sediments and provides key benefits to the crust community. Services provided include: sediment cohesion and resistance to erosion, moisture provision, protection from external harmful factors, as well as support to plant establishment and growth. EPM is the product of BSC microbial community, and it is constituted by exopolysaccharides (EPS) associated to other substances, organized in a three-dimensional structure having different levels of gelation, and degrees of condensation. Scope This review aims at focusing scientific attention, for the first time, on the characteristics and the roles of three operationally defined EPM fractions, one water soluble, one more adherent to cells and sediments, and one firmly attached to microbial cells. The latest results obtained by analyzing EPM of natural and induced (i.e, the result of cyanobacteria inoculation) BSCs are outlined, and the optimized extraction methodology is described in details. Conclusions The review underlines the complexity of investigating the characteristics and the role of microbial EPS, and its supra-structure (EPM), in natural conditions (as opposed to cultures in laboratory conditions), where the matrix is subjected to continuous microbial rearrangement due to biosynthetic, self- and cross-feeding processes, and where microbial activity affected by environmental parameters.

Soil surface cover is one of the most critical factors affecting soil water vapor transport, especially in drylands where water is limited, and the water movement occurs predominantly in the form of vapor instead of liquid. Biocrusts are an important living ground cover of dryland soils and play a vital role in modifying near‐surface soil properties and maintaining soil structure. The role of biocrusts in mediating soil water vapor transport during daytime water evaporation and nighttime condensation remains unclear. We investigated the differences in vapor diffusion properties, vapor adsorption capacity, and water evaporation between bare soil and three types of biocrusts (cyanobacterial, cyanobacterial‐moss mixed, and moss crusts) in the Chinese Loess Plateau. Our results showed that the three types of biocrusts had 5%–39% higher vapor diffusivity than bare soil. At the same level of ambient relative humidity and temperature, the initial vapor adsorption rates and cumulative adsorption amounts of the biocrusts were 10%–70% and 11%–85% higher than those of bare soil, respectively. Additionally, the late‐stage evaporation rate of cyanobacterial‐, cyanobacterial‐moss mixed‐, and moss‐biocrusts were 31%–217%, 79%–492%, and 146%–775% higher than that of bare soil, respectively. The effect of biocrusts on increasing vapor transport properties was attributed to the higher soil porosity, clay content, and specific surface area induced by the biocrust layer. All of these modifications caused by biocrusts on surface soil vapor transport properties suggest that biocrusts play a vital role in reshaping surface soil water and energy balance in drylands. Key Points Biocrusts increase water vapor diffusion properties, water vapor adsorption amount, and cumulative evaporation amount Modified soil properties of biocrust are a key factor influencing water vapor flux Reshaped vapor transport properties of biocrust control soil water and energy balance

Aims Biological soil crusts (BSCs) could improve severe environment ecological conditions by increasing soil moisture, soil nitrogen concentration, and so on. In order to control desertification and recover the destroyed soil fertility utilizing a new means using BSCs, the soil surface was artificially inoculated with Microcoleus vaginatus and Scytonema javanicum. Relationships between the development of the artificially induced biological soil crusts and the distribution and dynamic changes of nitrogen and phosphorus in the soil crusts have been analyzed. Methods Crusts of different ages were investigated by measuring soil physical and chemical factors, such as moisture, pH, total and available N content, and total and available P, which were correlated with the depths of the crusts. Results This study found that the types of color, shape, and species components of the algal crusts increased with crust development. Soil moisture, total N, available N, and available P increased gradually with crust growth. Soil with crusts was wetter than the controlled naked sandy soil, and a significant correlation was observed between biomass and total nitrogen (r= 0.946, P= 0.015). Soil pH was lower than that of control. The scytonemin on the soil surface was exceptionally higher than the other pigments, and all the pigments were mainly distributed at the soil surface level. Though the crusts were mainly distributed on soil surface, the available P was mainly stored below the crust layer. Conclusions Pearson correlation tests indicated that artificially inoculated biological crusts could improve soil fertility and micro-environment of the top soil: The development of artificially induced BSCs was very well, and this was favorable to inducing the following crust succession.

Biological soil crusts (biocrusts), comprised of mosses, lichens, and cyanobacteria, are key components to many dryland communities. Climate change and other anthropogenic disturbances are thought to cause a decline in mosses and lichens, yet few longterm studies exist to track potential shifts in these sensitive soil-surface communities. Using a unique long-term observational dataset from a temperate dryland with initial observations dating back to 1967, we examine the effects of 53 y of observed environmental variation and Bromus tectorum invasion on biocrust communities in a grassland never grazed by domestic livestock. Annual observations show a steep decline in N-fixing lichen cover (dominated by Collema species) from 1996 to 2002, coinciding with a period of extended drought, with Collema communities never able to recover. Declines in other lichen species were also observed, both in number of species present and by total cover, which were attributed to increasing summertime temperatures. Conversely, moss species gradually gained in cover over the survey years, especially following a large Bromus tectorum invasion at the study onset (ca. 1996 to 2001). These results support a growing body of studies that suggests climate change is a key driver in changes to certain components of late-successional biocrust communities. Results here suggest that warming may partially negate decades of protection from disturbance, with biocrust communities reaching a vital tipping point. The accelerated rate of ongoing warming observed in this study may have resulted in the loss of lichen cover and diversity, which could have long-term implications for global temperate dryland ecosystems.

AimsBiocrusts, the living skin of dryland ecosystems, contain diverse soil microorganisms that are essential to biocrust formation and the maintenance of multiple ecological functions including nitrogen fixation, carbon sequestration, soil stability, and rainfall redistribution. We know that biocrusts are important modulators of the soil microbiomes, however, much less is known about how local conditions influence biocrust adaptation and subsequently alter the soil microbiomes.MethodsTo understand the effects of microhabitat on bacterial communities via changes in biocrust traits, we collected biocrusts and analyzed soil microbiomes from eight representative microhabitats present in a semiarid ecosystem from the Chinese Northern Loess Plateau. These microhabitats were located a) outside plant canopy on level land, on shady gentle slope, and sunny gentle slope; b) under plant canopy on level land, on shady gentle, and sunny gentle slope; and c) outside plant canopy on shady and sunny steep slope, respectively. We then used structural equation modeling to investigate the relative contribution of microhabitat factors on important bacterial community metrics through quantifying the changes in biocrust traits.ResultsObserved microhabitat conditions significantly (P ≤ 0.033) altered the traits of biocrusts (e.g., thickness, biomass, and chlorophyll content), which were associated with significant changes in the soil bacterial community. For example, the bacterial richness in biocrusts developing under plant canopy, on shady slopes, and on gentle slopes was 20.1%, 19.9%, and 15.4% higher than that of the biocrusts developing outside plant canopy, on sunny slopes, and on steep slopes, respectively. We further showed that microhabitat conditions significantly impacted the network structure of bacterial communities under biocrusts, and structural equation modeling revealed that microhabitat metrics had strong indirect effects on network connectivity through changing biocrust traits.ConclusionsOur findings suggest that microhabitat factors can strongly influence soil bacterial communities via the changes in locally-adapted biocrust traits and soil properties. This knowledge is critical to understand the impacts of changing environments on biocrusts and associated soil bacterial communities, particularly as climate change progresses.

Biological soil crusts (BSCs) cover non-vegetated areas in most arid and semiarid ecosystems. BSCs play a crucial role in the redistribution of water and sediments and, ultimately, in the maintenance of ecosystem function. The effects of BSCs on water infiltration are complex. BSCs increase porosity and micro-topography, thus enhancing infiltration, but, at the same time, they can increase runoff by the secretion of hydrophobic compounds and clogging of soil pores upon wetting. BSCs confer stability on soil surfaces, reducing soil detachment locally; however, they can also increase runoff, which may increase sediment yield. Although the key role of BSCs in controlling infiltration—runoff and erosion is commonly accepted, conflicting evidence has been reported concerning the influence of BSCs on runoff generation. Very little is known about the relative importance of different BSC features such as cover, composition, roughness, or water repellency, and the interactions of these attributes in runoff and erosion. Because BSC characteristics can affect water flows and erosion both directly and indirectly, we examined the direct and indirect effects of different BSC features on runoff and erosion in a semiarid ecosystem under conditions of natural rainfall. We built structural equation models to determine the relative importance of BSC cover and type and their derived surface attributes controlling runoff and soil erosion. Our results show that the hydrological response of BSCs varies depending on rainfall properties, which, in turn, determine the process governing overland flow generation. During intense rainfalls, runoff is controlled not only by rainfall intensity but also by BSC cover, which exerts a strong direct and indirect influence on infiltration and surface hydrophobicity. Surface hydrophobicity was especially high for lichen BSCs, thus masking the positive effect of lichen crust on infiltration, and explaining the lower infiltration rates recorded on lichen than on cyanobacterial BSCs. Under low intensity, rainfall volume exerts a stronger effect than rainfall intensity, and BSC features play a secondary role in runoff generation, reducing runoff through their effect on surface micro-topography. Under these conditions, lichen BSCs presented higher infiltration rates than cyanobacterial BSCs. Our results highlight the significant protective effect against erosion exerted by BSCs at the plot scale, enhancing surface stability and reducing sediment yield in both high- and low-magnitude rainfall events.

Aims One of the most important questions of our time is how ecosystems will be transformed by climate change. Here, we used a five-year field experiment to investigate the effects of climate warming on the cover and function of a sub-Arctic alpine ecosystem in the highlands of Iceland dominated by biocrust, mosses and vascular plants. Methods We used Open Top Chambers (OTCs) to simulate warming; standard surface and Normalised Difference Vegetation Index (NDVI) analyses to measure plant cover and function; gas analyzers to monitor biocrust respiration; and the Tea Bag Index approach to estimate mass loss, decomposition and soil carbon stabilization rates. Results Contrary to our initial hypothesis of warming accelerating an ecological succession of plants growing on biocrust, we observed a warming-induced decreased abundance of vascular plants and mosses —possibly caused by high temperature summer peaks that resemble heat waves— and an increase in the cover of biocrust. The functional responses of biocrust to warming, including increased litter mass loss and respiration rates and a lower soil carbon stabilization rates, may suggest climate-driven depletion of soil nutrients in the future. Conclusion It remains to be studied how the effects of warming on biocrusts from high northern regions could interact with other drivers of ecosystem change, such as grazing; and if in the long-term global change could favor the growth of vascular plants on biocrust in the highlands of Iceland and similar ecosystems. For the moment, our experiment points to a warming-induced increase in the cover and activity of biocrust.

Biological soil crust (BSC) constitutes a consortium of cyanobacteria, algae, lichen, mosses, and heterotrophic microorganisms, forming a miniature ecosystem within the uppermost soil layer. The biomass of different organisms forming BSC and their activity changes along with succession. Previous studies focused primarily on BSC in hyper-arid/arid regions, whereas the ecophysiology of BSC in temperate climates is still not well recognized. In order to determine changes in overall microbial activity and photosynthetic biomass in BSC at different stages of the succession of inland sandy grasslands, we analyzed dehydrogenase activity and determined the content of photosynthetic pigments. We also compared these parameters between BSC developed on the dune ridges and aeolian blowouts in the initial stage of succession. Our study revealed a significant increase in both photosynthetic biomass and overall microbial activity in BSC as the succession of inland shifting sands progresses. We found that chl a concentration in BSC could be considered a useful quantitative indicator of both the presence of photoautotrophs and the degree of soil crust development in warm-summer humid continental climates. The photosynthetic biomass was closely related to increased microbial activity in BSC, which suggests that photoautotrophs constitute a major BSC component. Dune blowouts constitute environmental niches facilitating the development of BSC, compared to dune ridges. High biomass of microorganisms in the dune blowouts may be associated with a high amount of organic material and more favorable moisture conditions. We conclude that deflation fields are key places for keeping a mosaic of habitats in the area of shifting sands and can be a reservoir of microorganisms supporting further settlement of dune slopes by BSC.

Inoculation of soils with cyanobacteria is proposed as a sustainable biotechnological technique for restoration of degraded areas in drylands due to the important role that cyanobacteria and their exopolysaccharides (EPS) play in the environment. So far, few studies have analyzed the macromolecular and chemical characteristics of the polysaccharidic matrix in induced cyanobacterial biocrusts and the scarce existing studies have mainly focused on sandy soil textures. However, the characteristics of the cyanobacterial polysaccharidic matrix may greatly depend on soil type. The objective of this study was to examine the macromolecular distribution and monosaccharidic composition of the polysaccharidic matrix induced by inoculation of two cyanobacterial species common in arid environments, Phormidium ambiguum (non N-fixing) and Scytonema javanicum (N-fixing) in different soil types. S. javanicum promoted a higher release in the soil of the more soluble and less condensed EPS fraction (i. e., the loosely bound EPS fraction, LB-EPS), while P. ambiguum showed a higher release of the less soluble and more condensed EPS fraction (i. e., the tightly bound EPS fraction, TB-EPS). LB-EPSs were mainly composed of low MW molecules (< 50 kDa), while TB-EPSs were mainly composed of high MW molecules (1100–2000 kDa). The two EPS fractions showed a complex monosaccharidic composition (from 11 to 12 different types of monosaccharides), with glucose as the most abundant monosaccharide, in particular in the poorer soils characterized by lower organic C contents. In more C-rich soils, high abundances of galactose, mannose, and xylose were also found. Low abundance of uronic acids and hydrophobic monosaccharides, such as fucose and rhamnose, was found in the EPS extracted from the inoculated soils. Our results point to the influence of soil type on the macromolecular distribution and monosaccharide composition of the polysaccharidic matrix in induced biocrusts, which is likely to affect biocrust development and their role in soil structure and nutrient cycling in restored dryland soils.