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Globally, most bare-looking areas in dryland regions are covered by biocrusts which play a crucial role in modifying several soil surface properties and driving key ecosystem processes. These keystone communities face important threats (e.g. climate change) that place their conservation at risk and in turn the sustainability of the ecosystems they inhabit. Therefore, there is an urgent need to develop ecosystem management strategies to ensure their protection. However, to provide a solid path towards biocrust conservation, the understanding by stakeholders and governance structures of the ecological functions of these communities, their role as benefit providers, and the pressures threatening their important effects are indispensable. Whereas the ecological scope of biocrust has been widely studied in the last decades, the social dimension of their role remained unexplored. By reviewing literature in biocrusts from a social–ecological approach, here we identified knowledge gaps and new research areas that need to be addressed in order to produce scientific knowledge that better guides dryland conservation policies and actions. This research agenda is a prerequisite to advance biocrust conservation. [ABSTRACT FROM AUTHOR]

Cyanobacteria are key microbes in topsoil communities that have important roles in preventing soil erosion, carbon and nitrogen fixation, and influencing soil hydrology. However, little is known regarding the identity and distribution of the microbial components in the photosynthetic assemblages that form a cohesive biological soil crust (biocrust) in drylands of Europe. In this study, we investigated the cyanobacterial species colonizing biocrusts in three representative dryland ecosystems from the most arid region in Europe (SE Spain) that are characterized by different soil conditions. Isolated cyanobacterial cultures were identified by a polyphasic approach, including 16S rRNA gene sequencing, phylogenetic relationship determination, and morphological and ecological habitat assessments. Three well-differentiated groups were identified: heterocystous-cyanobacteria (Nostoc commune, Nostoc calcicola, Tolypothrix distorta and Scytonema hyalinum), which play an important role in N and C cycling in soil; nonheterocystous bundle-forming cyanobacteria (Microcoleus steenstrupii, Trichocoleus desertorum, and Schizothrix cf. calcicola); and narrow filamentous cyanobacteria (Leptolyngbya frigida and Oculatella kazantipica), all of which are essential genera for initial biocrust formation. The results of this study contribute to our understanding of cyanobacterial species composition in biocrusts from important and understudied European habitats, such as the Mediterranean Basin, a hotspot of biodiversity, where these species are keystone pioneer organisms. [ABSTRACT FROM AUTHOR]

Abstract: In recent years, soil inoculation with cyanobacteria has become one of the most promising biotechnological strategies for restoring soil functionality in degraded drylands because of their critical role in increasing soil fertility and preventing erosion. Nevertheless, in order to fully exploit this biotechnology on a large scale, it must still be shown whether inoculated cyanobacteria are capable of developing in soils with different physicochemical properties, and new candidates adapted to desert conditions must be explored. To evaluate the potential of cyanobacteria for restoring soil functions of degraded dryland soils, in this laboratory study, we analyzed the effect of inoculating three native N‐fixing species (Nostoc commune, Scytonema hyalinum, and Tolypothrix distorta), individually and as a consortium, on soil properties from three different semiarid ecosystems in southeast Spain. The biocrust colonization was monitored by determining chlorophyll a content (the typical surrogate used for biocrust biomass). Other methodologies, such as the analysis of soil spectral response and image classification were also applied for cover estimation of the biocrust. After 3 months, all inoculated soils showed cyanobacteria cover of up to 50%, lower albedo and higher chlorophyll a content. Cyanobacterial inoculation also improved soil functions, as they promoted a significant gain in total organic carbon and total nitrogen in all soils. Among inoculation treatments, Nostoc commune and the mixture of all three species promoted the most cyanobacteria coverage, chlorophyll content, and surface darkening, as well as organic carbon and total nitrogen gains in the soil, highlighting their excellent performance in biocrust development. [ABSTRACT FROM AUTHOR]

Biocrusts play crucial roles in hydrological processes by controlling soil water availability and regulating water redistribution from source to sink areas. Most studies have examined the influence of biocrusts on isolated components of the soil water balance, but few have addressed this matter from an integrated point of view, involving their influence on all components together. Such integration is crucial to elucidate the overall effects of biocrusts on the soil water balance. The aim of this study was to review the role of biocrusts in the soil water balance, by examining their influence on infiltration, evaporation and soil moisture at plot scale, in two contrasting ecosystems of SE Spain. Our results show that biocrust infiltration was higher in flat soils with sandy loam texture than in steep soils with silty loam texture. The influence of biocrusts on infiltration depended on rainfall intensity. Biocrusts increased infiltration with respect to biocrust-removed soils during low intensity rainfalls but showed similar or even lower infiltration than biocrust-removed soils during high-intensity events. As a result of the increase in infiltration and a decrease in evaporation during wet cold periods, biocrusts increased soil moisture when compared with biocrust-removed soils. However, during warm periods, biocrusts and biocrust-removed soils lost water very quickly, thus resulting in similar water losses and moisture content under both types of surfaces. We conclude that biocrusts increase water input by increasing infiltration and soil moisture, and reduce water output by reducing soil evaporation, thus eventually enhancing the available water to plants. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

• Humans consume much more energy than that merely needed to maintain metabolism. • Human exploitation generates tension with the ecosystem self-organization. • Human exploitation shifts the scale at which physical factors exert their control. • However, in turn, the landscape affects humans, generating adaptations. • The Ecodesert SI facilitates the understanding of Prof Puigdefabregas' contribution. [ABSTRACT FROM AUTHOR]

Highlights: [•] Moisture was monitored under two biological soil crusts and adjacent scalped soils. [•] Biological crust removal resulted in decreased moisture at 0.03m during soil wet periods. [•] Decrease in soil moisture was more noticeable in fine than in coarse-textured soils. [•] Soil water loss was faster in scalped than in biologically crusted soils. [•] Moisture was similar in soils with or without biological crusts during soil drying periods. [Copyright &y& Elsevier]

Summary: Runoff in arid and semiarid areas is characterized by high spatial and temporal variability. The spatial component is largely associated with the high spatial heterogeneity of soil surface attributes, such as vegetation and rock fragment covers, topography, and soil crust typology. Biological soil crusts (BSCs) are a common soil cover in arid and semiarid areas, and they play an essential role in local hydrological processes, since they affect many soil surface attributes associated with hydrologic properties. Although several publications have reported on the influence of BSCs on runoff at microplot spatial scales, only a few have examined their influence on larger spatial scales. Moreover, very few studies have analyzed the effect of BSCs on runoff under natural rain conditions. This is difficult, since a complex pattern of interactions among rainfall properties, BSC characteristics and some local characteristics, such as topography or type of soil is expected. In addition, in order to achieve a realistic model of how BSCs and rainfall affect runoff, it would be necessary to consider the level of human-driven degradation of BSCs. In this study, runoff was analyzed in plots with varying cover of cyanobacterial and lichen BSCs at microplot and small hillslope scales for two hydrological years in badlands in SE Spain. Structural equation modelling (SEM) was applied to test both direct and indirect relationships of BSC cover, slope gradient, rainfall characteristics and runoff. Our model showed that rainfall characteristics were the main factors controlling runoff yield. The slope positively affected runoff at small hillslope scales, but did not influence runoff at microplot scales. Runoff decreased at both scales with increased lichen-BSC cover. However, this effect was only significant during low-intensity events. Under high rainfall intensities, neither the BSC cover nor the slope had a causal effect on runoff. Our results suggest that incorporation of BSC-crusted surfaces in models should improve their capabilities for low-intensity rainfall events in semiarid areas similar to ours, but would be less important for high-intensity events. In addition, our approach, which takes direct and indirect relationships of factors affecting runoff into consideration, provides a very accurate picture of the process. [Copyright &y& Elsevier]

Abstract: Water and nutrients are scarce resources in arid and semiarid ecosystems. In these regions, biological soil crusts (BSCs) occupy a large part of the soil surface in the open spaces surrounding patches of vegetation. BSCs affect physicochemical soil properties, such as aggregate stability, water retention, organic carbon (OC) and nitrogen (N) content, associated with primary ecosystem processes like water availability and soil fertility. However, the way BSCs modify soil surface and subsurface properties greatly depends on the type of BSC. We hypothesised that physicochemical properties of soil crusts and of their underlying soils would improve with crust development stage. Physicochemical properties of various types of soil crusts (physical crusts and several BSC development stages) and of the underlying soil (soil layers 0–1 cm and 1–5 cm underneath the crusts) in two semiarid areas in SE Spain were analysed. The properties that differed significantly depending on crust development stage were aggregate stability, water content (WC) (at −33 kPa and −1500 kPa), OC and N content. Aggregate stability was higher under well-developed BSCs (cyanobacterial, lichen and moss crusts) than under physical crusts or incipient BSCs. WC, OC and N content significantly increased in the crust and its underlying soil with crust development, especially in the first centimetre of soil underneath the crust. Our results highlight the significant role of BSCs in water availability, soil stability and soil fertility in semiarid areas. [Copyright &y& Elsevier]

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 rainfall intensity), the first on dry soil and the second on wet soil, were carried out in microplots (0.25 m) 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, late-successional 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. [ABSTRACT FROM AUTHOR]

Arid and semi-arid ecosystems are characterized by patchy vegetation and variable resource availability. The interplant spaces of these ecosystems are very often covered by cyanobacteria-dominated biocrusts, which are the primary colonizers of terrestrial ecosystems and key in facilitating the succession of other biocrust organisms and plants. Cyanobacterial biocrusts regulate the horizontal and vertical fluxes of water, carbon and nutrients into and from the soil and play crucial hydrological, geomorphological and ecological roles in these ecosystems. In this paper, we analyze the influence of cyanobacterial biocrusts on water balance components (infiltration-runoff, evaporation, soil moisture and non-rainfall water inputs (NRWIs)) in representative semiarid ecosystems in southeastern Spain. The influence of cyanobacterial biocrusts, in two stages of their development, on runoff-infiltration was studied by rainfall simulation and in field plots under natural rainfall at different spatial scales. Results showed that cover, exopolysaccharide content, roughness, organic carbon, total nitrogen, available water holding capacity, aggregate stability, and other properties increased with the development of the cyanobacterial biocrust. Due to the effects on these soil properties, runoff generation was lower in well-developed than in incipient-cyanobacterial biocrusts under both simulated and natural rainfall and on different spatial scales. Runoff yield decreased at coarser spatial scales due to re-infiltration along the hillslope, thus decreasing hydrological connectivity. Soil moisture monitoring at 0.03 m depth revealed higher moisture content and slower soil water loss in plots covered by cyanobacterial biocrusts compared to bare soils. Non-rainfall water inputs were also higher under well-developed cyanobacterial biocrusts than in bare soils. Disturbance of cyanobacterial biocrusts seriously affected the water balance by increasing runoff, decreasing soil moisture and accelerating soil water loss, at the same time that led to a very significant increase in sediment yield. The recovery of biocrust cover after disturbance can be relatively fast, but its growth rate is strongly conditioned by microclimate. The results of this paper show the important influence of cyanobacterial biocrust in modulating the different processes supporting the capacity of these ecosystems to provide key services such as water regulation or erosion control, and also the important impacts of their anthropic disturbance. [ABSTRACT FROM AUTHOR]

Abstract Dryland ecosystems usually show low productivity, considerable water losses and land degradation problems associated with soil erosion. Most bare-looking areas in these ecosystems are covered by biocrusts, which represent an important nitrogen and carbon sink and provide essential ecosystem services. However, until now no studies analyzed the role of biocrusts in providing ecosystem services and compared these with the societal perception of biocrust-dominated ecosystems. In this study, we quantified the direct effect of biocrusts on soil water regulation, erosion control and climate and air quality in two different semiarid ecosystems dominated by biocrusts, and we explored the social importance and perceived vulnerability regarding the capacity of these ecosystems to provide services to society. Our results disclose an important role of biocrusts in providing four services, with indirect implications on other important ecosystem functions. However, interview partners neither recognized the capacity of the two analyzed ecosystems to provide supporting services nor their social importance. Survey respondents mostly valued provisioning services related to extraction activities, such as agriculture, which produce direct economic income but conversely threat biocrust sustainability. Thus, more effort is needed by scientists and policy makers to raise awareness on the key role of drylands with biocrust communities in preserving ecosystem functions that directly provide regulating services. Moreover, their protection is demanded not only for the provision of key ecosystem services, but also because biocrusts themselves should be considered as communities valued by humans. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

[Display omitted] • Biocrusts increase heat capacity by 2% in wet term but decrease it by 6% in dry term. • Moss crusts increase soil moisture by 500% due to their higher water-holding capacity. • Biocrusts reduce soil heat flux by 20%, verifying their effects on thermal properties. • Biocrusts decrease surface albedo by 32% and increase net solar radiation by 30% • Higher soil moisture and lower surface albedo are keys to raise biocrust temperature. Globally widespread, biocrusts form a "living skin" on most dryland soils, helping govern fundamental ecosystem functions, particularly water and heat balances in drylands. Nevertheless, the underlying mechanisms of biocrust effects on topsoil thermal properties and temperature (T) are still unclear. In this study, we continuously measured thermal properties of moss-dominated biocrusts and bare soil (aeolian sand) at 1 cm depth by utilizing heat pulse probes for one year on the Chinese Loess Plateau. The surface soil water content (θ), net all-wave radiation (J n), surface albedo (α), heat flux (G), and other properties were further measured to analyze their connections with soil thermal properties. Our results indicated that biocrusts decreased (F ≥ 1.91, P ≤ 0.031) heat capacity (C), thermal conductivity (λ), and thermal diffusivity (κ) by 1.6% (1.36 vs. 1.38 MJ m−3 K−1), 11.6% (0.62 vs. 0.69 W m−1 K−1), and 8.7% (0.62 vs. 0.69 × 10−7 m2 s−1) on average, respectively, in contrast to bare soil. Moreover, biocrusts variously regulated soil thermal properties in dry and wet seasons owing to the influences of rainfall events and θ. As compared with bare soil, the C , λ , and κ of biocrusts in dry season (Nov.–Apr.) were decreased by 5.7%, 19.0%, and 12.2%, respectively; while only the C was increased by 2.2% by biocrusts in wet season, but the λ and κ of biocrusts were similar to that of bare soil. Furthermore, biocrusts increased θ both in dry (by 5.0 times) and wet (by 3.5 times) seasons as compared with bare soil. Especially, biocrusts decreased α by 32.1% (0.19 vs. 0.28) and G (at 8 cm depth) by 20.0% (0.8 vs. 1.0 W m−2 s−1), and increased J n by 30.1% (73.0 vs. 56.1 W m−2) in comparison to bare soil. Lastly, combining with the higher θ and lower α as well as the lower soil thermal properties, biocrusts greatly increased T by 1.7 ℃ (up to 4.5 °C; P < 0.001) on average. In conclusion, moss-dominated biocrusts have great regulating effects on surface soil thermal properties and T , and these effects are mostly attributed to their lower bulk density and α but higher porosity and water-holding capacity, which further change surface θ and T regimes and play a vital role in soil water and heat balances as well as other critical processes in drylands. [ABSTRACT FROM AUTHOR]

[Display omitted] • Soil moisture (θ) measured by heat-pulse (HP) sensor fits well with the oven-drying θ. • HP sensor is more suitable in measuring near-surface θ than common dielectric sensors. • Surface θ measured by HP sensor is on average 0.05 cm3 cm−3 higher than other sensors. • Biocrusts have >76% greater surface θ than bare soil when measured using HP sensor. • Common dielectric sensors underestimate surface θ by 4%−28% in contrast to HP sensor. Understanding the physical, chemical, and biological processes and estimating water balance in drylands rely on accurate soil moisture (θ) measurements. Surface soil moisture is influenced by the presence and types of biocrusts because they strongly modify surface soil properties and hydrological processes. Nevertheless, it is still a challenge to precisely measure the θ of such thin biocrust layer and to evaluate biocrusts' roles in soil water balance. Due to the large sensing range (usually ≥ 5 cm), the most dielectric soil moisture sensors are incapable of accurately measuring near-surface (≤5 cm) θ , although they have been widely used for this purpose. Conversely, the heat-pulse (HP) sensor possibly performs better in measuring surface θ of biocrust layer because of its smaller sensing range (1.2 cm). In this study, we assessed the performance of HP sensor in investigating the surface θ differences between moss-dominated biocrusts and bare soil on the northern Chinese Loess Plateau. We measured surface θ of biocrusts and bare soil for one year at 1 cm depth using HP sensors, and at 2 or 5 cm depths using three common dielectric sensors based on alternative measurement principles. Our results showed that the relationship of θ and heat capacity from the HP sensor was quite well-fitted by a linear regression function (R 2 = 0.96), and the θ measured by HP sensor in the field agreed well with the oven-drying θ (root-mean-square error ≤ 0.015 cm3 cm−3 and mean deviation ≤ 0.007 cm3 cm−3). The HP sensor measured a 44% greater θ under biocrusts compared to bare soil throughout the experimental period, in contrast to lesser differences detected with the other sensors (17%–33%). Furthermore, the biocrust increased θ by 100% in the dry season (Nov.−Apr.) and by 40% in the wet season (May−Oct.) in contrast to bare soil based on the HP sensor. Moreover, the θ of biocrusts measured by HP sensor was 0.04 cm3 cm−3 higher than that of bare soil under the periods without rainfall, but the biocrusts increased θ by 0.05–0.10 cm3 cm−3 in contrast to bare soil after rainfall. The apparent differences among different types of sensors were reasonably attributed to their variable sensing ranges and volumes of measured soil. Although the installation depths were different among HP sensor and dielectric sensors, HP sensor was undoubtedly more suitable in measuring near-surface θ and sensitive enough to detect even non-rainfall water inputs (dew, fog, and water vapor sorption) into the soil. In conclusion, the θ differences between moss-dominated biocrusts and bare soil are mostly underestimated by common dielectric sensors compared to HP sensors. Therefore, biocrusts play a more vital role in surface water balance of drylands than we generally recognized before. [ABSTRACT FROM AUTHOR]

Abstract: Soil surface roughness has a strong influence on runoff and erosion, affecting surface storage capacity, water flow routing and velocity, and modifying runoff rates. It also reduces soil detachment by raindrops and the shear effect of water flow on the soil surface. In arid and semiarid ecosystems, biological soil crusts (BSCs) commonly appear in clearings between plants. Depending on the dominant component in the BSC community, the roughness of the soil surface may vary considerably, changing the hydrologic and erosive response of the soil. The aim of this study is to determine the effect of physical and biological crusts on soil surface roughness and their influence on runoff and erosion. For this purpose, we set up open plots containing different BSC types and treatments and recorded runoff and total erosion from all the events for 18months. Micro-topographic indexes were calculated from high-resolution digital surface models of the plots built from terrestrial laser scanner height data. After comparing different spatial resolutions and indexes, we concluded that only the local Random Roughness index in a 40-mm moving window provided a precise estimation of the roughness induced by BSCs, though it did not have a direct effect on runoff response. The best relationship between microtopography and runoff on biologically crusted soils was found for surface storage capacity, which appears as a powerful predictor of the runoff coefficient on long temporal scales. Sediment yield was not well predicted by any of the micro-topographic indexes studied. The only index that was significantly related to sediment yield was the local Random Roughness in a 40mm moving window, but even this explained only a third of the erosion variance. [Copyright &y& Elsevier]

In most of non-vegetated areas from drylands, soils are colonized by biocrusts, and although they represent a small fraction of the soil profile, they strongly affect several soil surface properties, such as porosity, available nitrogen and carbon content, hydrophobicity or micro-topography. The influence of BSCs on these soil properties has effects on numerous ecosystem processes, including water retention and runoff generation. Previous studies on the hydrological and erosive response of soils covered by biocrusts have highlighted the role of soil surface roughness as a key variable for understanding the influence of biocrusts on runoff and erosion, but biocrusts' effects on surface micro-topography varies depending on crust water content. Biocrusts can absorb large amounts of water in a short period of time, increasing their volume and modifying surface micro-topography, this change depending on biocruts type and development. A correct characterization of these surface variations may increase the knowledge about hydrological response of biocrusts, and for this reason, accurate ground level measurements of biologically crusted surfaces are essential. The objective of this study is to analyze the effect of wetting on surface micro-topography of biologically crusted soils. To achieve this objective, different crust types were scanned in the laboratory with high resolution laser scanner. Five samples were collected for each of the 4 different crust types (bare soil, cyanobacteria biocrust, and two different lichen biocruts). Two different scans were made in each sample, in dry and wet conditions. Random roughness (RR) was calculated for data from every scan, and the RR indexes obtained before and after wetting were compared. According with our initial hypothesis, an increase in surface height and surface roughness up to 0.24 and 0.20 mm respectively was observed in more developed lichen biocruts, under wet conditions respect to dry ones. These differences, despite being very subtle, could exert strong implications on runoff generation, and water evaporation, and show the complex interactions between biocruts, surface micro-topography and water fluxes. [ABSTRACT FROM AUTHOR]

• This biocrust special issue is a collection of 27 papers from 12 countries worldwide. • Various topics in soil-biocrust interactions in drylands are included and discussed. • Major topics include biocrust effects on soil hydrology, erosion, C, and biodiversity. • Biocrust and functional responses to physical disturbance and climate change are emphasized. • Biocrust conservation and restoration are considered important in combatting soil and land degradation. Biocrusts are photosynthetic biotic communities of cryptogams and microbes that aggregate minerals at the soil surface in many ecosystems. Due to their high tolerance to harsh environments, biocrusts are present in a wide range of habitats, but are especially representative ground covers in regions with restricted vegetation growth, such as drylands (hyperarid, arid, semiarid, and dry subhumid regions) where water is a limiting factor, or high latitude or altitude regions where cold is a limiting factor. Since biocrusts fulfill a large range of ecological roles particularly in modifying soil properties and regulating functions, their rehabilitation and management is believed to be a promising measure for combating land degradation. We organized this article collection to further highlight the importance of biocrusts and their fundamental roles in reshaping soil properties and multifunctionality in drylands and other ecosystems, and to elucidate the ways in which global change factors are influencing biocrust-soil systems. The special issue brings together 27 research articles pertinent to soil-biocrust interactions or biocrust response to global change and disturbance from 12 countries worldwide (10 papers from China, 6 papers from the USA, 2 papers from Spain, 2 papers from Australia, in addition to studies from Antarctica, Argentina, Brazil, Iceland, Iran, Mexico, Norway, South Africa, and Sweden). The discussed topics include biocrust roles in regulating soil hydrology (6 papers), reducing soil erosion (4 papers), affecting soil carbon fixation and respiration (2 papers), and influencing soil microbial biodiversity (5 papers). The responses of biocrusts themselves and their functions to trampling disturbance (2 papers), land use shifts (2 papers), and climate change (5 papers) are also emphasized. On the whole, we highlight the capability of biocrusts in reshaping most properties of surface soil, acting as engineers and architects of surface soil properties, functions, and processes in dryland or other harsh environments, and we recognize the necessity of their protection and consideration as valuable nature-based measures to combat soil and land degradation. [ABSTRACT FROM AUTHOR]