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Dry salt lakes are widely distributed in semi-arid regions. Dry lake beds have been found to have a greater capacity to release dust. However, the particle size characteristics of dry lakebed soils have rarely been studied. To reveal the soil grain size characteristics, we used surface and subsurface soil from dry lake beds in semi-arid regions with different soil crusts as the object of study. We used particle size parameters and class-standard deviation methods to study the particle size variation patterns and the degree of variability. The results show that the dry lake was mainly composed of clay (1.36%-43.96%) and silt (5.5%-67.94%) particles. Soil particle characteristics show the Mean size (Mz) of (0.65-8.89 f), poorly sorted (0.79-3.38 f) and a clear linear relationship between dry lake bed skewness and sorting. There is a high variability between soil particle size fractions across the lake bed, with soil crusts and soil moisture protecting the loss of subsurface soil particles, hence the differences between surface and subsurface soil particles. However, the destruction of the soil crust and the evaporation of soil moisture accelerates the wind erosion of the subsurface soil particles. In the dry lake area, vegetation construction has promoted the accumulation of sand content (73.28% -93.08%). We found that the fractions 0.19-9.15 µm, 9.15-36.99 µm, 36.99-366.93 µm, and 366.93-1214.72 µm are susceptible to respond to environmental changes and contribute to dust storms. Therefore, local governments should treat and manage dried-up lakes to reduce the impact of salt dust on the ecosystem. [ABSTRACT FROM AUTHOR]

Cyanobacteria are crucial primary producers in soil and soil crusts. However, their biodiversity in these habitats remains poorly understood, especially in the tropical and polar regions. We employed whole genome sequencing, morphology, and ecology to describe a novel cyanobacterial genus Argonema isolated from Antarctica. Extreme environments are renowned for their relatively high number of endemic species, but whether cyanobacteria are endemic or not is open to much current debate. To determine if a cyanobacterial lineage is endemic is a time consuming, elaborate, and expensive global sampling effort. Thus, we propose an approach that will help to overcome the limits of the sampling effort and better understand the global distribution of cyanobacterial clades. We employed a Sequencing Read Archive, which provides a rich source of data from thousands of environmental samples. We developed a framework for a characterization of the global distribution of any microbial species using Sequencing Read Archive. Using this approach, we found that Argonema is actually cosmopolitan in arid regions. It provides further evidence that endemic microbial taxa are likely to be much rarer than expected. [ABSTRACT FROM AUTHOR]

Cyanobacteria are crucial primary producers in soil and soil crusts. However, their biodiversity in these habitats remains poorly understood, especially in the tropical and polar regions. We employed whole genome sequencing, morphology, and ecology to describe a novel cyanobacterial genus Argonema isolated from Antarctica. Extreme environments are renowned for their relatively high number of endemic species, but whether cyanobacteria are endemic or not is open to much current debate. To determine if a cyanobacterial lineage is endemic is a time consuming, elaborate, and expensive global sampling effort. Thus, we propose an approach that will help to overcome the limits of the sampling effort and better understand the global distribution of cyanobacterial clades. We employed a Sequencing Read Archive, which provides a rich source of data from thousands of environmental samples. We developed a framework for a characterization of the global distribution of any microbial species using Sequencing Read Archive. Using this approach, we found that Argonema is actually cosmopolitan in arid regions. It provides further evidence that endemic microbial taxa are likely to be much rarer than expected. [ABSTRACT FROM AUTHOR]

Aim: Microbial diversity is one of the most important factors for maintaining the performance of multiple functions in soils (multifunctionality). However, existing studies typically consider the taxonomic richness or Shannon diversity of the entire community. We know little about the connection network of taxonomic and phylogenetic facets of diversity and their collective impact on multifunctionality. In this study, the linkages of effects of diversity within functional groups were disentangled in drylands. Location: The central Tibetan Plateau. Time period: Present. Major taxa studied: Phototrophs and diazotrophs (mainly cyanobacteria). Methods: Given the carbon‐ and nitrogen‐limited nature of drylands, we conducted a high‐throughput sequencing of C/N‐fixing functional groups from biocrusts and evaluated multiple facets of diversity (i.e., richness, evenness and phylogeny‐related trait dissimilarity), together with seven crucial variables of soil functioning to calculate multifunctionality. The relationships between multifaceted diversity and abundance with individual functions and multifunctionality were validated by a set of solid statistical analyses. Results: We found that the integrated biodiversity index was a stronger predictor of multifunctionality than richness. The divergent performance of different facets of diversity determined the idiosyncratic effect of each functional group on soil multifunctionality. Moreover, the evaluation of functional significance at the species level gave important clues on the trade‐offs and redundancy in each functional group, explaining the distinct patterns of effects of diversity. Richness was the dominant factor for diazotrophs to maximize multifunctionality, whereas phylogenetic dissimilarity was the essential one for phototrophs. Main conclusions: Our study demonstrated that multiple facets of diversity should be considered when assessing the effects of biodiversity. In contrast to the community level, within‐functional group measures might adequately capture the features of diversity that are most correlated with soil multifunctionality. Our results provide a perspective to bridge the gap between taxonomic and trait‐based approaches for elucidating the biodiversity–ecosystem function relationship. [ABSTRACT FROM AUTHOR]

Biological soil crusts (BSCs), are common on soil surface in arid and semiarid regions. They consist of cyanobacteria, algae, fungi, lichens and bryophytes associated with soil particles. BSCs have significant influence on primary ecosystem processes and are known as ecosystem engineers in drylands. In this study, the effect of BSCs on different soil properties in Kiamaky Wildlife Refuge in Jolfa county, East Azerbaijan province, Iran was investigated. Soil sampling was done in adjacent locations with BSCs, and bare substrate. The results showed that BSCs significantly increase the soil organic carbon, total nitrogen, microbial community population, soil respiration and microbial biomass carbon & nitrogen, as well as significantly decrease CaCO3 content. In addition, BSCs lightly increased electrical conductivity and availability of micronutrients in soil. The effect of BSCs significantly decreased along increasing depth. Generally, it was revealed that BSCs enhance the soil fertility, which can improve soil quality and ecosystem productivity in arid and semiarid regions. [ABSTRACT FROM AUTHOR]

Biological soil crusts (hereafter, “biocrusts”) dominate soil surfaces in nearly all dryland environments. To better understand the influence of water content on carbon (C) exchange, we assessed the ability of dual-probe heat-pulse (DPHP) sensors, installed vertically and angled, to measure changes in near-surface water content. Four DPHP sensors were installed in each of two research plots (eight sensors total) that differed by temperature treatment (control and heated). Responses were compared to horizontally installed water content measurements made with three frequency-domain reflectometry (FDR) sensors in each plot at 5-cm depth. The study was conducted near Moab, Utah, from April through September 2009. Results showed significant differences between sensor technologies: peak water content differences from the DPHP sensors were approximately three times higher than those from the FDR sensors; some of the differences can be explained by the targeted monitoring of biocrust material in the shorter DPHP sensor and by potential signal loss from horizontally installed FDR sensors, or by an oversampling of deeper soil. C-exchange estimates using the DPHP sensors showed a net C loss of 69 and 76 g C m −2 in control and heated plots, respectively. The study illustrates the potential for using the more sensitive data from shallow installations for estimating C exchange in biocrusts. [ABSTRACT FROM AUTHOR]

Background and aims: Rehabilitation of biological soil crusts (biocrusts) in degraded drylands may facilitate ecosystem recovery. In order to rehabilitate biocrusts, ex situ culture methods for biocrust organisms must be optimized so that biocrusts may be grown in sufficient quantities to be reintroduced into degraded areas. Our goal was to improve these culture methods. Methods: We cultured six biocrust lichens and mosses, alone or in combinations, in a full-factorial greenhouse experiment, also manipulating water quality and hydration schedule. Results: All cultures produced a multi-species biocrust. The lichen Collema grew fastest, increasing by up to 238 % over 5 months. The mosses Syntrichia caninervis, and Syntrichia ruralis also grew, whereas other lichen species failed to maintain growth. Species combinations featuring Collema and both mosses exhibited greater growth rates for all species, compared to monocultures. All species were either unaffected by water quality, or performed better when irrigated with purer water. Several species responded favorably to shorter dry periods. Conclusions: The lichen Collema is a promising restoration material because of its culturability, and its N-fixation ability. Initial species composition of a culture will likely affect its success, and complementarity among species may be exploitable in order to produce inoculum faster. [ABSTRACT FROM AUTHOR]

Background and Aims Many studies have analysed the mechanisms that determine plant coexistence in standing vegetation, but the determinants of soil seed bank species assemblies have rarely been studied. In gypsum soil communities, aerial vegetation and seed banks are tightly connected in space and time, but the mechanisms involved in their organization may differ. The aim of this study is to understand the relative importance of biotic and abiotic factors controlling soil seed bank composition and structure. Methods Persistent and complete (i.e. persistent plus transient) soil seed banks were investigated at two spatial scales in a very species-rich semi-arid community dominated by annuals. A water addition treatment equivalent to 50% annual increase in average precipitation (abiotic factor) was applied for two consecutive years, and the relationships of the soil seed bank to the biological soil crust (BSC), above-ground vegetation and the presence of Stipa tenacissima tussocks (biotic factors) were simultaneously evaluated. Key Results As expected, the standing vegetation was tightly related to seed abundance, species richness and composition in both seed banks. Remarkably, BSC cover was linked to a decrease in seed abundance and species richness in the persistent seed bank, and it even determined complete seed bank composition at the fine spatial scale. However, this effect disappeared at coarser scales, probably because of the high spatial heterogeneity induced by BSCs. In contrast to findings on standing vegetation, Stipa and the irrigation treatment for two consecutive years had no effect on soil seed banks. Conclusions Soil seed bank assemblies in our semi-arid plant community were the result of above-ground vegetation dynamics and of the direct filtering processes on seed fate operated by the spatially heterogeneous BSCs. Cover of BSCs was negatively correlated with seed abundance and species richness, and affected seed species composition in the soil. Changes in species composition and enrichment when the BSC cover is low suggest that BSCs promote a fine scale niche differentiation in the soil seed bank and thereby potentially enhance species coexistence and high species diversity in these communities. [ABSTRACT FROM AUTHOR]

Biological soil crusts (BSCs) have been recognized as a vital influence factor to desert terrestrial ecosystems under semiarid climate. However, their effects on soil moisture and temperature, which play very important roles in many ecological and hydrological processes, have not yet been well understood. To provide more insight into this issue, we conducted a five-year monitoring experiment for moisture (0–150 cm) and temperature (0–30 cm) of soil with or without moss-dominated BSCs in a semiarid ecosystem on the Loess Plateau of China. The results showed that: (1) the BSCs significantly increased soil moisture by up to 7.6% at 5 cm depth, and significantly decreased soil moisture by up to 3.1%, 6.1%, and 8.1% at 15, 30, and 50 cm depths, respectively; while they had nearly no influence on soil moisture at 70–150 cm depths; (2) the BSCs significantly decreased soil temperature by up to 11.8, 7.5, 5.4, and 3.2 °C at 0, 5, 15, 30 cm depths, respectively, under wet and hot conditions in summer; whereas they significantly increased soil temperature by up to 8.0, 3.7, 2.9, and 1.9 °C, respectively, under dry and cold conditions in winter; and (3) the effects of the BSCs on soil moisture and temperature were significantly correlated with each other, and both of them were significantly driven by solar radiation and precipitation. We concluded that soil moisture and temperature regimes were significantly changed by moss-dominated BSCs in semiarid ecosystems, however, their effects mostly depended on seasons and soil depths. [ABSTRACT FROM AUTHOR]

Biological soil crusts play important ecological functions in arid and semi-arid regions, while different crust successional patterns appeared in different regions. Therefore in this study, the environmental conditions between Shapotou (with cyanobacterial, lichen and moss crusts) and Dalate Banner (with only cyanobacterial and moss crusts) regions of China were compared to investigate why lichen crusts only appeared in Shapotou; at the same time, artificial moss inoculation was conducted to find out the environmental factors promoting crust succession to moss stage. The results showed lichen crusts always developed from cyanobacterial crusts, which provide not only the stable soil surface, but also the biomass basis for lichen formation; furthermore, addition of crust physicochemical characteristics (primarily silt content) play a facilitating effect on lichen emergence ( R 2 = 0.53). The inoculation experiment demonstrated early crust soil surface and enough water holding content (> 4%) provided the essential guarantee for moss germination. Our results show that there is heterogeneity in crust succession in different regions, which may be mainly affected by the ambient soil microenvironments. It is concluded that a positive feedback mechanism is expected between crust succession and ambient soil microenvironments; while a negative feedback mechanism forms between crust succession and free living cyanobacteria and algae. [ABSTRACT FROM AUTHOR]

Many arid and semiarid ecosystems have soils covered with well-developed biological soil crust communities (biocrusts) made up of mosses, lichens, cyanobacteria, and heterotrophs living at the soil surface. These communities are a fundamental component of dryland ecosystems, and are critical to dryland carbon (C) cycling. To examine the effects of warming temperatures on soil C balance in a dryland ecosystem, we used infrared heaters to warm biocrust-dominated soils to 2 °C above control conditions at a field site on the Colorado Plateau, USA. We monitored net soil exchange (NSE) of CO every hour for 21 months using automated flux chambers (5 control and 5 warmed chambers), which included the CO fluxes of the biocrusts and the soil beneath them. We observed measurable photosynthesis in biocrust soils on 12 % of measurement days, which correlated well with precipitation events and soil wet-up. These days included several snow events, providing what we believe to be the first evidence of substantial photosynthesis underneath snow by biocrust organisms in drylands. Overall, biocrust soils in both control and warmed plots were net CO sources to the atmosphere, with control plots losing 62 ± 8 g C m (mean ± SE) over the first year of measurement and warmed plots losing 74 ± 9 g C m. Between control and warmed plots, the difference in soil C loss was uncertain over the course of the entire year due to large and variable rates in spring, but on days during which soils were wet and crusts were actively photosynthesizing, biocrusts that were warmed by 2 °C had a substantially more negative C balance (i.e., biocrust soils took up less C and/or lost more C in warmed plots). Taken together, our data suggest a substantial risk of increased C loss from biocrust soils with higher future temperatures, and highlight a robust capacity to predict CO exchange in biocrust soils using easily measured environmental parameters. [ABSTRACT FROM AUTHOR]

Multi-temporal remote sensing information and spectral indices have been extensively used in studies to monitor ecosystem functioning and surface-energy budgets. However, most of these indices did not show good results in areas covered by sparse vegetation, like most of the Drylands. In these ecosystems, open spaces between plants are often covered by biological soil crusts (biocrusts), i.e. communities of cyanobacteria, algae, microfungi, lichens, mosses and other microorganisms growing in the uppermost millimeters of the soil. Due to their mostly dark color, biocrusts influence the spectral response of dryland surfaces, making it necessary to assess the sensibility of widely used spectral indices to variations in biocrusts cover. In this study we used spectra of biocrusts, bare soil and vegetation to analyze the effect of biocrust cover on the spectral response of heterogeneous areas. In a second approach we investigated the impact of biocrust water status on spectral characteristics. Based on spectral mixture analysis, we calculated the response of a wide range of vegetation/biocrust/bare soil landscape compositions, obtaining a total of 702 spectra. These were used to calculate the Normalized Difference Vegetation Index (NDVI), the Enhanced Vegetation Index (EVI), the Water Index (WI) and surface albedo, and the effect of biocrust cover and water status on these indices was analyzed. Biocrusts exerted a considerable effect on vegetation indices and surface albedo, whereas WI was mostly affected by vegetation type and cover. As biocrust cover increased, the value of NDVI and EVI also increased, whereas albedo decreased, and these effects were more important under low vegetation cover. Moreover, as biocrusts almost immediately turned dark after water pulses, the effect of biocrust cover on spectral indices increased already 30 min after wetting. Although these results varied depending on vegetation type, they demonstrate, that biocrusts largely affect the spectral response of dryland surfaces, and they illustrate how this effect is reinforced by water. Thus, biocrusts need to be considered in studies analyzing dryland phenology, productivity and water status. Moreover, in order to increase the accuracy of hydrological and climate forecast predictions, biocrust effects on surface albedo, both in a dry and wet stage, need to be included. [ABSTRACT FROM AUTHOR]

In almost all dryland systems, biological soil crusts (biocrusts) coexist alongside herbaceous and woody vegetation, creating landscape mosaics of vegetated and biocrusted patches. Results from past studies on the interaction between biocrusts and vascular plants have been contradictory. In the Gurbantunggut desert, a large temperate desert in northwestern China, well-developed lichen-dominated crusts dominate the areas at the base and between the sand dunes. We examined the influence of these lichen-dominated biocrusts on the germination, growth, biomass accumulation, and elemental content of five common plants in this desert: two shrubs ( Haloxylon persicum, Ephedra distachya) and three herbaceous plants ( Ceratocarpus arenarius, Malcolmia africana and Lappula semiglabra) under greenhouse conditions. The influence of biocrusts on seed germination was species-specific. Biocrusts did not affect percent germination in plants with smooth seeds, but inhibited germination of seeds with appendages that reduced or eliminated contact with the soil surface or prevented seeds from slipping into soil cracks. Once seeds had germinated, biocrusts had different influences on growth of shrub and herbaceous plants. The presence of biocrusts increased concentrations of nitrogen but did not affect phosphorus or potassium in tissue of all tested species, while the uptake of the other tested nutrients was species-specific. Our study showed that biocrusts can serve as a biological filter during seed germination and also can influence growth and elemental uptake. Therefore, they may be an important trigger for determining desert plant diversity and community composition in deserts. [ABSTRACT FROM AUTHOR]

Biological soil crust (BSC) communities control many functional processes in arid and semiarid ecosystems, where biological activity is closely influenced by soil wetting. Our goal was to analyze how the length of wetting events and the presence of BSC determine soil variables related to carbon (C) and nitrogen (N) cycling in a semiarid ecosystem. We applied three watering treatments (one, six and ten days) on soils from two microsites (BSC and bare ground) in a microcosm experiment. We analyzed multiple variables related to N and C cycling (N in microbial biomass [MB–N], dissolved organic nitrogen [DON], NH 4 + –N, NO 3 − –N, resin–NH 4 + –N and resin–NO 3 − –N, phenols and carbohydrates). For all treatments, minimum DON concentration appeared earlier in BSC than in bare ground, while maximum NO 3 − peaks appeared earlier in BSC than in the bare ground. Increases of NH 4 + were only perceptible under BSC. Our results showed that longer lasting wetting events kept higher mineral and organic N as well as labile organic C in soils under BSC, which suggest that longer wetting events may be related to an enhancement in the decomposition rate that compensate for nutrient losses associated to short lasting wetting events. This trend is much less obvious in bare ground than under BSC. Our data suggest that changes in the length of wetting events and the presence of BSC with climate changes could alter future soil community structure and function. [ABSTRACT FROM AUTHOR]

In dryland ecosystems, runoff is mainly generated in bare areas, which are also more susceptible to water erosion than vegetated areas. These bare areas are often covered and protected by biological soil crusts (BSCs), which modify numerous physicochemical surface properties involved in runoff and erosion processes. BSCs are considered as one of the most important stabilizing factors in the soil surface, but most previous research has concentrated only on patch or hillslope-scale effects of BSCs, and their effect at coarser scales has rarely been studied. In this article, we present a new approach based on previous surface cover quantification for including the effects of BSCs in physically-based runoff and erosion modeling. The Limburg Soil Erosion Model (LISEM) was used to parameterize and simulate the effects of BSCs on runoff and erosion in a small semiarid catchment characterized by fine-textured soils and predominantly covered by BSCs. Paired model simulations under two scenarios, with and without including the effects of BSCs, were run under different rainfall intensities to evaluate the effect of BSCs on runoff and erosion under different rainfall conditions. Runoff and erosion rates recorded in the field at the catchment outlet were predicted much more accurately when BSCs were included because there was less overestimation of runoff rate, maximum runoff peaks and erosion rates in the areas dominated by BSCs. The proposed approach enables BSCs to be included in spatially distributed runoff and erosion models, improving their predictions, and may be used for evaluating how the effect of human activity on BSCs affects catchment-scale water erosion. [ABSTRACT FROM AUTHOR]

Soil sealing and crusting are common, widespread processes in arid and semiarid soils that strongly affect runoff and erosion. In these soils, biological soil crusts (BSCs) are an essential surface component known to have an important role in water processes, and are also strong erosion protection agents. However, these crusts are also highly vulnerable to disturbances, which seriously affect their pedological and ecological roles. The aims of this study were to examine whether penetration resistance (PR) depends on BSC developmental stage and how removal of BSCs affects PR, under dry and wet soil conditions, and to analyse the relationship between PR and runoff and soil detachment in these soils. This research was conducted in two semiarid ecosystems representative of crusting processes in SE Spain (El Cautivo and Las Amoladeras, Almeria province). Four pairs of plots containing the main BSC types identified at both sites were selected and two treatments were considered, one in each plot in the pair: i) unaltered BSC and ii) BSC removal by scraping. Two intense rainfall simulation experiments were performed one after another in these plots. Runoff was measured during both, and water samples were collected for further determination of sediment yield. PR was measured in undisturbed BSC and scalped soil on dry soil prior to rainfall, on wet soil after the first and second rainfalls, and on dry soil seven months after BSC removal. Crust PR was higher at Las Amoladeras (3.13 ± 0.54 kg cm − 2 ) than at El Cautivo (1.52 ± 0.74 kg cm − 2 ) due to soil compaction from trampling by livestock in the former. PR varied with BSC development, with less developed cyanobacteria BSCs showing higher resistance (under dry and wet conditions) than more developed lichen BSCs. BSC removal was followed by the formation of a new seal (physical crust) upon raining, which however, did not result in increased PR. No significant relationship was found between PR and runoff, but there was a weak negative relationship between PR and soil detachment, because in BSCs, which had higher PRs, soil detachment rates were much lower than in recently scalped soils. Seven months after BSC removal, PR was significantly higher in scalped soils than for undisturbed BSCs. Our results show that PR was not a good indicator of hydrological and soil detachment response of BSCs or the soil sealing during rain immediately after BSC removal, but it was a good indicator of physical soil crust formation in scalped soils over time. [ABSTRACT FROM AUTHOR]

Biological soil crusts (biocrusts) occur across most of the world's drylands and are sensitive indicators of dryland degradation. Accounting for shifts in biocrust composition is important for quantifying integrity of arid and semi-arid ecosystems, but the best methods for assessing biocrusts are uncertain. We investigate the utility of surveying biocrust morphogroups, a reduced set of biotic classes, compared to species data, for detecting shifts in biocrust composition and making inference about dryland degradation., We used multivariate regression tree (MRT) analyses to model morphogroup abundance, species abundance and species occurrence data from two independent studies in semi-arid open woodlands of south-eastern Australia. We advanced the MRT method with a 'best subsets' model selection procedure, which improved model stability and prediction., Biocrust morphogroup composition responded strongly to surrogate variables of ecological degradation. Further, MRT models of morphogroup data had stronger explanatory power and predictive power than MRT models of species abundance or occurrence data. We also identified morphogroup indicators of degraded and less degraded sites in our study region., Synthesis and applications. Sustainable management of drylands requires methods to assess shifts in ecological integrity. We suggest that biocrust morphogroups are highly suitable for assessment of dryland integrity because they allow for non-expert, rapid survey and are informative about ecological function. Furthermore, morphogroups were more robust than biocrust species data, showed a strong response to ecological degradation and were less influenced by environmental variation, and models of morphogroup abundance were more predictive. [ABSTRACT FROM AUTHOR]

How soil cover types and rainfall patterns influence carbon (C) release in temperate desert ecosystems has largely been unexplored. We removed intact crusts down to 10 cm from the Shapotou region, China, and measured them in PVC mesocosms, immediately after rainfall. C release rates were measured in soils with four cover types (moss-crusted soil, algae-crusted soil, mixed (composed of moss, algae, and lichen)-crusted soil, and mobile dune sand). We investigated seven different rainfall magnitudes (0-1, 1-2, 2-5, 5-10, 10-15, 15-20, and >20 mm) under natural conditions. C release from all four BSCs increased with increasing rainfall amount. With a rainfall increase from 0 to 45 mm, carbon release amounts increased from 0.13 ± 0.09 to 15.2 ± 1.35 gC m in moss-crusted soil, 0.08 ± 0.06 to 6.43 ± 1.23 gC m in algae-crusted soil, 0.11 ± 0.08 to 8.01 ± 0.51 gC m in mixed-crusted soil, and 0.06 ± 0.04 to 8.47 ± 0.51 gC m in mobile dune sand, respectively. Immediately following heavy rainfall events (44.9 mm), moss-crusted soils showed significantly higher carbon release rates than algae- and mixed-crusted soils and mobile dune sands, which were 0.95 ± 0.02, 0.30 ± 0.03, 0.13 ± 0.04, and 0.51 ± 0.02 μmol CO m s, respectively. Changes in rainfall patterns, especially large rain pulses (>10 mm) affect the contributions of different soil cover types to carbon release amounts; moss-crusted soils sustain higher respiration rates than other biological crusts after short-term extreme rainfall events. [ABSTRACT FROM AUTHOR]

Abstract: Biological soil crusts (BSCs) are extensively developed and commonly regarded as a kind of vegetation in desertification areas around the world. The natural recovery process of BSCs after disturbance and their long-term impacts on the soil water conditions are important but not well understood. In order to provide more insights into this problem, we set up two treatments including BSCs (natural BSCs without disturbance) and disturbed BSCs (the top 30mm of surface soil, including the BSC layer, was severely disturbed and completely removed) in a semi-arid environment on the Loess Plateau of China. Over the succeeding years, the natural recovery process of BSCs was qualitatively described and the soil water content at 0–90cm depth of the two treatments was consecutively monitored. The results showed the following: (1) it is possible to recover natural moss-dominated BSCs after severe disturbance under natural conditions, and the recovery process to BSC full-coverage took approximately three years; (2) the BSC disturbance greatly decreased soil water content by up to 18% and the effects gradually weakened with time; (3) the BSC disturbance decreased surface soil water content (0–70cm) by up to 24% but increased deep soil water content (80–90cm) by up to 13%; and (4) the BSC disturbance decreased soil water storage at 0–90cm by 7.8mm, 4.4mm, 8.0mm, and 4.9mm in the second, third, fourth, and seventh years, respectively. We concluded that the BSC disturbance degraded soil water conditions in the three to four years following the disturbance. Therefore, the artificial destruction of natural moss-dominated BSCs in a semi-arid region on the Loess Plateau of China should not be recommended as a land management practice for the improvement of soil water conditions. [Copyright &y& Elsevier]

In arid and semiarid areas of the planet, biological soil crusts (BSCs) participate in functions critical to the ecosystem sustainability and are usually located in inter-vegetation spaces. These crusts are structurally and in terms of biomass dominated by mosses and lichens. Associated with them, a great variety of fungi, bacteria, and cyanobacteria have been found. Despite their ecological importance, major details of the functionalities of the microbial communities inhabiting BSCs and the underlying soil are still missing. Here, we explore the biomass, functionality, and protein-based phylogeny of the microbial communities associated with lichen-dominated BSCs under a semiarid climate. Two lichen-dominated BSCs (Fulgensia desertorum -dominated BSC, F-BSC; and Squamarina cartilaginea -dominated BSC, S-BSC) were sampled in a semiarid soil located in South-East Spain. Samples of the BSCs and the underlying soils were collected. The biomass and activity of the microbial community in both BSCs was higher than in the soil underlying them, as estimated by their PLFA content, basal respiration, and enzyme activities. The factor analysis of PLFAs and the putative phylogenetic analysis of the identified proteins indicated scarce differences in the composition of the microbial communities of the two biocrusts at the phylum level. In contrast, community-level physiological profiles (CLPPs) revealed that the microbial communities inhabiting BSCs metabolized substrates differently. The identification of ribulose-1,5 biphosphate carboxylase oxygenase (RuBisCO) peptides points to a specialization in carbon fixation which is carried out by some cyanobacterial species in F-BSC, such as Prochlorothrix hollandica and Microcoleus vaginatus, but not in S-BSC. The structure of the microbial community in the soil underlying each BSC was different, as estimated by factor analysis of PLFAs, but such differences did not correspond to changes in the functional structure, as indicated by the CLPPs. The multidisciplinary approach used in this study revealed remarkable differences between the microbial communities inhabiting F. desertorum and S. cartilaginea -dominated biological crusts, whereas the microbial communities in the soils underlying the two crusts behaved similarly. [ABSTRACT FROM AUTHOR]

Robust estimation of soil moisture using microwave remote sensing depends on extensive ground sampling for calibration and validation of the data. Soil surface sealing is a frequent phenomenon in dry environments. It modulates soil moisture close to the soil surface and, thus, has the potential to affect the retrieval of soil moisture from microwave remote sensing and the validation of these data based on ground observations. We addressed this issue using a physically-based modeling approach that accounts explicitly for surface sealing at the hillslope scale. Simulated mean soil moisture at the respective layers corresponding to both the ground validation probe and the radar beam's typical effective penetration depth were considered. A cyclic pattern was found in which, as compared to an unsealed profile, the seal layer intensifies the bias in validation during rainfall events and substantially reduces it during subsequent drying periods. The analysis of this cyclic pattern showed that, accounting for soil moisture dynamics at the soil surface, the optimal time for soil sampling following a rainfall event is a few hours in the case of an unsealed system and a few days in the case of a sealed one. Surface sealing was found to increase the temporal stability of soil moisture. In both sealed and unsealed systems, the greatest temporal stability was observed at positions with moderate slope inclination. Soil porosity was the best predictor of soil moisture temporal stability, indicating that prior knowledge regarding the soil texture distribution is crucial for the application of remote sensing validation schemes. [ABSTRACT FROM AUTHOR]

Abstract: Biological soil crusts (biocrusts) are ubiquitous in drylands globally. Lichens and mosses are essential biocrust components and provide a variety of ecosystem services, making their conservation and management of interest. Accordingly, understanding what factors are correlated with their distribution is important to land managers. We hypothesized that cover would be related to geologic and pedologic factors. We sampled 32 sites throughout the eastern Mojave Desert, stratifying by parent material and the age of the geomorphic surfaces. The cover of lichens and mosses on ‘available ground’ (L + Mav; available ground excludes ground covered by rocks or plant stems) was higher on limestone and quartzite-derived soils than granite-derived soils. Cover was also higher on moderately younger-aged geomorphic surfaces (Qya2, Qya3, Qya4) and cutbanks than on very young (Qya1), older-aged surfaces (Qia1, Qia2), or soils associated with coppice mounds or animal burrowing under Larrea tridentata. When all sites and parent materials were combined, soil texture was the most important factor predicting the occurrence of L + Mav, with cover positively associated with higher silt, very fine sand, and fine sand fractions and negatively associated with the very coarse sand fraction. When parent materials were examined separately, nutrients such as available potassium, iron, and calcium became the most important predictors of L + Mav cover. [Copyright &y& Elsevier]

In arid and semiarid regions, water infiltration is often affected by the soil surface conditions, such as the soil crusts and soil roughness. The objective of this laboratory study was to assess the effects of soil crusts and tillage treatments on soil infiltration in the Loess Plateau of China. A simulated rainfall storm at 80 mm h−1rate was applied to soil boxes set to slopes of 5°, 10°, 15° and 20° with two soil surface conditions (crusted and uncrusted) and three tillage treatments (contour tillage [CT], artificial digging [AD] and straight slope [SS]) to investigate the infiltration rates. The results show that the infiltration rate was always lower under the SS treatment than under the CT and AD treatments. The time interval to the steady state was shorter under the SS (10 min) than under the CT and AD treatments (15–20 min). The final infiltration rate was nearly the same under the CT treatment and AD treatment; whereas, the SS treatment resulted in the lowest final infiltration rate. The infiltration rates were always greater in soils without crusts than in soils with crusts. The slopes, soil crusts and tillage treatments had significant (p< 0.001) effects on the cumulative infiltration rates. The combined effects of the slopes and tillage treatments on the cumulative infiltration rates were much more significant (p< 0.01) than the other combined effects (p< 0.05). [ABSTRACT FROM AUTHOR]

Biological soil crusts (BSCs) cover >35% of the Earth’s land area and contribute to important ecological functions in arid and semiarid ecosystems, including erosion reduction, hydrological cycling, and nutrient cycling. Artificial rapid cultivation of BSCs can provide a novel alternative to traditional biological methods for controlling soil and water loss such as the planting of trees, shrubs, and grasses. At present, little is known regarding the cultivation of BSCs in the field due to lack of knowledge regarding the influencing factors that control BSCs growth. Thus, we determined the effects of various environmental factors (shade; watering; N, P, K, and Ca concentrations) on the growth of cyanobacteria-dominated BSCs from the Sonoran Desert in the southwestern United States. The soil surface changes and chlorophyll a concentrations were used as proxies of BSC growth and development. After 4 months, five factors were found to impact BSC growth with the following order of importance: NH4NO3 ≈ watering frequency>shading>CaCO3 ≈ KH2PO4. The soil water content was the primary positive factor affecting BSC growth, and BSCs that were watered every 5 days harbored greater biomass than those watered every 10 days. Groups that received NH4NO3 consistently exhibited poor growth, suggesting that fixed N amendment may suppress BSC growth. The effect of shading on the BSC biomass was inconsistent and depended on many factors including the soil water content and availability of nutrients. KH2PO4 and CaCO3 had nonsignificant effects on BSC growth. Collectively, our results indicate that the rapid restoration of BSCs can be controlled and realized by artificial “broadcasting” cultivation through the optimization of environmental factors. [ABSTRACT FROM AUTHOR]

Soil microbial communities in dryland ecosystems play important roles as root associates of the widely spaced plants and as the dominant members of biological soil crusts (biocrusts) colonizing the plant interspaces. We employed rRNA gene sequencing (bacterial 16S/fungal large subunit) and shotgun metagenomic sequencing to compare the microbial communities inhabiting the root zones of the dominant shrub, Larrea tridentata (creosote bush), and the interspace biocrusts in a Mojave desert shrubland within the Nevada Free Air CO2 Enrichment (FACE) experiment. Most of the numerically abundant bacteria and fungi were present in both the biocrusts and root zones, although the proportional abundance of those members differed significantly between habitats. Biocrust bacteria were predominantly Cyanobacteria while root zones harbored significantly more Actinobacteria and Proteobacteria. Pezizomycetes fungi dominated the biocrusts while Dothideomycetes were highest in root zones. Functional gene abundances in metagenome sequence datasets reflected the taxonomic differences noted in the 16S rRNA datasets. For example, functional categories related to photosynthesis, circadian clock proteins, and heterocyst-associated genes were enriched in the biocrusts, where populations of Cyanobacteria were larger. Genes related to potassium metabolism were also more abundant in the biocrusts, suggesting differences in nutrient cycling between biocrusts and root zones. Finally, ten years of elevated atmospheric CO2 did not result in large shifts in taxonomic composition of the bacterial or fungal communities or the functional gene inventories in the shotgun metagenomes. [ABSTRACT FROM AUTHOR]

Highlights: [•] Biological soil crusts (BSCs) decreased soil temperature by up to 11.8°C in hot and wet conditions in summer. [•] BSCs increased soil temperature by up to 1.2°C in cold and dry conditions in winter. [•] BSCs effects on soil temperature were positively correlated with air temperature and soil moisture. [•] BSCs effects on soil temperature decreased with soil depth. [•] BSCs effects on soil temperature are positive for decreasing susceptibility to desertification in semiarid ecosystem. [ABSTRACT FROM AUTHOR]

The well-developed biological soil crusts cover up to 40% of the soil surface in the alpine and subnival zones of the Tibetan Plateau, accounting for a vast area of Asia. We investigated the diversity and biomass of the phototrophic part ( Cyanobacteria) of the microbial community inhabiting biological soil crusts and uncrusted soils in their surroundings on the elevation gradient of 5200-5900 m a.s.l. The influence of soil physico-chemical properties on phototrophs was studied. The ability of high-altitude phototrophs to fix molecular nitrogen was also determined under laboratory conditions. The biological soil crust phototroph community did not differ from that living in uncrusted soil in terms of the species composition, but the biomass is three-to-five times higher. An increasing trend in the cyanobacterial biomass from the biological soil crusts with elevation was observed, with the genera Nostoc spp., Microcoleus vaginatus and Phormidium spp. contributing to this increase. Based on the laboratory experiments, the highest nitrogenase activity was recorded in the middle elevations, and the rate of nitrogen fixation was not correlated with the cyanobacterial biomass. [ABSTRACT FROM AUTHOR]

Abstract: Water redistribution has a profound influence on dryland ecosystem function. This hydrological function is largely regulated by ecosystem engineers including biological soil crusts (biocrusts) which produce run-off, and burrowing animals, such as the greater bilby, whose pits capture water. We estimated the relative importance of these two ecosystem engineers in determining infiltration rates in a system where dune slopes shed water to adjacent interdune swales to maximize overall productivity. Also, we determined which biocrust property was most hydrologically important: total cover, composition, patch aggregation or spatial heterogeneity. While both biocrusts and burrowing animals equally affected the overall infiltration through macro- and micropores (under ponding), only biocrusts were important for the infiltration specifically via micropores (under tension). Of the studied biocrust properties, community composition was the strongest influence such that the greater the prevalence of early successional biocrust patches, the greater the infiltration rate. Greater total cover of biocrusts reduced infiltration, and the spatial properties were relatively unimportant. Although bilbies and biocrusts comparably influenced infiltration under ponding at the microscale, realistic cover of bilby pits at the landscape scale is unlikely to strongly impair the hydrological function of dunes. Reintroduction of the endangered bilby may enhance nutrient cycling and plant recruitment via its seed and resource capturing pits, without a concomitant disruption of hydrological function. In contrast, removal of biocrusts caused by, e.g., livestock trampling, is expected to strongly enhance infiltration in the run-off areas, strongly reducing ecosystem productivity at the landscape scale. [Copyright &y& Elsevier]

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]

Abstract: Invasion by the exotic annual grass Bromus tectorum has increased the cover and connectivity of fine litter in the sagebrush steppes of western North America. This litter tends to cover biological soil crusts, which could affect their metabolism and growth. To investigate this possible phenomenon, biological soil crusts dominated by either the moss Bryum argenteum or the lichen Diploschistes muscorum were covered with B. tectorum litter (litter treatment) or left uncovered (control treatment) and exposed to natural field conditions. After periods of five and ten months, we removed the litter and compared the photosynthetic performance of biological soil crusts from the two treatments. Litter induced photosynthetic changes in our samples. In both B. argenteum and D. muscorum, biological soil crusts that had been covered with litter for ten months had lower rates of gross photosynthesis and lower chlorophyll content than control samples. Similarly in both biological soil crust types, litter reduced the rate of dark respiration. For D. muscorum, the reduction in dark respiration fully compensated for the decrease in gross photosynthesis, resulting in similar values of net photosynthesis in the two treatments. In contrast, for B. argenteum, net photosynthesis was four-times greater in the control than the litter treatment. Also under litter cover, D. muscorum showed three common adaptations to shade conditions: a decrease in the light compensation point, in the light intensity needed to achieve 95% of maximal net photosynthesis, and in the chlorophyll a/b ratio. None of these changes was apparent in B. argenteum. Overall, our results indicate that photosynthetic responses to the presence of litter varied among species of the crust biota and that the litter can reduce the photosynthetic capacity of biological soil crusts. These results help to explain field observations of decreases in biological soil crust cover and changes in biological soil crust composition with increases in litter cover, and suggest that the landscape-wide invasion by B. tectorum may have substantial effects on biological soil crust performance and therefore their capacity to function in semiarid shrublands. [Copyright &y& Elsevier]

Aims: Biological soil crusts (BSCs), composed of mosses, lichens, liverworts and cyanobacteria, are a key component of arid and semi-arid ecosystems worldwide, and play key roles modulating several aspects of the nitrogen (N) cycle, such as N fixation and mineralization. While the performance of its constituent organisms largely depends on moisture and rainfall conditions, the influence of these environmental factors on N transformations under BSC soils has not been evaluated before. Methods: The study was done using soils collected from areas devoid of vascular plants with and without lichen-dominated BSCs from a semi-arid Stipa tenacissima grassland. Soil samples were incubated under different temperature (T) and soil water content (SWC) conditions, and changes in microbial biomass-N, dissolved organic nitrogen (DON), amino acids, ammonium, nitrate and both inorganic N were monitored. To evaluate how BSCs modulate the resistance of the soil to changes in T and SWC, we estimated the Orwin and Wardle Resistance index. Results: The different variables studied were more affected by changes in T than by variations in SWC at both BSC-dominated and bare ground soils. However, under BSCs, a change in the dominance of N processes from a net nitrification to a net ammonification was observed at the highest SWC, regardless of T. Conclusions: Our results suggest that the N cycle is more resistant to changes in T in BSC-dominated than in bare ground areas. They also indicate that BSCs could play a key role in minimizing the likely impacts of climate change on the dynamics of N in semi-arid environments, given the prevalence and cover of these organisms worldwide. [ABSTRACT FROM AUTHOR]

Abstract: Many soils in arid and semi-arid regions of the world are affected by crusting, a process by which a compact layer or thin mantle of consolidated material is formed at the soil surface. Crusts can increase the resiliency of the soil to wind erosion, but characterization of soil crust formation to light precipitation events common to the Columbia Plateau region of the Inland Pacific Northwest is unknown. Our objective was to evaluate the effect of light rainfall events on crust formation of five soil types prominent in the Columbia Plateau. The five soils were Athena silt loam, Palouse silt loam, Ritzville silt loam, Walla Walla silt loam, and Warden sandy loam. Soil crusts were formed in the laboratory using a rainfall simulator that applied water at a rate of 2.5mm h−1 for various durations to achieve a total rainfall application of 0, 0.15, 0.30, 0.60 and 1.0mm. Crust strength was measured by a penetrometer while crust thickness was measured by a ruler. Silt and clay content of the five soils ranged from 23 to 66% and from 9 to 17%, respectively. An increase in crust thickness and strength was observed with an increase in rainfall for all soils. Crust thickness was similar among the soils whereas crust strength varied among soils at any given level of rainfall. For example, crust strength of soils when subject to 0.15–0.60mm of rainfall decreased in order of Walla Walla>Athena and Palouse>Ritzville and Warden. These results are consistent with a decreasing order of clay and silt contents. The strongest crust was formed on Palouse silt loam under the highest rainfall amount. A logarithm relationship adequately described the relationship between crust thickness and rainfall for the five soils (r 2 =1.00). This relationship over-predicted crust thickness based upon observations in the field, but performed better than the algorithm used by the Wind Erosion Prediction System. [Copyright &y& Elsevier]

Abstract: The natural landscapes of semiarid areas worldwide comprise a series of scattered patches of shrubs within a matrix of biologically crusted soils (BSC). As BSCs are considered ecosystem engineers the relationships between the BSC and the shrub patches determine system functioning. The objective of our study was to investigate long-term effects of agricultural practices on biological soil crusts and their influence on hydrological aspects of a semiarid ecosystem. During 1991, we experimentally simulated five of the area''s agricultural practices; 1) Scraping – the topsoil was removed to a depth of 2 cm, 2) Spraying – phototrophic organisms were chemically killed with herbicide, 3) Mowing – perennial vegetation was cut and spread to simulate grazing practices, 4) Car track – a heavy roller was used to simulate car-tracks, 5) Control – undisturbed natural plots. Sixteen years later, in 2007, these agricultural practices were found to have a long-term effect on the crusted soil surface and the related soil-surface properties. Mowing and car-track treatments led to decreased overland runoff and increased hydraulic conductivity, whereas scraping and spraying treatments led to increased overland runoff production and decreased hydraulic conductivity. We conclude that the practices had a long-term residual impact on BSC succession and related soil surface properties, which affected the hydrological processes and system functioning. [Copyright &y& Elsevier]

Biological soil crusts (biocrusts) cover soil surfaces in many drylands globally. The impacts of 10 years of elevated atmospheric CO2 on the cyanobacteria in biocrusts of an arid shrubland were examined at a large manipulated experiment in Nevada, USA. Cyanobacteria-specific quantitative PCR surveys of cyanobacteria small-subunit ( SSU) rRNA genes suggested a reduction in biocrust cyanobacterial biomass in the elevated CO2 treatment relative to the ambient controls. Additionally, SSU rRNA gene libraries and shotgun metagenomes showed reduced representation of cyanobacteria in the total microbial community. Taxonomic composition of the cyanobacteria was similar under ambient and elevated CO2 conditions, indicating the decline was manifest across multiple cyanobacterial lineages. Recruitment of cyanobacteria sequences from replicate shotgun metagenomes to cyanobacterial genomes representing major biocrust orders also suggested decreased abundance of cyanobacteria sequences across the majority of genomes tested. Functional assignment of cyanobacteria-related shotgun metagenome sequences indicated that four subsystem categories, three related to oxidative stress, were differentially abundant in relation to the elevated CO2 treatment. Taken together, these results suggest that elevated CO2 affected a generalized decrease in cyanobacteria in the biocrusts and may have favoured cyanobacteria with altered gene inventories for coping with oxidative stress. [ABSTRACT FROM AUTHOR]

Abstract: The ecology and functional role of biological soil crusts (BSCs) in arid and semi-arid zones have been extremely well studied. However, little is known about the biochemical properties related to the number and activity of the microbiota that form the crusts, even though information about these properties is very important for understanding many of the processes that affect the formations. In this study, several properties related to the activity and number of microorganisms (biomass-C, basal respiration, dehydrogenase activity and nitrogen mineralization potential) were determined at different depths (crusts, 0–0.5 cm; middle, 0.5–3 cm and deep, 3–5 cm layers) in two types of crusts (predominated by cyanobacteria and by lichens) in the Tabernas desert (Almeria, SE Spain). The absolute values of the above-mentioned properties and the values expressed relative to the total organic carbon (TOC) content were both much higher in the crust layers than in the surface horizons of soils under Mediterranean or Atlantic climates. A large part of the TOC in the BSCs was contained in the microbiota and another large part was readily metabolized during incubation of the crusts for 10 days at 25 °C. The net nitrogen mineralization rate was also high, and ammonification predominated in the crust layers, whereas nitrification predominated in the middle and deep layers. In all types of BSCs, the microbiota colonized the deep layers, although with greater intensity in the lichen-dominated BSCs than in the cyanobacterial BSCs. The results also indicate that hydrolytic enzymes are not stabilized on soil colloids and their activity depends only on the active microbiota. [Copyright &y& Elsevier]

Abstract: Arid and semiarid crusted ecosystems occupy a large extent of the Earth''s surface. In these ecosystems there is a delicate balance between C sequestration and biodegradation that could easily be altered due to human disturbance or global warming. These environments are characterized by the presence of biological soil crusts (BSCs) coexisting with shrubs in the interspaces. BSCs play an important role in the C-cycle in arid and semiarid areas, but there has been little research regarding C-storage in crusted soils. In this research, representative BSCs were studied in three different crusted semiarid ecosystems in Southern Spain. Chemical fractionation and characterization of the organic matter in BSCs and underlying soils were undertaken to compare the total amount and quality of humic substances and find surrogate indicators of the soil organic matter quality. After isolating the major organic fractions (particulate fraction, humic acid-like (HA) and fulvic acid (FA)), the macromolecular, HA fraction was studied by derivative visible spectroscopy and resolution-enhanced infrared (IR) spectroscopy. Our results show quantitative differences in organic matter fractions, in agreement with the structural characteristics of the HA-type substances. The more stable crusted ecosystem with more evolved vegetation cover tend to be associated with HAs with broadband IR profiles, high optical density and macromolecular condensation and presence of polycyclic fungal biomarker compounds. In contrast, in the crusted ecosystem with very low or almost absent vascular vegetation cover, the C sequestration depends almost exclusively on the activity of crust-inhabiting organisms. It is suggested that the organic matter in BSCs shares characteristics with aquatic humic matter derived from non-lignified plants, including structurally complex HA-type substances which may have been originated by abiotic condensation of unsaturated lipids and/or diagenetic alteration of aliphatic biomacromolecules. Although these humification mechanisms have not been extensively studied in terrestrial soils they may be very active mechanisms for C sequestration in some types of crusted ecosystems. In particular, the spectroscopic characteristics in the visible and IR ranges of the crust-isolated HA-type substances provide the above-indicated biogeochemical proxies informing on the organic matter stability and quality and their bearing on the potential of crusted ecosystems for maintaining their properties after external disturbance. [Copyright &y& Elsevier]

Dryland vegetation is inherently patchy. This patchiness goes on to impact ecology, hydrology, and biogeochemistry. Recently, researchers have proposed that dryland vegetation patch sizes follow a power law which is due to local plant facilitation. It is unknown what patch size distribution prevails when competition predominates over facilitation, or if such a pattern could be used to detect competition. We investigated this question in an alternative vegetation type, mosses and lichens of biological soil crusts, which exhibit a smaller scale patch-interpatch configuration. This micro-vegetation is characterized by competition for space. We proposed that multiplicative effects of genetics, environment and competition should result in a log-normal patch size distribution. When testing the prevalence of log-normal versus power law patch size distributions, we found that the log-normal was the better distribution in 53% of cases and a reasonable fit in 83%. In contrast, the power law was better in 39% of cases, and in 8% of instances both distributions fit equally well. We further hypothesized that the log-normal distribution parameters would be predictably influenced by competition strength. There was qualitative agreement between one of the distribution's parameters (μ) and a novel intransitive (lacking a 'best' competitor) competition index, suggesting that as intransitivity increases, patch sizes decrease. The correlation of μ with other competition indicators based on spatial segregation of species (the C-score) depended on aridity. In less arid sites, μ was negatively correlated with the C-score (suggesting smaller patches under stronger competition), while positive correlations (suggesting larger patches under stronger competition) were observed at more arid sites. We propose that this is due to an increasing prevalence of competition transitivity as aridity increases. These findings broaden the emerging theory surrounding dryland patch size distributions and, with refinement, may help us infer cryptic ecological processes from easily observed spatial patterns in the field. [ABSTRACT FROM AUTHOR]

Abstract: Biological soil crusts (BSCs) are important sources of carbon input to ecosystems in arid and semiarid regions, where vascular plants are restricted by the rigorous environment and limited soil water. Sand dune stabilization by revegetation can enhance colonization and development of BSCs on sandy soil surfaces, and convert BSCs from the early successional stage dominated by cyanobacteria and algae to the later stage dominated by lichens and mosses. This study estimated the carbon fixation by two successional stages of BSCs based on four years of field observations. Carbon fixation by two BSCs has been compared via estimating daily carbon fixation using measuring net photosynthesis with ten sampling plots per crust stage in situ, combining with calculating the wet daytime of crustal cryptogam thallus. However, diurnal carbon fixation of both BSCs was largely determined by crustal water content rather than photosynthesis photon flux and temperature. The range of optimal gravimetric water content for early BSCs was 1–3.5%, and 1–5% for the later BSCs. The annual carbon fixation was 11.36gCm−2 yr−1 for cyanobacteria–algae dominated crusts and 26.75gCm−2 yr−1 for lichen–moss dominated crusts. The latter had a higher carbon input due to a higher water-holding capacity, prolonging wet daytime and higher chlorophyll content, as well as higher light capture. These findings indicate the recovery of BSCs is expected to significantly increase carbon input into sandy desert ecosystems. [Copyright &y& Elsevier]

Abstract: Biological soil crusts (BSCs) cover up to 70% of the sparsely-vegetated open spaces in semiarid and arid regions throughout the world and fulfill a wide range of ecological functions. Previous investigations on BSCs mainly focused on the taxonomic or genetic diversity of their components and physical properties, while little attention was given to the functional diversity of microbial community, which plays crucial roles in carbon cycling. In order to distinguish between microbial functional diversity in BSCs with different developmental levels, MicroResp™ plates were used to determine the utilization rate of fifteen added carbon sources. Five types of crusts, A–E, were taken from the western Negev Desert (Israel) during the morning hours of March 2011. Abiotic and biotic variables were measured. The well-developed high-biomass BSCs exhibited pronounced increases in their chlorophyll a and organic-matter content. Higher activity and biomass of microbial communities, as well as higher density of viable microfungi, were also found in the well-developed BSCs. Marked increases in the utilization rates of aromatic acid, carbohydrates and carboxylic acid were consistent with the well-developed crusts. Although the utilization rates of the four carbon groups followed the order: carboxylic acid > amino acid > carbohydrates and aromatic acid, the redundancy analysis revealed that crust A preferred consuming amino acids while crusts C and E preferred carboxylic acids and carbohydrates. No significant differences in functional diversity were observed between BSCs with different developmental levels. More carbon sources, as well as molecular and stable isotopic approaches, should be used in further studies. [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]

The study of species coexistence and community assembly has been a hot topic in ecology for decades. Disentangling the hierarchical role of abiotic and biotic filters is crucial to understand community assembly processes. The most critical environmental factor in semi-arid environments is known to be water availability, and perennials are usually described as nurses that create milder local conditions and expand the niche range of several species. We aimed to broaden this view by jointly evaluating how biological soil crusts (BSCs), water availability, perennial species (presence/absence of Stipa tenacissima) and plant-plant interactions shape a semi-arid annual plant community. The presence and cover of annual species was monitored during three years of contrasting climate. Water stress acted as the primary filter determining the species pool available for plant community assembly. Stipa and BSCs acted as secondary filters by modulating the effects of water availability. At extremely harsh environmental conditions, Stipa exerted a negative effect on the annual plant community, while at more benign conditions it increased annual community richness. Biological soil crusts exerted a contradictory effect depending on climate and on the presence of Stipa, favoring annuals in the most adverse conditions but showing repulsion at higher water availability conditions. Finally, interactions among co-occurring annuals shaped species richness and diversity of the final annual plant assembly. This study sheds light on the processes determining the assembly of annual communities and highlights the importance of Biological Soil Crusts and of interactions among annual plants on the final outcome of the species assembly. [ABSTRACT FROM AUTHOR]

Abstract: Dictating the length of time during which water is available, evaporation plays a cardinal role in ecosystem ecology and structure. This is especially so in arid and semi arid zones, where precipitation is highly limited. Covering the surface with a distinct 1–10cm thick layer, biocrusts (called also biological soil crusts or microbiotic crusts) may affect the evaporation rate and hence the water loss of the underlying parent material. Owing to their wide occurrence in arid and semi arid regions, examination of their impact upon the evaporation rate of the underlying soil is of special importance. The evaporation of water from sand covered by four types of cyanobacterial crusts and one moss-dominated crust in the Negev Desert was examined along with bare sand. The crusts and sand were subjected to wetting by rain and sprinkling (in Petri dishes and within 25cm diameter plots), and the crust and subcrust temperatures and moisture content were measured. In addition the albedo of the crusts was also measured. Whereas thick and high-biomass crusts tended to retain higher amounts of water in the Petri dishes, they were found to increase subsurface temperatures (by up to 3.9°C at 5cm depth) under field conditions, subsequently decreasing the 0–10cm moisture content. Moisture content at the crusted plots was significantly lower than that of non-crusted plot, with crusted plots having an evaporation rate of 1.24–1.57 (with average of 1.38) times higher than the bare sand. These findings were supported by field measurements under natural rain precipitation. In both cases, the field data under sprinkling and rain precipitation showed that the non-crusted surfaces retained their available moisture at the upper 10cm of the soil profile for 5–7days longer than that of the crusted surfaces. The findings may explain contradictory data published in the literature regarding field and lab experiments, and may point to the negative effect that biocrusts may have upon the soil water budget and hence on water availability for higher plants. [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]

While microbial communities of aerosols have been examined, little is known about their sources. Nutrient composition and microbial communities of potential dust sources, saline lake sediments ( SLS) and adjacent biological soil crusts ( BSC), from Southern Australia were determined and compared with a previously analyzed dust sample. Multivariate analyses of fingerprinting profiles indicated that the bacterial communities of SLS and BSC were different, and these differences were mainly explained by salinity. Nutrient concentrations varied among the sites but could not explain the differences in microbial diversity patterns. Comparison of microbial communities with dust samples showed that deflation selects against filamentous cyanobacteria, such as the Nostocales group. This could be attributed to the firm attachment of cyanobacterial filaments to soil particles and/or because deflation occurs mainly in disturbed BSC, where cyanobacterial diversity is often low. Other bacterial groups, such as Actinobacteria and the spore-forming Firmicutes, were found in both dust and its sources. While Firmicutes-related sequences were mostly detected in the SLS bacterial communities (10% of total sequences), the actinobacterial sequences were retrieved from both (11-13%). In conclusion, the potential dust sources examined here show highly diverse bacterial communities and contain nutrients that can be transported with aerosols. The obtained fingerprinting and sequencing data may enable back tracking of dust plumes and their microorganisms. [ABSTRACT FROM AUTHOR]

Biological soil crusts (BSCs) are thought to be important in the fertility of arid lands as gateways for carbon (C) and nitrogen (N). Studies on the Colorado Plateau have shown that an incomplete internal N cycle operates in BSCs that results in significant exports of dissolved organic N, ammonia and nitrate into the bulk soil through percolating water, thus mechanistically explaining their role as a N gateway. It is not known if this pattern is found in other arid regions. To examine this, we measured rates of major biogeochemical N-transformations in a variety of BSCs collected from the Colorado Plateau and the Mojave, Sonoran and Chihuahuan Deserts. Dinitrogen fixation and aerobic ammonia oxidation were prominent transformations at all sites. We found anaerobic ammonia oxidation (anammox) rates to be below the detection limit in all cases, and at least 50-fold smaller than rates of N-fixation, making it an irrelevant process for these BSCs. Heterotrophic denitrification was also of little consequence for the flow of N, with rates at least an order of magnitude smaller than those of N-fixation. Thus we could confirm that despite the demonstrable differences in microbial community composition and soil material, BSCs from major biogeographic regions in arid North America displayed a remarkably consistent pattern of internal N cycling. The implications for arid land fertility drawn from previous studies in the Colorado Plateau appear applicable to BSCs across other arid regions of the Southwestern United States. [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]

Molecular methodologies were used to investigate fungal assemblages of biological soil crusts (BSCs) from arid lands in the southwestern United States. Fungal diversity of BSCs was assessed in a broad survey that included the Chihuahuan and Sonoran deserts as well as the Colorado Plateau. At selected sites samples were collected along kilometer-scale transects, and fungal community diversity and composition were assessed based on community rRNA gene fingerprinting using PCR-denaturing gradient gel electrophoresis (DGGE). Individual phylotypes were characterized through band sequencing. The results indicate that a considerable diversity of fungi is present within crusted soils, with higher diversity being recovered from more successionally mature BSCs. The overwhelming majority of crust fungi belong to the Ascomycota, with the Pleosporales being widespread and frequently dominant. Beta diversity patterns of phylotypes putatively representing dominant members of BSC fimgal communities suggest that these assemblages are specific to their respective geographic regions of origin. [ABSTRACT FROM AUTHOR]

Abstract: Biological soil crusts (BSCs) are highly specialized topsoil microbial communities commonly found in arid and semiarid environments, permeated by a polymeric matrix of polysaccharides. BSCs can in principle influence edaphic properties such as texture, pore formation and water retention, which in turn determine water distribution and biological activity in dry lands. This paper investigates the influence of biotic and abiotic factors on BSC hydraulic conductivity, a parameter gauging the ease with which water can move through the pore spaces. Texture, phototroph abundance, microbial composition, and extracellular carbohydrate content were considered as potentially relevant parameters in a correlational study of BSC samples that spanned 1.5 orders of magnitude in hydraulic conductivity. A newly developed, non-destructive extraction method enabled us to directly quantify the specific role of extracellular polysaccharides on soil permeability on a variety of samples. Hydraulic conductivity showed a strongest correlation with texture (positive with sand content, negative with silt and clay). A weaker negative correlation with carbohydrate content, especially with polysaccharides having a molecular weight < 100 kDa, was also detected. In multiple regression analyses texture (silt content) was sufficient to explain most of the variation in hydraulic conductivity However, experimental removal of polymeric carbohydrates, resulted invariably in a substantial decrease in hydraulic conductivity for any given sample (between 1.7 and 3.3 fold). Our results suggest that while soil texture determines overall hydraulic conductivity in BSCs, the presence of exopolysaccharides can significantly enhance it, likely by conferring a spongy structure to a BSC thus increasing the number of waterways within it. [Copyright &y& Elsevier]

To better understand the key role of biological soil crusts (BSCs) in redistribution processes of limited water resources in semi-arid sandy ecosystems, surface runoff over BSCs in a sandy area in the southern Mu Us sandy area was observed in this study. The results indicated that runoff occurred twice among six rainfall events during the experimental period. Runoff yields varied among types of BSCs, and that they increased in the following order: light algae crusts, dark algae crusts and moss crusts. These findings showed that runoff yields were increasing with the development of BSCs. The percentage of runoff in individual rainfall events increased from light algae crusts to moss crusts, which indicated that the redistribution ratios of different types of BSCs differed. Surface runoff of BSCs may be influenced by rainfall, rain intensity, degree of water saturation of the BSCs before the rain and degree of development of BSCs. Formation of BSCs on the surface of sand dunes changed the spatial distribution pattern of water in this semi-arid sandy ecosystem, which increased the heterogeneity of resources such as water and nutrients. Therefore, making appropriate disturbances of BSCs during storm season is beneficial to maintaining the balance of natural resources in the semi-arid sandy ecosystem. [ABSTRACT FROM AUTHOR]

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]