Biocrust Communities Research Articles

Contribution of biological soil crusts to soil organic matter composition and stability in temperate forests

Biological soil crusts (biocrusts) play an important role in the biogeochemical C-, N- and P-cycles but there is limited knowledge about their effect on the composition and stability of organic matter (OM) in soil. In pine and beech dominated temperate forests, the different compartments of biocrust, crust-adhering soil and crust-free soil were analysed by pyrolysis field ionization mass spectrometry (Py-FIMS) and C K-edge X-ray absorption near edge structure (XANES) spectroscopy. Phototrophic biocrust communities had been determined morphologically. Predominant tree species affected OM composition and stability but an overriding effect of biocrust organisms was observed. In biocrusts, the OM mineralization potential was high due to a high carbohydrate/ketone ratio and a high proportion of the fatty acid C22:0. A high proportion of free fatty acids in combination with a high carbohydrate proportion was identified as a relative marker for biocrusts and additional high proportions of lignin dimers were specific for samples under pine. In biocrust under pine, OM in soil compartments was characterized by large proportions of long-chain aliphatics from plant cuticular material. In A-horizon substrate under beech, OM compound classes were not affected by phototrophic species richness, suggesting functional redundancy of these biocrust organisms with regard to OM composition. In contrast to A-horizon substrate, OM in C-horizon substrate was characterized by a higher proportion of easily decomposable material. Under pine, forest management intensity (SMI) affected OM by stand age and density due to effects on phototrophic biocrust species. We conclude that compartment, vegetation, horizon substrate and SMI affected OM composition and stability, and biocrusts hence contributed to soil OM diversity in forest ecosystems.

Biogeography of global drylands.

Despite their extent and socio-ecological importance, a comprehensive biogeographical synthesis of drylands is lacking. Here we synthesize the biogeography of key organisms (vascular and nonvascular vegetation and soil microorganisms), attributes (functional traits, spatial patterns, plant-plant and plant-soil interactions) and processes (productivity and land cover) across global drylands. These areas have a long evolutionary history, are centers of diversification for many plant lineages and include important plant diversity hotspots. This diversity captures a strikingly high portion of the variation in leaf functional diversity observed globally. Part of this functional diversity is associated with the large variation in response and effect traits in the shrubs encroaching dryland grasslands. Aridity and its interplay with the traits of interacting plant species largely shape biogeographical patterns in plant-plant and plant-soil interactions, and in plant spatial patterns. Aridity also drives the composition of biocrust communities and vegetation productivity, which shows large geographical variation. We finish our review by discussing major research gaps, which include: studying regular vegetation spatial patterns; establishing large-scale plant and biocrust field surveys assessing individual-level trait measurements; knowing whether the impacts of plant-plant and plant-soil interactions on biodiversity are predictable; and assessing how elevated CO2 modulates future aridity conditions and plant productivity.

Null‐model analysis and changes in species interactions in biocrusts along a successional gradient in the Tengger Desert, northern China

AbstractAimsIn arid regions, the succession of biological soil crusts (biocrusts) usually accompanies substantial turnover of community composition. How interspecific interactions among biocrust‐forming mosses, lichens, algae and cyanobacteria change along the successional gradient is largely unknown. According to the stress gradient hypothesis (SGH), the frequency or intensity of facilitative and competitive interactions within the community will vary along an environmental stress gradient, with an increasing role of competition relative to facilitation under more benign environmental conditions. In the process of biocrust succession, decreasing soil stress also occurs because of slow soil amelioration, which meets the assumption of the SGH. Therefore, we hypothesize that the changes in facilitative and competitive interactions within biocrust communities will follow the prediction of the SGH and that the nature of biocrust interactions will also change.LocationSand‐binding vegetation belts for the Baotou–Lanzhou railway at the southeast fringe of the Tengger Desert in the Shapotou region of the Ningxia Hui Autonomous Region, western China.MethodsWe conducted a field evaluation at five adjacent sand‐binding vegetation belts built in chronosequence to study the successional gradient. Small‐scale patterns of co‐occurrence and null models were used to evaluate the community‐level interspecific interactions among the biocrust‐forming mosses, lichens, algae and cyanobacteria. In addition, an accessory method of species‐pair co‐occurrence based on null models was also applied to potentially identify the nature of the interspecific interactions within the biocrusts and for the causal investigation of these patterns.ResultsA strong spatial segregation of co‐occurrence patterns occurred between cyanobacteria and algae and species of mosses and lichens at early succession, and occurred among species of mosses and lichens at late succession, while the weak spatial aggregation between lichen species only occurred at middle succession.ConclusionsNegative interspecific interactions were prevalent within biocrusts throughout succession. The co‐occurrence patterns at both the community and species‐pair levels are consistent, supporting the unimodal version of the SGH that species interactions shift toward competition under extremely stressed conditions. Given the lack of research on interspecific interactions in biocrusts, our study is important for obtaining a deeper understanding of biocrust succession.

Connectivity of Edaphic and Endolithic Microbial Niches in Cold Mountain Desert of Eastern Pamir (Tajikistan).

Simple SummaryMicrobial communities play a significant role in the functioning of the ecosystem in the cold mountain desert of Eastern Pamir, a region that is characterized by extreme environments with limited nutrient and water availability and sparse vegetation. This study aims to investigate the taxonomic composition and structure of bacterial endolithic (rock matrix colonizing) assemblages and soil-inhabiting communities that occur in the Eastern Pamir Mountains, and to compare the composition and structure of both types of communities looking for general patterns and trying to identify the connectivity between these types of microbial assemblages. The taxonomic composition of Pamirian endolithic communities and soil-inhabiting microbial consortia (biocrust communities) exhibit similarities to communities from cold and hot deserts, but they also harbor taxa, which are possibly novel species or which, so far, lack reference sequences in the databases. The endolithic communities occurring in three subregions of Eastern Pamir are characterized by high heterogeneity and cannot be grouped according to the subregion, while the distribution of Cyanobacteria in the soil crusts demonstrates the similarities between the communities within the subregion reflecting regional differences. We also found connectivity between both habitats concerning the occurrence the same taxa (e.g., Cyanobacteria).Microbial communities found in arid environments are commonly represented by biological soil crusts (BSCs) and endolithic assemblages. There is still limited knowledge concerning endoliths and BSCs occurring in the cold mountain desert of Pamir. The aim of the study was to investigate the composition and structure of endolithic bacterial communities in comparison to surrounding BSCs in three subregions of the Eastern Pamir (Tajikistan). The endolithic and BSC communities were studied using culture-independent and culture-dependent techniques. The structure of the endolithic bacterial communities can be characterized as Actinobacteria–Proteobacteria–Bacteroidetes–Chloroflexi–Cyanobacteria, while the BSCs’ can be described as Proteobacteria–Actinobacteria–Bacteroidetes–Cyanobacteria assemblages with low representation of other bacteria. The endolithic cyanobacterial communities were characterized by the high percentage of Chroococcidiopsaceae, Nodosilineaceae, Nostocaceae and Thermosynechococcaceae, while in the BSCs were dominated by Nodosilineaceae, Phormidiaceae and Nostocaceae. The analysis of 16S rRNA genes of the cyanobacterial cultures revealed the presence of possibly novel species of Chroococcidiopsis, Gloeocapsopsis and Wilmottia. Despite the niches’ specificity, which is related to the influence of microenvironment factors on the composition and structure of endolithic communities, our results illustrate the interrelation between the endoliths and the surrounding BSCs in some regions. The structure of cyanobacterial communities from BSC was the only one to demonstrate some subregional differences.

Divergent responses of moss- and lichen-dominated biocrusts to warming and increased drought in arid desert regions

Global warming coupled with increased drought is predicted to have a significant negative impact on desert ecosystems. In arid desert regions, a large proportion of the ground surface is covered by specialized organisms such as mosses and lichens that form biocrusts, which play a vital role in ecosystems. However, the long-term effects of warming and drought on these key biotic components of desert ecosystems remain poorly understood. Using a manipulative experiment conducted for 12 years in the Tengger Desert, northwestern China, we evaluated how both mosses and lichens in biocrust communities responded to 0.5°C and 1.5°C increases in temperature coupled with 5% and 8% reductions in total annual precipitation, respectively, using two groups of open-top chambers to approximate climate change conditions that are predicted to occur in this study region. Furthermore, surface soil carbon uptake by the biocrusts was also evaluated. Twelve years of warming coupled with increased drought resulted in a significant decrease in the cover and biomass of mosses but did not change the cover or biomass of lichens. These changes in the mosses were positively correlated with the duration and intensity of the treatments. Warming coupled with reduced precipitation significantly reduced the carbon uptake of the moss-dominated biocrusts by reducing the availability of moisture content. However, lichen carbon uptake was insensitive to the warming and reduced precipitation treatments. The reduction in cover and biomass of moss-dominated biocrusts might be attributed to large amounts of carbon loss, which further alters biocrust multifunctionality in desert ecosystems. In addition, our findings suggest that coupled warming and drought conditions could increase the dominance of lichens in biocrust communities to partly maintain the multifunctionality of biocrusts in this desert ecosystem.

Biocrust cyanobacterial composition, diversity, and environmental drivers in two contrasting climatic regions in Brazil

Biological soil crusts or biocrusts have critical ecological roles in dryland ecosystems including soil stabilization, erosion control and nutrient cycling. Global environmental changes are expected to impact terrestrial ecosystems, including biocrust communities. Thus, a growing number of studies have focused on investigating the diversity of biocrust-forming organisms including cyanobacteria. Despite the increasing interest in understanding biocrust cyanobacteria, there are still several knowledge gaps, particularly in areas from South America, where studies are limited. Here, we studied the composition, abundance, and environmental drivers of cyanobacterial biocrust from two important biomes in Brazil, i.e. Caatinga and Pampa, which are subject to desertification and anthropogenic pressures, respectively. Ten samples at three sites were explored at each biome (n = 60) and cyanobacterial biocrust community composition and diversity was analyzed through morphological evaluation and Next Generation Sequencing targeting the 16S rRNA gen. Soil and climatic variables, e.g. organic matter, soil texture, nitrogen and phosphorus content, temperature, and solar irradiance, were also determined at each site. Our results showed that biocrust cyanobacteria had distinct taxonomic compositions and assemblages at each biome. Similarly, cyanobacterial composition across sites within each biome differed substantially. Soil temperature and pH were identified as the main factors explaining such biotic structures. Caatinga sites, characterized by more arid environments, presented a higher abundance of N-fixing cyanobacteria, e.g. Scytonema. In contrast, Pampa, with higher rainfall regimes, showed a larger abundance of biocrust-forming bacteria such as Microcoleus and Leptolyngbya. The outcomes of this research are expected to provide a basis to enhance biocrusts and conservation efforts and promote the use of biocrust cyanobacteria as global change indicators.

Relative humidity predominantly determines long‐term biocrust‐forming lichen cover in drylands under climate change

Abstract Manipulative experiments typically show a decrease in dryland biocrust cover and altered species composition under climate change. Biocrust‐forming lichens, such as the globally distributed Diploschistes diacapsis, are particularly affected and show a decrease in cover with simulated climate change. However, the underlying mechanisms are not fully understood, and long‐term interacting effects of different drivers are largely unknown due to the short‐term nature of the experimental studies conducted so far. We addressed this gap and successfully parameterised a process‐based model for D. diacapsis to quantify how changing atmospheric CO2, temperature, rainfall amount and relative humidity affect its photosynthetic activity and cover. We also mimicked a long‐term manipulative climate change experiment to understand the mechanisms underlying observed patterns in the field. The model reproduced observed experimental findings: warming reduced lichen cover, whereas less rainfall had no effect on lichen performance. This warming effect was caused by the associated decrease in relative humidity and non‐rainfall water inputs, which are major water sources for biocrust‐forming lichens. Warming alone, however, increased cover because higher temperatures promoted photosynthesis during early morning hours with high lichen activity. When combined, climate variables showed non‐additive effects on lichen cover, and effects of increased CO2 levelled off with decreasing levels of relative humidity. Synthesis. Our results show that a decrease in relative humidity, rather than an increase in temperature, may be the key factor for the survival of the lichen D. diacapsis under climate change and that effects of increased CO2 levels might be offset by a reduction in non‐rainfall water inputs in the future. Because of a global trend towards warmer and drier air and the widespread global distribution of D. diacapsis, this will affect lichen‐dominated dryland biocrust communities and their role in regulating ecosystem functions worldwide.

The fate of biological soil crusts after fire: A meta-analysis

Fire is a global disturbance that is predicted to increase in frequency and severity in many parts of the world due to climate change. Biological soil crust (biocrust) communities are often overlooked in fire studies despite having a substantial effect on ecological function and the adjacent communities. The goal of this study is to synthesize and analyze existing data elucidating the recovery of biocrust cover following fire at the global-scale and suggest avenues for future research. We performed a meta-analysis of studies from 1984 to 2019 to address the response of biocrust after fire and determine the moderating factors governing their response. Overall, fire reduced biocrust cover by 50% and had a significantly negative effect on biocrusts classified as cyanobacteria or algal dominated. Additionally, as time since fire increased, total biocrust cover increased but this response was modulated by biocrust type indicating compliance with traditional biocrust successional models. However, there was significant unexplained heterogeneity within the meta-analysis. This reflects a critical need for more studies specifically addressing the effect of fire on biocrust communities as they are an ecosystem engineer in drylands around the world. We suggest more thorough characterization of biocrust organisms through field and laboratory studies to understand the mechanisms of biocrust response to fire. Additionally, research is needed across a broader geographic range to represent the known distribution of biocrust communities, particularly as fires increase in severity and scope.

Resilience and alternative stable states after desert wildfires

AbstractImproving models of community change is a fundamental goal in ecology and has renewed importance during global change and increasing human disturbance of the biosphere. Using the Mojave Desert (southwestern United States) as a model system, invaded by nonnative plants and subject to wildfire disturbances, we examined models of resilience, alternative stable states, and convergent‐divergent trajectories for 36 yr of plant community change after 31 wildfires in communities dominated by the native shrubs Larrea tridentata or Coleogyne ramosissima. Perennial species richness on average was fully resilient within 23 yr after disturbance in both community types. Perennial cover was fully resilient within 25 yr in the Larrea community, but recovery was projected to require 52 yr in the Coleogyne community. Species composition shifts were persistent, and in the Coleogyne community, the projected compositional recovery time of 550 yr and increasing resembled a deflected trajectory toward potential alternative states. Disturbed sites contained a perennial species composition of predominately short‐statured forbs, subshrubs, and grasses, contrasting with the larger‐statured shrub and tree structure of undisturbed sites. Auxiliary data sets characterizing species recruitment, annual plants including nonnative grasses, biocrust communities, and soils showed persistent differences between disturbed and undisturbed sites consistent with positive feedbacks potentially contributing to alternative stable states. Resprouting produced limited resilience for the large shrubs L. tridentata and Yucca spp. important to population persistence but did not forestall long‐term reduced abundance of the species. The nonnative annual grass Bromus rubens increased on disturbed sites over time, suggesting persistently abundant nonnative plant fuels and reburn potential. Biocrust cover on disturbed sites was half and species richness a third of amounts on undisturbed sites. Soil nitrogen was 30% greater on disturbed sites and no significant trend was evident for it to decline on even the oldest burns. Disturbed desert plant communities simultaneously supported all three models of resilience, alternative stable states, and convergent‐divergent trajectories among community measures (e.g., species richness, composition), timeframes since disturbance, and spatial resolutions. Accommodating expression within ecosystems of multiple models, including those opposing each other, may help broaden theoretical models of ecosystem change.

A symbiotic nutrient exchange within the cyanosphere microbiome of the biocrust cyanobacterium, Microcoleus vaginatus.

Microcoleus vaginatus plays a prominent role as both primary producer and pioneer in biocrust communities from dryland soils. And yet, it cannot fix dinitrogen, essential in often nitrogen-limited drylands. But a diazotroph-rich "cyanosphere" has been described in M. vaginatus, hinting that there exists a C for N exchange between the photoautotroph and heterotrophic diazotrophs. We provide evidence for this by establishing such a symbiosis in culture and by showing that it is selective and dependent on nitrogen availability. In natural populations, provision of nitrogen resulted in loss of diazotrophs from the cyanosphere of M. vaginatus compared to controls, but provision of phosphorus did not. Co-culturing of pedigreed cyanosphere diazotroph isolates with axenic M. vaginatus resulted in copious growth in C and N-free medium, but co-culture with non-cyanosphere diazotrophs or other heterotrophs did not. Unexpectedly, bundle formation in M. vaginatus, diacritical to the genus but not seen in axenic culture, was restored in vitro by imposed nitrogen limitation or, even more strongly, by co-culture with diazotrophic partners, implicating this trait in the symbiosis. Our findings provide direct evidence for a symbiotic relationship between M. vaginatus and its cyanosphere and help explain how it can be a global pioneer in spite of its genetic shortcomings.

High Arctic biocrusts: characterization of the exopolysaccharidic matrix

Biocrusts can be found in a wide array of habitats, where they provide important ecosystem services. These microbial associations are particularly important in High Arctic environments, where biocrust colonize the newly exposed barren soil after glacier retreat and significantly contribute to soil stabilization and nutrient cycling. Starting from incipient, structurally simple biolayers, they develop in complexity, increasing from the glacier terminus. Starting from a simple community structure, mainly constituted by cyanobacteria, heterotrophic bacteria and fungi immersed in a self-secreted extracellular polymeric matrix (cyanobacterial crusts), they later may recruit mosses and lichens (moss crusts and lichen crusts, respectively). The extracellular polymeric matrix protects the biocrust community from abiotic constraints, notably drought and freezing stress, from external physical harming factors, and from predation. The physicochemical characteristics of the extracellular matrix are related to several of its properties, such as its soil-stabilizing effect and water retention. We analysed the chemical (monosaccharidic composition) and macromolecular (molecular weight distribution) properties of the extracellular polymeric matrix of biocrusts with different morphologies collected in northwestern Spitsbergen, Norway. The uronic acid content and molecular weight (MW) distribution of the extracellular polysaccharidic matrices (EPMs) appeared in accordance with the developmental stages of the biocrusts. The MW distribution also showed significant differences between the samples, possibly reflecting differences in microbial enzymatic activities leading to the degradation of high-MW polymers into smaller compounds. The MW distribution profiles presented some important differences, reflecting differences in environmental conditions and, probably, the seasonal variance in microbial community composition that is known to characterize the environment examined in the present study.

Biocrusts Modulate Responses of Nitrous Oxide and Methane Soil Fluxes to Simulated Climate Change in a Mediterranean Dryland

Little is known about the role of biocrusts in regulating the responses of N2O and CH4 fluxes to climate change in drylands. Here, we aim to help filling this knowledge gap by using an 8-year field experiment in central Spain where temperature and rainfall are being manipulated (~ 1.9°C warming, 33% rainfall reduction and their combination) in areas with and without well-developed biocrust communities. Areas with initial high cover of well-developed biocrusts showed lower N2O emissions, enhanced CH4 uptake and higher abundances of functional genes linked to N2O and CH4 fluxes compared with areas with poorly developed biocrusts. Moreover, biocrusts modulated the responses of gases emissions and related functional genes to warming and rainfall reductions. Specifically, we found under rainfall exclusion and its combination with warming a sharp reduction in N2O fluxes (~ 96% and ~ 197%, respectively) only under well-developed biocrust cover. Warming and its combination with rainfall exclusion reduced CH4 consumption in areas with initial low cover of well-developed biocrust, whereas rainfall exclusion enhanced CH4 uptake only in areas with high initial cover of well-developed biocrusts. Similarly, the combination of warming and rainfall exclusion increased the abundance of the nosZ gene compared to the rainfall exclusion treatment and increased the abundance of the pmoA gene compared to the control, but only in areas with low biocrust cover. Taken together, our results indicate that well-developed biocrust communities could counteract the impact of warming and altered rainfall patterns on soil N2O and CH4 fluxes, highlighting their importance and the need to preserve them to minimize climate change impacts on drylands.

Dual community assembly processes in dryland biocrust communities

Abstract Biocrusts are critical components of drylands where they regulate a wide range of ecosystem functions, however, their response to the world‐wide phenomenon of shrub encroachment and to livestock grazing, the most extensive land use in drylands, is not well studied. Grazing by livestock and increases in shrub cover could influence biocrust communities directly via trampling or shading, or indirectly, by altering biotic interactions amongst biocrust taxa. The extent of these changes in biocrust cover, diversity and composition are poorly known. We used linear models and structural equation modelling to examine the direct effects of grazing and shrubs on biocrust community composition and the indirect effects mediated by changes in species interactions. Biocrust richness and cover increased with increasing shrub cover at the site level. This pattern occurred despite the negative response we found (lower cover and richness) under shrub patches versus open areas, which was consistent irrespective of the grazing level. Functional diversity and evenness were similar between shrubs and open at low grazing intensity, but at high grazing functional diversity was greater in the open. Competition between biocrust species was an important driver of their community assembly irrespective of shrub cover, grazing intensity or patch type. Structural equation models showed that the effects of grazing and shrub cover on functional evenness, functional diversity and richness were controlled by biotic interactions within the shrub microsites. In the open, however, these effects were either direct or mediated by changes in cover. Biocrust cover, species richness and functional diversity increase with shrub cover at the site scale, despite the negative effects at the microsite level. We demonstrate here that drivers of community assembly differ markedly at small spatial scales. Though biocrust communities were directly driven by environmental filtering in the open, biotic interactions played a fundamental role in their assembly when growing beneath shrubs. A free plain language summary can be found within the Supporting Information of this article.

Bacterial Communities of Mojave Desert Biological Soil Crusts Are Shaped by Dominant Photoautotrophs and the Presence of Hypolithic Niches

Biocrust communities are often heterogeneous and affected by small-scale physical and environmental factors. The presence of moss in biocrusts, for example, is likely to create a set of physical conditions distinct from those found in biocrusts that lack mosses. Similarly, hypolithic organisms living under and on the belowground surface of translucent and opaque stones (typically quartz) experience a distinct environment relative to surrounding soil surface organisms. To understand the impact of these factors on biocrust bacterial community composition, we performed 16S rRNA sequencing from surface and hypolithic biocrusts with and without the common biocrust moss, Syntrichia caninervis. While alpha diversity indices did not differ significantly between any of the biocrust habitats sampled, we did observe differences in overall community composition. Cyanobacterial abundance and diversity decreased in the presence of moss and in surface samples compared to hypolithic biocrusts, while Proteobacteria showed the opposite pattern. Acidobacteria were significantly more abundant in hypolithic niches, and Patescibacteria were found to be restricted to moss-dominated surface biocrusts. Notably, bacterial community composition was found to shift significantly between surface and hypolithic biocrusts lacking moss (Adonis, R2 = 0.122, p = 0.002) and between surface moss and non-moss biocrusts (R2 = 0.107, p = 0.002). These findings support the idea that even at small spatial scales (e.g., within < 10 cm), desert biocrust bacterial community composition varies based on the habitat structure and cohabitants.

Effects of nitrogen deposition on the spatial pattern of biocrusts and soil microbial activity in a semi‐arid Mediterranean shrubland

Abstract Human activities have doubled, or even tripled, the amount of nitrogen (N) fixed globally, which alters the functioning and reduces the biodiversity of terrestrial ecosystems. In this study, we used a 10‐year N addition experiment in a semi‐arid Mediterranean shrubland in central Spain to evaluate the effects of increased N deposition on biocrust cover, soil microbial enzymatic activity and their spatial patterns. We collected highly replicated, spatially explicit data to carry out Moran's I correlograms at the plot level and used linear mixed models to evaluate the effects of N addition on cover, microbial activity and Moran's I values for each distance class of correlograms. High N deposition (50 kg N ha−1 year−1) reduced the cover of the foliose lichen Cladonia foliacea, an important component of biocrusts, and generated significant alterations and losses of the spatial pattern of biocrusts and soil microbial enzymatic activity. Furthermore, we found indication that N addition altered the relationship between biocrust cover and microbial enzymatic activity, suggesting that the presence of well‐developed biocrust communities modulates the effects of high N deposition on soil microbial activity. Synthesis. Overall, our study suggests that an increase in N deposition may have important consequences for the structure and functioning of Mediterranean ecosystems and that some of these consequences may be due to alterations in the way in which different soil communities (e.g. moss‐lichen biocrusts and soil microorganisms) interact with one another. A free Plain Language Summary can be found within the Supporting Information of this article.

Inoculation and habitat amelioration efforts in biological soil crust recovery vary by desert and soil texture

As dryland degradation continues, it is increasingly important to understand how to effectively restore biocrust communities. Potential techniques include the addition of biocrust inoculum to accelerate biocrust recovery. Enhanced erosion typical of degraded environments creates a challenge for these approaches, due to loss by wind or water and burial by saltating particles. To retain and protect added inoculum, the inclusion of habitat‐amelioration techniques can improve recovery rates. This study tested three different types of inoculum (field‐collected, greenhouse‐cultivated, and laboratory‐cultivated biocrust) coupled with two treatments to augment soil stability and ameliorate habitat limitations: soil surface polyacrylamide additions and installation of straw barriers. This was done across two deserts (Great Basin and Chihuahuan) and separated into generally coarse‐ or finer‐textured soils in each desert, with results monitored for 3 years (2015, 2016, 2017). While the inoculum type, coupled with habitat ameliorations, occasionally enhanced biocrust growth across years and treatments, in other cases, it made no difference compared to natural recovery rates. Rather, the desert location and soil texture groupings were the most prominent factors in determining recovery trajectories. Recovery proportions were similar in the finer‐textured sites in both the Great Basin and the Chihuahuan deserts, while the coarser‐textured site in the Great Basin did show some recovery over time and the Chihuahuan coarser‐textured site did not. This study demonstrates the importance of understanding site potential and identifying key limitations to biocrust recovery for successful restoration projects.

Grazing Disturbance Promotes Exotic Annual Grasses by Degrading Soil Biocrust Communities

Exotic invasive plants threaten ecosystem integrity, and their success depends on a combination of abiotic factors, disturbances, and interactions with existing communities. In dryland ecosystems, soil biocrusts (communities of lichens, bryophytes, and microorganisms) can limit favorable microsites needed for invasive species establishment, but the relative importance of biocrusts for landscape-scale invasion patterns remains poorly understood. We examine effects of livestock grazing in habitats at high risk for invasion to test the hypothesis that disturbance indirectly favors exotic annual grasses by reducing biocrust cover. We present some of the first evidence that biocrusts increase site resistance to invasion at a landscape scale and mediate the effects of disturbance. Biocrust species richness, which is reduced by livestock grazing, also appears to promote native perennial grasses. Short mosses, as a functional group, appear to be particularly valuable for preventing invasion by exotic annual grasses. Our study suggests that maintaining biocrust communities with high cover, species richness, and cover of short mosses can increase resistance to invasion. These results highlight the potential of soil surface communities to mediate invasion dynamics and suggest promising avenues for restoration in dryland ecosystems.

Characteristics of Wetting-Induced Bacteriophage Blooms in Biological Soil Crust.

Biological soil crusts (biocrusts) are photosynthetic "hot spots" in deserts and cover ∼12% of the Earth's terrestrial surface, and yet they face an uncertain future given expected shifts in rainfall events. Laboratory wetting of biocrust communities is known to cause a bloom of Firmicutes which rapidly become dominant community members within 2 days after emerging from a sporulated state. We hypothesized that their bacteriophages (phages) would respond to such a dramatic increase in their host's abundance. In our experiment, wetting caused Firmicutes to bloom and triggered a significant depletion of cyanobacterial diversity. We used genome-resolved metagenomics to link phage to their hosts and found that the bloom of the genus Bacillus correlated with a dramatic increase in the number of Caudovirales phages targeting these diverse spore-formers (r = 0.762). After 2 days, we observed dramatic reductions in the relative abundances of Bacillus, while the number of Bacillus phages continued to increase, suggestive of a predator-prey relationship. We found predicted auxiliary metabolic genes (AMGs) associated with sporulation in several Caudovirales genomes, suggesting that phages may influence and even benefit from sporulation dynamics in biocrusts. Prophage elements and CRISPR-Cas repeats in Firmicutes metagenome-assembled genomes (MAGs) provide evidence of recent infection events by phages, which were corroborated by mapping viral contigs to their host MAGs. Combined, these findings suggest that the blooming Firmicutes become primary targets for biocrust Caudovirales phages, consistent with the classical "kill-the-winner" hypothesis.IMPORTANCE This work forms part of an overarching research theme studying the effects of a changing climate on biological soil crust (biocrust) in the Southwestern United States. To our knowledge, this study was the first to characterize bacteriophages in biocrust and offers a view into the ecology of phages in response to a laboratory wetting experiment. The phages identified here represent lineages of Caudovirales, and we found that the dynamics of their interactions with their Firmicutes hosts explain the collapse of a bacterial bloom that was induced by wetting. Moreover, we show that phages carried host-altering metabolic genes and found evidence of proviral infection and CRISPR-Cas repeats within host genomes. Our results suggest that phages exert controls on population density by lysing dominant bacterial hosts and that they further impact biocrust by acquiring host genes for sporulation. Future research should explore how dominant these phages are in other biocrust communities and quantify how much the control and lysis of blooming populations contributes to nutrient cycling in biocrusts.

Spectral Diversity Successfully Estimates the α-Diversity of Biocrust-Forming Lichens

Biocrusts, topsoil communities formed by mosses, lichens, liverworts, algae, and cyanobacteria, are a key biotic component of dryland ecosystems worldwide. Experiments carried out with lichen- and moss-dominated biocrusts indicate that climate change may dramatically reduce their cover and diversity. Therefore, the development of reproducible methods to monitor changes in biocrust diversity and abundance across multiple spatio-temporal scales is key for evaluating how climate change may impact biocrust communities and the myriad of ecosystem functions and services that rely on them. In this study, we collected lichen-dominated biocrust samples from a semi-arid ecosystem in central Spain. Their α-diversity was then evaluated using very high spatial resolution hyperspectral images (pixel size of 0.091 mm) measured in laboratory under controlled conditions. Support vector machines were used to map the biocrust composition. Traditional α-diversity metrics (i.e., species richness, Shannon’s, Simpson’s, and Pielou’s indices) were calculated using lichen fractional cover data derived from their classifications in the hyperspectral imagery. Spectral diversity was calculated at different wavelength ranges as the coefficient of variation of different regions of the reflectance spectra of lichens and as the standard deviation of the continuum removal algorithm (SD_CR). The accuracy of the classifications of the images obtained was close to 100%. The results showed the best coefficient of determination (r2 = 0.47) between SD_CR calculated at 680 nm and the α-diversity calculated as the Simpson’s index, which includes species richness and their evenness. These findings indicate that this spectral diversity index could be used to track spatio-temporal changes in lichen-dominated biocrust communities. Thus, they are the first step to monitor α-diversity of biocrust-forming lichens at the ecosystem and regional levels, a key task for any program aiming to evaluate changes in biodiversity and associated ecosystem services in drylands.

Grazing disturbance promotes exotic annual grasses by degrading soil biocrust communities.

Exotic invasive plants threaten ecosystem integrity, and their success depends on a combination of abiotic factors, disturbances, and interactions with existing communities. In dryland ecosystems, soil biocrusts (communities of lichens, bryophytes, and microorganisms) can limit favorable microsites needed for invasive species establishment, but the relative importance of biocrusts for landscape-scale invasion patterns remains poorly understood. We examine effects of livestock grazing in habitats at high risk for invasion to test the hypothesis that disturbance indirectly favors exotic annual grasses by reducing biocrust cover. We present some of the first evidence that biocrusts increase site resistance to invasion at a landscape scale and mediate the effects of disturbance. Biocrust species richness, which is reduced by livestock grazing, also appears to promote native perennial grasses. Short mosses, as a functional group, appear to be particularly valuable for preventing invasion by exotic annual grasses. Our study suggests that maintaining biocrust communities with high cover, species richness, and cover of short mosses can increase resistance to invasion. These results highlight the potential of soil surface communities to mediate invasion dynamics and suggest promising avenues for restoration in dryland ecosystems.