Climate change and physical disturbance cause similar community shifts in biological soil crusts

eBryoSoil: a citizen science application to monitor changes in soil ecosystems

Biological soil covers (BSCs) play a pivotal role in ecosystem functioning by enhancing soil stability, mediating nutrient cycling, and influencing soil hydrology. Recognized as ecosystem engineers, they can physically modify, maintain, or create habitats, facilitating plant community development. Through these intricate interactions, BSCs contribute significantly to ecological processes, highlighting their importance in the overall health and functionality of the ecosystems of the Iberian Peninsula. Here we present the results obtained from the contributions of the citizen scientists uploaded from November 2019 to January 2021 with eBryoSoil, an app that allows citizens to participate in mapping the BSC communities across the Iberian Peninsula. Here, we emphasize the importance of habitats and consequently, their interaction with climatic variables for the persistence of BSCs (lichens and bryophytes). Conservation efforts targeted at preserving diverse habitats are essential to ensure the continued presence of lichen and bryophyte communities. Despite challenges posed by the SARS-CoV-2 outbreak, this citizen science project demonstrated success in utilizing a specifically tailored app to gather valuable information on BSC communities, providing insights into their vulnerability to climate change. This program serves as an illustrative example of how citizen science can effectively identify and study vulnerable habitats, offering a blueprint for future studies focused on understudied organisms.

Soil carbon and nitrogen cycling at the atmosphere-soil interface: Quantifying the responses of biocrust-soil interactions to global change

In drylands, where water scarcity limits vascular plant growth, much of the primary production occurs at the soil surface. This is where complex macro- and microbial communities, in an intricate bond with soil particles, form biological soil crusts (biocrusts). Despite their critical role in regulating C and N cycling in dryland ecosystems, there is limited understanding of the fate of biologically fixed C and N from biocrusts into the mineral soil, or how climate change will affect C and N fluxes between the atmosphere, biocrusts, and subsurface soils. To address these gaps, we subjected biocrust-soil systems to experimental warming and drought under controlled laboratory conditions, monitored CO2 fluxes, and applied dual isotopic labeling pulses (13CO2 and 15N2). This allowed detailed quantification of elemental pathways into specific organic matter (OM) pools and microbial biomass via density fractionation and phospholipid fatty acid analyses. While biocrusts modulated CO2 fluxes regardless of the temperature regime, drought severely limited their photosynthetic C uptake to the extent that the systems no longer sustained net C uptake. Furthermore, the effect of biocrusts extended into the underlying 1 cm of mineral soil, where C and N accumulated as mineral-associated OM (MAOM<63μm). This was strongly associated with increased relative dominance of fungi, suggesting that fungal hyphae facilitate the downward C and N translocation and subsequent MAOM formation. Most strikingly, however, these pathways were disrupted in systems exposed to warming, where no effects of biocrusts on the elemental composition of the underlying soil nor on MAOM were determined. This was further associated with reduced net biological N fixation under combined warming and drought, highlighting how changing climatic conditions diminish some of the most fundamental ecosystem functions of biocrusts, with detrimental repercussions for C and N cycling and the persistence of soil organic matter pools in dryland ecosystems.

Prescribed fire regimes influence responses of fungal and bacterial communities on new litter substrates in a brackish tidal marsh

Processes modifying newly deposited litter substrates should affect fine fuels in fire-managed tidal marsh ecosystems. Differences in chemical composition and dynamics of litter should arise from fire histories that generate pyrodiverse plant communities, tropical cyclones that deposit wrack as litter, tidal inundation that introduces and alters sediments and microbes, and interactions among these different processes. The resulting diversity and dynamics of available litter compounds should affect microbial (fungal and bacterial) communities and their roles in litter substrate dynamics and ecosystem responses over time. We experimentally examined effects of differences in litter types produced by different fire regimes and litter loads (simulating wrack deposition) on microbial community composition and changes over time. We established replicated plots at similar elevations within frequent tidal-inundation zones of a coastal brackish Louisiana marsh. Plots were located within blocks with different prescribed fire regimes. We deployed different measured loads of new sterilized litter collected from zones in which plots were established, then re-measured litter masses at subsequent collection times. We used DNA sequencing to characterize microbial communities, indicator families, and inferred ecosystem functions in litter subsamples. Differences in fire regimes had large, similar effects on fungal and bacterial indicator families and community compositions and were associated with alternate trajectories of community development over time. Both microbial and plant community compositional patterns were associated with fire regimes, but in dissimilar ways. Differences in litter loads introduced differences in sediment accumulation associated with tidal inundation that may have affected microbial communities. Our study further suggests that fire regimes and tropical cyclones, in the context of frequent tidal inundation, may interactively generate substrate heterogeneities and alter microbial community composition, potentially modifying fine fuels and hence subsequent fires. Understanding microbial community compositional and functional responses to fire regimes and tropical cyclones should be useful in management of coastal marsh ecosystems.

Effects of mixed biocrusts on soil nutrients and bacterial community structure: a case study from Hilly Loess Plateau, China

The ecological function of biological crusts in arid and semi-arid areas is of great importance. Bacteria, as a crucial microbial group in biological crusts, play a key role in the formation, nutrient cycling, and regulation of these crusts. However, the succession of biological crusts and the diversity of bacterial communities, along with key environmental factors in the Loess Plateau's hilly and gully areas, remain unclear. This study investigated soil bacterial abundance and diversity in bare soil (BS), alga-lichen mixed crust (MC), and alga-lichen mixed crust subsoil (MCS) using high-throughput sequencing methods. It explored the relationship between the bacterial community in biological crusts and key environmental factors. The results indicated that the Chao1, Shannon index, and phylogenetic diversity of bacteria significantly increased with the succession of biological crusts. There were notable differences in the community composition and structure of bacteria at different stages of crust development, with Rubrobacteria and Cyanobacteriia dominating in MCS. Effective phosphorus, available potassium, nitrogen, pH, and total organic carbon were identified as key environmental factors affecting soil bacterial communities. In summary, the succession of biological crusts alters soil physicochemical characteristics and creates different ecological niches for bacterial communities. Soil nutrients and pH play a crucial role in the selection of bacterial species and the shaping of bacterial communities in the Loess Plateau's hilly and gully areas.

Cyanotoxin accumulation and growth patterns of biocrust communities under variable environmental conditions

Biocrusts dominate the soil surface in deserts and are composed of diverse microbial communities that provide important ecosystem services. Cyanobacteria in biocrusts produce many secondary metabolites, including the neurotoxins BMAA, AEG, DAB, anatoxin-a(S) (guanitoxin), and the microcystin hepatotoxins, all known or suspected to cause disease or illness in humans and other animals. We examined cyanobacterial growth and prevalence of these toxins in biocrusts at millimeter-scales, under a desert-relevant illumination gradient. In contrast to previous work, we showed that hydration had an overall positive effect on growth and toxin accumulation, that nitrogen was not correlated with growth or toxin production, and that phosphorus enrichment negatively affected AEG and BMAA concentrations. Excess illumination positively correlated with AEG, and negatively correlated with all other toxins and growth. Basic pH negatively affected only the accumulation of BMAA. Anatoxin-a(S) (guanitoxin) was not correlated with any tested variables, while microcystins were not detected in any of the samples. Concerning toxin pools, AEG and BMAA were good predictors of the presence of one another. In a newly conceptualized scheme, we integrate aspects of biocrust growth and toxin pool accumulations with arid-relevant desertification drivers.

Deterministic processes influence bacterial more than fungal community assembly during the development of biological soil crusts in the desert ecosystem

Biological soil crusts (biocrusts) constitute a crucial biological component of the soil surface in arid and semi-arid ecosystems. Understanding the variations in soil microbial community assembly across biocrust successional stages is essential for a deeper comprehension of microbial biodiversity and desert ecosystem functioning. However, knowledge about the mechanisms of microbial community assembly and the factors influencing its development remains limited. In this study, we utilized amplicons sequencing to assess the compositions of bacterial and fungal communities in bare sand and three types of biocrusts (light cyanobacterial biocrusts, dark cyanobacterial biocrusts, and moss crusts). Subsequently, we analyzed the ecological processes shaping microbial community composition and structure, along with the influencing factors. Our results revealed a significant increase in bacterial diversity and no significant changes in fungal diversity during biocrust development. The relative abundances of the copiotrophic bacteria (e.g., Actinobacteria, Acidobacteria, and Bacteroidetes) showed significant increases, while oligotrophic bacteria (e.g., Proteobacteria and Firmicutes) decreased over time. Moreover, the relative abundances of Ascomycota, which exhibit strong resistance to adverse environmental conditions, significantly decreased, whereas Basidiomycota, known for their ability to degrade lignin, significantly increased throughout biocrust development. Additionally, stochastic processes (dispersal limitation and drift) predominantly drove the assemblies of both bacterial and fungal communities. However, the relative importance of deterministic processes (homogeneous selection) in bacterial assembly increased during biocrust development. Structural equation modeling indicated that bacterial community assembly was primarily related to soil water content, whereas fungal community assembly was primarily related to total organic carbon. These findings provide a scientific foundation for investigating the formation and development of biocrusts, and further insights into the conservation and sustainable management of biocrust resources under future climate change scenarios.

Organizing a global list of cyanobacteria and algae from soil biocrusts evidenced great geographic and taxonomic gaps

Biocrusts determine soil stability and resiliency, with a special role played by oxygenic photoautotrophic microorganisms in these communities. We evaluated temporal and geographic trends in studies focused on these microorganisms in biocrusts. Two databases were surveyed to obtain scientific articles published from 1998 to 2020 containing the terms 'biocrusts,' 'algae,' and 'cyanobacteria.' Although interest in biocrusts has increased recently, their ecological importance is still little explored. The scientific articles that mentioned a species list of cyanobacteria and/or algae revealed a very heterogeneous geographic distribution of research. Biocrusts have not been explored in many regions and knowledge in the tropics, where these communities showed high species richness, is limited. Geographic gaps were detected and more detailed studies are needed, mainly where biocrust communities are threatened by anthropogenic impacts. Aiming to address these knowledge gaps, we assembled a taxonomic list of all algae and cyanobacteria found in these articles, including information on their occurrence and ecology. This review is an updated global taxonomic survey of biocrusts, which importantly reveals their high species richness of oxygenic photoautotrophic microorganisms. We believe this database will be useful to future research by providing valuable taxonomic and biogeographic information regarding algae and cyanobacteria in biocrusts.

Impact of changing climate on bryophyte contributions to terrestrial water, carbon, and nitrogen cycles

Bryophytes, including the lineages of mosses, liverworts, and hornworts, are the second-largest photoautotroph group on Earth. Recent work across terrestrial ecosystems has highlighted how bryophytes retain and control water, fix substantial amounts of carbon (C), and contribute to nitrogen (N) cycles in forests (boreal, temperate, and tropical), tundra, peatlands, grasslands, and deserts. Understanding how changing climate affects bryophyte contributions to global cycles in different ecosystems is of primary importance. However, because of their small physical size, bryophytes have been largely ignored in research on water, C, and N cycles at global scales. Here, we review the literature on how bryophytes influence global biogeochemical cycles, and we highlight that while some aspects of global change represent critical tipping points for survival, bryophytes may also buffer many ecosystems from change due to their capacity for water, C, and N uptake and storage. However, as the thresholds of resistance of bryophytes to temperature and precipitation regime changes are mostly unknown, it is challenging to predict how long this buffering capacity will remain functional. Furthermore, as ecosystems shift their global distribution in response to changing climate, the size of different bryophyte-influenced biomes will change, resulting in shifts in the magnitude of bryophyte impacts on global ecosystem functions.

Environmental conditions play a key role in controlling the composition and diversity of Colombian biocrust microbiomes

Drylands soils worldwide are naturally colonized by microbial communities known as biocrusts. These soil microbiomes render important ecosystem services associated with soil fertility, water holding capacity, and stability to the areas they cover. Because of the importance of biocrusts in the global cycling of nutrients, there is a growing interest in describing the many microbial configurations these communities display worldwide. However, comprehensive 16S rRNA genes surveys of biocrust communities do not exist for much of the planet: for example, in the continents of South America and the northern part of Africa. The absence of a global understanding of biocrust biodiversity has lead us to assign a general importance to community members that may, in fact, be regional. Here we report for the first time the presence of biocrusts in Colombia (South America) through 16S rRNA genes surveys across an arid, a semi-arid and a dry subtropical region within the country. Our results constitute the first glance of the Bacterial/Archaeal communities associated with South American biocrust microbiomes. Communities where cyanobacteria other than Microcoleus vaginatus prevail, despite the latter being considered a key species elsewhere, illustrate differentiable results in these surveys. We also find that the coastal biocrust communities in Colombia include halo-tolerant and halophilic species, and that niche preference of some nitrogen fixing organisms deviate from previously described global trends. In addition, we identified a high proportion (ranging from 5 to 70%, in average) of cyanobacterial sequences that did not match any formally described cyanobacterial species. Our investigation of Colombian biocrusts points to highly diverse communities with climatic regions controlling taxonomic configurations. They also highlight an extensive local diversity to be discovered which is central to better design management and restoration strategies for drylands soils currently undergoing disturbances due to land use and global warming. Finally, this field study highlights the need for an improved mechanistic understanding of the response of key biocrust community members to changes in moisture and temperature.

Remote sensing and spectroscopy of lichens

Lichens are combinations of two symbiotic organisms, a green alga or cyanobacterium and a fungus. They grow in nearly all terrestrial ecosystems and survive in habitats, which are very dry or cold, or too poor in nutrients to maintain vegetation growth. Because lichens grow on visible surfaces and exhibit spectral properties, which are clearly different from, for example, vegetation, it is possible to distinguish them in remote sensing data. In this first systematic review article on remote sensing of lichens, we analyze and summarize which lichen species or genera, and in which habitats and geographical regions, have been remotely sensed, and which remote sensing or spectroscopic technologies have been used. We found that laboratory or in situ measured spectra of over 70 lichen species have been reported to date. We show that studies on remote sensing of lichens fall under seven broad themes: (1) collection of lichen spectra for quantification of lichen species or characteristics, (2) pollution monitoring with lichens as ecological indicators, (3) geological and lithological mapping, (4) desert and dryland monitoring, (5) animal habitat monitoring, (6) land cover or vegetation mapping, and (7) surface energy budget modeling.

Distinguishing natural and anthropogenic contributions to biological soil crust distribution in China's drylands

Desertification caused by natural factors and human activities seriously threatens dryland biological communities. However, the impact of these factors on non-vascular plants in drylands has not been fully documented. This study proposed a framework to distinguish the natural and anthropogenic contributions to the distribution of the biological soil crust (BSC) coverage. The 20 model-simulated environmental datasets, including climate, soil characteristics and terrain, were selected to explore the internal relationship between these environmental drivers and BSC coverage. Random forest classification and regression models were developed to calculate the BSC coverage in the drylands of China under natural conditions. By subtracting the predicted natural BSC coverage from the observed BSC coverage, the spatial distribution of changes in BSC coverage attributed to human activities was mapped. The results showed that in the limited vegetation areas of China's drylands, human activities had a positive impact on BSC coverage in only 11.3 % of the regions while having a negative effect on 25.4 % of the regions. Moreover, human activities led to a 33 % reduction in BSC coverage in these regions. The positive impacts of large-scale ecological restoration projects on BSC coverage in the drylands of China were limited due to land use changes caused by human economic activities. This framework provides support for assessing regional variations in anthropogenic impacts on dryland BSC communities and contributes to the development of appropriate dryland management policies.

Mapping biocrust distribution in China's drylands under changing climate

Biological soil crusts (biocrusts) are widely distributed in global drylands and have multiple significant roles in regulating dryland soil and ecosystem multifunctionality. However, maps of their distribution over large spatial scales are uncommon and sometimes unreliable, because our current remote sensing technology is unable to efficiently discriminate between biocrusts and vascular plants or even bare soil across different ecosystem and soil types. The lack of biocrust spatial data may limit our ability to detect risks to dryland function or key tipping points. Here, we indirectly mapped biocrust distribution in China's drylands using spatial prediction modeling, based on a set of occurrences of biocrusts (379 in total) and high-resolution soil and environmental data. The results showed that biocrusts currently cover 13.9 % of China's drylands (or 5.7 % of China's total area), with moss-, lichen-, and cyanobacterial-dominated biocrusts each occupying 5.7 % to 10.7 % of the region. Biocrust distribution is mainly determined by soil properties (soil type and contents of gravel and nitrogen), aridity stress, and altitude. Their most favorable habitat is arenosols with low contents of gravel and nitrogen, in climate with a drought index of 0.54 and an altitude of about 500 m. By 2050, climate change will lead to a 5.5 %-9.0 % reduction in biocrust cover. Lichen biocrusts exhibit a high vulnerability to climate change, with potential reductions of up to 19.0 % in coverage. Biocrust cover loss is primarily caused by the combined effects of the elevated temperature and increased precipitation. Our study provides the first high-resolution (250 × 250 m) map of biocrust distribution in China's drylands and offers a reliable approach for mapping regional or global biocrust colonization. We suggest incorporating biocrusts into Earth system models to identify their significant impact on global or regional-scale processes under climate change.

Balanced biogeographic and local environmental effects determine the patterns of microbial diversity in biocrusts at multi-scales

Introduction

Biodiversity maintenance and its underlying mechanisms are central issues of ecology. However, predicting the composition turnovers of microbial communities at multiple spatial scales remains greatly challenging because they are obscured by the inconsistent impacts of climatic and local edaphic conditions on the assembly process.

Methods

Based on the Illumina MeSeq 16S/18S rRNA sequencing technology, we investigated soil bacterial and eukaryotic communities in biocrusts with different successional levels at a subcontinental scale of Northern China.

Results

Results showed that irrespective of spatial scale, bacterial α diversity increased but eukaryotic diversity decreased with the primary succession, whereas both β diversities decreased at the subcontinental scale compared with smaller scales, indicating that the biogeographic pattern of soil microorganisms was balanced by successional convergence and distance decay effect. We found that the convergence of bacterial and eukaryotic communities was attributed to the turnovers of generalist and specialist species, respectively. In this process, edaphic and climatic factors showed unique roles in the changes of diversity at local/subcontinental scales. Moreover, the taxonomic diversity tended to be more susceptible to climatic and edaphic conditions, while biotic factors (photosynthesis and pigments) were more important to phylogenetic diversity.

Conclusion

Taken together, our study provided comprehensive insights into understanding the pattern of microbial diversity at multiple spatial scales of drylands.

Cultivating Resilience in Dryland Soils: An Assisted Migration Approach to Biological Soil Crust Restoration

Land use practices and climate change have driven substantial soil degradation across global drylands, impacting ecosystem functions and human livelihoods. Biological soil crusts, a common feature of dryland ecosystems, are under extensive exploration for their potential to restore the stability and fertility of degraded soils through the development of inoculants. However, stressful abiotic conditions often result in the failure of inoculation-based restoration in the field and may hinder the long-term success of biocrust restoration efforts. Taking an assisted migration approach, we cultivated biocrust inocula sourced from multiple hot-adapted sites (Mojave and Sonoran Deserts) in an outdoor facility at a cool desert site (Colorado Plateau). In addition to cultivating inoculum from each site, we created an inoculum mixture of biocrust from the Mojave Desert, Sonoran Desert, and Colorado Plateau. We then applied two habitat amelioration treatments to the cultivation site (growth substrate and shading) to enhance soil stability and water availability and reduce UV stress. Using marker gene sequencing, we found that the cultivated mixed inoculum comprised both local- and hot-adapted cyanobacteria at the end of cultivation but had similar cyanobacterial richness as each unmixed inoculum. All cultivated inocula had more cyanobacterial 16S rRNA gene copies and higher cyanobacterial richness when cultivated with a growth substrate and shade. Our work shows that it is possible to field cultivate biocrust inocula sourced from different deserts, but that community composition shifts toward that of the cultivation site unless habitat amelioration is employed. Future assessments of the function of a mixed inoculum in restoration and its resilience in the face of abiotic stressors are needed to determine the relative benefit of assisted migration compared to the challenges and risks of this approach.

Biological soil crusts on agricultural soils of mesic regions promote microbial cross-kingdom co-occurrences and nutrient retention

Introduction

Biological soil crusts (biocrusts) are known as biological hotspots on undisturbed, nutrient-poor bare soil surfaces and until now, are mostly observed in (semi-) arid regions but are currently poorly understood in agricultural systems. This is a crucial knowledge gap because managed sites of mesic regions can quickly cover large areas. Thus, we addressed the questions (i) if biocrusts from agricultural sites of mesic regions also increase nutrients and microbial biomass as their (semi-) arid counterparts, and (ii) how microbial community assemblage in those biocrusts is influenced by disturbances like different fertilization and tillage regimes.

Methods

We compared phototrophic biomass, nutrient concentrations as well as the abundance, diversity and co-occurrence of Archaea, Bacteria, and Fungi in biocrusts and bare soils at a site with low agricultural soil quality.

Results and Discussion

Biocrusts built up significant quantities of phototrophic and microbial biomass and stored more nutrients compared to bare soils independent of the fertilizer applied and the tillage management. Surprisingly, particularly low abundant Actinobacteria were highly connected in the networks of biocrusts. In contrast, Cyanobacteria were rarely connected, which indicates reduced importance within the microbial community of the biocrusts. However, in bare soil networks, Cyanobacteria were the most connected bacterial group and, hence, might play a role in early biocrust formation due to their ability to, e.g., fix nitrogen and thus induce hotspot-like properties. The microbial community composition differed and network complexity was reduced by conventional tillage. Mineral and organic fertilizers led to networks that are more complex with a higher percentage of positive correlations favoring microbe-microbe interactions. Our study demonstrates that biocrusts represent a microbial hotspot on soil surfaces under agricultural use, which may have important implications for sustainable management of such soils in the future.

Biocrusts intensify water redistribution and improve water availability to dryland vegetation: insights from a spatially-explicit ecohydrological model

Biocrusts are ecosystem engineers in drylands and structure the landscape through their ecohydrological effects. They regulate soil infiltration and evaporation but also surface water redistribution, providing important resources for vascular vegetation. Spatially-explicit ecohydrological models are useful tools to explore such ecohydrological mechanisms, but biocrusts have rarely been included in them. We contribute to closing this gap and assess how biocrusts shape spatio-temporal water fluxes and availability in a dryland landscape and how landscape hydrology is affected by climate-change induced shifts in the biocrust community. We extended the spatially-explicit, process-based ecohydrological dryland model EcoHyD by a biocrust layer which modifies water in- and outputs from the soil and affects surface runoff. The model was parameterized for a dryland hillslope in South-East Spain using field and literature data. We assessed the effect of biocrusts on landscape-scale soil moisture distribution, plant-available water and the hydrological processes behind it. To quantify the biocrust effects, we ran the model with and without biocrusts for a wet and dry year. Finally, we compared the effect of incipient and well-developed cyanobacteria- and lichen biocrusts on surface hydrology to evaluate possible paths forward if biocrust communities change due to climate change. Our model reproduced the runoff source-sink patterns typical of the landscape. The spatial differentiation of soil moisture in deeper layers matched the observed distribution of vascular vegetation. Biocrusts in the model led to higher water availability overall and in vegetated areas of the landscape and that this positive effect in part also held for a dry year. Compared to bare soil and incipient biocrusts, well-developed biocrusts protected the soil from evaporation thus preserving soil moisture despite lower infiltration while at the same time redistributing water toward downhill vegetation. Biocrust cover is vital for water redistribution and plant-available water but potential changes of biocrust composition and cover can reduce their ability of being a water source and sustaining dryland vegetation. The process-based model used in this study is a promising tool to further quantify and assess long-term scenarios of climate change and how it affects ecohydrological feedbacks that shape and stabilize dryland landscapes.

Patterns in biocrust recovery over time in semiarid southeast Spain

Biological soil crusts (biocrusts) are communities of microorganisms, fungi, algae, lichens and mosses inhabiting on the soil surface and within the uppermost soil millimetres. They play an important ecological role in drylands, determining physical and chemical soil properties and reducing soil erosion. Studies on biocrust natural recovery establish highly variable recovery times. The different objectives and methodologies of experimentation and analysis, strongly influence these predictions. The main purpose of this research is to analyze the recovery dynamics of four biocrust communities and their relationship with microclimatic variables. In 2004, in Tabernas Desert, some of us removed the biocrust in central 30 cm × 30 cm area of three 50 cm × 50 cm plots in each of four biocrust communities (Cyanobacteria, Squamarina, Diploschistes, and Lepraria), installing a microclimatic station in each one with sensors for temperature and humidity of the soil and air, dew point, PAR and rain. Yearly, the 50 cm × 50 cm plots were photographed, and the cover of every species was monitored in every 5 cm × 5 cm cell of a 36-cells grid covering the removed central area. We analyzed different functions to fit the cover recovery, the differences in cover recovery speed between communities, the recovery dynamics from the spatial analysis of the plot, the changes in dissimilarity and biodiversity and the possible relationships with the climatic variables. The recovery of the biocrust cover fits to a sigmoidal function. The community dominated by Cyanobacteria developed faster than those dominated by lichens. The Squamarina and Diploschistes communities recovered faster than that of Lepraria and appears to be influenced by the surrounding undisturbed areas. Species-based dissimilarity between consecutive inventories fluctuated and decreased over time, while biodiversity increases in a similar way. The speed of recovery of the biocrust in each community, along with the order in which the species appeared, support the hypothesis about the succession, which would include three phases: firstly Cyanobacteria, then Diploschistes and/or Squamarina and finally Lepraria. The relationship between biocrust recovery and microclimate is complex and this work highlights the need to carry out further research on this topic and on biocrust dynamics in general.

Landscape characteristics shape surface soil microbiomes in the Chihuahuan Desert

Introduction

Soil microbial communities, including biological soil crust microbiomes, play key roles in water, carbon and nitrogen cycling, biological weathering, and other nutrient releasing processes of desert ecosystems. However, our knowledge of microbial distribution patterns and ecological drivers is still poor, especially so for the Chihuahuan Desert.

Methods

This project investigated the effects of trampling disturbance on surface soil microbiomes, explored community composition and structure, and related patterns to abiotic and biotic landscape characteristics within the Chihuahuan Desert biome. Composite soil samples were collected in disturbed and undisturbed areas of 15 long-term ecological research plots in the Jornada Basin, New Mexico. Microbial diversity of cross-domain microbial groups (total Bacteria, Cyanobacteria, Archaea, and Fungi) was obtained via DNA amplicon metabarcode sequencing. Sequence data were related to landscape characteristics including vegetation type, landforms, ecological site and state as well as soil properties including gravel content, soil texture, pH, and electrical conductivity.

Results

Filamentous Cyanobacteria dominated the photoautotrophic community while Proteobacteria and Actinobacteria dominated among the heterotrophic bacteria. Thaumarchaeota were the most abundant Archaea and drought adapted taxa in Dothideomycetes and Agaricomycetes were most abundant fungi in the soil surface microbiomes. Apart from richness within Archaea (p = 0.0124), disturbed samples did not differ from undisturbed samples with respect to alpha diversity and community composition (p ≥ 0.05), possibly due to a lack of frequent or impactful disturbance. Vegetation type and landform showed differences in richness of Bacteria, Archaea, and Cyanobacteria but not in Fungi. Richness lacked strong relationships with soil variables. Landscape features including parent material, vegetation type, landform type, and ecological sites and states, exhibited stronger influence on relative abundances and microbial community composition than on alpha diversity, especially for Cyanobacteria and Fungi. Soil texture, moisture, pH, electrical conductivity, lichen cover, and perennial plant biomass correlated strongly with microbial community gradients detected in NMDS ordinations.

Discussion

Our study provides first comprehensive insights into the relationships between landscape characteristics, associated soil properties, and cross-domain soil microbiomes in the Chihuahuan Desert. Our findings will inform land management and restoration efforts and aid in the understanding of processes such as desertification and state transitioning, which represent urgent ecological and economical challenges in drylands around the world.

Droughting a megadrought: Ecological consequences of a decade of experimental drought atop aridification on the Colorado Plateau

Global dryland vegetation communities will likely change as ongoing drought conditions shift regional climates towards a more arid future. Additional aridification of drylands can impact plant and ground cover, biogeochemical cycles, and plant-soil feedbacks, yet how and when these crucial ecosystem components will respond to drought intensification requires further investigation. Using a long-term precipitation reduction experiment (35% reduction) conducted across the Colorado Plateau and spanning 10 years into a 20+ year regional megadrought, we explored how vegetation cover, soil conditions, and growing season nitrogen (N) availability are impacted by drying climate conditions. We observed large declines for all dominant plant functional types (C₃ and C₄ grasses and C₃ and C₄ shrubs) across measurement period, both in the drought treatment and control plots, likely due to ongoing regional megadrought conditions. In experimental drought plots, we observed less plant cover, less biological soil crust cover, warmer and drier soil conditions, and more soil resin-extractable N compared to the control plots. Observed increases in soil N availability were best explained by a negative correlation with plant cover regardless of treatment, suggesting that declines in vegetation N uptake may be driving increases in available soil N. However, in ecosystems experiencing long-term aridification, increased N availability may ultimately result in N losses if soil moisture is consistently too dry to support plant and microbial N immobilization and ecosystem recovery. These results show dramatic, worrisome declines in plant cover with long-term drought. Additionally, this study highlights that more plant cover losses are possible with further drought intensification and underscore that, in addition to large drought effects on aboveground communities, drying trends drive significant changes to critical soil resources such as N availability, all of which could have long-term ecosystem impacts for drylands.

Protist Diversity Responses to Experimental N Deposition in Biological Crusts of a Semiarid Mediterranean Ecosystem

Biological soil crusts (BSC) are associations of different macro and microorganisms and aggregated soil particles located on the surface of soils in many different habitats. BSC harbour a diverse and complex community of ciliates and testate amoebae. These phagotrophic protists play an important role in C and N recycling in soil ecosystems but have not been frequently studied in BSC. In this context, the effects of three increasing N inputs on ciliates and testate amoebae in crusts from a semi-arid Mediterranean ecosystem were evaluated. A field experiment with artificial N-deposition was designed to mimic the effects caused by anthropogenic N depositions. The results have shown that the protist populations of these semi-arid Mediterranean environments have lower species richness than other soil environments. The increase in N produces a net loss of diversity in the populations studied and shifts in the community structure. It has also been shown that some ciliates and testate amoebae, due to their population responses to increased N concentrations, could potentially be used as bio-indicators of N contamination in these BSCs.

Climate dictates microbial community composition and diversity in Australian biological soil crusts (biocrusts)

The soil surface of drylands can typically be colonized by cyanobacteria and other microbes, forming biological soil crusts or 'biocrusts'. Biocrusts provide critical benefits to ecosystems and are a common component of the largely arid and semi-arid Australian continent. Yet, their distribution and the parameters that shape their microbial composition have not been investigated. We present here the first detailed description of Australia's biocrust microbiome assessed from 15 sites across the continent using 16S rRNA sequencing. The most abundant bacterial phyla from all sites were Cyanobacteria, Proteobacteria, Actinobacteria, Chloroflexi and Bacteroidetes. Cyanobacterial communities from northern regions were more diverse and unclassified cyanobacteria were a noticeable feature of northern biocrusts. Segregation between northern and southern regions was largely due to the differential abundance of Microcoleus spp., with M. paludosus dominating in the north and M. vaginatus dominating in the south. The geographical shifts in bacterial composition and diversity were correlated to seasonal temperatures and summer rainfall. Our findings provide an initial reference for sampling strategies to maximize access to bacterial genetic diversity. As hubs for essential ecosystem services, further investigation into biocrusts in arid and semi-arid regions may yield discoveries of genetic mechanisms that combat increases in warming due to climate change.

Biocrusts Modulate Climate Change Effects on Soil Organic Carbon Pools: Insights From a 9-Year Experiment

Accumulating evidence suggests that warming associated with climate change is decreasing the total amount of soil organic carbon (SOC) in drylands, although scientific research has not given enough emphasis to particulate (POC) and mineral-associated organic carbon (MAOC) pools. Biocrusts are a major biotic feature of drylands and have large impacts on the C cycle, yet it is largely unknown whether they modulate the responses of POC and MAOC to climate change. Here, we assessed the effects of simulated climate change (control, reduced rainfall (RE), warming (WA), and RE + WA) and initial biocrust cover (low (< 20%) versus high (> 50%)) on the mineral protection of soil C and soil organic matter quality in a dryland ecosystem in central Spain for 9 years. At low initial biocrust cover levels, both WA and RE + WA increased SOC, especially POC but also MAOC, and promoted a higher contribution of carbohydrates, relative to aromatic compounds, to the POC fraction. These results suggest that the accumulation of soil C under warming treatments may be transitory in soils with low initial biocrust cover. In soils with high initial biocrust cover, climate change treatments did not affect SOC, neither POC nor MAOC fraction. Overall, our results indicate that biocrust communities modulate the negative effect of climate change on SOC, because no losses of soil C were observed with the climate manipulations under biocrusts. Future work should focus on determining the long-term persistence of the observed buffering effect by biocrust-forming lichens, as they are known to be negatively affected by warming.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10021-022-00779-0.

Continuous monitoring of chlorophyll a fluorescence and microclimatic conditions reveals warming-induced physiological damage in biocrust-forming lichens

PURPOSE

Biocrust communities, which are important regulators of multiple ecosystem functions in drylands, are highly sensitive to climate change. There is growing evidence of the negative impacts of warming on the performance of biocrust constituents like lichens in the field. Here, we aim to understand the physiological basis behind this pattern.

METHODS

Using a unique manipulative climate change experiment, we monitored every 30 minutes and for 9 months the chlorophyll a fluorescence and microclimatic conditions (lichen surface temperature, relative moisture and photosynthetically active radiation) of Psora decipiens, a key biocrust constituent in drylands worldwide. This long-term monitoring resulted in 11,847 records at the thallus-level, which allowed us to evaluate the impacts of ~2.3 °C simulated warming treatment on the physiology of Psora at an unprecedented level of detail.

RESULTS

Simulated warming and the associated decrease in relative moisture promoted by this treatment negatively impacted the physiology of Psora, especially during the diurnal period of the spring, when conditions are warmer and drier. These impacts were driven by a mechanism based on the reduction of the length of the periods allowing net photosynthesis, and by declines in Yield and Fv/Fm under simulated warming.

CONCLUSION

Our study reveals the physiological basis explaining observed negative impacts of ongoing global warming on biocrust-forming lichens in the field. The functional response observed could limit the growth and cover of biocrust-forming lichens in drylands in the long-term, negatively impacting in key soil attributes such as biogeochemical cycles, water balance, biological activity and ability of controlling erosion.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s11104-022-05686-w.

The long-term effect of removing the UV-protectant usnic acid from the thalli of the lichen Cladonia foliacea

Terricolous lichens are abundant in semi-arid areas, where they are exposed to high irradiation. Photoprotection is essential for the algae as the photobiont provides the primer carbon source for both symbionts. The UV-protectant lichen metabolites and different quenching procedures of the alga ensure adequate photoprotection. Since the long-term effect of diminishing UV-protectant lichen metabolites is unknown, a major part of lichen secondary metabolites was removed from Cladonia foliacea thalli by acetone rinsing, and the lichens were then maintained under field conditions to investigate the effect on both symbionts for 3 years. Our aim was to determine if the decreased level of UV-protectant metabolites caused an elevated photoprotection in the algae and to reveal the dynamics of production of the metabolites. Photosynthetic activity and light protection were checked by chlorophyll a fluorescence kinetics measurements every 6 months. The concentrations of fumarprotocetraric and usnic acids were monitored by chromatographic methods. Our results proved that seasonality had a more pronounced effect than that of acetone treatment on the function of lichens over a long-term scale. Even after 3 years, the acetone-treated thalli contained half as much usnic acid as the control thalli, and the level of photoprotection remained unchanged in the algae. However, the amount of available humidity was a more critical limiting environmental factor than the amount of incoming irradiation affecting usnic acid production. The lichenicolous fungus Didymocyrtis cladoniicola became relatively more abundant in the acetone-treated samples than in the control samples, indicating a slight change caused by the treatment.

Biocrusts increase the resistance to warming-induced increases in topsoil P pools

Ongoing global warming and alterations in rainfall patterns driven by climate change are known to have large impacts on biogeochemical cycles, particularly on drylands. In addition, the global increase in atmospheric nitrogen (N) deposition can destabilize primary productivity in terrestrial ecosystems, and phosphorus (P) may become the most limiting nutrient in many terrestrial ecosystems. However, the impacts of climate change on soil P pools in drylands remain poorly understood. Furthermore, it is unknown whether biocrusts, a major biotic component of drylands worldwide, modulate such impacts.Here we used two long-term (8-10 years) experiments conducted in Central (Aranjuez) and SE (Sorbas) Spain to test how a ~2.5°C warming, a ~30% rainfall reduction and biocrust cover affected topsoil (0-1 cm) P pools (non-occluded P, organic P, calcium bound P, occluded P and total P).Warming significantly increased most P pools-except occluded P-in Aranjuez, whereas only augmented non-occluded P in Sorbas. The rainfall reduction treatment had no effect on the soil P pools at any experimental site. Biocrusts increased most soil P pools and conferred resistance to simulated warming for major P pools at both sites, and to rainfall reduction for non-occluded and occluded P in Aranjuez. Synthesis. Our findings provide novel insights on the responses of soil P pools to warming and rainfall reduction, and highlight the importance of biocrusts as modulators of these responses in dryland ecosystems. Our results suggest that the observed negative impacts of warming on dryland biocrust communities will decrease their capacity to buffer changes in topsoil P driven by climate change.

Contrasting seasonal patterns and factors regulating biocrust N2-fixation in two Florida agroecosystems

Biocrusts are communities of microorganisms within the top centimeter of soil, often dominated by phototrophic dinitrogen-fixing (N₂-fixing) organisms. They are common globally in arid ecosystems and have recently been identified in agroecosystems. However, unlike natural ecosystem biocrusts, agroecosystem biocrusts receive regular fertilizer and irrigation inputs. These inputs could influence seasonal biocrust N₂-fixation and their relationship with soil nutrients in perennial agroecosystems, which is of particular interest given crop management requirements. In this study, biocrust and adjacent bare soil N₂-fixation activity was measured in the field during the summer, fall, spring, and winter seasons in a Florida citrus orchard and vineyard using both acetylene reduction assays and ¹⁵N₂ incubations. Samples were analyzed for microbial and extractable carbon (MBC, EC), nitrogen (MBN, EN), and phosphorus (MBP, EP). In both agroecosystems, biocrusts had greater microbial biomass and extractable nutrients compared to bare soil. The citrus and grape biocrusts were both actively fixing N₂, despite crop fertilization, with rates similar to those found in natural arid and mesic systems, from 0.1 to 142 nmol of C₂H₄ g^–1 of biocrust dry weight h^–1 (equivalent to 1–401 μmol m^–2h^–1). Lower soil temperatures and higher EC:EN ratios were associated with higher N₂-fixation rates in citrus biocrusts, while higher soil moisture and higher EP were associated with higher N₂-fixation rates in grape biocrusts. The N₂-fixation activity of these agroecosystem biocrusts indicates the possibility of biocrusts to enhance N cycling in perennial agroecosystems, with potential benefits for crop production.

Ecology and responses to climate change of biocrust-forming mosses in drylands

Interest in understanding the role of biocrusts as ecosystem engineers in drylands has substantially increased during the past two decades. Mosses are a major component of biocrusts and dominate their late successional stages. In general, their impacts on most ecosystem functions are greater than those of early-stage biocrust constituents. However, it is common to find contradictory results regarding how moss interactions with different biotic and abiotic factors affect ecosystem processes. This review aims to (i) describe the adaptations and environmental constraints of biocrust-forming mosses in drylands, (ii) identify their primary ecological roles in these ecosystems, and (iii) synthesize their responses to climate change. We emphasize the importance of interactions between specific functional traits of mosses (e.g. height, radiation reflectance, morphology, and shoot densities) and both the environment (e.g. climate, topography, and soil properties) and other organisms to understand their ecological roles and responses to climate change. We also highlight key areas that should be researched in the future to fill essential gaps in our understanding of the ecology and the responses to ongoing climate change of biocrust-forming mosses. These include a better understanding of intra- and interspecific interactions and mechanisms driving mosses' carbon balance during desiccation-rehydration cycles.

Biological soil crusts and how they might colonize other worlds: insights from these Brazilian ecosystem engineers

When bryophytes, lichens, eukaryotic algae, cyanobacteria, bacteria, and fungi live interacting intimately with the most superficial particles of the soil, they form a complex community of organisms called the biological soil crust (BSC or biocrust). These biocrusts occur predominantly in drylands, where they provide important ecological services such as soil aggregation, moisture retention, and nitrogen fixation. Unfortunately, many BSC communities remain poorly explored, especially in the tropics. This review summarizes studies about BSCs in Brazil, a tropical megadiverse country, and shows the importance of ecological, physiological, and taxonomic knowledge of biocrusts. We also compare Brazilian BSC communities with others around the world, describe why BSCs can be considered ecosystem engineers, and propose their use in the colonization of other worlds.

Functional diversity regulates the effects of habitat degradation on biocrust phylogenetic and taxonomic diversities

Biocrusts are major contributors to dryland diversity, functioning, and services. However, little is known about how habitat degradation will impact multiple facets of biocrust diversity and measurable functional traits. We evaluated changes in taxonomic, functional, and phylogenetic diversity of biocrust-forming lichens along a habitat degradation gradient related to the presence of linear infrastructure (i.e., a road) and a profound agricultural driven transformation. To do so, we selected 50 remnants of a Mediterranean shrubland. We considered several surrogates of habitat quality and causal disturbance on the various diversity facets of biocrusts by using structural equation modeling, hypothesizing that habitat degradation primarily affects functional diversity, which in turn regulates changes in taxonomic and phylogenetic diversities, and also that taxonomic and phylogenetic diversities are coupled. Fragment connectivity, distance to linear infrastructure (i.e., a road) and, particularly, soil fertility (i.e., soil P concentration), had mostly negative effects on biocrust functional diversity, which in turn affected both taxonomic and phylogenetic diversities. However, we found no direct effects of habitat degradation variables on the taxonomic and phylogenetic diversities. We also found that increases in phylogenetic diversity had a positive effect on taxonomic diversity along the habitat degradation gradient. Our results indicate that functional diversity of biocrusts is strongly affected by habitat degradation, which may profoundly alter their contribution to ecosystem functioning and services. Furthermore, functional diversity regulates the response of biocrust taxonomic and phylogenetic diversity to habitat degradation. These findings indicate that habitat degradation alters and simplifies the diversity of functional traits of biocrust-forming lichens, leading to biodiversity loss, with important consequences for the conservation of global drylands biodiversity.

Molecularly cleavable bioinks facilitate high-performance digital light processing-based bioprinting of functional volumetric soft tissues

Digital light processing bioprinting favors biofabrication of tissues with improved structural complexity. However, soft-tissue fabrication with this method remains a challenge to balance the physical performances of the bioinks for high-fidelity bioprinting and suitable microenvironments for the encapsulated cells to thrive. Here, we propose a molecular cleavage approach, where hyaluronic acid methacrylate (HAMA) is mixed with gelatin methacryloyl to achieve high-performance bioprinting, followed by selectively enzymatic digestion of HAMA, resulting in tissue-matching mechanical properties without losing the structural complexity and fidelity. Our method allows cellular morphological and functional improvements across multiple bioprinted tissue types featuring a wide range of mechanical stiffness, from the muscles to the brain, the softest organ of the human body. This platform endows us to biofabricate mechanically precisely tunable constructs to meet the biological function requirements of target tissues, potentially paving the way for broad applications in tissue and tissue model engineering.

Water Availability, Soil Characteristics, and Confounding Effects on the Patterns of Biocrust Diversity in the Desert Regions of Northern China

The species diversity of biocrusts is an important community characteristic in determining their multiple ecosystem functions. Hence, understanding the diversity patterns of biocrusts and their environmental drivers is of fundamental importance. However, explain variables often correlated with each other; thus, the confounding effects among them may arise and result in spurious causal relationships and biased ecological inferences. In this study, we investigated the richness of three biocrust-forming components (mosses, lichens, and cyanobacteria-algae) and their environmental variables across six desert regions of northern China. A comparison between conventional redundancy analysis (RDA) and structural equation model (SEM) was conducted to study the environmental driver-richness relationship and the confounding effects. Our results showed that three latent variables related to water availability, soil texture, and soil salinity and sodicity, could account for the main environmental variations and explain the diversity patterns of biocrusts at the intracontinental scale. Water availability was positively and negatively related to the richness of mosses and cyanobacteria-algae, respectively, while soil texture was positively related to the richness of lichens. In addition, environmental variables confounded with each other caused distinct driver-richness relationships between results of RDA and SEM. Therefore, we suggest that future multivariable studies should utilize path analysis in conjunction with conventional canonical ordination to facilitate more rigorous ecological inferences.

What is a biocrust? A refined, contemporary definition for a broadening research community

Studies of biological soil crusts (biocrusts) have proliferated over the last few decades. The biocrust literature has broadened, with more studies assessing and describing the function of a variety of biocrust communities in a broad range of biomes and habitats and across a large spectrum of disciplines, and also by the incorporation of biocrusts into global perspectives and biogeochemical models. As the number of biocrust researchers increases, along with the scope of soil communities defined as ‘biocrust’, it is worth asking whether we all share a clear, universal, and fully articulated definition of what constitutes a biocrust. In this review, we synthesize the literature with the views of new and experienced biocrust researchers, to provide a refined and fully elaborated definition of biocrusts. In doing so, we illustrate the ecological relevance and ecosystem services provided by them. We demonstrate that biocrusts are defined by four distinct elements: physical structure, functional characteristics, habitat, and taxonomic composition. We describe outgroups, which have some, but not all, of the characteristics necessary to be fully consistent with our definition and thus would not be considered biocrusts. We also summarize the wide variety of different types of communities that fall under our definition of biocrusts, in the process of highlighting their global distribution. Finally, we suggest the universal use of the Belnap, Büdel & Lange definition, with minor modifications: Biological soil crusts (biocrusts) result from an intimate association between soil particles and differing proportions of photoautotrophic (e.g. cyanobacteria, algae, lichens, bryophytes) and heterotrophic (e.g. bacteria, fungi, archaea) organisms, which live within, or immediately on top of, the uppermost millimetres of soil. Soil particles are aggregated through the presence and activity of these often extremotolerant biota that desiccate regularly, and the resultant living crust covers the surface of the ground as a coherent layer. With this detailed definition of biocrusts, illustrating their ecological functions and widespread distribution, we hope to stimulate interest in biocrust research and inform various stakeholders (e.g. land managers, land users) on their overall importance to ecosystem and Earth system functioning.

Responses of Biocrust and Associated Soil Bacteria to Novel Climates Are Not Tightly Coupled

Climate change is expanding drylands even as land use practices degrade them. Representing ∼40% of Earth’s terrestrial surface, drylands rely on biological soil crusts (biocrusts) for key ecosystem functions including soil stability, biogeochemical cycling, and water capture. Understanding how biocrusts adapt to climate change is critical to understanding how dryland ecosystems will function with altered climate. We investigated the sensitivity of biocrusts to experimentally imposed novel climates to track changes in productivity and stability under both warming and cooling scenarios. We established three common gardens along an elevational-climate gradient on the Colorado Plateau. Mature biocrusts were collected from each site and reciprocally transplanted intact. Over 20 months we monitored visible species composition and cover, chlorophyll a, and the composition of soil bacterial communities using high throughput sequencing. We hypothesized that biocrusts replanted at their home site would show local preference, and biocrusts transplanted to novel environments would maintain higher cover and stability at elevations higher than their origin, compared to at elevations lower than their origin. We expected responses of the visible biocrust cover and soil bacterial components of the biocrust community to be coupled, with later successional taxa showing higher sensitivity to novel environments. Only high elevation sourced biocrusts maintained higher biocrust cover and community stability at their site of origin. Biocrusts from all sources had higher cover and stability in the high elevation garden. Later successional taxa decreased cover in low elevation gardens, suggesting successional reversal with warming. Visible community composition was influenced by both source and transplant environment. In contrast, soil bacterial community composition was not influenced by transplant environments but retained fidelity to the source. Thus, responses of the visible and soil bacterial components of the biocrust community were not coupled. Synthesis: Our results suggest biocrust communities are sensitive to climate change, and loss of species and function can be expected, while associated soil bacteria may be buffered against rapid change.

Physical Disturbance Reduces Cyanobacterial Relative Abundance and Substrate Metabolism Potential of Biological Soil Crusts on a Gold Mine Tailing of Central China

As the critical ecological engineers, biological soil crusts (biocrusts) are considered to play essential roles in improving substrate conditions during ecological rehabilitation processes. Physical disturbance, however, often leads to the degradation of biocrusts, and it remains unclear how the physical disturbance affects biocrust microorganisms and their related metabolism. In this study, the photosynthetic biomass (indicated by chlorophyll a), nutrients, enzyme activities, and bacterial communities of biocrusts were investigated in a gold mine tailing of Central China to evaluate the impact of physical disturbance on biocrusts during the rehabilitation process of gold mine tailings. The results show that physical disturbance significantly reduced the photosynthetic biomass, nutrient contents (organic carbon, ammonium nitrogen, nitrate nitrogen, and total phosphorus), and enzyme activities (β-glucosidase, sucrase, nitrogenase, neutral phosphatase, and urease) of biocrusts in the mine tailings. Furthermore, 16S rDNA sequencing showed that physical disturbance strongly changed the composition, structure, and interactions of the bacterial community, leading to a shift from a cyanobacteria dominated community to a heterotrophic bacteria (proteobacteria, actinobacteria, and acidobacteria) dominated community and a more complex bacterial network (higher complexity, nodes, and edges). Altogether, our results show that the biocrusts dominated by cyanobacteria could also develop in the tailings of humid region, and the dominants (e.g., Microcoleus) were the same as those from dryland biocrusts; nevertheless, physical disturbance significantly reduced cyanobacterial relative abundance in biocrusts. Based on our findings, we propose the future work on cyanobacterial inoculation (e.g., Microcoleus), which is expected to promote substrate metabolism and accumulation, ultimately accelerating the development of biocrusts and the subsequent ecological restoration of tailings.

Decline in biological soil crust N-fixing lichens linked to increasing summertime temperatures

Across many global drylands, biocrusts form a protective barrier on the soil surface and fill many critical roles in these harsh yet fragile environments. Previous short-term research suggests that climate change and invasive plant introduction can damage and alter biocrust communities, yet few long-term observations exist. Using a globally unique long-term record of continuous biocrust surveys from a rare never-grazed, protected grassland on the US Colorado Plateau, we found lichen species diversity and cover to be negatively correlated with increasing summer air temperatures, while moss species showed more sensitivity to variation in precipitation and invasive grass cover. These results suggest that dryland systems may be at a critical tipping point where ongoing warming could result in biological soil crust degradation.

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

Non-Toxic Increases in Nitrogen Availability Can Improve the Ability of the Soil Lichen Cladonia rangiferina to Cope with Environmental Changes

Climate change and atmospheric nitrogen (N) deposition on drylands are greatly threatening these especially vulnerable areas. Soil biocrust-forming lichens in drylands can provide early indicators of these disturbances and play a pivotal role, as they contribute to key ecosystem services. In this study, we explored the effects of different long-term water availability regimes simulating climate changes and their interaction with N addition on the physiological response of the soil lichen Cladonia rangiferina. Three sets of this lichen were subjected to control, reduced watering, and reduced watering and N addition (40 kg NH₄NO₃ ha⁻¹ year⁻¹) treatments for 16 months. Finally, all samples were subjected to daily hydration cycles with N-enriched water at two levels (40 and 80 kg NH₄NO₃ ha⁻¹ year⁻¹) for 23 days. We found that reduced watering significantly decreased the vitality of this lichen, whereas N addition unexpectedly helped lichens subjected to reduced watering to cope with stress produced by high temperatures. We also found that long-term exposure to N addition contributed to the acclimation to higher N availability. Overall, our data suggest that the interactions between reduced watering and increased N supply and temperature have an important potential to reduce the physiological performance of this soil lichen.

Multiple resource limitation of dryland soil microbial carbon cycling on the Colorado Plateau

Understanding interactions among biogeochemical cycles is increasingly important as anthropogenic alterations of global climate and of carbon (C), nitrogen (N), and phosphorus (P) cycles interactively affect the Earth system. Ecosystem processes in the dryland biome, which makes up over 40% of Earth's terrestrial surface, are often distinctively sensitive to small changes in resource availability, likely because levels of many resources are low. However, data also suggest that simultaneous changes in the availability of multiple resources may be necessary to affect a response in these low-resource systems, offering an opportunity to test patterns and controls of co-limitation, serial limitation, and individual limitation in soil environments. While drylands may play a governing role in key aspects of Earth's C cycle, and while an improved understanding of resource limitation could substantially improve our forecasts of dryland responses to change, our understanding of interacting controls on soil C cycle processes remains notably poor in these dry systems. Here, we address multiple fundamental hypotheses of resource controls over ecosystem function to test how water, C, N, and P regulate soil C cycling individually and interactively in a dryland ecosystem on the Colorado Plateau. Using a series of laboratory incubations, we found that while water, C, and N limited C cycling through serial limitation, water alone resulted in an extremely small respiratory response from target organisms, whereas water + C resulted in a dramatic increase in soil C cycling, suggesting a degree of functional co-limitation. Nitrogen additions alone resulted in no changes to soil C cycling, but when N was added in concert with water and C, N greatly increased soil C cycling rates relative to additions of water and C without N. Phosphorus additions had no effect on the C cycle either alone or synergistically. These patterns were consistent with the stoichiometry of the system, and interactions among resources were surprising in ways that inform our understanding of critical theories in ecology, such as the Transient Maxima Hypothesis, supporting the suggestion that multiple resource limitation explains pulse-dynamic C cycling in drylands better than water limitation alone.

Disturbance to biocrusts decreased cyanobacteria, N-fixer abundance, and grass leaf N but increased fungal abundance

Interactions between plants and soil microbes influence plant nutrient transformations, including nitrogen (N) fixation, nutrient mineralization, and resource exchanges through fungal networks. Physical disturbances to soils can disrupt soil microbes and associated processes that support plant and microbial productivity. In low resource drylands, biological soil crusts ("biocrusts") occupy surface soils and house key autotrophic and diazotrophic bacteria, non-vascular plants, or lichens. Interactions among biocrusts, plants, and fungal networks between them are hypothesized to drive carbon and nutrient dynamics; however, comparisons across ecosystems are needed to generalize how soil disturbances alter microbial communities and their contributions to N pools and transformations. To evaluate linkages among plants, fungi, and biocrusts, we disturbed all unvegetated surfaces with human foot trampling twice yearly in dry conditions from 2013-2019 in cyanobacteria-dominated biocrusts in Chihuahuan Desert grassland and shrubland ecosystems. After five years, disturbance decreased the abundances of cyanobacteria (especially Microcoleus steenstrupii clade) and N-fixers (Scytonema sp., and Schizothrix sp.) by >77% and chlorophyll a by up to 55%, but conversely, increased soil fungal abundance by 50% compared to controls. Responses of root-associated fungi differed between the two dominant plant species and ecosystem types, with a maximum of 80% more aseptate hyphae in disturbed than control plots. Although disturbance did not affect ¹⁵ N tracer transfer from biocrusts to the dominant grass, Bouteloua eriopoda, disturbance increased available soil N by 65% in the shrubland, and decreased leaf N of B. eriopoda up to 16%, suggesting that although rapid N transfer during peak production was not affected by disturbance, over the long term, plant nutrient content was disrupted. Altogether, the shrubland may be more resilient to detrimental changes due to disturbance than grassland, and these results demonstrate that disturbances to soil microbial communities have potential to cause substantial changes in N pools by reducing and reordering biocrust taxa.

Soil Chemistry and Nutrients Influence the Distribution of Aerobic Anoxygenic Phototrophic Bacteria and Eukaryotic Phototrophic Microorganisms of Physical Soil Crusts at Different Elevations on the Tibetan Plateau

Photosynthetic microorganisms are widely distributed in the soil and play an important role in plant-free soil crusts. However, the distribution and environmental drivers of phototrophic microbial communities in physical soil crusts, where the abundance of cyanobacteria is low, are scarcely understood. Here, we performed high-throughput sequencing of pufM and 18S rRNA genes in soil crusts at different elevations on the Tibetan Plateau and used the data combined with environmental variables to analyze the diversity and structure of phototrophic microbial communities. We found that the dominant taxa of aerobic anoxygenic phototrophic bacteria (AAPB) and eukaryotic phototrophic microorganisms (EPM) were shown to shift with elevation. The phototrophic microbial diversity showed a single-peak pattern, with the lowest diversity of AAPB and highest diversity of EPM at middle elevations. Moreover, the elevation and soil property determined the phototrophic microbial community. Soil salts, especially Cl^-, were the most important for AAPB. Likewise, soil nutrients, especially carbon, were the most important for EPM. The relationship between high-abundance taxa and environmental variables showed that Rhizobiales was significantly negatively correlated with salt ions and positively correlated with chlorophyll. Rhodobacterales showed the strongest and significant positive associations with Cl^-. Chlorophyceae and Bacillariophyceae were positively correlated with CO₃^2-. These results indicated that salinity and soil nutrients affected the diversity and structure of microbial communities. This study contributes to our understanding of the diversity, composition, and structure of photosynthetic microorganisms in physical soil crusts and helps in developing new approaches for controlling desertification and salinization and improving the desert ecological environment.

Interannual climate variability mediates changes in carbon and nitrogen pools caused by annual grass invasion in a semiarid shrubland

Exotic plant invasions alter ecosystem properties and threaten ecosystem functions globally. Interannual climate variability (ICV) influences both plant community composition (PCC) and soil properties, and interactions between ICV and PCC may influence nitrogen (N) and carbon (C) pools. We asked how ICV and non-native annual grass invasion covary to influence soil and plant N and C in a semiarid shrubland undergoing widespread ecosystem transformation due to invasions and altered fire regimes. We sampled four progressive stages of annual grass invasion at 20 sites across a large (25,000 km² ) landscape for plant community composition, plant tissue N and C, and soil total N and C in 2013 and 2016, which followed 2 years of dry and wet conditions, respectively. Multivariate analyses and ANOVAs showed that in invasion stages where native shrub and perennial grass and forb communities were replaced by annual grass-dominated communities, the ecosystem lost more soil N and C in wet years. Path analysis showed that high water availability led to higher herbaceous cover in all invasion stages. In stages with native shrubs and perennial grasses, higher perennial grass cover was associated with increased soil C and N, while in annual-dominated stages, higher annual grass cover was associated with losses of soil C and N. Also, soil total C and C:N ratios were more homogeneous in annual-dominated invasion stages as indicated by within-site standard deviations. Loss of native shrubs and perennial grasses and forbs coupled with annual grass invasion may lead to long-term declines in soil N and C and hamper restoration efforts. Restoration strategies that use innovative techniques and novel species to address increasing temperatures and ICV and emphasize maintaining plant community structure-shrubs, grasses, and forbs-will allow sagebrush ecosystems to maintain C sequestration, soil fertility, and soil heterogeneity.

Cyanobacterial persistence and influence on microbial community dynamics over 15 years in induced biocrusts

Biocrusts provide numerous ecological functions in drylands. Recovering biocrusts via cyanobacterial inoculation recently gathered interest for ecological restoration, yet it still lacks long-term experiments to unravel biocrust community dynamics. To examine how cyanobacterial inoculants influenced local microbial community and biocrust development, we observed a 2 km² (Qubqi Desert, China) inoculation experiment after 10 and 15 years, following biocrust formation. Our results revealed that biocrust development was in line with ecological regime shift, providing evidence for biocrust community succession, from cyanobacteria- to moss-dominated types. Associated with biocrust development, microbial communities differed significantly with less specialists compared to shifting sands. Cyanobacterial community analysis showed that Microcoleus vaginatus and Scytonema javanicum are an ideal inoculating model, as they were still dominating the community after 15 years since inoculation, while other nitrogen-fixing cyanobacteria occurred profusely with biocrust development. Biocrust community composition combined with thickness, Chl-a and exopolysaccharide measurements revealed the large variation of cyanobacterial ecological functions along biocrust development, suggesting a main function shift: from carbon fixation associated with exopolysaccharide secretion in bare sandy soils to nitrogen fixation in developed biocrusts. This large-scale field study verifies that cyanobacterial inoculation accelerates biocrust development and forwards succession, shaping the biocrust community composition over a long time.

Incorporating Biogeochemistry into Dryland Restoration

Dryland degradation is a persistent and accelerating global problem. Although the mechanisms initiating and maintaining dryland degradation are largely understood, returning productivity and function through ecological restoration remains difficult. Water limitation commonly drives slow recovery rates within drylands; however, the altered biogeochemical cycles that accompany degradation also play key roles in limiting restoration outcomes. Addressing biogeochemical changes and resource limitations may help improve restoration efforts within this difficult-to-restore biome. In the present article, we present a synthesis of restoration literature that identifies multiple ways biogeochemical understandings might augment dryland restoration outcomes, including timing restoration around resource cycling and uptake, connecting heterogeneous landscapes, manipulating resource pools, and using organismal functional traits to a restoration advantage. We conclude by suggesting ways to incorporate biogeochemistry into existing restoration frameworks and discuss research directions that may help improve restoration outcomes in the world's highly altered dryland landscapes.

Distinct patterns of abundant and rare subcommunities in paddy soil during wetting-drying cycles

Wetting-drying cycles typically result in a wide range of soil moistures and redox potentials (Eh) that significantly affect the soil microbial community. Although numerous studies have addressed the effects of soil moisture on soil microbial community structure and composition, the response of active microbes to the fluctuation in soil Eh is still largely unknown; this is especially true for the ecological roles of abundant and rare taxa. To explore the dynamics of active and total microbial communities in response to wetting-drying cycles, we conducted a microcosm experiment based on three wetting-drying cycles and 16S rRNA transcript (active) and 16S rRNA gene (total) amplicon sequencing. We found that both active and total microbial communities during three wetting-drying cycles were clustered according to the number of wetting-drying cycles (temporal factor) rather than soil moisture or Eh. Dynamics of the active microbial community, however, were redox dependent during the first wetting-drying cycle. In addition, rare taxa in the active microbial community exhibited more obvious differences than abundant ones during three wetting-drying cycles. Species turnover of abundant and rare taxa of total and active microbes, rather than species richness, explained the highest percentage of community variation. Rare taxa exhibited the most marked temporal turnover during three wetting-drying cycles. Members of Rhodospirillaceae were the major contributor to the resilience of abundant taxa of active microbes during the first wetting-drying cycle. Overall, these findings expand our current understanding of underlying assembly mechanisms of soil microbial communities responding to wetting-drying cycles.

Biological Soil Crust Bacterial Communities Vary Along Climatic and Shrub Cover Gradients Within a Sagebrush Steppe Ecosystem

Numerous studies have examined bacterial communities in biological soil crusts (BSCs) associated with warm arid to semiarid ecosystems. Few, however, have examined bacterial communities in BSCs associated with cold steppe ecosystems, which often span a wide range of climate conditions and are sensitive to trends predicted by relevant climate models. Here, we utilized Illumina sequencing to examine BSC bacterial communities with respect to climatic gradients (elevation), land management practices (grazing vs. non-grazing), and shrub/intershrub patches in a cold sagebrush steppe ecosystem in southwestern Idaho, United States. Particular attention was paid to shifts in bacterial community structure and composition. BSC bacterial communities, including keystone N-fixing taxa, shifted dramatically with both elevation and shrub-canopy microclimates within elevational zones. BSC cover and BSC cyanobacteria abundance were much higher at lower elevation (warmer and drier) sites and in intershrub areas. Shrub-understory BSCs were significantly associated with several non-cyanobacteria diazotrophic genera, including Mesorhizobium and Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium. High elevation (wetter and colder) sites had distinct, highly diverse, but low-cover BSC communities that were significantly indicated by non-cyanobacterial diazotrophic taxa including families in the order Rhizobiales and the family Frankiaceae. Abiotic soil characteristics, especially pH and ammonium, varied with both elevation and shrub/intershrub level, and were strongly associated with BSC community composition. Functional inference using the PICRUSt pipeline identified shifts in putative N-fixing taxa with respect to both the elevational gradient and the presence/absence of shrub canopy cover. These results add to current understanding of biocrust microbial ecology in cold steppe, serving as a baseline for future mechanistic research.

Resistance, Resilience, and Recovery of Dryland Soil Bacterial Communities Across Multiple Disturbances

Dryland ecosystems are sensitive to perturbations and generally slow to recover post disturbance. The microorganisms residing in dryland soils are especially important as they contribute to soil structure and nutrient cycling. Disturbance can have particularly strong effects on dryland soil structure and function, yet the natural resistance and recovery of the microbial components of dryland soils has not been well documented. In this study, the recovery of surface soil bacterial communities from multiple physical and environmental disturbances is assessed. Samples were collected from three field sites in the vicinity of Moab, UT, United States, 6 to 7 years after physical and climate disturbance manipulations had been terminated, allowing for the assessment of community recovery. Additionally, samples were collected in a transect that included three habitat patches: the canopy zone soils under the dominant shrubs, the interspace soils that are colonized by biological soil crusts, and edge soils at the plot borders. Field site and habitat patch were significant factors structuring the bacterial communities, illustrating that sites and habitats harbored unique soil microbiomes. Across the different sites and disturbance treatments, there was evidence of significant bacterial community recovery, as bacterial biomass and diversity were not significantly different than control plots. There was, however, a small number of 16S rRNA gene amplicon sequence variants that distinguished particular treatments, suggesting that legacy effects of the disturbances still remained. Taken together, these data suggest that dryland bacterial communities may possess a previously unappreciated potential to recover within years of the original disturbance.