Crust Organisms Research Articles

The enigmatic enrichment of potassium and magnesium in runoff and floodwater in the Negev: Do biocrusts hold the key?

Regardless of lithology and plant cover, chemical composition of floodwater in the Negev show a consistent enrichment in K+ and Mg2+ ions, which could not have been explained by the rock or clay minerals or (due to the scarcity of plants) by plant decomposition. Hypothesizing that rock-dwelling (lithobionts) or soil (loess)-dwelling biocrusts may shed light on the phenomena, we conducted sprinkling experiments in the Negev Highlands. Sprinkling was conducted on 4 types of lithobionts: cyanobacteria which inhabit the south-facing bedrock (ENC), epilithic lichens, inhabiting the inclined (EPIi) and the flat (EPIf) north-facing bedrocks, and endolithic lichens (ENL) inhabiting south-facing boulders. Additional sprinkling took place on two types of soil biocrusts, a mixed crust composed of cyanobacteria, lichens and mosses at the north-facing footslope and a cyanobacterial crust at the more xeric south-facing footslope. The runoff water (of 5 and for 4 plots for each lithobiont and soil biocrust type, respectively) was analyzed for the ionic composition of Na+, K+, Mg2+, Ca2+, NH4+, Cl−, SO42−, and NO3−, whereas HCO3− was calculated. In comparison to rainwater, all habitats (except for K+ in ENL) showed high enrichment ratios (ERs) in K+ and Mg2+, which, unlike the high ERs of the other ions (such as SO42− that may stem from gypsum dissolution), could not have been explained by the rock lithology, clay or dust composition. It is suggested that following wetting, K+, serving for osmoregulation of cells, is released by the crust organisms, being thus responsible for K+ enrichment, while chlorophyll degradation is responsible for Mg2+ enrichment. It is suggested that rock- and soil-dwelling microorganisms may explain K+ and Mg2+ enrichment in runoff and floodwater and subsequently in groundwater of the Negev, and possibly in other arid zones worldwide, affecting in turn the quality of irrigation and drinking water.

Facilitating restoration of degraded biological soil crusts using mixed culture inoculation

In arid and semiarid ecosystems, the biological soil crust (biocrust) community is ecologically critical and it is advantageous to restore damaged biocrust. Coastal sage scrub (CSS) is a unique and imperiled valuable habitat. At present, studies of biocrust restoration in CSS habitat do not exist. This study examined the feasibility of isolating and culturing a mix of endemic CSS cyanobacteria and green algae to inoculate native CSS soil, thereby facilitating recovery of disturbed biocrust. To gauge whether inoculation increases soil function, we measured culture growth, chlorophyll a, extractable polysaccharides, and stability. Growth of biocrust organisms and increases in functionality were compared among three inoculation treatments (mixed culture, ground biocrust, and uninoculated control) with either autoclaved or non-autoclaved native soil to determine the extent that native crust organisms can regrow without inoculation. A novel genus and species of cyanobacteria was isolated and classified as Trichotorquatus maritimus. The mixed culture included a green alga, possibly Chlamydomonas callunae. Mixed inoculum added to non-autoclaved native soil significantly improved the soil functions. After two months, the mixed inoculum increased chlorophyll levels, soil stability, and increased extractable polysaccharides. It may be possible in the future to restore biocrust in CSS using mixed culture inoculation.

Spatial distribution and the influencing factors of organic carbon of biological crusts on regional scale in Mu Us sandy land, China

As an important soil cover in deserts, biological crusts play a central role in ecosystem function such as nutrient cycling, nitrogen fixation, and carbon sequestration. Many biological crust organisms could fix CO2 through photosynthesis to improve soil organic carbon content. There is a knowledge gap in the origin of soil organic carbon (SOC) from biological crusts on a regional level, which restricts the prediction of soil carbon pool. Based on 45 plots in the Mu Us sandy land (42200 km2), we measured the SOC content and soil organic carbon density (SOCD) of two types of typical biological crusts (moss crusts, algal crusts) and their underlying soils, and analyzed together with the climate data, soil and vegetation factors to investigate the spatial distribution characteristics and controlling factors of organic carbon of biological crusts at the regional scale. The results showed that: 1) biological crusts significantly increased SOC and SOCD compared with bare ground. Moss crusts and the underlying SOC (4.93 g·kg-1) and SOCD (0.41 kg·m-2) were higher than those of algal crusts (1.89 g·kg-1, 0.18 kg·m-2). 2) On the regional scale, the SOC and SOCD of biological crusts had clear spatial distribution characteristics, demonstrating a banded distribution and block mosaic from northeast to central and west to southeast. 3) The SOC and SOCD of biological crusts and their underlying soils were mainly affected by climate, soil and vegetation conditions, while the main controlling factors depended on the types of biological crusts. The SOC and SOCD of moss crust were controlled by annual maximum temperature and potential evapotranspiration, whereas those of algal crusts were controlled by water vapor pressure.

Soil biocrusts affect metabolic response to hydration on dunes in west Queensland, Australia

Soil biocrusts, formed from communities of microbes and their extracellular products are a common feature of dryland soil surfaces. Biocrust organisms are only intermittently metabolically active, but due to their ubiquity they make a significant contribution to the carbon cycle. Quantification of the controls and insights into the interlinked process of photosynthesis and respiration are essential to enhancing our understanding of the carbon cycle in the world’s drylands. Yet, there have been relatively few field studies investigating controls on both biocrust photosynthesis and respiration. We undertook field-based experiments at two dune sites during the dry season in Diamantina National Park in Queensland, Australia to determine how biocrust hydration and illumination affect soil CO2 flux and photosynthesis. Static chambers and an infra-red gas analyser were used to quantify soil CO2 flux, and a fluorometer and a CFImager were used to determine a range of photosynthetic parameters in the field and laboratory respectively. When dry, biocrust photosynthetic activity was not detected and soil CO2 flux was very low irrespective of biocrust cover. Hydration led to a large and immediate increase in CO2 flux, which was more pronounced in the presence of biocrusts and on the dune with thinner biocrusts. Hydration also initiated the onset of photosynthesis in some biocrusts, which was greatest under low light conditions and sustained with further hydration. There were only infrequent periods of net CO2 uptake to the soil, occurring when CO2 uptake due to photosynthetic activity was less than background soil CO2 flux. Chlorophyll fluorescence imaging indicated biocrust spatial heterogeneity was evident at the cm scale where microtopography creates a myriad of environments for different crust organisms. Our findings demonstrate that biocrusts are highly spatially heterogenetic at both landscape and small scale, which suggests the maintenance of biocrust spatial diversity is likely to be key to imparting resilience to changing climate and disturbance. As well as reaffirming the importance of biocrusts for the carbon cycle in dryland dune soils the study demonstrates that biocrust respiration and photosynthesis respond differently to hydration and shading. This adds an unpredictability to the distribution of soil carbon stocks and the gaseous exchanges of CO2 between the surface and atmosphere. Future changes to precipitation and increased temperatures are likely to reduce soil moisture across much of the Australian interior and consequently biocrusts may experience a decline in biomass, structure, and function which could have significant repercussions beyond carbon stocks.

Lichens Bite the Dust - A Bioweathering Scenario in the Atacama Desert.

SummaryBioweathering mediated by microorganisms plays a significant role in biogeochemical cycles on global scales over geological timescales. Single processes induced by specific taxa have been described but could rarely be demonstrated for complex communities that dominate whole landscapes. The recently discovered grit crust of the coastal Atacama Desert, which is a transitional community between a cryptogamic ground cover and a rock-bound lithic assemblage, offers the unique chance to elucidate various bioweathering processes that occur simultaneously. Here, we present a bioweathering scenario of this biocenosis including processes such as penetration of the lithomatrix, microbial responses to wet-dry cycles, alkalinolysis, enzyme activity, and mineral re-localization. Frequently occurring fog, for example, led to a volume increase of microorganisms and the lithomatrix. This, together with pH shifts and dust accumulation, consequently results in biophysical breakdown and the formation of a terrestrial protopedon, an initial stage of pedogenesis fueled by the grit crust.

Reproduction and Dispersal of Biological Soil Crust Organisms

Biological soil crusts (BSCs) consist of a diverse and highly integrated community of organisms that effectively occupy and collectively stabilize soil surfaces. BSCs vary in terms of soil chemistry and texture as well as the environmental parameters that combine to support unique combinations of organisms – including cyanobacteria dominated, lichen-dominated, and bryophyte-dominated crusts. The list of organismal groups that make up BSC communities in various and unique combinations include – free living, lichenized, and mycorrhizal fungi, chemoheterotrophic bacteria, cyanobacteria, diazotrophic bacteria and archaea, eukaryotic algae, and bryophytes. The various BSC organismal groups demonstrate several common characteristics including – desiccation and extreme temperature tolerance, production of various soil binding chemistries, a near exclusive dependency on asexual reproduction, a pattern of aerial dispersal over impressive distances, and a universal vulnerability to a wide range of human-related perturbations. With this publication, we provide literature-based insights as to how each organismal group contributes to the formation and maintenance of the structural and functional attributes of BSCs, how they reproduce, and how they are dispersed. We also emphasize the importance of effective application of molecular and microenvironment sampling and assessment tools in order to provide cogent and essential answers that will allow scientists and land managers to better understand and manage the biodiversity and functional relationships of soil crust communities.

Biological Soil Crust Response to Late Season Prescribed Fire in a Great Basin Juniper Woodland

Expansion of juniper on U.S. rangelands is a significant environmental concern. Prescribed fire is often recommended to control juniper. To that end, a prescribed burn was conducted in a Great Basin juniper woodland. Conditions were suboptimal; fire did not encroach into mid- or late-seral stages and was patchy in the early-seral stage. This study evaluated the effects of the burn on biological soil crusts of early-seral juniper. Fire reduced moss cover under sagebrush and in shrub interspaces. Mosses were rare under juniper; their cover was unaffected there. Lichens were uncommon under juniper and sagebrush and therefore not significantly impacted there. Their cover was greater in shrub interspaces, but because the fire was spotty and of low intensity, the effects of burning were minimal. Compared with unburned plots, the biomass of cyanobacteria was diminished under juniper and sagebrush; it was reduced in the interspaces in both burned and unburned plots, presumably in response to generally harsher conditions in the postburn environment. Nitrogen fixation rates declined over time in juniper plots and interspaces but not in sagebrush plots. Although fire negatively affected some biological soil crust organisms in some parts of the early-seral juniper woodland, the overall impact on the crusts was minimal. If the intent of burning is to reduce juniper, burning of early-seral juniper woodland is appropriate, as most affected trees were killed. Control of sagebrush can likewise be accomplished by low-intensity, cool season fires without eliminating the crust component. Intense fire should be avoided due to the potential for greater encroachment into the shrub interspaces, which contain the majority of biological soil crust organisms. Burning early-seral juniper may be preferred for controlling juniper encroachment on rangeland.

Dew formation on the surface of biological soil crusts in central European sand ecosystems

Abstract. Dew formation was investigated in three developmental stages of biological soil crusts (BSC), which were collected along a catena of an inland dune and in the initial substrate. The Penman equation, which was developed for saturated surfaces, was modified for unsaturated surfaces and used for prediction of dewfall rates. The levels of surface saturation required for this approach were predicted using the water retention functions and the thicknesses of the BSCs. During a first field campaign (2–3 August 2011), dewfall increased from 0.042 kg m−2 for the initial sandy substrate to 0.058, 0.143 and 0.178 kg m−2 for crusts 1 to 3, respectively. During a second field campaign (17–18 August 2011), where dew formation was recorded in 1.5 to 2.75-h intervals after installation at 21:30 CEST, dewfall increased from 0.011 kg m−2 for the initial sandy substrate to 0.013, 0.028 and 0.055 kg m−2 for crusts 1 to 3, respectively. Dewfall rates remained on low levels for the substrate and for crust 1, and decreased overnight for crusts 2 and 3 (with crust 3 > crust 2 > crust 1 throughout the campaign). Dew formation was well reflected by the model response. The suggested mechanism of dew formation involves a delay in water saturation in near-surface soil pores and extracellular polymeric substances (EPS) where the crusts were thicker and where the water capacity was high, resulting in elevated vapor flux towards the surface. The results also indicate that the amount of dewfall was too low to saturate the BSCs and to observe water flow into deeper soil. Analysis of the soil water retention curves revealed that, despite the sandy mineral matrix, moist crusts clogged by swollen EPS pores exhibited a clay-like behavior. It is hypothesized that BSCs gain double benefit from suppressing their competitors by runoff generation and from improving their water supply by dew collection. Despite higher amounts of dew, the water availability to the crust community decreases with crust development, which may be compensated by ecophysiological adaptation of crust organisms, and which may further suppress higher vegetation or mosses.

Biological Soil Crusts in the Mojave Desert, USA: Micromorphology and Pedogenesis

Biological soil crusts (BSCs) are bio-sedimentary complexes that play critical ecological roles in arid landscapes; however, the interactions between component biota and sediments are poorly understood. A detailed micromorphological investigation of BSC development and crust microstructure in the Muddy Mountains Wilderness Area, Nevada, examined features in thin section using petrographic microscopy, light microscopy, scanning electron microscopy, and energy dispersive x-ray spectroscopy. The >1800 microscopic observations were linked to crust macroscale features and soil geomorphology. Complex bio-sedimentary structures of BSCs reflect a dynamic genetic history and diverse formative processes, including: (i) stabilization and authigenic mineral precipitation; (ii) wetting–drying and expansion–contraction; (iii) dust capture; (iv) microscale mass wasting; and (v) vesicular (Av) horizon formation. A new conceptual model for hot deserts illustrates how these processes co-develop with BSC succession, during countless wet–dry cycles, to build up pinnacle microtopography while simultaneously forming Av horizons in the bio-rich and bio-poor zones. Complex surficial and internal bio-sedimentary structures, which vary as a function of crust morphology, trap surface water for uptake by crust organisms, while dust influx provides a source of nutrients. These phenomena influence landscape-scale water dynamics and biogeochemical cycling, increasing the availability of soil resources during times of biotic stress. Biological soil crusts uniquely facilitate the accumulation, morphology, and ecosystem function of dust and should, therefore, be considered critical agents in arid pedogenesis and landscape development.

Surveying for biotic soil crust lichens of shrub steppe habitats in the Columbia Basin.

Arid lands of eastern Oregon and Washington support a great diversity of biotic soil crust organisms that are often overlooked in biotic inventories. Land managers are increasingly interested in incorporating these species in their surveys so that they can protect unique habitats and prevent uncommon species from becoming threatened. Current monitoring guidelines for rangeland health sometimes take into account the percent cover by soil crusts; however, there is very little guidance for surveying for uncommon species. Furthermore, because soil crust species and habitats are unfamiliar to most botanists, finding these species presents special challenges. We outline suggestions for future surveys with these goals.

Warming and increased precipitation frequency on the Colorado Plateau: implications for biological soil crusts and soil processes

Changes in temperature and precipitation are expected to influence ecosystem processes worldwide. Despite their globally large extent, few studies to date have examined the effects of climate change in desert ecosystems, where biological soil crusts are key nutrient cycling components. The goal of this work was to assess how increased temperature and frequency of summertime precipitation affect the contributions of crust organisms to soil processes. With a combination of experimental 2°C warming and altered summer precipitation frequency applied over 2 years, we measured soil nutrient cycling and the structure and function of crust communities. We saw no change in crust cover, composition, or other measures of crust function in response to 2°C warming and no effects on any measure of soil chemistry. In contrast, crust cover and function responded to increased frequency of summer precipitation, shifting from moss to cyanobacteria-dominated crusts; however, in the short timeframe we measured, there was no accompanying change in soil chemistry. Total bacterial and fungal biomass was also reduced in watered plots, while the activity of two enzymes increased, indicating a functional change in the microbial community. Taken together, our results highlight the limited effects of warming alone on biological soil crust communities and soil chemistry, but demonstrate the substantially larger effects of altered summertime precipitation.

Carbon Exchange Responses of Cyanobacterial-Algal Crusts to Dehydration, Air Temperature, and CO2 Concentration

Biological soil crusts occur in dry land worldwide. However, studies on the carbon exchange of crusts with different histories of crust development are rare, especially in terms of the responses of carbon exchange to increasing atmospheric CO2 concentrations and decreasing water contents of crust organisms. In this study, we classified sites in Shapotou in the Tengger Desert as recent (site revegetated in 1981 in mobile sand dunes) and old (site revegetated in 1956) successional stages, and systematically analyzed the changes in the carbon exchange of cyanobacterial-algal crusts developed in the two sites to dehydration, air temperature (15 and 25 °C), and CO2 concentration (200–2000 ppm). The old crusts had a higher carbon exchange rate than the recent crusts during dehydration, and the maximum carbon exchange rates were 3.49 and 2.45 µmol · m−2 · s−1 for the old and recent crusts, respectively, at an approximately 60% water content of dry crust organisms. The carbon exchange rate was higher at 25°C than at 15°C for both crusts. No difference in the CO2 saturation points between the two crusts and between the two temperatures was observed, however, the saturation point significantly increased with the increase in the water content of the crust organisms, from 224 to 1384 ppm, when the water content of the crust organisms was increased from 20% to 100%. The water content of the crust organisms was the regulating factor for the carbon exchange rate of both the old and recent cyanobacterial-algal crusts.

Functional profiles reveal unique ecological roles of various biological soil crust organisms

Summary 1. At the heart of the body of research on biodiversity effects on ecosystem function is the debate over whether different species tend to be functionally singular or redundant. When we consider ecosystem multi-function, the provision of multiple ecosystem functions simultaneously, we may find that seemingly redundant species may in fact play unique roles in ecosystems. 2. Over the last few decades, the significance of biological soil crusts (BSCs) as ecological boundaries and ecosystem engineers, and their multi-functional nature, has become increasingly well documented. We compiled ‘functional profiles’ of the organisms in this understudied community, to determine whether functional singularity emerges when multiple ecosystem functions are considered. 3. In two data sets, one representing multiple sites around the semi-arid regions of Spain (regional scale), and another from a single site in central Spain (local scale), we examined correlations between the abundance or frequency of BSC species in a community, and multiple surrogates of ecosystem functioning. There was a wide array of apparent effects of species on specific functions. 4. Notably, in gypsiferous soils and at regional scale, we found that indicators of carbon (C) and phosphorus cycling were apparently suppressed and promoted by the lichens Diploschistes diacapsis and Squamarina lentigera, respectively. The moss Pleurochaete squarrosa appears to promote C cycling in calcareous soils at this spatial scale. At the local scale in gypsiferous soils, D. diacapsis positively correlated with carbon cycling, but negatively with nitrogen cycling, whereas numerous lichens exhibited the opposite profile. 5. We found a high degree of functional singularity, i.e. that species were highly individualistic in their effects on multiple functions. Many functional attributes were not easily predictable from existing functional grouping systems based primarily on morphology. 6. Our results suggest that maintaining species-rich BSC communities is crucial to maintain the overall functionality of ecosystems dominated by these organisms, and that dominance and the outcome of competition could be highly influential in the determination of such functionality.

Biological soil crust distribution is related to patterns of fragmentation and landuse in a dryland agricultural landscape of southern Australia

The dryland agricultural landscape of north-west Victoria, Australia, includes isolated remnants of eucalypt woodland that are exposed to ongoing disturbance from sheep grazing and cropping activity. Biological soil crusts are a functionally important feature of these woodland communities. We used a modern form of regression (boosted regression tree (BRT) models) to investigate relationships between crust abundance and environmental and landscape variables. We also investigated whether the use of broad morphological groups of crust organisms is more informative than simply measuring total crust cover. Remnant size was the single most influential variable for crust abundance, with negligible crust cover in small patches (<5 ha). The BRT model also identified relationships between crust abundance and available P, soil C and perennial grass. We argue that disturbance from stock grazing and camping is the mechanism driving these relationships. Other variables related to crust abundance were proximity to the windward edge, litter cover and tree cover. Morphological groups showed a differential response to some variables, suggesting assessment of total cover may mask important patterns in community structure. Crust disturbance represents a serious issue for maintenance of ecosystem function in the study region, particularly loss of crusts from small remnants because the majority of remnants are small.

Soil crusts and disturbance benefit plant germination, establishment and growth on nutrient deficient sand

Abstract The influence of cyanobacterial soil crusts combined with mechanical soil disturbance on germination, establishment, growth and nutrition of Lepidium sativum seedlings on nutrient poor sand was analyzed under controlled conditions in a growth chamber. Marked release of nitrate and ammonia into the soil was only found in established and older (aging) crusts. Analyses of the 15 N signatures of the crust and the seedling biomass revealed, that there was clear evidence that the established seedlings took up considerable amounts of the released nitrogen. This positively affected plant performance. Mechanical disturbance of the crust led to a temporal increase of nitrogen release but later to a considerable decrease of the released nitrogen components. Undisturbed crusts had a low stimulating effect on seed germination. In contrast and independent of the presence or absence of crusts mechanical soil disturbance led to significantly higher seed germination rates. It is concluded that the occurrence of small-scale disturbance events of moderate intensity at an intermediate frequency, which is the typical disturbance regime in many natural soil crust habitats, seems to be necessary for the successful coexistence of crust organisms and higher plants in nutrient poor environments.

Soil lichens have species-specific effects on the seedling emergence of three gypsophile plant species

Despite advancements in our knowledge on the importance of biological soil crusts (BSC) in arid and semiarid environments, little is known on the effects of specific crust organisms on vascular plant seed emergence. We conducted a growth chamber experiment to evaluate the effects of two BSC-forming lichens ( Squamarina lentigera and Diploschistes diacapsis) on the emergence of three specialist vascular plants from gypsum Mediterranean habitats ( Campanula fastigiata, Reseda stricta and Helianthemum squamatum). The design of the experiment included three disturbance levels (intact, disturbed and removed) and two sterilization levels (live vs. autoclaved) of BSC-forming lichens. Specifically, we evaluated three hypotheses: (i) lichens will reduce the emergence of vascular plants, (ii) the magnitude of the negative effect of lichens on emergence will be species-specific, and (iii) the two species of lichens evaluated will exert differential effects on seed emergence. Emergence in the presence of lichens was lower than that obtained in tests conducted under optimum lab conditions (reduction of 50%). The effects of crust disturbance and sterilization on seed emergence were species-specific, and also varied with the lichen species serving as substrate. When germinated with Squamarina, a disturbance×sterilization interaction was found in Campanula. Disturbance increased and reduced the emergence of this species when the lichen was dead and alive, respectively. When Helianthemum was sown on Squamarina and Diploschistes, disturbance reduced and increased, respectively, the emergence of this species. The emergence of Reseda was not affected by any treatment when sown on Diploschistes, but disturbance reduced it when shown on Squamarina. Our results highlight the importance of species-specific interactions between vascular plants and BSC-forming soil lichens. The potential mechanisms underlying these interactions are discussed.

Structure of biological soil crust communities inCallitris glaucophyllawoodlands of New South Wales, Australia

Abstract:Question:What is the nature of the relationships between cover, diversity and abundance of biological soil crusts, cover and diversity of vascular plants, and annual rainfall, soil texture and forestry practices inCallitris glaucophyllawoodlands? Location: Arid and semi‐aridCallitris glaucophylla‐domi‐nated woodlands of eastern Australia.Methods:We documented soil crust‐forming mosses, lichens and liverworts at 83 woodland sites along a gradient of declining rainfall. Linear and non‐linear regression were used to examine relationships between soil crust species and attributes of vascular plant communities, and a similarity matrix (species abundance X sites) was subjected to Non‐metric Multi‐Dimensional Scaling (MDS), and Analysis of Similarities (ANOSIM) to show the degree of association between groups of taxa, and soil texture, rainfall classes and forestry practices.Results: We collected 86 taxa. Mosses were dominated by the familyPottiaceae, and lichens were dominated by squamulose forms. Average annual rainfall was highly correlated with soil crust community composition, and loamy soils supported a greater cover and diversity of taxa compared with sandy soils. Increases in tree cover were associated with significant, though weak, increases in abundance, but not diversity, of crusts. Crusts tended to be more diverse in areas that (1) had a sparse cover of ground‐storey plants; (2) were relatively stable ‐ as indicated by the proportion of perennial and/or native plants; (3) had more stable soil surfaces; and (4) were unlogged. Litter cover, overstorey thinning, and livestock grazing had no appreciable effect on crust diversity or cover.Conclusions:Callitris glaucophyllawoodlands provide substantial habitat for soil crust organisms, and the dense tree cover and closed canopies ofCallitrisdo not appear to have a major influence on the structure of biological crust communities. Unlike other woodland systems, relatively few patches would be required to reserve a high diversity of crust species.

Structure of biological soil crust communities in Callitris glaucophylla woodlands of New South Wales, Australia

Question: What is the nature of the relationships between cover, diversity and abundance of biological soil crusts, cover and diversity of vascular plants, and annual rainfall, soil texture and forestry practices in Callitris glaucophylla woodlands? Location: Arid and semi-arid Callitris glaucophylla-dominated woodlands of eastern Australia. Methods: We documented soil crust-forming mosses, lichens and liverworts at 83 woodland sites along a gradient of declining rainfall. Linear and non-linear regression were used to examine relationships between soil crust species and attributes of vascular plant communities, and a similarity matrix (species abundance ∞ sites) was subjected to Non-metric Multi-Dimensional Scaling (MDS), and Analysis of Similarities (ANOSIM) to show the degree of association between groups of taxa, and soil texture, rainfall classes and forestry practices. Results: We collected 86 taxa. Mosses were dominated by the family Pottiaceae, and lichens were dominated by squamulose forms. Average annual rainfall was highly correlated with soil crust community composition, and loamy soils supported a greater cover and diversity of taxa compared with sandy soils. Increases in tree cover were associated with significant, though weak, increases in abundance, but not diversity, of crusts. Crusts tended to be more diverse in areas that (1) had a sparse cover of ground-storey plants; (2) were relatively stable – as indicated by the proportion of perennial and/or native plants; (3) had more stable soil surfaces; and (4) were unlogged. Litter cover, overstorey thinning, and livestock grazing had no appreciable effect on crust diversity or cover. Conclusions: Callitris glaucophylla woodlands provide substantial habitat for soil crust organisms, and the dense tree cover and closed canopies of Callitris do not appear to have a major influence on the structure of biological crust communities. Unlike other woodland systems, relatively few patches would be required to reserve a high diversity of crust species.

Activation of photosynthesis and resistance to photoinhibition in cyanobacteria within biological desert crust.

Filamentous cyanobacteria are the main primary producers in biological desert sand crusts. The cells are exposed to extreme environmental conditions including temperature, light, and diurnal desiccation/rehydration cycles. We have studied the kinetics of activation of photosynthesis during rehydration of the cyanobacteria, primarily Microcoleus sp., within crust samples collected in the Negev desert, Israel. We also investigated their susceptibility to photoinhibition. Activation of the photosynthetic apparatus, measured by fluorescence kinetics, thermoluminescence, and low temperature fluorescence emission spectra, did not require de novo protein synthesis. Over 50% of the photosystem II (PSII) activity, assembled phycobilisomes, and photosystem I (PSI) antennae were detected within less than 5 min of rehydration. Energy transfer to PSII and PSI by the respective antennae was fully established within 10 to 20 min of rehydration. The activation of a fraction of PSII population (about 20%-30%) was light and temperature-dependent but did not require electron flow to plastoquinone [was not inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea]. The cyanobacteria within the crusts are remarkably resistant to photoinhibition even in the absence of protein synthesis. The rate of PSII repair increased with light intensity and with time of exposure. Consequently, the extent of photoinhibition in high-light-exposed crusts reached a constant, relatively low, level. This is in contrast to model organisms such as Synechocystis sp. strain PCC 6803 where PSII activity declined continuously over the entire exposure to high illumination. Ability of the crust's organisms to rapidly activate photosynthesis upon rehydration and withstand photoinhibition under high light intensity may partly explain their ability to survive in this ecosystem.

Application of the Herbicide, Simazine, and its Effect on Soil Surface Parameters and Vegetation in a Patchy Desert Landscape

Semiarid regions are characterized by spatial heterogeneity with two patch types: (a) shrub patches with an annual vegetation understorey, and (b) biological soil crusts. Runoff as overland flow from the crust patch is a source of water and nutrients for the shrub patch and, thus, influences annual plant productivity. The aim of this study was to quantify the effects of the herbicide, simazine [2-chloro-4,6-bis (ethylamino)-s-triazine], a photosynthesis inhibitor, on biological soil crusts and the consequences for the patchy desert landscape, in the northern Negev Desert, Israel. We studied the direct effect of the herbicide on the factors that compose the crusts (polysaccharides production, moss density, chlorophyll level), indirect effects on plant productivity (vegetation index and annual biomass), and resources (water, soil, organic matter and nutrients) on the landscape. We carried out a 2-year experiment, which entailed applying simazine on 20 (0.5 m2) plots with combinations of crust and shrub patches, and 20 control plots. Results indicated that 41.3% of the runoff water that flow overland from the crust patches was intercepted and absorbed by the shrub patches. Destruction of live crust components, such as cyanobacteria and soil algae, reduce polysaccharide production (from 0.12 to 0.06 µg mL−1 of crust extraction) and moss density (from 66 to 4 moss caulidia cm−2), which led to soil (12 g m−2 yr−1), organic matter (0.5 g m−2 yr−1) and nitrate (600 µg m−2yr−1) erosion. Degradation of shrub patch productivity was due to the removal of annual vegetation by simazine. We conclude that applying simazine to biological soil crusts in this fragile semiarid ecosystem, affected the crust organisms and in turn, the flow of water, soil and nutrients.