Microbiotic Crusts Research Articles

Building a Conceptual Model for the Environmental Fate of the Fungicide Benzovindiflupyr.

Degradation of the fungicide benzovindiflupyr was slow in standard regulatory laboratory studies in soil and aquatic systems, suggesting it is a persistent molecule. However, the conditions in these studies differed significantly from actual environmental conditions, particularly the exclusion of light, which prevents potential contributions from the phototrophic microorganisms that are ubiquitous in both aquatic and terrestrial environments. Higher tier laboratory studies that include a more comprehensive range of degradation processes can more accurately describe environmental fate under field conditions. Indirect aqueous photolysis studies with benzovindiflupyr showed that the photolytic half-life in natural surface water can be as short as 10 days, compared with 94 days in pure buffered water. Inclusion of a light-dark cycle in higher tier aquatic metabolism studies, to include the contribution of phototrophic organisms, reduced the total system half-life from >1 year in dark test systems to as little as 23 days. The relevance of these additional processes was confirmed in an outdoor aquatic microcosm study in which the half-life of benzovindiflupyr was 13-58 days. In laboratory soil degradation studies, the degradation rate of benzovindiflupyr was significantly faster in cores with an undisturbed surface microbiotic crust, incubated in a light-dark cycle (half-life of 35 days), than in regulatory studies with sieved soil in the dark (half-life >1 year). A radiolabeled field study validated these observations, showing residue decline with a half-life of approximately 25 days over the initial 4 weeks. Conceptual models of environmental fate based on standard regulatory studies may be incomplete, and additional higher tier laboratory studies can be valuable in elucidating degradation processes and improving the prediction of persistence under actual use conditions. Environ Toxicol Chem 2023;00:1-15. © 2023 SETAC.

Slip slidin' away: Bristle-driven gliding by Tetradesmus deserticola (Chlorophyta) in microfluidic chambers.

Microalgae within the Scenedesmaceae are often distinguished by spines, bristles, and other wall characteristics. We examined the dynamic production and chemical nature of bristles extruded from the poles of Tetradesmus deserticola previously isolated from microbiotic crust. Rapidly growing cells in a liquid growth medium were established in polydimethylsiloxane microfluidic chambers specially designed to maintain aerobic conditions over time within a chamber 6-12 μm deep. This geometry enabled in-focus imaging of single cells over long periods. Differential interference contrast (DIC) imaging revealed that after multiple fission of mother cells, the newly released, lemon-shaped daughter cells began extruding bristles from each pole. In some instances, the bristles became stuck to either the glass floor or polydimethylsiloxane (PDMS) walls of the chamber, and the force by which the new bristle was extruded was sufficient to propel the cells across the field of view at ~1.2 μm · h-1 . Confocal fluorescence and DIC imaging of cells stained with pontamine fast scarlet and calcofluor, and treated with proteinase K, suggested that bristles are proteinaceous and may also host carbohydrate modifications. The polar bristles extruded by this desert-derived T. deserticola may simply be relics of bristles produced by an aquatic ancestor for flotation or predator deterrence. But, their tendency to attach to glass (silicate) and/or PDMS surfaces suggests a potential role in tethering cells in place or binding soil particles. T. deserticola is closely related to T. obliquus, which is of interest for biofuels development; extruded bristles in T. deserticola may offer tethers for industrial use of these stress-tolerant algae.

Plant Species Composition and Forage Production 14 Yr After Biosolids Application and Grazing Exclusion

This paper examines the effects of a single surface application of biosolids (at 20 dry Mg ha−1) on plant species composition, forage quality and quantity, and C stocks after 14 yr of rest in rangelands of the Central Interior of British Columbia. More than two times the aboveground biomass of grasses and the percent cover of plant litter were found in the biosolids treatment relative to the control, along with reductions in bare soil and microbiotic crust cover. Significantly greater plant uptake of all macronutrients (C, N, P, K, S, Ca, and Mg); most micronutrients (B, Cu, Mn, Mo, and Zn); and Al occurred in the biosolids treatment. P and Cu were the only two nutrients to be more concentrated in the biosolids-treated forage relative to the control forage, while N, Mg, and protein were more concentrated in the control forage. No significant difference in forage digestibility was found between biosolids and control treatments. Bluebunch wheatgrass, the late-seral native grass species, had significantly increased cover and aboveground biomass in the biosolids treatment relative to the control; however, between 2006 and 2016, non-native Kentucky bluegrass had reached > 25% cover in the biosolids plots, perhaps restricting the full recovery of bluebunch wheatgrass. Our findings indicate that biosolids application to ungrazed rangeland can increase long-term forage production and reduce bare soil. However, at our study site biosolids application also led to a long-term shift in the plant community composition away from the late-seral (i.e., bluebunch wheatgrass) trajectory, and the effects of this shift on rangeland health and productivity require further investigation.

Soil Photosynthetic Microbial Communities Mediate Aggregate Stability: Influence of Cropping Systems and Herbicide Use in an Agricultural Soil.

Edaphic cyanobacteria and algae have been extensively studied in dryland soils because they play key roles in the formation of biological soil crusts and the stabilization of soil surfaces. Yet, in temperate agricultural crop soils, little is understood about the functional significance of indigenous photosynthetic microbial communities for various soil processes. This study investigated how indigenous soil algae and cyanobacteria affected topsoil aggregate stability in cereal cropping systems. Topsoil aggregates from conventional and organic cropping systems were incubated in microcosms under dark or photoperiodic conditions with or without a treatment with an herbicide (isoproturon). Physicochemical parameters (bound exopolysaccharides, organic carbon) and microbial parameters (esterase activity, chlorophyll a biomass, and pigment profiles) were measured for incubated aggregates. Aggregate stability were analyzed on the basis of aggregate size distribution and the mean weight diameter (MWD) index, resulting from disaggregation tests. Soil photosynthetic microbial biomass (chl a) was strongly and positively correlated with aggregate stability indicators. The development of microalgae crusts in photoperiodic conditions induced a strong increase of the largest aggregates (>2 mm), as compared to dark conditions (up to 10.6 fold and 27.1 fold, in soil from organic and conventional cropping systems, respectively). Concomitantly, the MWD significantly increased by 2.4 fold and 4.2 fold, for soil from organic and conventional cropping systems. Soil microalgae may have operated directly via biochemical mechanisms, by producing exopolymeric matrices surrounding soil aggregates (bound exopolysaccharides: 0.39–0.45 μg C g−1 soil), and via biophysical mechanisms, where filamentous living microbiota enmeshed soil aggregates. In addition, they may have acted indirectly by stimulating heterotrophic microbial communities, as revealed by the positive effect of microalgal growth on total microbial activity. The herbicide treatment negatively impacted soil microalgal community, resulting in significant decreases of the MWD of the conventional soil aggregates (up to −42% of the value in light treatment). This study underscores that indigenous edaphic algae and cyanobacteria can promote aggregate formation, by forming photosynthetic microbiotic crusts, thus improving the structural stability of topsoil, in temperate croplands. However, the herbicide uses can impair the functional abilities of algal and cyanobacterial communities in agricultural soils.Originality/SignificanceEdaphic algal and cyanobacterial communities are known to form photosynthetic microbial crusts in arid soils, where they drive key ecosystem functions. Although less well characterized, such communities are also transiently abundant in temperate and mesic cropped soils. This microcosm study investigated the communities’ functional significance in topsoil aggregate formation and stabilization in two temperate cropping systems. Overall, our results showed that the development of indigenous microalgal communities under our experimental conditions drove higher structural stability in topsoil aggregates in temperate cropland soils. Also, herbicide use affected photosynthetic microbial communities and consequently impaired soil aggregation.

One-time application of biosolids to ungrazed semiarid rangelands: 14 yr soil responses

Biosolids have been shown to improve forage production and soil quality on semiarid rangelands in the short term, but less is known about longer-term impacts of one-time biosolids applications. The objective of this study was to determine the effects of a single, surface biosolids application (at 20 dry Mg ha−1) on stability of soil aggregates, bulk density, total carbon (C) and nitrogen (N), permanganate-oxidizable carbon (POXC), polysaccharides, pH, nutrient availability, and soil water content (all at 0–7.5 cm depth) 14 yr following application to ungrazed rangelands in the Central interior of British Columbia. Fourteen years following the biosolids application, aboveground plant biomass was almost two times greater with biosolids application than on control, while exposed mineral soil and microbiotic crust significantly decreased in biosolids plots. Despite differences in aboveground biomass, there was no difference in total soil C and N, POXC, and polysaccharides between biosolids and control plots. Biosolids-amended soil did exhibit significantly greater mean weight diameter of water-stable aggregates, lower pH, increased spring soil water content, and increased availability of Fe3+, Zn2+, Cu2+, and phosphate ions. These findings indicate that the long-term improvements to soil on ungrazed rangeland are possible even from a single biosolids application.

A model suite of green algae within the Scenedesmaceae for investigating contrasting desiccation tolerance and morphology.

Microscopic green algae inhabiting desert microbiotic crusts are remarkably diverse phylogenetically, and many desert lineages have independently evolved from aquatic ancestors. Here we worked with five desert and aquatic species within the family Scenedesmaceae to examine mechanisms that underlie desiccation tolerance and release of unicellular versus multicellular progeny. Live cell staining and time-lapse confocal imaging coupled with transmission electron microscopy established that the desert and aquatic species all divide by multiple (rather than binary) fission, although progeny were unicellular in three species and multicellular (joined in a sheet-like coenobium) in two. During division, Golgi complexes were localized near nuclei, and all species exhibited dynamic rotation of the daughter cell mass within the mother cell wall at cytokinesis. Differential desiccation tolerance across the five species, assessed from photosynthetic efficiency during desiccation/rehydration cycles, was accompanied by differential accumulation of intracellular reactive oxygen species (ROS) detected using a dye sensitive to intracellular ROS. Further comparative investigation will aim to understand the genetic, ultrastructural and physiological characteristics supporting unicellular versus multicellular coenobial morphology, and the ability of representatives in the Scenedesmaceae to colonize ecologically diverse, even extreme, habitats.

Drought effect on biocrust resilience: High-speed winds result in crust burial and crust rupture and flaking.

Once established, biocrusts (known also as biological soil crusts or microbiotic crusts) are thought to be relatively resilient to wind erosion, with crust burial being considered as the main mechanism responsible for crust death. Thus far, to the best of our knowledge, crust flaking and rupture under natural conditions were not reported. We report herein a two-year study during two severe drought years (2010-2012) in a dunefield in the Negev Desert during which in addition to crust burial, crust rupture and flaking also took place. As for crust burial, it took place under sand sheets or coppice dunes (mounds). Subsequent removal of the coppice dunes by wind resulted in crust disintegration and erosion of the formerly buried crust and the formation of patches devoid of crusts termed herein 'erosion cirques'. As for crust flaking and rupture, it is explained by a large change in the properties of the extracellular polymeric substances (EPS) composing the crust. The EPS adherence and viscoelastic properties were monitored using a quartz crystal microbalance with dissipation monitoring (QCD-M) technology. EPS adherence and viscoelastic properties deduced from the QCM-D experiments suggest that crust coherence and elasticity, mediated by the EPS, were affected by droughts. Although crust flaking affected up to 25% of the interdunal surface, it is suggested that with continuous rain shortage, further crust flaking is likely to take place under continuous drought-driven dry surface conditions. This positive feedback mechanism, during which initially eroded crusts trigger additional crust erosion, may have severe consequences on the structure and function of drought-prone ecosystems, and may endanger the stability of dunefields, causing dust storms, triggering dune encroachment and declining air quality.

The role of crust thickness in runoff generation from microbiotic crusts

AbstractContrary to humid areas where runoff takes place following the saturation of the soil column, runoff in arid and semiarid zones takes place when rain intensities exceed the infiltration capability of the upper soil crust, whether physical crust or microbiotic crust (MC). This type of overland flow, known as Hortonian overland flow (HOF), is not fully understood, especially in the case of MC. In particular, little is known regarding the effect of crust thickness and its fine (silt and clay) content on runoff generation, with some scholars claiming that runoff generation is positively correlated with crust thickness and fine content. In an attempt to determine the effect of crust thickness and to assess the role played by the silt and clay on runoff generation, a set of field and lab experiments were undertaken on MCs inhabiting sand dunes in the Negev Desert (Israel). These included sprinkling experiments coupled with measurements of the physical (thickness, silt and clay) and biological (chlorophyll, protein, total carbohydrates) properties of 0.5–10‐mm‐thick crusts. The data showed that runoff generation took place on surfaces as thin as ~0.5–0.7 mm only, and was not correlated with the fine (silt and clay) content. The implications for HOF and for arid ecosystems are discussed. Copyright © 2014 John Wiley & Sons, Ltd.

The use of microbiotic crusts as biomarkers for ponding, subsurface flow and soil moisture content and duration

Previous work at the Hallamish dunefield in the western Negev Desert defined and mapped 5 mature types of microbiotic crusts (MCs). Crust definition and sampling were based on 4×4m pixels. Yet within these pixels, microhabitats with different crust types could be noted. This was also the case within crust type A that extends over the south-facing slope and the sandy interdune. There, close examination mainly at <0.5×0.5m pixels revealed a high variability in crust types, as was also the case at fine-grained playa surfaces, scattered within the sandy interdune. These microhabitats belonged to two groups: habitats that received water from an additional source (runoff or subsurface flow), i.e., water-enriched habitats (WE) and habitats having a better water regime due to shading (SH), such as by shrubs.Daylight wetness duration and crust biomass were monitored during 1991–1994 in 12 habitats. WE habitats, which exhibited the highest biomass also exhibited longer wetness duration in comparison to SH habitats. Both groups exhibited higher biomass and wetness duration than the control. When the relation between daylight wetness duration and the chlorophyll content of the crust was compared, a positive linear relation with a good correlation (r2=0.74) was found. The findings indicate that crust biomass may be used as a biomarker for surface wetness duration and may thus point to habitats subjected to particular hydrological processes such as water ponding and subsurface flow. They may also point to soil patches having high moisture content and therefore long wetness duration. Higher moisture content that lasted for a longer duration was indeed recorded during preliminary measurements that were carried out at 0–40cm soil patches subjected to subsurface flow (as indicated by their MC) during the end of the winter season of 1994/95 and 1996/97. The findings may attest to differential rates of pedogenesis, thus highlighting the use of MCs as possible biomarkers for small soil habitats with variable pedogenesis.

The effect of biocrusts on evaporation from sand dunes in the Negev Desert

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

Contrasting effects of microbiotic crusts on runoff in desert surfaces

Microbiotic crusts (MCs) play an important role in surface hydrology by altering runoff yield. In order to study the crust's role on water redistribution, rainfall and runoff were measured during 1998–2000 at three sites within the northern Chihuahuan Desert, New Mexico, USA: the Sevilleta National Wildlife Refuge (SEV), the White Sands National Monument (WS), and the Jornada Experimental Range (JER). Whereas quartz and gypsum sand characterize the SEV and WS sites, respectively, both of which have high infiltration rates, silty alluvial deposits characterize the JER site. Runoff was measured in four pairs of 1.8–6.4 m 2 plots having MCs, one of which was scalped in each pair. No runoff was generated at WS, whether on the crusted or the scalped plots. Runoff was however generated at SEV and JER, being higher on the crusted plots at SEV and lower on the JER plots. The results were explained by the combined effect of (a) parent material and (b) the crust properties, such as species composition, microrelief (surface roughness) and exopolysaccharide (EPS) content (reflected in the ratio of carbohydrates to chlorophyll). Whereas the effective rainfall, the fines and the EPS content were found to explain runoff initiation, the effective rainfall and the crust microrelief were found to explain the amount of runoff at SEV and JER where runoff generation took place. The findings attest to the fundamental role of the parent material and the crust's species composition and properties on runoff and hence to the complex interactions and the variable effects that MCs have on dryland hydrology.

Soil microbial metabolic profiles in two geomorphological units in a semistable sand-dune ecosystem

The playas and microbiotic crust-covered interdunes are two geomorphological units in the Hallamish sand-dune field, Israel. As a depositional unit, a lack of vegetation and microbiotic crusts, as well as high contents of fine particles and salinity, may lead to low microbial activity and functional diversity at the playa. The present study was initiated in order to illuminate the spatiotemporal changes in the activity, biomass and metabolic profiles of soil microbial communities at the two habitats. Soil samples were collected from the 0–50-cm depth at the playa and interdune during the wet and dry seasons of 2010. Soil moisture and organic-carbon contents recorded at the playa were 2- to above 10-fold higher than at the crust-covered interdune throughout the study period. Nevertheless, opposite trends were recorded for fungal and bacterial CFUs, as well as microbial basal respiration and biomass, in most soil depths at the playa compared to the crust-covered interdune. The total substrate utilization rates of soil microbial communities fluctuated between 4.9 and 6.4 and between 3.9 and 14.9 μg CO2–C g−1 dry soil h−1 at the playa and interdune during the wet season, while 3-fold higher values were recorded at the two habitats during the dry season. The total substrate utilization rates, carbohydrates and carboxylic-acid utilization rates, and functional diversity of soil microbial communities at the playa, were lower than at the crust-covered interdune at most soil depths, while higher utilization rates of aromatic acid were recorded in at least three soil layers at the playa.

Hydrological behaviour of microbiotic crusts on sand dunes: Example from NW China comparing infiltration in crusted and crust-removed soil

Biological soil crusts are important microbiotic communities which are considered a key factor in combating desertification in terms of sand dune remobilitation. Because of their greater concentration of clay, silt and hydrophobic organic matter, they create a physical discontinuity in surface profile which potentially contribute to variability in soil hydrology of arid and semiarid land and should be considered in the development of hydrological and erosion models. However, until recently limited inconclusive research has been conducted to determine the functional relationship between soil and microbiotic crusts. We investigated the influence of microbiotic crust on hydrological behaviour of crusted sand dunes in NW China. The infiltration data on a crusted and crust-removed sand dune were obtained with dish permeameter which was not moved during the measurement at a sequence of nominal pressure head values, h, of −1, −4, −7, −10, −18 cm. Hydraulic conductivity K( h) was measured by steady-state infiltration experiments and piece-wise application of the exponential relation between K and h, according to Wooding's analytical solution of a three-dimensional flow field. The transient experimental infiltration data, along with initial and final water contents were used for numerical analysis within sand profiles without and with microbiotic crust. Specifically, we provided evidences that the main physical and hydrological consequences occurring locally, always cause lower unsaturated fluxes in the sand beneath the crusted layer. Nevertheless, due to the effective rainfalls regime, it has insignificant effects on the local soil water regimes with unchanged storage and deep fluxes.

Geologic composition influences distribution of microbiotic crusts in the Mojave and Colorado Deserts at the regional scale

Abiotic and biotic factors influencing distribution of microbiotic crusts within hot deserts, such as the Mojave and Colorado Deserts, are poorly known. Our objective was to examine microbiotic crust distribution with reference to soil and parent material characteristics as well as plant functional groups in wilderness areas of Joshua Tree National Park (JTNP). A total of 75 sites were visually assessed for crust abundance and plant community composition; soil physical and chemical factors also were measured. Microbiotic crusts of JTNP, in particular lichen and moss crusts, were not as well-developed or as widely distributed as in other arid regions of North America. Algal crusts were most prevalent, lichen crusts were sparse, and crusts containing mosses were rare, with average percent land surface absolute (and relative) cover for these three cover categories being 11.4% (17.4%), 1.7% (2.7%), and 0.02% (0.02%), respectively. Previously reported individual drivers of crust development, such as pH, electrical conductivity (EC), and soil texture, did not appear to strongly influence crust development in this study of the Mojave and Colorado Deserts. Proximity to granitic bedrock and grusy granitic soils associated with it were the key determinants of microbiotic crust distribution in the wilderness areas of JTNP. In particular, crusts were best developed in grusy granitic soils. Overall, our study emphasized the importance of geology in driving crust distribution and its potential value as a predictor of where crusts may occur in the hot deserts of North America.

Habitat‐specific demography across dune fixation stages in a semi‐arid sandland: understanding the expansion, stabilization and decline of a dominant shrub

Summary 1. Maintaining viable populations in different habitats requires physiological, morphological and demographic adaptations of plants. In sandland environments, plants experience substantial variation in growing conditions during the dune fixation process, with high sand mobility in early stages and denser vegetation cover in later stages. 2. We studied the changes in demography of a dominant shrub, Artemisia ordosica, at three stages of dune fixation: semi‐fixed dunes, fixed dunes and fixed dunes covered with microbiotic crust. Demographic data from three annual censuses were used to parameterize integral projection models (IPMs) to conduct comparative demographic analyses. 3. Plant growth and reproduction decreased strongly as dunes became more fixed. Shrinkage in plant height occurred very frequently, particularly in the fixed dunes with microbiotic crust. Population growth rate (λ) declined substantially with dune fixation: from rapid expansion in semi‐fixed dunes (λ = 1.35–1.09) to moderate decline in fixed dunes with microbiotic crust (λ = 0.94–0.89). 4. Elasticity analysis revealed that survival was a key vital rate for population growth in all habitats. Growth and fecundity were of higher importance in the semi‐fixed habitat than in the other two habitats where shrinkage became an important factor determining λ. Seedlings and small plants were critical for population growth in semi‐fixed dunes, whereas moderate to large‐sized plants were most important in the other habitats. 5. Results of life table response experiments showed that the observed strong decrease in λ during dune fixation was mainly caused by reduction in fecundity, but with additional and considerable contributions from reduced plant growth and increased occurrence of shrinkage. Thus, populations in semi‐fixed dunes are able to expand rapidly due to a much higher fecundity compared to those in other habitats. 6. Synthesis. Artemisia ordosica adopts different life history strategies along the dune fixation process. Fast expansion in semi‐fixed dunes is enabled by high seed production and effective recruitment, while populations at later dune fixation stages are maintained through frequent plant shrinkage. Integral projection models are highly appropriate tools for analysing such life history changes as they are based on statistical comparisons of vital rates across habitats.

Properties and spatial distribution of microbiotic crusts in the Negev Desert, Israel

Abstract Playing a cardinal role in surface stabilization and in carbon and nitrogen fixation, microbiotic crusts play a crucial role in arid regions where they may serve as useful biomarkers for wind power and wetness duration. This is especially the case on relatively unstable and infertile sand dunes in the Negev Desert where high correlations between the crust chlorophyll content and the daytime wetness duration were found. Yet, only scarce data are available as to the possible link between the chlorophyll content and other physical (color, thickness, strength, crack density, surface roughness and infiltrability) and biological (protein, carbohydrate, organic matter and species composition) factors, which determine, in turn, the crust type and its effect upon geomorphological and ecological processes. No data are available on crust type distribution. These were the aims of the current research. When a cluster analysis was performed, five types of microbiotic crusts were defined, four of which were cyanobacterial (A–D) and one moss-dominated crust (E). The crusts differed in their physical and biological properties. They showed an increase in chlorophyll content, protein, carbohydrates and organic matter from A to E, with concomitant increase in species diversity, thickness, roughness and strength, but with some variables (crack density and infiltrability) showing a reversed trend at the moss-dominated crust. The increase in the biomass components of the crust and the gradual change of the physical properties are explained by the improved physical conditions (primarily wetness duration), which facilitates longer hours of photosynthetic activity and consequently the introduction of additional, more mesic species such as green algae, lichens and mosses. Extended wetness duration was found to shift the crust type from cyanobacterial to moss-dominated crust. The spatial distribution of the crusts, as verified by crust mapping, coincided with the daylight surface duration, which in turn was controlled by topography (aspect, angle and slope position). It implies that whereas initial physical conditions dictates species composition and thus crust type, the crust type in turn is responsible for characterizing the physical properties of the surface, which may largely affect ecological and geomorphological processes.

Effect of woody vegetation clearing on nutrient and carbon relations of semi-arid dystrophic savanna

Throughout the savanna biome, woody vegetation is cleared to increase productivity of herbaceous pasture. While clearing can result in increased pasture production of semi-arid dystrophic savannas in the short term, it is uncertain whether production is sustained in the long term. There is insufficient knowledge of how clearing affects soil nutrient and organic carbon (SOC) stocks. Using cleared-uncleared site pairs, we evaluated techniques for time-integrated assessment of nutrient and carbon relations in Australian savanna. Short-term in situ resin incubation showed that soil at cleared sites had a higher time-integrated availability of ammonium and nitrate, indicating that nitrogen (N) may turn over faster and/or is taken up slower at cleared sites than uncleared savanna. Nitrate and ammonium availability was approximately 2-fold higher in spring than in summer, likely due to greater uptake and/or loss of nitrate during summer rains. Nitrate was a prominent N source for evergreen trees, especially before summer rain, pointing to a role of trees as permanent N sinks. Stable isotope signatures of soil and vegetation indicate that N input occurs via N2 fixing microbiotic crusts and Acacia species. 30 years after clearing, SOC contained more C4 grass-derived carbon than uncleared savanna, but this shift in C source was not associated with the net C gain often observed in grasslands. Interactions between altered nutrient and C relations and composition of the understorey should be assessed in context of introduced buffelgrass (Cenchrus ciliaris) which had higher macronutrient concentrations than native grasses. Heterogeneity of the studied soils highlights the need for replication at several spatial scales to infer long-term dynamics with space-for-time chronosequences. We conclude that the techniques presented here are useful for gaining knowledge of the biogeochemical processes governing savannas and the systems that result from clearing.

Water repellency and pore clogging at early successional stages of microbiotic crusts on inland dunes, Brandenburg, NE Germany

Abstract Biological soil crusts play a key role for hydrological processes in many open landscapes. They seal the topsoil and generate surface run-off. Utilization of the mineral substrate at early stages of microbiotic crusts was investigated using scanning electron microscopy (SEM), water repellency indices were measured using the ethanol/water microinfiltrometer method, and steady state water flow was determined on the dry crusts and after 300, 600, 1200 and 1800 s of wetting, thus allowing to follow pore clogging through swelling of extracellular polymeric substances (EPS). It was found that water repellency increases with initial crust development where filamentous cyanobacteria and filamentous green algae were dominating, but decreases as coccal algae, bryophytes and fungi associated with bryophytes emerged. Swelling of EPS took place immediately after wetting, and its influence on steady state water flow was most pronounced when filamentous cyanobacteria and algae partially filled in the matrix pores and enmeshed sand grains, still leaving micropore channels available for free water infiltration, but prior to appearance of coccal algae, bryophytes and associated fungi which formed a dense cover on the surface. It was concluded that a new phase of crust succession was accompanied by easier wettability but slower infiltration. Transition from hydrophobicity to pore clogging as ruling mechanisms causing water run-off may occur during wetting of individual biological crusts, but also during crust succession over time.

Microbiotic crusts as biomarkers for surface stability and wetness duration in the Negev Desert

AbstractMicrobiotic crusts play an important role in arid and semi‐arid regions. Yet, very little information exists regarding the factors that impact their development. In an attempt to assess the main factors that may determine their growth, measurements of the amount of fines (silt and clay), rain, moisture content, wetness duration and wind erosion and deposition were carried out along a 12 station transect within a partially crusted dune field in the western Negev Desert and compared to the crust cover and chlorophyll content. Surface stability was the only variable that exhibited significant relationship with crust cover while daylight wetness duration exhibited strong positive relationship (r2 = 0·92–0·99) with the crust's chlorophyll content. The data point out that microbiotic crusts may serve as a useful biomarker for surface stability. While wetness duration and wind will control crust cover and the crust chlorophyll content in semi‐stable habitats (with absolute annual change in sand level of 2–3 mm), stable habitats (absolute change &lt;1 mm) will be controlled primarily by moisture, while habitats with low surface stability (absolute change of tens and hundreds of millimeters) will be primarily controlled by wind. Furthermore, owing to the strong positive relationship between daylight wetness duration and the crust's chlorophyll content, the crust may serve as a useful biomarker for the quantification of surface wetness duration. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Effects of Different Dune Positions and Microbiotic Crusts on Soil Seed Banks in the Gurbantonggut Desert

In this paper,the soil seed banks in different microbiotic crusts and at different sand dune positions in the Gurbantonggut Desert are studied using germination method.The results show that the seed density,species richness(Margalet index) and species diversity(Simpson's diversity index and Shannon-Winner index) are higher on algal crust than that on lichen and moss crusts.The seed density and diversity are decreased with the evolution from algal crust to lichen and moss crusts,and they are the lowest on the top of sand dunes without crust.The seed density and diversity of seed bank on algal crust are significantly higher than that on top sand.Comparing with the algal crust,there is no significant difference of seed density and diversity between the lichen and moss crusts and the topsoil without crust.The seed density and diversity on algal crust are significantly higher on leeward slope than that on windward slope.