Cyanobacterial Soil Crusts Research Articles

Contributions of hypolithic communities to surface soil organic carbon across a hyperarid-to-arid climate gradient

Carbon (C) in soils accounts for a substantial and dynamic portion of the global C cycle, with concentrations of soil organic carbon (SOC) typically closely linked to vascular plant productivity. However, C fixed by non-vascular photosynthetic organisms may account for a sizeable proportion of SOC in locations where vascular plants are not abundant. In the hyperarid Namib Desert, vascular plant growth is very limited and largely ephemeral. Extreme abiotic conditions even limit establishment of cyanobacterial soil crusts, with photoautotrophic cyanobacteria largely restricted to the undersides of translucent quartz clasts that buffer environmental extremes. The importance of these ‘hypoliths’ in enhancing SOC pools, and how this may vary with climate and clast physical characteristics, remains unknown. We worked across a rainfall and fog gradient in the central Namib to assess quartz clast size and distribution, factors affecting the probability of hypolithic colonization, and the landscape-level influence of hypoliths on surface SOC pools. Clast colonization increased with clast thickness and with increasing mean annual rainfall. SOC and chlorophyll a (a proxy for cyanobacterial biomass) concentrations were greater under colonized quartz clasts than under non-quartz clasts or in bare soil. Landscape-level SOC, estimated by combining SOC concentration with the distribution and colonization of quartz clasts, was greatest at the site with the highest rainfall and lowest at a mid-gradient site with moderate rainfall and fog. Climate change scenarios that promote quartz colonization have the potential to double SOC pools at the mid-gradient site, although SOC changes would be more muted at other sites. Low vascular plant and biocrust cover in the Namib Desert allows hypolithic communities to play an outsized role in SOC pools; a loss of all hypolithic cyanobacteria would lead to 10–20% declines in surface SOC pools. Future climate change has the potential to shift surface SOC if it alters cyanobacterial colonization of quartz.

Growth of cyanobacterial soil crusts during diurnal freeze-thaw cycles.

Various Nostoc spp. and related cyanobacteria are able to survive extreme temperatures and are among the most successful colonists of high-elevation sites being exposed due to glacial retreat. It is unclear, however, if cyanobacteria can grow during the extreme freeze-thaw cycles that occur on a year-round basis at high-elevation, peri-glacial sites or if they only grow during the rare periods when freeze-thaw cycles do not occur. We conducted several experiments to determine if cyanobacteria that form biological soil crusts (BSCs) at high-elevation sites (> 5,000 m.a.s.l.) in the Andes can grow during diurnal freeze-thaw cycles on a par with those that occur in the field. Here we show that a soil crust that had been frozen at -20°C for five years was able to increase from 40% to 100% soil coverage during a 45-day incubation during which the soil temperature cycled between -12°C and 26°C every day. In a second, experiment an undeveloped soil with no visible BSCs showed a statistically significant shift in the bacterial community from one containing few cyanobacterial sequences (8% of sequences) to one dominated (27%) by Nostoc, Microcoleus, and Leptolyngbya phylotypes during a 77-day incubation with daily freeze-thaw cycles. In addition, counts of spherical Nostoc-like colonies increased significantly on the soil surface during the experiment, especially in microcosms receiving phosphorus. Taken together these results show that freeze-thaw cycles alone do not limit the growth of BSCs in high-elevation soils, and provide new insight into how life is able to thrive in one of the most extreme terrestrial environments on Earth.

Effects of Cyanobacterial Soil Crusts on Surface Roughness and Splash Erosion

AbstractSoil surface roughness (SSR) modifies interactions and feedback processes between terrestrial and atmospheric systems driven by both the abiotic and biotic components of soils. This paper compares SSR response to a low‐intensity multiday rainfall event for soils with and without early successional stage cyanobacteria‐dominated biological soil crusts (CBCs). A rainfall simulator was used to apply 2, 5, and 2 mm of rain separated by a 24‐hr period over 3 days at an intensity of 60 mm/hr. Changes in SSR were quantified using geostatistically derived indicators calculated from semivariogram analysis of high‐resolution laser scans. The CBCs were stronger and splash erosion substantially less than from the physical soil crusts. Prior to rainfall treatment, soils with CBCs had greater SSR than those without. The rainfall treatments caused the physical crusted soils to increase SSR and spatial patterning due to the translocation of particles, soil loss, and the development of raindrop impact craters. Rainfall caused swelling of cyanobacterial filaments but only a slight increase in SSR, and raindrop impact cratering and splash loss were low on the soils with CBCs. There is no relationship between random roughness and splash erosion, but an increase in splash loss was associated with an increase in topographic roughness and small‐scale spatial patterning. A comparison of this study with other research indicates that for rainfall events up to 100 mm, the effectiveness of CBCs in reducing soil loss is >80% regardless of the rainfall amount and intensity, which highlights their importance for landscape stabilization.

Resource utilization of microalgae from biological soil crusts: Biodiesel production associated with desertification control

With the continuing consumption of resources and increasingly prominent environmental issues, microalgal resource utilization has received extensive attention. In this study, based on the microalgal investigation in desert biological soil crusts (BSCs) using pyrosequencing technology, the cultivated crust microalgae were further isolated in order to obtain high quality microalgae for resource utilization. The results showed that with crust development and succession, microalgal diversity gradually decreased, including the number of operational taxonomic units (OTUs) and genus, although Microcoleus always was the dominant genera. Pyrosequencing obtained 630 OTUs of cyanobacteria, 25 OTUs of green algae and 9 OTUs of diatom; however, part of cultivated microalgae still could not yet be detected due to the DNA extraction preferences and errors caused by PCR amplification. After isolation, four strains were purified and cultivated, including two filamentous cyanobacteria Microcoleus vaginatus BSC-06 and Scytonema javanicum BSC-39, and two unicellular green algae Chlorella sp. BSC-24 and Monoraphidium dybowskii BSC-81. The two green algae grew fast (>250 mg L-1 d-1), and achieved high lipid productivity up to 75–85 mg L-1 d-1, with lipid content of 28.7–39.0%, thus was considered as promising feedstock for biodiesel production. In addition, the two crust cyanobacteria could be used to construct artificial cyanobacterial soil crusts in desertification control, although their biomass accumulation was not as high as that in the green algae. Ultimately, combining biodiesel production with desertification control would not only improve desert environments, but also provide ideal places for the local microalgal resource exploitation, further promoting desert socioeconomic development.

Renitrogen Fixation Activity in Cyanobacterial Biological Soil Crusts with Domination of the Stigonema Genus Species in Mountain and Plain North- East European Tundra Ecosystems

To model global processes which happen under the climate change, we should know the input of cyanobacterial communities to nitrogen balance in high latitude regions. It is widely known that the Arctic and mountain ecosystems are highly sensitive to changes of the climate. The forecasted increase in temperature in high latitudes, undoubtedly, will cause shifts in scale and rate of biogeochemical N-cycles and result in transformations of plant communities. The reliable forecasting of changes in the functional organization of ecosystems is only possible if vast and broad experimental data is taken into account in the models. Both seasonal and daily values of N2-fixation process has to be evaluated in various plant communities from different zones under variety of conditions. For balanced estimations, the various types of arctic ecosystems have to be included in the studies. The N2-fixation activity is barely studied on large territories of plain and mountain tundra of the NortheastEuropean Arctic. The article sums up results of a research on nitrogenase activity ratesin the soil crusts dominated byStigonema species.The species of the genus arewidespreadinthestudied regions, they have capability to fix nitrogen and they are important contributors to the N-inputin cryogenic disturbed soils. The nitrogen-fixing activity of investigated crusts was studied and the measured rates were 0.59-0.80 mg С2Н4 m-2 h-1under 15°С (frequent midday temperature in July)and 0.97-1.64 mgC2H4m-2 h-1 under 21°С(optimal temperature for N2-fixation in the Arctic). Crusts with the dominance of Stigonema genus from different regions of the European North-East have close rates of N2-fixing activity in latitudinal and altitudinal gradients under similar environmental conditions.The obtained values might be used to calculate the input of crustswith Stigonemato the N-balancein mountain and plain tundras for calculating global balance for high latitudes.

Effects of freeze–thaw cycles on the soil nutrient balances, infiltration, and stability of cyanobacterial soil crusts in northern China

Freeze–thaw fluctuation is a natural phenomenon, which is frequently encountered by biological soil crusts (BSCs) in late autumn and early spring in cold deserts. The objective of our study was to investigate the effects of freeze–thaw cycles (FTCs) on the soil nutrient balances, infiltration, and stability of cyanobacterial soil crusts (CSCs) in the temperate desert region. A controlled incubation experiment was carried out to study the effects of diurnal freeze–thaw cycles (FTCs) on total soil carbon (TC), total soil nitrogen (TN), soil TC/TN, hydraulic conductivity, and strength of light and dark cyanobacterial crusts, respectively. Six successive diurnal FTCs were applied as three temperature regimes (i.e., six successive mild FTCs (mild), six successive severe FTCs (severe), three successive mild FTCs followed by three successive severe FTCs (medium)). The experiment intended to simulate natural temperature changes in one of the temperate regions of northern China. Compared with dark CSCs cores, light CSCs cores lost a greater proportion of nitrogen. For both crust cores, severe FTCs decreased TC and TN more than mild FTCs. However, TC and TN remained relative constant when CSCs cores were treated with severe FTCs after experiencing mild FTCs. TC and TN of both CSCs cores decreased in the earlier FTCs and then remained stable in the later FTCs. TC/TN increased significantly for light CSCs, but only changed slightly for dark CSCs after successive FTCs. The effects of FTCs on the hydraulic conductivity and strength of CSCs were not consistent with our expectations that FTCs would increase hydrological conductivity and decrease strength. These effects depended on crust type, FTC number, and freeze/thaw intensity. Increase in hydraulic conductivity and decrease in strength only occurred in severe treatment in the dark CSCs during the later FTCs. Light CSCs are more sensitive to FTCs than dark CSCs. Mild FTCs decrease less TC and TN than severe FTCs and mostly increase the stability of the CSCs. However, severe FTCs may decrease TC and TN drastically, thereby, degrading the BSCs.

Microstructures and photosynthetic diurnal changes in the different types of lichen soil crusts

In the revegetation region of Shapotou (at the southeastern edge of Tengger Desert), biological soil crusts (BSCs) generally develop and succeed from cyanobacterial soil crusts (CSCs) to lichen soil crusts (LSCs). Based on the different dominant lichens on crust surface, LSCs were further divided into cyanolichen soil crusts (CLSCs) and green algae lichen soil crusts (GLSCs). In this study, the microstructures and photosynthetic diurnal changes in both CLSCs and GLSCs were investigated using the microscopic observation and chlorophyll fluorescence techniques. The results showed the different types of LSCs have different surface morphological characteristics and inner structures due to the different biological compositions. Compared with the CLSCs, GLSCs had a higher photosynthetic efficiency, including the maximum photosynthetic efficiency (Fv/Fm) and effective photosynthetic efficiency (ΦPSII), which in the final analysis was considered to be closely correlated with the different photosynthetic characteristics between different photobiont types (green algae and cyanobacteria). In addition, similar to CSCs, a midday depression of photosynthetic efficiency was detected in LSCs, although the depression extents were different in different types of LSCs. In conclusion, the occurrence of midday depression of photosynthetic efficiency might be a stress effect and also an adaptation strategy of LSCs to adversity, especially the high light intensity and temperature at noon.

Response of cyanobacterial soil crusts to moisture and nutrient availability

Cyanobacterial soil crusts play an important role in arid ecosystems and in stabilising dryland soils . Such arid regions are prone to extreme environmental conditions such as prolonged drought and short-lived intense rainfall and flood events, yet the impact of such events on cyanobacterial growth, and hence their role in landscape stability, is poorly-understood. This paper presents a series of laboratory experiments which determine cyanobacterial soil crust response to moisture and nutrient availability. Photosynthetic yield measurements indicate a rapid response in cyanobacterial response following hydration and suggest moisture content > 14% (c. 1.5 mm rainfall equivalent) is necessary to sustain such activity. The addition of nutrients did not trigger any significant increase in crust metabolic activity or photosynthetic response compared with deionised water over a 7-day period. However, over longer periods (> 4 weeks) nutrient-enriched rainfall promoted an increase in cyanobacterial metabolic growth measures (chlorophyll a and ‘greenness’) compared with deionised water. From the results we can infer that the magnitude and frequency of hydration from rainfall and flood events in arid regions play an important role in the maintenance of cyanobacterial soil crusts. Predicted changes in dryland hydrological regimes , for example changing frequency and intensity of rainfall in the Australian arid zone as a consequence of climate shifts, may therefore have an impact on the ability of cyanobacterial crusts to promote landscape stability. • Moisture content > 14% is necessary to sustain cyanobacterial photosynthetic activity. • Nutrients promoted an increase in cyanobacterial biomass over longer periods > 4 weeks. • Moisture frequency and magnitude are important to landscape stability via crusts.

Species diversity, biomass and long-term patterns of biological soil crusts with special focus on Cyanobacteria of the Acacia aneura Mulga Lands of Queensland, Australia

The Australian Mulga Lands bioregion covers 251,640 km 2 and is characterised by Acacia aneura mulga and Eucalyptus populnea woodlands. Equally important to land managers are the extensive perennial grasslands. Since the late 1800's almost the entire bioregion was settled by pastoralists and grazed by sheep or cattle on large leasehold stations. Well-established cyanobacterial soil crusts are an important part of this semi-arid ecosystem, providing a nutrient-rich layer closely integrated into the soil surface. Crust diversity from twelve Mulga Land sites was 61 taxa, comprising 23 cyanobacteria, 18 chlorolichens, 5 cyanolichens, 6 liverworts, 6 mosses, 2 eukaryotic algae and 1 micro-fungus. Notably the soil crusts at this site and others across the Mulga Lands were dominated by Porphyrosiphon notarisii and the N-fixing Scytonema hofman-bangii. Cyanobacterial diversity was well represented at the study site where diversity accounted for > 50% of the 23 species. Biomass of crusts from the study site ranged between 91–117 mg m –2 chlorophyll a, similar to several records from across Queensland. Morphologically, cyanobacteria frequently appeared stunted in size compared to other records, indicative of frequent droughts and prolonged periods of desiccation. The cyanobacteria from the study site were resilient to drought although recovered at different rates. Porphyrosiphon notarisii tolerated and recovered from drought exceptionally well while Gloeocapsopsis cf. dvorakii was least drought tolerant. Nostoc commune and Microcoleus spp. were abundant in degraded sites or where soil chemistry was more neutral or alkaline. This study illustrated a highly diverse cyanobacterial crust community across the Mulga Lands dominated by drought tolerant cyanobacteria. This paper presents the first taxonomic descriptions based on morphological features of the key terrestrial cyanobacteria found commonly across the Mulga Lands.

Physiological responses of soil crust‐forming cyanobacteria to diurnal temperature variation

The optimum growth of soil crust-forming cyanobacterial species occurs between 21 and 30 °C. When the temperature decreases below -5 °C, the liquid water in the cyanobacterial cells may freeze. In the natural environment, the temperature gradually decreases from autumn to winter, and the diurnal temperatures fluctuate enormously. It was hypothesized that the physiology of cyanobacterial cells changes in later autumn to acclimatize the cells to the upcoming freezing temperatures. In the present study, an incubation experiment in growth chambers was designed to stimulate the responses of cyanobacterial cells to diurnal temperature variations before freezing in late autumn. The results showed that "light" cyanobacterial soil crusts are more tolerant to diurnal temperature fluctuations than "dark" cyanobacterial soil crusts. After the first diurnal temperature cycle between 24 and -4 °C, the malondialdehyde (MDA) contents increased and the photosynthetic activity decreased. The superoxide dismutase activity increased, more extracellular polysaccharides (EPS) were secreted and the ratios of the light-harvesting and light-screening pigments decreased. With increasing numbers of diurnal temperature cycles, the MDA contents and photosynthetic activity gradually returned to their initial levels. Our results suggest that there are at least three pathways by which crust-forming cyanobacteria acclimate to the diurnal temperature cycles in the late autumn in the Hopq Desert, Northwest China. These three pathways include increased secretion of EPS, regulation of the ratios of light-harvesting and light-screening pigments, and activation of the antioxidant system. The results also indicate that late autumn is a critical period for the protection and restoration of the cyanobacterial soil crusts in the Hopq Desert.

Deposition of sand over a cyanobacterial soil crust increases nitrogen bioavailability in a semi-arid woodland

The movement of sand by erosion is a common feature of drylands during droughts and periods of sparse vascular plant cover. We examined the effects of sand deposition on the bioavailability of N in cyanobacterial-dominant soil crusts during and after a severe drought. Crusts were sampled from two depths on stony and stone-free surfaces with and without sandy deposits. All sites supported an extensive cover (up to 51%) of N-fixing cyanobacteria and cyanolichens. During drought, sand-covered crusts had up to three-times more mineral N (NH 4 + and NO 3 −) and twice the mineralisable N, at both depths, than sand-free samples. Mineralisable N was always greater in the surface soil layer both during and after drought. During the drought, two common N-fixing cyanobacteria ( Scytonema cf. hofman-bangii, Stigonema ocellatum) were significantly more abundant on uncovered than sand-covered surfaces. Increased N bioavailability likely results from autolysis and subsequent breakdown of N-enriched cyanobacterial cell material mediated by changes in the soil surface microenvironment. Our work suggests that landscape-level processes of sand deposition have a marked effect on soil nutrient pools by enhancing the accumulation of plant-available N on cyanobacterial crusted surfaces. Inappropriate land management or the loss of cyanobacterial soil crusts during drought would compromise the long-term bioavailability of soil N.

Carbohydrates in cyanobacterial soil crusts as a source of carbon in the southwest Kalahari, Botswana

Semiarid ecosystems, also known as drylands, contain small amounts of carbon (C). To date only few studies have evaluated soil C dynamics in these ecosystems. Cyanobacterial soil crusts are considered a major source of organic C in semiarid ecosystems through photosynthesis, increasing soil organic C (SOC) pool as carbohydrates. This study considers cyanobacterial soil crusts as a source of C in the southwest Kalahari. Cyanobacterial soil crusts are well adapted to drylands because extracellular polysaccharide (EPS) secretions provide stabilization of soil and resistance to desiccation. The carbohydrate and chlorophyll a content were evaluated in the upper soil profile on Kalahari Sands and pan soils. Topsoil carbohydrate concentration decreased exponentially with depth. The carbohydrate content produced in cyanobacterial soil crusts in the southwest Kalahari represents up to 75% of the total SOC and is thus an essential component for the fertility of Kalahari Sand soils.

Grazing and drought reduce cyanobacterial soil crusts in an Australian Acacia woodland

Cyanobacteria-dominant soil crusts are important components of unvegetated interspaces in Australia's semi-arid rangelands. We studied the changes in crust cover and floristics at increasing distances up to 500m from three stock watering points in western Queensland's Mulga bioregion between 2002 and 2005. Sampling times corresponded with pre-drought (2002), drought (2003), early post-drought (2004) and late post-drought (2005) periods. Cyanobacterial cover increased markedly with increasing distance from water, with the largest changes occurring within 200m of water. The change in cover with distance from water was most pronounced before the drought compared with during or after drought. Crust composition also changed in relation to distance from water, and the abundance of three cyanobacteria (Scytonema sp., Stigonema ocellatum, Porphyrosiphon notarissi) was greater away from water. The diversity of crust species increased significantly with distance from water, and was greatest in the late post-drought period. Although cyanobacterial crusts were present close to water (<100m) before the drought, cover close to water had not recovered to pre-drought levels 2 years after drought. Our results indicate that excessive damage by stock trampling has an immediate impact on crusts and reduces their capacity to recover after drought. Changes in the severity or intensity of drought may therefore accelerate degradation processes and compromise the ability of crusts to recover after grazing.

Nitrogen isotope composition of soils, C 3 and C 4 plants along land use gradients in southern Africa

This paper provides values for nitrogen isotopic abundances of southern African soils and plants along land use gradients of varying aridity. The δ 15N values of soils and plants were generally higher in sites with greater land use intensity, except in the most arid site where δ 15N decreased with land use intensity. The enrichment in 15N with land use intensity agrees with the expected effects of grazing and cultivation on N cycling processes, including increased volatilization of ammonium, exports of plant material, and decreased N 2 fixation by the destruction of cyanobacterial soil crusts. Gross mineralization and nitrification rates were more affected by local heterogeneity in the soils than by aridity or land use. In general, C 3 plants had significantly higher δ 15N than C 4 plants from the same location, suggesting different N use by the two plant types. This study suggests that land use intensity affects N cycling processes that may result in different and opposite changes of ecosystem δ 15N, as those observed between the most arid, and the other semi arid sites analyzed. In addition to the 15N enrichment caused by the loss of gaseous and plant 14N, changes in tree and grass cover may affect soil δ 15N by the differential uptake of soil N with different isotopic abundances.

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.

Spatial and temporal distribution of cyanobacterial soil crusts in the Kalahari: Implications for soil surface properties

Localised patterns of erosion and deposition in vegetated semi-arid rangelands have been shown to influence ecological change and biogeochemical cycles. In the flat, vegetated Kalahari rangelands of Southern Africa the factors regulating erodibility of the fine sand soils and the erosivity of wind regimes require further investigation. This paper reports on the spatial and temporal patterns of cyanobacterial soil crust cover from ten sites at five sampling locations in the semi-arid Kalahari and discusses the likely impact on factors regulating surface erodibility and erosivity. Cyanobacterial soil crust cover on Kalahari Sand varied between 11% and 95% of the ground surface and was higher than previously reported. Cover was inversely related to grazing with the lowest crust cover found close to boreholes and the highest in the Game Reserve and Wildlife Management Zone. In grazed areas, crusts form under the protective canopies of the thorny shrub Acacia mellifera. Fenced plot data showed that crusts recover quickly from disturbance, with a near complete surface crust cover forming within 15 months of disturbance. Crust development is restricted by burial by wind blown sediment and by raindrop impact. Crusts had significantly greater organic matter and total nitrogen compared to unconsolidated surfaces. Crusts also significantly increased the compressive strength of the surface (and thus decreased erodibility) and changed the surface roughness. Establishing exactly how these changes affect aeolian erosion requires further process-based studies. The proportion of shear velocity acting on the surface in this complex mixed bush–grass–crust environment will be the key to understanding how crusts affect erodibility.

Distribution and characteristics of cyanobacterial soil crusts in the Molopo Basin, South Africa

This paper provides an analysis of the physical and chemical characteristics of cyanobacterial soil crusts in the Molopo Basin, South Africa. It details the influence that soil type, livestock disturbance and shrubs have on the spatial distribution of crusts. Four morphologically distinct cyanobacterial crusts were identified and crust cover ranged from 24% to 55%. Crust cover was significantly higher and characterized by darkened type 3 and 4 crusts on Ironstone soils compared to Kalahari Sand. More frequently disturbed sites had the least crust cover and had predominantly type 1 and 2 crusts. Type 3 and 4 crusts are more common on the less disturbed sites and under the canopies of Acacia mellifera where soils are protected from livestock disturbance. Total nitrogen concentrations were significantly higher in crusts compared to unconsolidated soil. There is also a strong correlation between the pH and NH 4 +-N concentrations in crusts and the soil immediately below the crust, suggesting that crusts have an influence on some of the properties of the underlying soil. If the A. mellifera can utilize additional nitrogen from crusts it may provide a competitive advantage to their establishment in formerly grass-dominated grazing lands.

Cyanobacterial soil crusts and woody shrub canopies in Kalahari rangelands

AbstractIntensive grazing of Kalahari rangelands has led to woody plant encroachment, notably of Acacia mellifera and Grewia flava. The mechanisms causing this process, and the ecological stability of woody plant encroached ecosystems, remain uncertain. Past studies suggest that canopy–soil relations may enhance woody plant competitive dominance. This study aims to investigate one element of this ecological change by examining the spatial distribution of cyanobacterial soil crusts in two vegetation sub‐habitats at sites of different disturbance. Crust burial by litter was also assessed to analyse the dynamics of canopy–crust relations. Our results show there is enhanced cyanobacterial crust cover under A. mellifera canopies and that unlike G. flava canopies, the crust cover remains under A. mellifera even at highly disturbed sites. This canopy–crust association suggests A. mellifera encroachment exhibits intrinsic resilience because of the crusts ability to stabilize the soil surface and increase nutrient retention. Crust burial by litter that accumulates under larger woody plants restricts crust development under canopies. Disturbance restricts crust development in plant interspaces and under G. flava. These two mechanisms combine to restrict crust development to an observed 40% threshold, with nonlinear models required to explain spatial patterns of crust dynamics.

Title: Importance of nitrogen fixation in soil crusts of southern African arid ecosystems: acetylene reduction and stable isotope studies

Cyanobacterial soil crusts may be important in arid and semi-arid ecosystems because of their ability to fix atmospheric nitrogen (N 2). These crusts are very sensitive to trampling by animals, and their destruction can decrease ecosystem N inputs, affecting the productivity of the region. The objective of this study was to quantify the nitrogen-fixing activity in soil crusts during the wet season in southern African ecosystems using in situ acetylene reduction assays. The average acetylene reduction rates for each site ranged from 88 to 535 nmol m −2 h −1, were highly variable, and were lower than previously reported for other arid areas. All soil samples showed acetylene reduction activity; however, soils with crusts supported higher rates than did “non-crusty” soils under litter, moss, or sand. High values of 15N natural abundance (δ 15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/denitrification and ammonia volatilization or atmospheric deposition of 15N-enriched nitrate or ammonium may have caused shifts in δ 15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha −1 year −1, orders of magnitude lower than values estimated in other studies. The anomalous wet conditions experienced during the year of the study may have increased the temporal availability of soil mineral N and decreased N fixation rates. However, the presence of N fixation activity in all crusts analysed and their ability to survive at high temperature and after long dry periods may provide ecosystem resilience, facilitating ecosystem recovery after severe droughts.

Temporal variation in community composition, pigmentation, and F(v)/F(m) of desert cyanobacterial soil crusts.

Summers on the Colorado Plateau (USA) are typified by harsh conditions such as high temperatures, brief soil hydration periods, and high UV and visible radiation. We investigated whether community composition, physiological status, and pigmentation might vary in biological soil crusts as a result of such conditions. Representative surface cores were sampled at the ENE, WSW, and top microaspects of 20 individual soil crust pedicels at a single site in Canyonlands National Park, Utah, in spring and fall of 1999. Frequency of cyanobacterial taxa, pigment concentrations, and dark adapted quantum yield [F(v)/F(m)] were measured for each core. The frequency of major cyanobacterial taxa was lower in the fall compared to spring. The less-pigmented cyanobacterium Microcoleus vaginatus showed significant mortality when not in the presence of Nostoc spp. and Scytonema myochrous (Dillw.) Agardh. (both synthesizers of UV radiation-linked pigments) but had little or no mortality when these species were abundant. We hypothesize that the sunscreen pigments produced by Nostoc and Scytonema in the surface of crusts protect other, less-pigmented taxa. When fall and spring samples were compared, overall cyanobacterial frequency was lower in fall, while sunscreen pigment concentrations, chlorophyll a concentration, and F(v)/F(m) were higher in fall. The ratio of cyanobacterial frequency/chlorophyll a concentrations was 2-3 times lower in fall than spring. Because chlorophyll a is commonly used as a surrogate measure of soil cyanobacterial biomass, these results indicate that seasonality needs to be taken into consideration. In the fall sample, most pigments associated with UV radiation protection or repair were at their highest concentrations on pedicel tops and WSW microaspects, and at their lowest concentrations on ENE microaspects. We suggest that differential pigment concentrations between microaspects are induced by varying UV radiation dosage at the soil surface on these different microaspects.