Role Of Biological Soil Crusts Research Articles

Exploring the role of biological soil crusts in protecting rammed earth sites: A case study of the Shanhaiguan Great Wall in Bohai Bay

Exploring the role of biological soil crusts in protecting rammed earth sites: A case study of the Shanhaiguan Great Wall in Bohai Bay

The role of biological soil crusts in vascular plant recruitment in the Loess Plateau region, China

The role of biological soil crusts in vascular plant recruitment in the Loess Plateau region, China

Microbial survival strategies in biological soil crusts of polymetallic tailing wetlands

The role of biological soil crusts (biocrusts) in ecological restoration is dominated by microbiota. This has been extensively studied in arid ecosystems characterized by nutrient deficiencies and poor soil substrates, including metal(loid)-rich mining regions. However, in the prior study areas, water constraints obscured the toxicity and restriction of succession by heavy metals, which were essential for soil remediation in mining regions using biocrusts. Accordingly, in the current study, we characterized biocrusts from a typical polymetallic tailing wetland and performed pot incubation to evaluate microbial community succession and microbial co-occurrence patterns. Results showed that adequate water and heavy-metal stress produced thicker and more hydrophobic biocrusts than did arid conditions, promoting stronger microbial activity in tailing wetlands. Moreover, the bioinformatic analyses during pot incubation suggest three potential survival strategies. (1) Cyanobacteria variation improved the succession level of cyanobacterial crusts. Even under heavy-metal stress, succession from light to dark cyanobacterial crusts still occurred, with Coleofasciculaceae decreasing 63% and Scytonema increasing 59% compared with the control groups. (2) Keystone taxa variation maintained high inorganic nitrogen turnover. Organisms potentially capable of inorganic nitrogen turnover, including Chloroflexi A4b and SBR1031, maintained a high relative abundance accounting for 42% (15/36) of the keystone taxa. (3) Variations in the interactions among microbiota created closer-knit microbial networks. Compared with the control groups, fewer nodes (15.2%) maintained higher average clustering coefficient values (1.8%) through 14% more positive interactions in the groups exposed to heavy-metal stress at the end of the incubation experiment. This study provides insights into the distinctive survival strategies of biocrusts in metal-rich extreme environments, offering theoretical support for biocrust-mediated ecological restoration in humid metal-contaminated regions.

Divergent effects of biological soil crusts on soil respiration between bare patches and shrub patches under simulated rainfall in a desert ecosystem in Northwest China

The important role of biological soil crusts (biocrusts) in determining the soil carbon balance in dryland ecosystems under future climate change has been widely explored. However, when assessing the effects of biocrusts on soil respiration, few studies have considered biocrusts in bare soil patches and shrub soil patches. A field experiment was performed with five simulated rainfall addition events (0, 5, 15, 25, and 40 mm) in shrub and bare soil patches in June (early growing season, EGS), July (middle growing season, MGS) and September (late growing season, LGS) in 2016 and 2017. Our objective was to examine whether biocrusts at shrub and bare soil patches have the same effect on soil respiration under altered rainfall regimes. The results showed that the removal of biocrusts from bare patches reduced soil respiration by 15 % after rainfall amounts of 5–40 mm and increased the subsurface soil temperature and decreased the soil moisture after various rainfall treatments. However, in shrub patches, the removal of biocrusts led to a significant increase in soil respiration of 57 % and subsequent increases in subsurface soil temperature and moisutre. The discrepancy in the soil respiration response between the two types of patches might be attributable to the increase in soil infiltration after biocrust removal, with a greater stimulating increase in soil carbon emissions stimulated in the shrub patches than in the bare patches due to more the greater amounts of roots distributed under the former shrub patches. Our results indicate that the destruction of biocrusts in shrub patches may reduce the ability of biocrust ecosystems to become a net carbon sinks, with potentially negative consequences for ecosystem function and sustainability in water-limited regions.

Biological soil crusts determine soil properties and salt dynamics under arid climatic condition in Qara Qir, Iran

Biological soil crusts are a thin layer within the soil system but strongly determine the infiltration, runoff and water and solute movement. Little is known about the role of biological soil crusts on soil solute dynamics in arid ecosystems and the objective of this paper is to determine in Qara Qir rangeland how biological soil crusts control the water and salt distribution along the soil profile. Rainfall simulation experiments were carried out at five locations, and measurements of the soil at 0–5, 5–10, 10–20, 20–30, 30–50 and 50–80 cm depth were done before, 48 h and 21 days after the rainfall simulations. Soil particle size distribution, bulk density, water content, organic carbon and electrical conductivity were measured at each of the 270 samples (3 seasons × 3 times × 5 sites × 6 depths). Biological soil crusts increased soil organic carbon, soil water content, and infiltration rate; and biological soil crusts decreased soil bulk density, clay fraction, electrical conductivity, and other saline-sodic properties, especially in the upper layers (0–10 cm). Large pores in soils covered by biological soil crusts enhanced the preferential flows, infiltration and solute transport. Biological soil crusts not only directly affected the soil surface, but also influenced soil properties, and consequently determined spatio-temporal soil salinity distribution. Biological soil crusts act as a soil salinity reducing agent and contribute to the soil quality improvement under arid climatic conditions. Biological soil crusts can be considered as a soil conservation strategy and actively used in soil rehabilitation and ecosystems restoration.

Warming effects on soil respiration in moss-dominated crusts in the Tengger Desert, northern China

Despite the important role of biological soil crusts in the soil carbon cycles of desert ecosystems, the responses of soil respiration in biological soil crust-dominated areas to warming are not well understood. The goal of this study was to investigate the expected increases in temperature on soil respiration both diurnally and seasonally in biological soil crust-dominated areas. We used open-top chambers to simulate warming in the Shapotou region in the Tengger Desert, northern China. An automated soil respiration system was used to measure the soil respiration rates in moss-dominated crusts. The measured environmental variables included the precipitation, volumetric soil water content, air temperature and soil temperature at depths of 0, 5, 10, 20, and 50 cm. The response of soil respiration to warming is a function of soil moisture following rainfall in desert ecosystems. Our results showed that 1.5 °C of simulated warming significantly decreased soil respiration, indicating that the inhibition of soil respiration was likely due to the reduction in soil water content at a relatively high temperature. Over daily cycles, hourly soil respiration rates have commonly been related to hourly temperatures. The observed diel hysteresis between hourly soil respiration and temperature resulted in semielliptical hysteresis loops, and the temperature often lagged behind soil respiration for several hours. The lag times between soil respiration and temperature were significantly and positively related to the depth of the soil temperature measurements. The proximate reason for the diel hysteresis between soil respiration and temperature was likely a mismatch between the depth of CO2 production and the depth of the temperature measurements. Our results indicate that warming increases the response of soil respiration to soil water availability in biological soil crust-dominated desert ecosystems. Therefore, the accelerated drying effect of warming on soil respiration and diel soil respiration patterns between soil respiration and temperature at different depths should be considered in future soil carbon cycle models for biological soil crust-dominated desert ecosystems.

The Role of Biological Soil Crusts in Nitrogen Cycling and Soil Stabilization in Kangerlussuaq, West Greenland

Biological soil crusts (biocrusts) naturally coexist with vascular plants in many dryland ecosystems. Although most studies of dryland biocrusts have been conducted in warm deserts, dryland biocrusts also exist in the Arctic, where they may be an important source of nitrogen (N) and carbon (C) to nutrient-limited environments. In Kangerlussuaq, Greenland, wind-driven soil erosion has created a heterogeneous landscape where biocrusts dominate distinct patches of soil but are absent from the surrounding shrub and graminoid tundra. Prior to this study, little was known about the physical development and nutrient cycling of west Greenland biocrusts and their role in maintaining landscape heterogeneity. We characterized the physical properties, lichen assemblages, and nutrient concentrations of biocrusts and underlying soils along gradients in biocrust development and age. We found that biocrusts took 180 ± 40 years to fully develop and that biocrusts became thicker and soil penetration resistance increased as they developed. The N-fixing lichen Stereocaulon sp. was found throughout the study region at all stages of biocrust development. Natural 15N abundance suggests that Stereocaulon sp. obtains about half of its N from biological fixation and that some biologically fixed N is incorporated into the underlying soils over time. Although the N and C concentrations of underlying soils increased slightly with biocrust development, nutrient concentrations under the most developed biocrusts remained low compared to the surrounding vegetated tundra. Our results suggest that biocrusts are a persistent feature and play an important role in maintaining the high spatial heterogeneity of the Kangerlussuaq terrestrial landscape.

The effect of biological soil crusts on soil moisture dynamics under different rainfall conditions in the Tengger Desert, China

AbstractRainfall is considered as the dominant water replenishment in desert ecosystems, and the conversion of rainfall into soil water availability plays a central role in sustaining the ecosystem function. In this study, the role of biological soil crusts (BSCs), typically formed in the revegetated desert ecosystem in the Tengger Desert of China, in converting rainfall into soil water, especially for the underlying soil moisture dynamics, was clarified by taking into account the synthetic effects of BSCs, rainfall characteristics, and antecedent soil water content on natural rainfall conditions at point scale. Our results showed that BSCs retard the infiltration process due to its higher water holding capacity during the initial stage of infiltration, such negative effect could be offset by the initial wet condition of BSCs. The influence of BSCs on infiltration amount was dependent on rainfall regime and soil depth. BSCs promoted a higher infiltration through the way of prolonged water containing duration in the ground surface and exhibited a lower infiltration at deep soil layer, which were much more obvious under small and medium rainfall events for the BSCs area compared with the sand area. Generally, the higher infiltration at top soil layer only increased soil moisture at 0.03 m depth; in consequence, there was no water recharge for the deep soil, and thus, BSCs had a negative effect on soil water effectiveness, which may be a potential challenge for the sustainability of the local deep‐rooted vegetation under the site specific rainfall conditions in northwestern China.

Precipitation and topsoil attributes determine the species diversity and distribution patterns of crustal communities in desert ecosystems

Despite the important role of biological soil crusts in desert ecosystems, the responses of the crustal composition and distribution to changes in both climatic and edaphic factors at different spatial scales are not well understood. The goal of our research was to study the drivers of crustal community diversity at different scales, determine how crustal diversity and distribution patterns are related to abiotic and biotic factors, and identify the characteristics of crustal communities along a precipitation gradient. We investigated the distribution and characteristics of crustal communities based on 457 soil samples that were randomly collected from six desert regions of northern China. These regions represent a precipitation gradient from 450 mm in the east to 80–100 mm in the west. The measured environmental variables included soil moisture, pH, clay and silt content, soil organic carbon (SOC), total N, total P, total K, CaCO3 content, annual and perennial plant cover and crustal variables, including species richness, crustal cover and biomass. An ANOVA analysis, a stepwise regression and a redundancy analysis were used to analyse and interpret the response of biological soil crusts to environmental factors at two spatial scales. Our results showed that the moss distribution and biomass were strongly related to the topsoil moisture content. The highest diversity of cyanobacteria and algae was observed at sites with relatively dry topsoil, and lichen diversity was promoted at sites with fine-textured soils. In addition, the cover and biomass of moss were positively correlated with perennial plant cover and higher topsoil moisture, which was expected because plant cover provides shade for the moss. The cover and biomass of lichens, cyanobacteria and algae were positively correlated with that of annual plants because these plants may create an ideal habitat and provide nutrients. The physiochemical properties of the topsoil greatly influenced the distribution pattern of the crustal communities at the regional scale. The species richness and biomass of the crustal mosses were positively correlated with precipitation, whereas the species richness and biomass of cyanobacteria and algae were negatively correlated with precipitation at the landscape scale. Rainfall largely determined the species richness and cover of the crustal cyanobacteria and mosses, whereas soil properties had a greater impact on the crustal lichens. Future changes to rainfall regimes could result in the conversion of biological soil crusts and further contribute to changes in the structure and functioning of desert ecosystems.

Role of biological soil crusts in affecting soil evolution and salt geochemistry in hyper-arid Atacama Desert, Chile

Eight soil profiles, either barren or covered with biological soil crusts (BSCs), were sampled at multiple depths in a remote valley in the Atacama Desert, Chile, to explore the impact of BSCs on soil evolution and salt geochemistry. BSC sites had thicker soil profiles compared to non-BSC sites, which could be a result of past geological processes and also partially accounted for by the establishment of BSCs that can help limit erosion and promote significant soil accumulation. The greater abundances of fine particles below BSCs could essentially be attributed to the ability of BSCs to trap more fine grains. The generally higher salt ion inventories but smaller salt ion concentrations in soil profiles beneath BSCs were probably due to that BSCs can promote the retention of salt ions and insoluble dust simultaneously, but the latter to a greater extent. Nitrate oxygen isotopes indicated the predominant existence of atmospheric nitrate in the soils, suggesting biological processes of BSCs may not significantly impact the ion accumulation or distribution in the underlying soils. A mechanism of consistent soil accumulation via the retention of atmospheric deposition and occasional interruption of subsurface collapsing due to erosion processes was proposed to explain the development of soil profiles and different landscape features as well as the role of BSCs in soil evolution in this study area. The soil ages were estimated to be around 440ky based on the proposed soil accumulation mechanism.

Ecohydrological role of biological soil crusts across a gradient in levels of development

AbstractThough biological soil crusts (biocrusts) form abundant covers in arid and semiarid regions, their competing effects on soil hydrologic conditions are rarely accounted for in models. This study presents the modification of a soil water balance model to account for the presence of biocrusts at different levels of development (LODs) and their impact on one‐dimensional hydrologic processes during warm and cold seasons. The model is developed, tested, and applied to study the hydrologic controls of biocrusts in the context of a long‐term manipulative experiment equipped with meteorological and soil moisture measurements in a Colorado Plateau ecosystem near Moab, Utah. The climate manipulation treatments resulted in distinct biocrust communities, and model performance with respect to soil moisture was assessed in experimental plots with varying LOD as quantified through a field‐based roughness index (RI). Model calibration and testing yielded excellent comparisons to observations and smooth variations of biocrust parameters with RI approximated through simple regressions. The model was then used to quantify how LOD affects soil infiltration, evapotranspiration, and run‐off under calibrated conditions and in simulation experiments with gradual modifications in biocrust porosity and hydraulic conductivity. Simulation results show that highly developed biocrusts modulate soil moisture nonlinearly with LOD by altering soil infiltration and buffering against evapotranspiration losses, with small impacts on run‐off. The nonlinear and threshold variations of the soil water balance in the presence of biocrusts of varying LOD helps explain conflicting outcomes of various field studies and sheds light on the ecohydrological role of biocrusts in arid and semiarid ecosystems.

Biological soil crusts of temperate forests: Their role in P cycling

To elucidate the ecological role of biological soil crusts (BSCs) in the P-cycle and to disclose vegetation (pine vs. beech), soil horizon (A horizon vs. C horizon) and land use intensity effects, we examined BSCs collected from a temperate forest in Schorfheide-Chorin (Germany). Total C, N and P of the three sample compartments crust, crust-adhering soil underneath and crust-free soil were quantified and inorganic and organic P (Pi, Po) contents in five sequentially extracted P fractions of the different compartments were determined. In addition, P species were characterized using P K-edge XANES (X-ray absorption near edge structure) and 31P NMR (nuclear magnetic resonance) spectroscopy. BSC biodiversity of algae was morphologically determined using enrichment cultivation. Results showed an accumulation of total P in the BSC with high shares of Po. Proportions of labile and moderately labile Po pools were higher in BSC than crust-free soil in expense of residual P indicating weathering of Fe/Al-P species by BSC organisms. Vegetation affected the C/N ratio and proportion of labile Pi in total P of BSC; they were significantly higher under beech compared with pine. XANES results revealed BSC weathering of mainly Fe-P under beech and Al-P under pine. Soil horizon affected the composition of BSC organisms; there were more filamentous algae in BSCs on C horizon than A horizon. Moderately labile Pi concentrations were higher in BSC on C horizon compared with A horizon while the share of the labile Po pool in total P was lower on C horizon than A horizon. Increasing forest management intensity decreased the share of moderately labile P and Po in total P as well as the monoester-P/diester-P ratio in pine BSC. While in BSC under pine changes occurred in microbial diversity, under beech changes occurred in algal richness and life form. We conclude that BSCs in Central European forests are particularly involved in the transformation of Pi to Po fractions and respond differently to management intensity depending on the predominating tree species.

Effects of Near Soil Surface Characteristics on the Soil Detachment Process in a Chronological Series of Vegetation Restoration

The effects of near soil surface characteristics on the soil detachment process might be different at different stages of vegetation restoration. This study was performed to investigate the effects of near soil surface factors on the soil detachment process by overland flow. Soil samples were collected from two natural grasslands of different ages and subjected to flow scouring. The results indicated that the effects of near soil surface characteristics on soil detachment were substantial during vegetation restoration. The total reduction in soil detachment capacity of the 1-yr-old grassland was 98.1%, and of this total, 7.9, 30.0, and 60.2% was attributed to the litter, biological soil crusts (BSCs), and plant roots, respectively. In the 24-yr-old grassland, the soil detachment capacity decreased by 99.0%, of which 13.2, 23.5, and 62.3% were due to litter, BSCs, and plant roots, respectively. Combined with previously published data for a 7-yr-old grassland, the influence of plant litter on soil detachment was demonstrated to increase with restoration time, but soil detachment was also affected by the litter type and composition. The role of BSCs was greater than that of plant litter during the early stages of vegetation recovery. However, its contribution weakened with time. The influence of plant roots accounted for half to two-thirds of the total near soil surface factors, of which >72.6% was attributed to the physical binding effects of roots. The correction coefficients of near soil surface characteristics for rill erodibility were determined for the Water Erosion Prediction Project model.

The role of biological soil crusts in soil moisture dynamics in two semiarid ecosystems with contrasting soil textures

Summary The interplant soil surfaces in most arid and semiarid ecosystems are covered by biological soil crusts (BSCs). These crusts regulate water inputs and losses through soils and play major roles in local hydrological regimes. In recent years, the role of BSCs in infiltration and runoff has gained increasing importance and better knowledge of their effects on these processes has been acquired. However, the role of BSCs in other important components of the water balance, such as evaporation or soil moisture has hardly been studied, so their effects on these processes remain unknown. The aim of this study was to explore the influence of BSCs on soil moisture regimes in the top layer of the soil in two semiarid ecosystems in SE Spain with different particle-size distributions. At both study sites, soil moisture was monitored at 0.03 and 0.10 m under two types of BSCs, a cyanobacteria-dominated BSC and a lichen-dominated BSC, and in adjacent soils where they had been removed. Our results showed that during wet soil periods, removal of BSCs led to decreased soil moisture, especially in the upper layer (0.03 m), compared to soils covered by BSCs. Decrease in soil moisture was more noticeable after removal of lichens than cyanobacterial BSCs, and more so in fine than in coarse-textured soils. Soil water loss was also generally faster in soils with no BSCs than in soils covered by them. However, no difference was found in soil moisture under either crusted or scalped soils during soil drying periods. The type of BSC influenced soil moisture differently depending on soil water content. During wet soil periods, soil water loss was faster and soil moisture lower under cyanobacterial than under lichen BSCs. On the contrary, during soil drying periods, soils covered by lichens lost water faster and showed lower moisture than those covered by cyanobacteria. Our results show the major role of the presence of BSCs, as well as the types, in soil water content in semiarid ecosystems.

Biological and microbial activity in biological soil crusts from the Tabernas desert, a sub-arid zone in SE Spain

The ecology and functional role of biological soil crusts (BSCs) in arid and semi-arid zones have been extremely well studied. However, little is known about the biochemical properties related to the number and activity of the microbiota that form the crusts, even though information about these properties is very important for understanding many of the processes that affect the formations. In this study, several properties related to the activity and number of microorganisms (biomass-C, basal respiration, dehydrogenase activity and nitrogen mineralization potential) were determined at different depths (crusts, 0–0.5 cm; middle, 0.5–3 cm and deep, 3–5 cm layers) in two types of crusts (predominated by cyanobacteria and by lichens) in the Tabernas desert (Almeria, SE Spain). The absolute values of the above-mentioned properties and the values expressed relative to the total organic carbon (TOC) content were both much higher in the crust layers than in the surface horizons of soils under Mediterranean or Atlantic climates. A large part of the TOC in the BSCs was contained in the microbiota and another large part was readily metabolized during incubation of the crusts for 10 days at 25 °C. The net nitrogen mineralization rate was also high, and ammonification predominated in the crust layers, whereas nitrification predominated in the middle and deep layers. In all types of BSCs, the microbiota colonized the deep layers, although with greater intensity in the lichen-dominated BSCs than in the cyanobacterial BSCs. The results also indicate that hydrolytic enzymes are not stabilized on soil colloids and their activity depends only on the active microbiota.

Runoff generation fostered by physical and biological crusts in semi-arid sandy soils

Biological soil crusts occur extensively in semi-arid regions. In the western part of Niger, they are associated with various types of physical soil crusts in fallows and in the “tiger bush ecosystem” (a landscape with a typical banded pattern consisting of densely vegetated bands of small trees and shrubs alternating with bare soil bands). Despite their widespread occurrence, not much research has been done regarding their role on runoff generation in the Sahelian environment. The present study aims at: (i) highlighting the relative contribution of biological soil crusts in infiltration and runoff on crusted soil in respect with other soil factors; (ii) assessing the impact of underlying physical crusts associated with a wide range of microbial cover; and (iii) gathering the data required for an evaluation of the potential of microbial cover in water redistribution in an arid environment and for modelling purposes. The study was based on runoff measurements performed in situ at the level of ten plots, 1 m 2 each, under two simulated rainfalls. Those experiments were coupled with the visual characterization and image analysis of soil surface conditions as well as micromorphological examinations, using a light microscope, both on the field and at the laboratory. Visual estimation of crust cover was found consistent with the estimation based on the image analysis. Biological soil crusts capped the summit of pre-existing physical soil crusts, i.e. structural crusts, formed in situ by the impact of water drop and erosion crusts, resulting from water or wind erosion of structural crusts. The percentage of microbial cover in the 1-m 2 plots range from 39 to 80% (55% in average) on structural crusts compared to 4 to 29% (17% in average) on erosion crusts. Runoff coefficient varied with both the level of microbial cover and the type of physical soil crust. Higher runoff coefficients were found where biological soil crusts had developed on erosion crusts as compared to structural crusts. Runoff coefficients recorded from erosion crusts increased with the level of microbial cover while negative correlations were found on structural crusts. They show that the clogging effect of microbial cover for the plasmic layer of erosion crusts is higher than that of the sand layer of structural crusts. Separating biological soil crusts according to the types of the underlying physical crusts explain, therefore, the contradictory findings related to their role on infiltration and runoff generation. Those data will help to evaluate the potential role of biological soil crusts in the modelling of water redistribution in the arid environment of the western part of Niger.

Characteristics of surface runoff in a sandy area in southern Mu Us sandy land

To better understand the key role of biological soil crusts (BSCs) in redistribution processes of limited water resources in semi-arid sandy ecosystems, surface runoff over BSCs in a sandy area in the southern Mu Us sandy area was observed in this study. The results indicated that runoff occurred twice among six rainfall events during the experimental period. Runoff yields varied among types of BSCs, and that they increased in the following order: light algae crusts, dark algae crusts and moss crusts. These findings showed that runoff yields were increasing with the development of BSCs. The percentage of runoff in individual rainfall events increased from light algae crusts to moss crusts, which indicated that the redistribution ratios of different types of BSCs differed. Surface runoff of BSCs may be influenced by rainfall, rain intensity, degree of water saturation of the BSCs before the rain and degree of development of BSCs. Formation of BSCs on the surface of sand dunes changed the spatial distribution pattern of water in this semi-arid sandy ecosystem, which increased the heterogeneity of resources such as water and nutrients. Therefore, making appropriate disturbances of BSCs during storm season is beneficial to maintaining the balance of natural resources in the semi-arid sandy ecosystem.

Spatial distribution and physiology of biological soil crusts from semi-arid central Spain are related to soil chemistry and shrub cover

Despite the critical role of biological soil crusts (BSCs) in arid and semi-arid ecosystem function, few studies are found concerning the most important environmental variables affecting their distribution and physiology. This study seeks to determine soil and microenvironmental factors affecting the spatial distribution and pigment production of BSC-forming lichens and mosses in open patches of a semi-arid Mediterranean kermes oak thicket. We measured late-successional BSC cover, shrub cover, distance to nearest kermes oak (to test for effects of kermes oak thicket microenvironment on BSC), and pigment concentration of one lichen ( Cladonia foliacea) and one moss ( Pleurochaete squarrosa) species in the Nature Reserve El Regajal-Mar de Ontígola (Central Spain). At the macroscale (>0.5 m), results showed that BSC distribution and pigments were tightly coupled to a suite of soil properties, in particular soil pH, Fe, and Ca. Specifically, soil pH had a positive relationship with the cover of five individual BSC-forming lichen species and was negatively related to pigment production in C. foliacea. When pH was excluded from the analysis, Ca appeared as the main soil variable and was correlated with total BSC cover and total lichen cover. The micronutrient Fe had a significant positive relationship with the concentration of eight pigments in P. squarrosa and was also coupled with the cover of two BSC-forming lichens. Manganese, previously proposed as a key limiting micronutrient for BSCs, affected lichen diversity in a negative way. At the microscale (∼0.5 m), kermes oak microenvironment, shrub cover, and moss cover were determinants of BSC distribution, and total lichen and total BSC cover were overrepresented on N and E-facing shrub microsites. Our findings suggest that soil chemical variability and microsite diversity created by neighbouring vegetation affect BSC distribution in complex and essential ways and that studies aiming to explore BSC-environment relationships should be conducted at various spatial scales. Studies based on species- or group-specific responses are, thus, inadequate to unveil the main factors determining the distribution of the diverse organisms that constitute BSCs and/or to propose potential tools aiming to restore BSC in arid and semiarid ecosystems.

Assessment of physical and hydrological properties of biological soil crusts using X-ray microtomography and modeling

Biological soil crusts (BSCs) are formed by aggregates of soil particles and communities of microbial organisms and are common in all drylands. The role of BSCs on infiltration remains uncertain due to the lack of data on their role in affecting soil physical properties such as porosity and structure. Quantitative assessment of these properties is primarily hindered by the fragile nature of the crusts. Here we show how the use of a combination of non-destructive imaging X-ray microtomography (XMT) and Lattice Boltzmann method (LBM) enables quantification of key soil physical parameters and the modeling of water flow through BSCs samples from Kalahari Sands, Botswana. We quantify porosity and flow changes as a result of mechanical disturbance of such a fragile cyanobacteria-dominated crust. Results show significant variations in porosity between different types of crusts and how they affect the flow and that disturbance of a cyanobacteria-dominated crust results in the breakdown of larger pore spaces and reduces flow rates through the surface layer. We conclude that the XMT–LBM approach is well suited for study of fragile surface crust samples where physical and hydraulic properties cannot be easily quantified using conventional methods.

Long‐term effects of revegetation on soil hydrological processes in vegetation‐stabilized desert ecosystems

AbstractPlanting of sand‐binding vegetation in the Shapotou region on the southeastern edge of the Tengger Desert began in 1956. The revegetation programme successfully stabilized formerly mobile dunes in northern China, permitting the operation of the Baotou‐Lanzhou railway. Long‐term monitoring has shown that the revegetation programme produced various ecological changes, including the formation of biological soil crusts (BSCs). To gain insight into the role of BSCs in both past ecological change and current ecological evolution at the revegetation sites, we used field measurements and HYDRUS‐1D model simulations to investigate the effects of BSCs on soil hydrological processes at revegetated sites planted in 1956 and 1964 and at an unplanted mobile dune site. The results demonstrate that the formation of BSCs has altered patterns of soil water storage, increasing the moisture content near the surface (0–5 cm) while decreasing the moisture content in deeper layers (5–120 cm). Soil evaporation at BSC sites is elevated relative to unplanted sites during periods when canopy coverage is low. Rainfall infiltration was not affected by BSCs during the very dry period that was studied (30 April to 30 September 2005); during periods with higher rainfall intensity, differences in infiltration may be expected due to runoff at BSC sites. The simulated changes in soil moisture storage and hydrological processes are consistent with ongoing plant community succession at the revegetated sites, from deep‐rooted shrubs to more shallow‐rooted herbaceous species. Copyright © 2009 John Wiley & Sons, Ltd.