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Biological soil crusts (biocrusts) are a prevalent form of organic cover in drylands across the world, and they play a crucial role in shaping the ecohydrological processes and functioning of dryland ecosystems. However, in desert steppe ecosystems, the ecohydrological influences of biocrusts and their pathways have not yet been thoroughly understood. Here, we examined the influences of three types of biocrusts (cyanobacterial, cyanobacterial‐moss mixed, and moss biocrusts) on soil water retention and infiltration in a typical desert steppe of northwestern China, and we also analysed the possible pathways through which biocrusts affect them. Accordingly, the field capacity (θf, indicating soil water‐holding capacity) and saturated hydraulic conductivity (Ks, indicating soil infiltrability) of each biocrust‐covered soil and bare soil were measured both at 0–5 and 5–10 cm depths, on Arenosols (loamy sand texture) and Calcisols (silty loamy texture), respectively. Our results showed that the presence of biocrusts increased surface soil (0–5 cm) water‐holding capacity while decreased soil infiltrability, creating a remarkable impact on soil water retention and infiltration. As compared to bare soil, the θf of cyanobacterial, cyanobacterial‐moss mixed, and moss biocrusts were increased by 16.5%, 21.4%, and 21.7% on Arenosols and by 9.6%, 18.9%, and 22.8% on Calcisols, while their Ks were reduced by 43.2%, 56.2%, and 38.2% on Arenosols, and by 49.0%, 68.7%, and 36.0% on Calcisols, respectively. The development of biocrusts on sandy textured soil had a more pronounced impact on both soil water retention and infiltration than loamy textured soil. We found that biocrusts mainly changed soil water retention and infiltration by increasing the contents of clay and organic matter. More specifically, the increase in clay content led to a more significant effect compared to the increase in organic matter content on Arenosols, while the opposite influence was observed on Calcisols. Among the three types of biocrusts, the soil infiltrability of the mixed biocrusts was the lowest, perhaps due to the interaction between the accumulation of clay particles by moss and the secretion of extracellular polymeric substances by cyanobacteria. Our study highlights the remarkable ecohydrological impacts of biocrusts on desert steppe ecosystems, and the results imply that it is of great necessary to integrate biocrusts and their influences into our understanding of ecohydrological processes in global drylands. [ABSTRACT FROM AUTHOR]

Background: The rock cut slope (RCS) could cause damage to regional ecological functions and landscapes and requires recovery. Biological soil crusts (BSCs) are pioneer and dominant colonizers during the initial recovery stage. To accelerate the natural recovery of RCS, the development process and influencing agents of BSC should be revealed. Thus, the area index of crevices (IR), BSC coverage (COV) and biomass (BM), soil weight (SW), and major soil nutrients [organic carbon (OC), total nitrogen (TN) and total phosphorus (TP)] content, collected from 164 quadrats on 13 RCSs in the mountainous area of west Sichuan Province, China, were measured, to explore the effect of crevice of RCS on BSC development. Results: Soil OC, TN and TP on RCSs ranged from 18.61 to 123.03 g kg−1, 0.96 to 6.02 g kg−1 and 0.52 to 2.46 g kg−1, respectively, and were approximately to or higher than those on natural slopes. The OC, TN and TP contents in soils elevated unsystematically with recovery time of RCSs. BSCs on RCS distributed along crevices generally and firstly. During the first 13 years of natural recovery, COV, BM and SW ranged from 6.5 to 28.2%, 14.43 to 67.25 g m−2, and 127.69 to 1277.74 g m−2, respectively. COV, BM and SW increased linearly with IR on RCSs. The positive correlation between COV and BM and IR was insignificantly impacted by bedrock, slope aspect and altitude within the recovery time less than 13 years. COV and BM on RCSs increased significantly when the recovery time is more than 27 years. Conclusions: Crevice on RCSs could be a major environmental factor which is conducive to BSC development and soil accumulation through creating a space for water and soil particle. Furthermore, with the increase of recovery time of RCSs, BSCs may grow and reach a stable state with the promotion of soil nutrients, plant growth and microbial activity. These results provide a development process of BSC that from inside to outside the crevices on RCSs. In the areas with stable rock strata and a low risk of geological disasters, purposeful improvement in crevice density on RCS may effectively accelerate BSC development. [ABSTRACT FROM AUTHOR]

[Display omitted] • Moss biocrusts show significant temporal variability in greenness. • Moss biocrust greenness increases 48% in wet season compared to dry season. • Climatic factors (moisture, humidity, and wind) influence greenness more than soil. • Biocrust greenness correlates positively with biocrust gross primary productivity. • Biocrust greenness serves as a reliable indicator for biocrust carbon-fixation rates. Biocrusts are common living covers found across drylands worldwide, and their photosynthesis substantially contributes to carbon input in these ecosystems. However, direct monitoring of biocrusts' photosynthetic carbon fixation is challenging due to their scattered distribution, dark pigments, and lower CO 2 exchange rate. Current studies have limited monitoring frequency and the results are difficult to extend to a broader spatial scale. Greenness typically functions as an indicator of both vegetation cover and plant photosynthesis. Monitoring biocrust greenness is expected to be able to provide insights into their environmental responses and estimate carbon fixation rates, particularly for moss biocrusts. Here, we monitored greenness, soil moisture and temperature, and gross primary productivity (GPP) of moss biocrusts, as well as climatic factors over two years in the northern China's Loess Plateau. The results indicated that moss biocrust greenness was highly sensitive to changing environmental conditions and exhibited significant temporal variability. Moss biocrust greenness exhibited a notably higher value in wet season (0.384) than in dry season (0.259). According to these greenness values, we proposed moss biocrust activity and classified it into high (>0.403), medium (0.334–0.403), low (0.271–0.333), and dormancy (<0.270). Soil moisture, relative humidity, and wind speed were the three main determinates of moss biocrust greenness. Higher soil moisture and relative humidity were linked to increased greenness, whereas elevated wind speed led to a reduction. In dry seasons, soil moisture contributed 23.4 % to the variation in moss biocrust greenness, while in wet seasons relative humidity contributed 16.1 %. A significant and positive correlation between moss biocrust greenness and GPP was found, and annual GPP prediction based on greenness using machine learning performed well. Our study highlights the significance of moss biocrust greenness and their potential as indicators for moss biocrust photosynthetic carbon fixation rates and possibly other ecological functions in drylands. [ABSTRACT FROM AUTHOR]

Dryland ecosystems exist in various vegetation landscapes with contrasting compositions of vascular plants and biocrusts (surface‐soil mats comprised of soil particles, cyanobacteria, lichens, mosses and fungi), and they are very vulnerable to ongoing global change. However, compared to vascular plants, the responses of biocrusts and their coexistence with vascular plants to global change remain elusive.We conducted a 5‐year experiment involving multi‐level precipitation and nitrogen addition in a desert shrubland of northern China to examine these responses.We found that shrubs, perennial herbs and annuals exhibited facilitative, competitive and insignificant effects on biocrusts respectively. Biocrusts coexisted with vascular plants below a threshold of herbaceous productivity of approximately 70 g/m2 and tended to be replaced by herbs beyond this threshold, resulting from the balance between the shade tolerance of biocrusts and herb‐induced light limitation. Increased precipitation and nitrogen enrichment altered biocrust–vascular plant composition by influencing this balance and tended to shift from a biocrust‐dominated to a vascular plant‐dominated state when the threshold was crossed.Synthesis. Our results suggest that biocrusts can coexist with dwarf shrubs and annuals, but cannot withstand perennial herbs when the herbaceous productivity threshold was exceeded. Our findings provide new insights into the threshold and mechanisms of biocrust–vascular plant coexistence, which may greatly contribute to managing dryland ecosystems for global change. [ABSTRACT FROM AUTHOR]

Soil nematodes are the most abundant animals on Earth and play critical roles in regulating numerous ecosystem processes, from enhancing primary productivity to mineralizing multiple nutrients. In dryland soils, a rich community of microphyte organisms (biocrusts) provide critical habitats for soil nematodes, but their presence is being threatened by increasing aridity induced by global climate change. Despite its importance, how types of biocrusts and aridity index influence soil nematode community in dryland mountain ecosystems remains largely unknown. To fill these knowledge gaps, we conducted a field survey with contrasting aridity indexes (0.2, 0.4, and 0.6) and three types of biocrusts (cyanobacterial, cyanobacterial-moss mixed, and moss crusts) in the topsoil (0-5 cm) from the northern Chinese Loess Plateau. We found that the abundance (number of individuals per gram of soil), richness (number of Operational Taxonomic Units; OTUs), and diversity (number of different species) of soil nematodes were remarkably higher under biocrusts than in bare soils, regardless of aridity index and types of biocrusts. Our results also showed that the same variables had the highest values in moss crusts compared to cyanobacterial and cyanobacterial-moss mixed crusts. Structural equation modelling further revealed that biocrust types and traits (i.e., biocrust thickness, chlorophyll content, shear force, and penetration resistance) are the most important factors associated with both nematode abundance and richness. Together, our findings indicate that biocrusts, especially moss cover, and less stressful aridity conditions favor soil nematodes community in dryland mountain regions. Such knowledge is critical for anticipating the distribution of these animals under climate change scenarios and, ultimately, the numerous ecosystem services supported by soil nematodes., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Soil erosion stands as the preeminent environmental concern globally, attaining heightened significance, particularly within islands where land resources prove notably scarce. Biological soil crusts, referred to as biocrusts, assume a pivotal ecological role in soil conservation. Notably, they augment the horizontal stability of the substrate through the exudation of microbial extracellular polymeric substances (EPS), thereby shielding the soil against shear stress, exemplified in the form of water erosion. While extant research has delved into the anti-erosion mechanisms of biocrusts in arid landscapes, a conspicuous lacuna persists in the exploration of coral island environments. In this study, we collected and assessed 30 samples encompassing dark biocrusts, light biocrusts, and bare soil to scrutinize the potential anti-erosion efficacy of tropical coral island biocrusts within the South China Sea. Employing a cohesive strength meter, we quantified soil shear stress across various stages of biocrust development, revealing a discernible enhancement in soil erosion resistance during the formation of biocrusts. Relative to the exposed bare soil, the soil shear stress exhibited an escalation from 0.33 N m -2 to 0.61 N m -2 and 1.31 N m -2 in the light biocrusts and dark biocrusts, respectively. Mechanistically, we assayed microbial EPS contents, exposing a positive correlation between EPS and soil anti-erodibility, encompassing extracellular protein and polysaccharide. Concurrently, bacterial abundance displayed a significant augmentation commensurate with biocrust formation and development. In pursuit of elucidating the origin of EPS, high-throughput amplicon sequencing was executed to identify microorganisms contributing to biocrust development. Correlation analysis discerned Cyanobacteria, Chloroflexi, Deinococcota, and Patescibacteria as potential microbials fostering EPS production and fortifying erosion resistance. Collectively, our study presents the first evidence that biocrust from tropical coral reef island in the South China Sea promotes resistance to soil erosion, pinpointing key EPS-producing microbials against soil erosion. The findings would provide insights for island environment restoration., Competing Interests: Declaration of competing interest The authors declare that they have no conflict of interest., (Copyright © 2024. Published by Elsevier Ltd.)

Biocrusts are known to regulate soil respiration in drylands. However, how biocrusts influence the changes in soil respiration across seasons and soil depths is far less understood. This knowledge gap hampers our ability to accurately predict the impacts of biocrust development or disturbance on soil carbon (C) balance in drylands, which covers almost half of the planet. Here, we used solid-state CO 2 sensors in a cold-winter desert on the Loess Plateau in northern China to investigate the impacts of biocrusts on C flux across seasons and soil depths. In particular, we measured C fluxes from 5, 15, and 30 cm depths in biocrust-covered and bare soils during two continuous years and multiple seasons. Our high-time-resolution approach indicated that biocrusts release 55% (i.e., 490 g C m−2) more C per year than bare soil. This result is attributed to the improvements of organic C pools, microbial abundance and diversity, and hydrothermal environments induced by biocrusts. Soil C flux of biocrusts have significant seasonal variations due to the differences of biocrust responses to soil temperature and water content and their effects on microbial activities in different seasons. Meanwhile, the seasonal variability of C flux in biocrusts was higher than that in bare soil (coefficient of variation = 0.48 vs. 0.43). Biocrusts contributed the most to annual C efflux in cold season (winter: 32%) followed by warm season (summer: 29%), and the reason is that the duration of winter during cold season (averaged 175 days over two years) is about 3 times longer than that of summer during warm season (67 days) in the cold desert. Furthermore, although the total soil C efflux was mainly from 0 to 15 cm depth (>95%), biocrusts still increased the contribution (3%) of deeper soils (15−30 cm) to total C efflux compared with bare soil. Our findings highlight the fundamental role of biocrusts as major C regulators in cold deserts by providing high temporal resolution and soil depth data. This knowledge is critical to understand the consequences of biocrust development, or biocrust disturbance, for C cycling when accurately assessing the contribution of cold deserts to terrestrial ecosystem under global environmental changes. [Display omitted] • In cold deserts, biocrusts release 490 g C m−2 more per year compared with bare soil. • Seasonal changes of C flux in biocrusts are more dramatic than those in bare soil. • Cold season C efflux is the main contributor to annual C efflux in cold deserts. • Biocrusts increase contribution of deep soils to total C efflux compared with bare soil. • Biocrusts are regulators of seasonal and depth variability of C flux in cold deserts. [ABSTRACT FROM AUTHOR]

The Loess Plateau in China is one of the most severely eroded regions of the world. Since the implementation of “Grain for Green” ecological restoration project, biological soil crusts (biocrusts) were widely distributed in this region, which significantly affected surface runoff. Numerous studies have explored the effect of biocrusts on runoff. However, the conclusions were still widely different. In the Loess Plateau region, rainfall is mostly concentrated in June to September, and the rainfall duration is not fixed, which may affect the runoff characteristics of biocrustal slopes. This study investigated characteristics of runoff from biocrustal slope in different rainfall durations in the Loess Plateau region by using artificial simulated rainfall experiment. The experiment was conducted in the revegetated grassland of northern Shaanxi Provence, China. The experiment site was about 80 m×20 m, and the slope gradient was approximately 15°. The biocrust types were mainly moss crust and moss cyanobacteria mixed crust in this site and their average coverage was 79.2%. The dimensions of the experimental plots were 10 m×2.1 m (length×width).Canopy of higher plants in the plots was removed with scissors. According to the range of local biocrust coverage, two treatments were set: 1) slopes with undisturbed biocrust as a high coverage biocrust (the average biocrust coverage were 79.2%); 2) the slopes with removal of a part of the biocrusts by shovels, which simulated the low biocrust cover situation (the average biocrust coverage were 43.6%). Meanwhile, ploughing plots were set as the control group. The rainfall intensity was set as 90mm/h and the duration was1 hour. The results showed that the initial runoff time of biocrust slope was significantly reduced compared to the bare soil slope. The initial runoff yield time of bare soil was 1.59-3.04 times that of the biocrust slopes. There was a significant negative correlation between biocrust coverage and initial runoff generation time; Conclusion of the influence of biocrusts on runoff yield was contradictory during the first 15 min and 60 min. For 90 mm/h rainfall intensity, runoff from biocrust slope increased by 75.42% compared to bare soil when the rainfall duration was the 15 min. While, runoff from biocrust slope was decreased by 52.42% compared to the bare soil when the rainfall lasted to 60 min; the infiltration rate of soil moisture was affected by biocrusts. The infiltration rate of bare soil slope with 60 min rainfall was 34.30% lower than that with 15 min. The infiltration rate of high coverage biocrust slope with 60 min rainfall was only 6.38% lower than that with 15 min, which may cause the difference of runoff yield between bare soil slope and biocrust slope; the effect of biocrust on slope infiltration and runoff is closely related to rainfall duration. Different periods of rainfall are likely to lead to inconsistent conclusions. Therefore, the duration of rainfall experiment considering the factors of biocrust should be no less than 45 min. The study provides scientific evidences for explaining the differences in the effect of biocrusts on infiltration and runoff, and further clarifies the hydrological effect of biocrusts in arid and semi-arid areas. [ABSTRACT FROM AUTHOR]

• Microhabitats are important in shaping soil prokaryotic communities in deserts. • Alpha diversity differs in soil depths, biocrust stages, with/without shrub cover. • With/without shrub cover do not change prokaryotic community compositions. • Cyanobacteria dominates in biocrusts and in cyano- and lichen crusts than others. • Soil pH and TP respectively drive patterns of alpha and beta diversities. The influence of microhabitat factors on prokaryotic communities within desert ecosystems, particularly those residing in biological soil crusts (biocrusts), remains poorly understood. This study aimed to elucidate the relative importance of different microhabitats, including soil depths, biocrust developmental stages, and shrub cover, in shaping soil prokaryotic communities. To achieve this, we conducted an in-depth investigation of soil prokaryotic communities using high-throughput amplicon sequencing in the Tengger desert, China. Our results showed that difference of soil properties, prokaryotic alpha, beta diversities and abundances of taxa and functional groups primarily differed between soil depths and among biocrust developmental stages. Furthermore, the impact of shrub cover on the aforementioned parameters was found to be modified by biocrust development. Specifically, we observed higher soil nutrient levels in the upper biocrust layer compared to the subsoil, with a consistent increasing trend during biocrust development in both soil layers. Conversely, soil pH displayed contrasting patterns. Prokaryotic alpha diversity exhibited a V-shaped pattern with biocrust development, initially decreasing and then increasing, and was higher in the subsoil compared to the biocrust layer. Additionally, although prokaryotic alpha diversity showed differences between shrub cover or not, the specific effect depended on biocrust stages. Cyanobacteria, phototrophic and nitrogen-fixing prokaryotes were more abundant in the upper biocrust layer, following a unimodal pattern with biocrust development, initially increasing, and then decreasing; while Actinobacteriota and Acidobacteriota showed higher abundance in the subsoil, with varying trends between the two soil depths during biocrust development. Soil pH and total P emerged as key regulators of prokaryotic communities across different microhabitats. This study contributes to our comprehensive understanding of the complex dynamics governing prokaryotic communities in desert ecosystems. [ABSTRACT FROM AUTHOR]

Biological soil crusts (biocrusts) are widely distributed in global drylands and have multiple significant roles in regulating dryland soil and ecosystem multifunctionality. However, maps of their distribution over large spatial scales are uncommon and sometimes unreliable, because our current remote sensing technology is unable to efficiently discriminate between biocrusts and vascular plants or even bare soil across different ecosystem and soil types. The lack of biocrust spatial data may limit our ability to detect risks to dryland function or key tipping points. Here, we indirectly mapped biocrust distribution in China's drylands using spatial prediction modeling, based on a set of occurrences of biocrusts (379 in total) and high-resolution soil and environmental data. The results showed that biocrusts currently cover 13.9 % of China's drylands (or 5.7 % of China's total area), with moss-, lichen-, and cyanobacterial-dominated biocrusts each occupying 5.7 % to 10.7 % of the region. Biocrust distribution is mainly determined by soil properties (soil type and contents of gravel and nitrogen), aridity stress, and altitude. Their most favorable habitat is arenosols with low contents of gravel and nitrogen, in climate with a drought index of 0.54 and an altitude of about 500 m. By 2050, climate change will lead to a 5.5 %-9.0 % reduction in biocrust cover. Lichen biocrusts exhibit a high vulnerability to climate change, with potential reductions of up to 19.0 % in coverage. Biocrust cover loss is primarily caused by the combined effects of the elevated temperature and increased precipitation. Our study provides the first high-resolution (250 × 250 m) map of biocrust distribution in China's drylands and offers a reliable approach for mapping regional or global biocrust colonization. We suggest incorporating biocrusts into Earth system models to identify their significant impact on global or regional-scale processes under climate change., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Background and aims: 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. Methods: 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. Results: 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. Conclusions: 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. [ABSTRACT FROM AUTHOR]

• Annual soil erosion rates decreased with tree species richness. • Tree diversity reduced soil erosion by affecting tree canopy and biological soil crust development. • Restoring species-rich plantations may be beneficial for soil erosion control. Biodiversity plays a crucial role in forest ecosystem sustainability. However, it is unclear how tree diversity and especially the relationship between diversity and ecosystem functioning affect soil erosion. Based on a forest biodiversity and ecosystem functioning experiment established in subtropical China (BEF China), we measured soil erosion at four tree species richness levels (monocultures, 8 tree species, 16 tree species and 24 species stands) during the rainy seasons from 2013 to 2015. The result showed that mean annual soil erosion rates were detected to decrease with tree species richness significantly over the observed three years. Leaf area index (LAI) and biological soil crusts (BSCs) were the two main factors driving soil erosion within tree stands of different species richness. Positive effects of tree species richness on tree canopy structure and BSCs might drive the reduction of soil erosion in the earlier successional stage after afforestation of tree plantations. Therefore, we highlight the important influence of tree species richness on soil erosion control, hydrologic processes and thus sustainable ecology services. [ABSTRACT FROM AUTHOR]

Purpose: Global warming is expected to have profound effects on terrestrial carbon (C) fluxes, consequently influencing future climate. Biocrusts are important sources of C in the C cycle of desert ecosystems, where vascular plants are restricted by limited soil moisture. This study was conducted in order to evaluate the expected increases in temperature on soil respiration in biocrust-dominated areas.Materials and methods: In a field warming experiment, we evaluated the impact of increased temperature on soil respiration in biocrust-dominated (moss-crusted and lichen-crusted) areas in Shapotou, China. In addition, the impacts of precipitation, soil temperature, and moisture on soil respiration were investigated.Results and discussion: The effect of warming on soil respiration varied with soil water availability. Our results showed that soil respiration in moss-crusted and lichen-crusted areas in the warming treatment was significantly lower than that in the control. The observed inhibition of soil respiration by the increase in soil temperature was likely due to the reduction in soil moisture caused by the increased water evaporation rate under higher soil temperature. Warming also decreased cumulative C release in moss-crusted and lichen-crusted areas. Moreover, cumulative C release showed marked seasonal variations, with the highest C release occurring in summer and the lowest in winter. Over the seasonal cycle, soil respiration rates were positively correlated with precipitation, soil temperature, and volumetric soil water content.Conclusions: The results of this study indicate that warming may increase the sensitivity of soil respiration to water availability in biocrust-dominated areas in desert ecosystems, suggesting that biocrust should be considered in projections of future C budget. [ABSTRACT FROM AUTHOR]

[Display omitted] • Biocrusts have higher water-holding ability and capacity as compared with bare soils. • Biocrusts decrease homogeneous matrix flow and infiltration depth during penetration. • Biocrusts increase soil moisture both at surface and subsurface soils after infiltration. • Biocrusts raise evaporation rate by 27%−39% and cumulative evaporation amount by 14% • Depth-restricted infiltration and wetter surface soil enhance biocrusts evaporation. Globally widespread biocrusts form a "living skin" on most dryland soils, helping govern fundamental ecosystem functions, particularly hydrological processes. However, the underlying mechanisms of biocrust effects on soil water infiltration and evaporation are still controversial. Biocrusts possibly regulate infiltration and evaporation due to their unique physicochemical properties and influence on surface soil moisture (θ), and these assumptions were verified in this study conducted on the Loess Plateau of China. The differences in vertical soil moisture distribution and flow patterns of infiltration water between moss-dominated biocrusts and bare soil were investigated through dye tracing experiments. The evaporation characteristics of biocrusts and bare soil were also obtained by weighing micro-lysimeters in both simulated and in-situ experiments. Results showed that biocrusts retarded deep soil water infiltration below ∼ 15 cm and significantly (F = 48.15, P < 0.001) decreased the stained area ratio (percentage of stained area to total area at the same soil depth) by 16% in contrast to bare soil. Because the infiltrated water was mostly retained in shallow soil (0–15 cm) in biocrust-covered soil, their ratios of low interaction flow (stained path width < 20 mm) and high interaction flow (20 mm < stained path width < 200 mm) were significantly higher (F ≥ 7.82, P ≤ 0.007) than those of bare soil. After the dye tracing experiments, the biocrusts modestly increased θ by 17% throughout the soil profile (0–50 cm), but they remarkably increased θ by 33%−78% in the top 20 cm of soil. Additionally, the biocrusts increased soil daily evaporation rate (E) by 27%−39% (with only the exception of the beginning) and the cumulative evaporation amount by 14% in contrast to bare soil over an 8-day field measurement. More importantly, the biocrust-affected soil properties, such as the contents of fine particles and organic matter, as well as saturated water content and field capacity, were closely correlated with E (| ρ| ≥ 0.52, P ≤ 0.08). Consequently, it was concluded that biocrusts restrict the depth of rainwater infiltration and increase surface θ , which may greatly enhance surface soil evaporation in the Chinese Loess Plateau. [ABSTRACT FROM AUTHOR]

Artificial enclosure has been an important and practical measure widely used to restore vegetation in the ecologically degraded area. In this study, using a standard plot investigation method, we investigated the changes in biomass and species diversity of the plant communities in the desertified grassland located in Ningxia Autonomous Region, China, after enclosure for 0, 5, 15 and 25 years with fences made up of cement piles and barbed wires, with a complete exclusion of interferences from any activities including grazing. We also analysed the factors driving the changes in these parameters. The results showed that with the increase in the enclosure duration, the soil electrical conductivity and nutrient contents were significantly increased but the thickness of biological crust in the deserted area was not changed regularly and the soil pH values were changed only slightly. Furthermore, while no significant differences in plant biomass, species abundance and a-diversity were seen, significant difference in b-diversity was seen among different enclosure years. Principal component analysis showed the significant differences in species compositions in the deserted grassland among various enclosure years. The redundancy analysis revealed that the available nitrogen, available potassium, soil organic carbon and soil electrical conductivity were the major factors affecting the plant species compositions in the deserted area in the studied region. These findings can be of practical significance for formulating the measures to effectively curb the deterioration of ecological environment in grassland. [ABSTRACT FROM AUTHOR]

Summary Sand burial and dew deposition are two fundamental phenomena profoundly influencing biological soil crusts in desert areas. However, little information is available regarding the effects of sand burial on dew deposition on biological soil crusts in desert ecosystems. In this study, we evaluated the effects of sand burial at depths of 0 (control), 0.5, 1, 2 and 4 mm on dew formation and evaporation of three dominant moss crusts in a revegetated area of the Tengger Desert (Northern China) in 2010. The results revealed that sand burial significantly decreased the amount of dew deposited on the three moss crust types by acting as a semi-insulator retarding the dew formation and evaporation rates. The changes in surface temperature cannot fully explain the variations of the formation and evaporation rates of dew by moss crusts buried by sand. The extension of dew retention time was reflected by the higher dew ratios (the ratio of dew amount at a certain time to the maximum value in a daily course) in the daytime, and may to some extent have acted as compensatory mechanisms that diminished the negative effects of the reduction of dew amount induced by sand burial of moss crusts. The resistances to reduction of dewfall caused by sand burial among the three moss crusts were also compared and it was found that Bryum argenteum crust showed the highest tolerance, followed by crusts dominated by Didymodon vinealis and Syntrichia caninervis . This sequence corresponds well with the successional order of the three moss crusts in the revegetated area, thereby suggesting that resistance to reduction of dewfall may act as one mechanism by which sand burial drives the succession of moss crusts in desert ecosystems. This side effect of dew reduction induced by sand burial on biological soil crusts should be considered in future ecosystem construction and management of desert area. [ABSTRACT FROM AUTHOR]

• NA of biocrusts is mainly determined by surface soil water. • NA of cyanobacteria- was higher than that of moss-dominated crust when not restricted by water. • NA of moss- was related with micro-biomass C, but oxidable C for cyanobacteria-dominated crust. In arid desert regions, a large proportion of the ground surface is covered by specialized organisms such as cyanobacteria and mosses that form biological soil crusts (BSCs), which have multifunctionality in the ecosystem via stabilizing soil and fixing nitrogen (N) and carbon (C). Climate change, especially variations in rainfall will alter BSC species composition, cover and biomass, shaping the N-fixation of BSCs. Although BSCs are main contributors of N in desert ecosystems, it is not clear how nitrogenase activity (NA) of BSCs responds to changes in rainfall amount and frequency to function in the N-cycle. To address this important knowledge gap, based on long-term rainfall monitoring and predicted scenarios, we designed five levels of rainfall amount ranging from decreased by 50% to increased by 50%, and five rainfall frequencies ranging from increased by 100% to decreased by 80%, to simulate the potential variation of rainfall using rainout shelters. The NA of bare soil and cyanobacteria- and moss-dominated crust soil (three soil types) was determined by acetylene reduction assay. The soil water content (SWC), oxidable C and microbial biomass C and N were also determined. The NA of the three soil types was positively correlated with their SWC when rainfall amount decreased. In the case of increased rainfall frequency, NA of bare soil and moss-dominated crust was positively correlated with SWC, but negatively correlated for cyanobacteria-dominated crust. However, with increased rainfall amount combined with decreased rainfall frequency, there was no significant relationship between NA and SWC. In addition, NA of BSC was higher than that of bare soil with increased rainfall amount combined with decreased rainfall frequency, while that of bare soil was higher than that of BSC for decreased rainfall amount combined with increased rainfall frequency. In the case of increased frequency combined with decreased rainfall amount, the NA was higher for moss-dominated than cyanobacteria-dominated crust, but higher for cyanobacteria-dominated than moss-dominated crust for reduced rainfall frequency combined with increased rainfall amount. Meanwhile, NA of both bare soil and moss-dominated crust was correlated with microbial biomass C; and NA of cyanobacteria-dominated crust was positively correlated with SWC as well as oxidable C. Our findings indicated that future changes in rainfall regime will strongly affect NA of BSCs and bare soil in arid desert regions, and consequently change the soil fertility and net primary production. [ABSTRACT FROM AUTHOR]

[Display omitted] • Biocrusts increase evaporation rate by 10% on average in laboratory experiments. • All types of biocrusts increase evaporation rate by >42% in field experiments. • Biocrusts increase soil moisture (θ) by 9–44% compared with bare soil at 0–10 cm. • Biocrusts decrease soil temperature by 4.7–4.9 °C at 0–10 cm depth after rainfalls. • Reshaped soil properties and increased θ of biocrusts are key factors of increasing E. Soil evaporation is a three-stage process and plays a vital role in soil physical, chemical, and biological processes, as well as global climate change. As an important living skin, biocrusts strongly influence most soil properties and support fundamental ecosystem functions. Although there are many studies concerning biocrust effects on surface soil hydrological processes, it still remains unclear how biocrusts impact soil evaporation and their potential effects on the water budget in drylands. In this study, we selected aeolian sandy soil (bare soil) and three types of biocrusts, specifically cyanobacteria (cyano), cyanobacteria and moss mixed crusts (cyano-moss), and moss crusts (moss), on the northern Chinese Loess Plateau. By utilizing weighing micro-lysimeters in simulated and ∼4 months in-situ field evaporation experiments, we investigated all evaporation stages containing stages I–II and III, as well as the soil evaporation rate (E) of all treatments. The soil moisture (θ) and temperature (T) at 2 cm and 10 cm depths were also measured in field. Our results showed that biocrust types strongly influenced E in the ranked order of moss > cyano-moss > cyano > bare soil. The cumulative evaporation amount (E c) of all types of biocrusts was significantly higher than that of bare soil in simulated experiments. Particularly, moss increased E c by 15.7% compared with bare soil. The results of in-situ field measurements illustrated that the E of cyano, cyano-moss, and moss was increased by 42.4%, 55.5%, and 46.9%, respectively, compared with the bare soil, and these increasing effects became more pronounced after rainfall events. Additionally, all types of biocrusts markedly increased θ and decreased T at 0–10 cm depth of soil, but cyano and cyano-moss increased T (by up to 5.7 °C) at 2 cm depth during the non-rainfall periods. The increasing effects of biocrusts on E were reasonably attributed to their higher contents of fine particles and organic matter, as well as their higher field capacity in contrast to bare soil. All these regulations of biocrusts on soil properties further reshaped soil water and temperature regimes and finally changed soil evaporation. In conclusion, biocrusts may greatly intensify surface soil water loss due to their higher E. However, their greater θ and lower T in comparison to bare soil may partially offset these negative effects in return, and they may have feedback impacts on surface soil water and heat balances in drylands. [ABSTRACT FROM AUTHOR]

Characteristics of water vapour condensation, including the onset, duration, and amount of water vapour condensation on moss-dominated biological soil crust (BSC) and dune sand were studied under simulated conditions with varying air temperature and relative humidity. The simulations were performed in a plant growth chamber using an electronic balance recording the weight of condensation. There was a positive linear correlation between the water vapour condensation and relative humidity while the mean temperature was negatively linearly related to amounts of water vapour condensation for both soil surfaces. The amount of water vapour condensation on BSC and dune sand can be described by the difference between air temperature and dew point with an exponential function, indicating that when the difference of air temperature and dew point exceeds a value of 35.3C, there will be zero water vapour condensed on BSC. In contrast, when the difference of air temperature and dew point exceeds a value of 20.4C, the water vapour condensation will be zero for dune sand. In general, when the air is fully saturated with water and the dew point is equal to the current air temperature, the water vapour condensed on BSC attained its maximum value of 0.398 mm, whereas it was 0.058 mm for dune sand. In comparison, water vapour condensed on BSC was at a relatively high temperature and low relative humidity, while we did not detect water vapour condensation on the dune sand under the similar conditions. Physical and chemical analyses of the samples pointed to a greater porosity, high content of fine particles, and high salinity for BSC compared to the dune sand. These results highlight that soil physicochemical properties are the likely factors influencing the mechanism of water vapour condensation under specific meteorological conditions, as onset was earlier and the duration was longer for water vapour condensation on BSC in comparison with that of dune sand. This contributed to the greater amount of vapour absorbed on BSC compared to the dune sand under an identical meteorological condition. The feedback of water vapour condensation on BSC formation and its contribution to sustain the revegetation desert ecosystems was discussed. [ABSTRACT FROM AUTHOR]

Dryland ecosystems are often nitrogen-limited, and small nitrogen inputs may produce large responses to dryland ecological processes, such as gaseous nitrogen emission. The effect of increased anthropogenic nitrogen deposition on N 2 O and NO emissions in desert ecosystems is unclear, especially in non-growing seasons when the surface is covered with snow. In this study, nitrogen applications were performed on biological soil crusts (lichen crust and moss crust, bare sand for control) in the Gurbantunggut Desert, Northwest China. We measured the fluxes of N 2 O and NO and related nitrogen cycle functional gene abundances in winter for three-years period. Nitrogen addition significantly affected N 2 O emissions and increased the abundances of key functional gene for nitrogen cycle, while it only slightly influenced NO emissions. These effects of nitrogen addition depended on composition of biological soil crusts. For bare sand and lichen crust, nitrogen addition significantly increased N 2 O emissions, whereas for moss crust, only a negligible effect was observed. Meanwhile, significant differences in nitrogen cycle functional gene abundances were found among different composition of biological soil crusts. Abundance of amoA, narG, and nosZ genes were highly related to N 2 O and NO emissions. Thus, our results indicate that gaseous nitrogen emissions were generally increased by nitrogen addition through their effects on related functional microbial groups. The effects were regulated by composition of biological soil crusts which can buffer the effects of increasing nitrogen addition during winter., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Biological soil crusts (biocrusts) cover up to 60-70% of the soil surface in grasslands rehabilitated since the Grain for Green project was implemented in the hilly Loess Plateau region in 1999, which exerted significant impacts on runoff and soil loss from revegetated grasslands. In the study, field plots were used to investigate runoff and soil loss in sites of a 4- and a 13-year revegetated grassland, with each exhibiting an early and a later successional biocrust, respectively. The objectives of the study were to (1) examine the role of biocrusts on runoff and soil loss during their early and later successional stages in a semiarid region under water erosion, (2) determine the influence of biocrusts on soil antiscourability with different runoff intensities, and (3) isolate the effects of biocrust patches and vascular plant canopies on runoff and soil loss from revegetated grasslands. Treatments used in both sites included (1) retaining biocrusts and plant canopies intact (CP), (2) retaining biocrusts without plant canopies (CNP), (3) retaining plant canopies without biocrusts (PNC), and (4) removing both biocrusts and plant canopies (NCP). The simulated scouring water flux was designed as 7.8, 12.0, and to reflect local rainfall conditions. The results indicated that the runoff yield was increased by biocrust patches in their well-development stage. Runoff was increased by 15.1% when plant canopies were retained and 16.0% when plant canopies were removed in the 13-year revegetated grassland with the scouring water flux. Compared with biocrust patches, plant canopies reduced runoff by 11.3% (with biocrusts) and 8.4% (biocrusts was removed) with the same scouring water flux. No significant difference was found in runoff yield with respect to the four treatments in the 4-year revegetated grassland. In contrast, 92% of the sediments were reduced for the formation of biocrusts in their early successional stage (cyanobacteria-dominated biocrusts) in the 4-year revegetated grassland with respect to CNP compared with NCP at the scouring intensity. No sediment was generated on either CP or CNP treatments in grassland revegetated for 13 years (moss-dominated biocrusts) with the same intensity of simulated runoff. Compared with biocrusts, plant canopies had a limited influence on soil loss. This amounted to reductions of 45 and 10% in soil loss for grasslands that revegetated for 4 and 13 years, respectively. The results of the study suggest that biocrusts play an important role in soil loss control from water erosion in semiarid regions, although there was a potential increase in runoff yield. [ABSTRACT FROM AUTHOR]

We examined the effects of moss-dominated biocrusts on soil infiltration properties in Wangjiaqiao watershed of the Three Gorges Reservoir Area. Five levels of coverage (1%-20%, 20%-40%, 40%-60%, 60%-80% and 80%-100%) were set, with a nearby bare land as the control. We collected soil samples and conducted infiltration process observation by double cutting ring method. The results showed that biocrusts could appreciably increase soil cohesion, porosity, clay content, water-stable aggregates and organic carbon of topsoil, but significantly reduce soil bulk density and sand content. Biocrusts promoted soil water infiltration, with the initial infiltration rate ( I i ), stable infiltration rate ( I f ), average infiltration rate and cumulative infiltration amount being two times or more of that in bare land. Withbiocrust coverage increasing, soil infiltration properties firstly increased and then decreased, and peaked at 40%-60% coverage level. Results of path analysis indicated that I i was mainly affected by biocrust coverage, soil bulk density, and organic carbon content, while I f was mainly affected by biocrust coverage and soil bulk density. The simulation results of four infiltration models demonstrated that Horton model was the best fitting on the water infiltration process of biocrusted soil in the Three Gorges Reservoir Area.

Landscape indices can quantitatively describe the distribution characteristics of biological soil crusts (biocrusts). However, there are too many landscape indices, with high redundancy. We investigated 58 plots of biocrusts with different distribution patterns in the Hegou watershed of Wuqi County, Shaanxi Province, located in the hilly Loess Plateau. First, we calculated 15 common landscape indices, and selected representative landscape indices that could describe the biocrust landscape pattern and had specific ecological significance, based on correlation analysis, factor analysis, and sensitivity analysis. The reliability and rationality of the representative landscape indices were verified with data of the different biocrusts coverage in the Yingwoshanjian watershed of Yangjing Town, Dingbian County, Shaanxi Province. The results showed that 10 of the 15 landscape indices had significant correlations. Total edge (TE) and edge density (ED) were not significantly correlated with number of patches (NP), patch density (PD), clumpiness (CLUMPY), and interspersion juxtaposition index (IJI), respectively. The percentage of landscape (PLAND), ED, patch cohesion index (COHESION), and splitting index (SPLIT) described the spatial distribution characteristics of biocrust from coverage, length, connectivity, and fragmentation, respectively. The cumulative contribution of the three common factors represented in describing the spatial distribution of biocrusts was 91.6%. The study identified the representative landscape indices that could quantify the complexity of biocrusts distribution and thus would provide a theoretical basis for studying the pattern evolution of biocrusts and their relationship with ecological processes.

Abstract: Physical and chemical properties (including coverage, thickness, hardness, moisture, particle size distribution, organic matter and nutrient contents etc.) of biological soil crust and 0–5.0cm surface soil under the crust in three types of vegetation (semi-shrub Artemisia frigida, shrub Salix gordejevii and tree Populus simonii) were surveyed in 2005 and 2006 in Horqin Sand Land to understand the effects of different vegetation types on development of soil crust and surface soil properties under crust. The results showed that 1) no physical soil crust or biological soil crust (BSC) formed on mobile dunes without vegetation, though most ground surface in originally mobile dunes had been covered by BSC fifteen years after planted vegetation was established; 2) BSC development was best in shrub S. gordejevii sites, second in semi-shrub A. frigida sites, and weakest in tree P. simonii sites; 3) development of BSC was better in moss crust distributed mainly beneath or near plant canopies than in lichen crust distributed mainly between plant canopies in all three vegetation types; 4) surface soil properties 0–5.0cm under BSC were improved significantly in all three vegetation types compared to those in mobile sand land. The magnitude of improvement was greater in S. gordejevii sites than in A. frigida and P. simonii sites, and greater in moss crust sites than lichen crust sites, but the magnitude of improvement decreased significantly with increasing surface soil depth; 5) based on the above-mentioned results, we suggest that S. gordejevii should be favored in future planting in Horqin Sand Land. [Copyright &y& Elsevier]

• Decreased biocrust coverage changes the flow pattern to supercritical flow. • Biocrust coverage decreased the sediment via decreasing the runoff rate and velocity. • Sediment decreased as an exponential function of biocrust coverage at slope scale. • The results provide a support for inclusion of biocrusts in erosion models. In dryland ecosystems, soil erosion poses severe threats to ecosystem productivity and human well-being. Biological soil crusts (biocrusts) are ubiquitous living covers in these ecosystems and play an important role in soil stabilization and erosion prevention. However, the quantitative relationship between biocrust coverage and soil water erosion as well as the driving mechanism have not yet been fully discussed, which hinders the development of soil erosion models that consider the effect of biocrusts. Accordingly, we set up 19 biocrust plots (10 m × 2.1 m) with coverage ranging from 9.3% to 80.0% on the Loess Plateau of China and conducted a series of simulated rainfall experiments. The quantitative relationship between runoff and sediment yield and biocrust coverage was assessed, and further complex correlations among these variables were analyzed using structural equation modeling (SEM). We found that (1) the runoff rates and sediment yield were significantly related to biocrust coverage. The runoff rates and sediment yield decreased with increasing biocrust coverage via a logarithmic function and an exponential function, respectively. The runoff rate and sediment yield rate from the plot with 9.3% biocrust coverage were 3.2 and 51.5 times higher, respectively, than those from the plot with 78.4% biocrust coverage. (2) The flow pattern and regime of overland flow were significantly affected by biocrust coverage. The overland flow was shifted from tranquil flow to supercritical flow when the biocrust coverage declined to 12.3%. The flow velocity, stream power, Darcy-Weisbach resistance coefficient, Froude number, and Reynolds number were strongly correlated with biocrust coverage (R 2 = 0.548–0.830). (3) Biocrust coverage both directly and indirectly affected sediment yield. The anchoring effect of biocrusts directly protected the surface soil from raindrop impacts and runoff scouring (direct effect = −0.59). Additionally, biocrust cover had an indirect negative effect (−0.38) on sediment yield by decreasing the flow velocity and runoff rate with path coefficients of −0.81 and −0.78, respectively. Our results confirm that soil hydrological and erosive behavior have nonlinear relationships with biocrust coverage at the hill-slope scale, and they also reveal that variation in biocrust coverage is one of the main driving factors affecting soil hydrological and erosive behaviors. These results would improve soil water erosion estimation on biocrust-covered slopes and provide guidance for revising soil erosion models to include the effectiveness of biocrusts. [ABSTRACT FROM AUTHOR]

This study was undertaken at Horqin Sand Land, Inner Mongolia, Northern China. Field samples of biological soil crusts (BSCs) and underlying topsoil (0–5 cm under BSC) were taken in areas of different dune stabilization stages, and their physicochemical properties were analyzed, including particle size distribution, bulk density, organic matter, nitrogen, phosphorus, electrical conductivity (EC), pH, and CaCO3 content. The results revealed that semi-mobile dunes, semi-fixed dunes and fixed dunes had developed a physical crust, algae crust and moss crust, respectively. The thickness, hardness, water content, fine fraction and nutrient contents of BSCs were gradually increasing along the dune stabilization gradient. Meanwhile, BSC establishment and development enhanced the bulk density, silt and clay content, and nutrients of the topsoil under it, in an increasing tend from semi-mobile dune to fixed dune. Organic matter concentrations and other nutrients in the 0–5 cm topsoil layer under BSCs were significantly higher compared to unconsolidated soil (control). Moreover, there were strong significant positive correlations between topsoil and BSCs’ organic matter, total nitrogen, available nitrogen, available phosphorus, CaCO3, and <0.05 mm particle content, suggesting that BSCs have an influence on some of the properties of the underlying topsoil. [ABSTRACT FROM AUTHOR]

Biological soil crusts are a universal and common feature in arid and semi-arid regions and their appearance profoundly affects soil surface properties which may greatly change the seed dispersal, germination and establishment. To date, only a handful of experiments have exerted to investigate the effects of crusts on vascular plants and the conclusions from these studies are variable. In this study, we investigate the influences of two different crusts universally spreading in southeastern part of the Tengger Desert with four chronosequences (24, 41, 50 years old in artificial vegetation area and natural vegetation crusts) on vascular plants. Crusts were placed at three different sites to simulate different environmental factors (wind velocity and soil crust moisture), we set two soil moisture regimes for crusts to investigate how vascular plants respond under different moisture regimes in crusts. Emergence densities of vascular plants were significantly higher in moss crust than in algae crust. With the development of crusts, seed emergence increased in moss crust while decreased in algae crust. As for effects of moisture, our results showed that soil moisture had a significant effect on seed emergence in both types of crusts at all developing phases. Crusts with higher moisture had more seedlings than those with lower moisture. The above results indicated that the appearance of crusts changed the surface soil properties, which had greatly influenced the entrapment and lodgement of seeds in the study area, thus subsequently influence seed emergence through affecting natural factors. [ABSTRACT FROM AUTHOR]

The biological soil crust serves as one of the biological factors contributing to the sand fixation in the Gurbantunggut Desert, the largest fixed and semi-fixed desert in China. This study was conducted to investigate the microstructure and formation of biological soil crusts which develop as a result of occurrence of cryptogams. One year after removal of biological soil crusts, the exposed surface could be fixed by bacteria, which make sand particles cohere by exopolysaccharides. The exopolysaccharides were mainly composed of glucose, mannitol, arabinose and galactose. The intension of pressure for this kind of crust is 13.42± 1.38 Pa. After four-year recovery of the exposed sandy surface, the biological soil crust resulting from the colonization of soil surface by communities of filamentous cyanobacteria were mainly dominated by Microcoleus, which occurred as a cluster of filaments surrounded by a gelatinous sheath. At this developmental stage, the main contributors for sand fixation were changed from bacteria to filamentous cyanobacteria. Microscopic examination of this kind of crust revealed an intricate network of filamentous cyanobacteria and extracellular polymer secretions, which binds and entraps mineral particles and finer particles on the filament surface. These effects enhance soil cohesion and resistance to erosion. The intension of pressure for this kind of crust is 32.53±3.08 Pa. [ABSTRACT FROM AUTHOR]

[Display omitted] • Biocrusts increase mean soil water content by 15–165% across whole potential sections. • Biocrusts increase field capacity by 9%–256% on aeolian sand and loess soil. • Soil water retention curve (SWRC) of biocrusts was well fitted by VG and LND models. • Fitting error of SWRC models is mostly distributed in high and low potential ranges. • Biocrust effects on soil water holding ability may be linked to its microscopic pores. As ecosystem engineers in drylands, biocrusts possibly generate great influences on surface soil water holding capacity and availability, but these effects have not yet been sufficiently investigated, especially through analyzing and modeling their soil water retention curve (SWRC). On the Loess Plateau of China, the naturally developed moss biocrusts and surface bare soil were sampled in five replicates on loess soil and aeolian sand respectively, and their SWRCs from saturation (soil water potential = 0 hPa) to dry (soil water potential = −15,000 hPa) were measured with a sandbox (low water potential range) in combination with a pressure plate (high water potential range). The measured SWRCs were compared to the van Genuchten (VG), Brooks and Corey (BC), and Log-Normal Distribution (LND) models, and the model with best performance was adopted to further determine the effects of biocrusts on surface soil water holding capacity and water availability through comparing the differences in soil water content (θ) at specific water potentials between the biocrusts and uncrusted soil. Our results showed that (i) the biocrusts always had higher θ as compared with the bare soil across the whole water potential range, and the increasing effects of the biocrusts was much stronger on aeolian sand (165%) in comparison to that on loess soil (15%). (ii) As compared with the bare soil, the saturated water content, field capacity, wilting point, and available water content of the aeolian sand increased by 52%, 256% (0.192 vs. 0.054 cm3 cm−3), 65%, and 1000% by the biocrusts, respectively; while that of the loess soil increased by 28%, 9% (0.240 vs.0.220 cm3 cm−3), 27%, and 8% by the biocrusts, respectively. (iii) The increased water holding capacity of the biocrusts was possibly attributed to their higher percentage (18% vs. 16%) of microscopic pores (0.3–5 μm) on the loess soil and their increasing (16% vs. 1%) microscopic pores and decreasing (18% vs. 50%) medium pores (30–75 μm) on the aeolian sand. (iv) The VG and LND models performed better (R 2 > 0.98, RMSE ≤ 0.019 cm3 cm−3, RE ≤ 7.4%) than the BC model in simulating the SWRCs of the biocrusts and bare soil, and their fitting errors were mostly distributed in the high (−500 to −1000 hPa) and low (<−5000 hPa) water potential ranges. As indicated by SWRCs, we conclude that biocrusts greatly increase surface soil water holding capacity and water availability, leading in turn to a positive feedback mechanism of the biocrusted layer, which may affect surface hydropedological process (e.g., infiltration and evaporation) in drylands. [ABSTRACT FROM AUTHOR]

Biological soil crusts (BSCs), known as ecological engineers, play an important role in soil organic carbon (SOC) sequestration in dryland ecosystems. Although numerous individual studies had been conducted, the global patterns of the changes in SOC concentration following BSCs establishment remain unclear. In this study, we performed a comprehensive meta-analysis of 184 independent observations at 47 sites to quantify the responses of SOC and other soil variables to BSCs establishment and identify the underlying mechanisms. Our results showed that BSCs generally increased SOC by 70.9% compared to the controls (uncrusted soil), and the positive effects of BSCs on SOC in deserts (120.3%) were stronger than those in grasslands (32.7%). Mosses and lichens had a stronger positive effect on SOC than algae crusts (67.5%, 82.8%, and 58.2% respectively). Mixed crusts accumulated more SOC (181.6%) than single (moss, lichen and algae) crusts. The presence of BSCs considerably increased total nitrogen (TN) (+80.7%), total phosphorus (TP) (+20.3%), available N (+62.7%), and available P (+14.3%). Significant relationships were observed among the effect size of SOC and climate and soil N and P in both desert and grassland. The random forest analysis showed that TN could be considered as a determinant of the concentration of SOC, followed by climate (P < 0.01). Our study shows that the capacity of the BSCs to fix and store C could be regulated by soil N and P dynamics, indicating a major finding opening new ways to promote soil recovery and formation. Our findings highlight the remarkable contribution of mixed crusts to soil C pools; this contribution needs to be incorporated into regional and global models to predict the effects of human disturbance on drylands worldwide and for assessing the soil C budget., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021. Published by Elsevier B.V.)

Hydrological model is an effective tool for hydrological research. The initial abstraction ratio ( λ ) is a key parameter of SCS-CN model, a commonly used runoff model of great significance to simulate the hydrological process at the watershed scale. In order to examine the effects of biological soil crusts (biocrusts) on λ and improve the accuracy of the model used in the restored grasslands where biocrusts widely presented in the Loess Plateau region, we firstly determined the relationship between the amount of the potential maximum infiltration ( S ) and the amount of the actual infiltration ( F ), and then investigated the effects of biocrust coverage on λ by using the simulated rainfall experiment in the Yingwoshanjian watershed in Dingbian County, Shaanxi Province. The revised model was verified by the runoff results of the simulated rainfall experiments in the Zhifanggou watershed in Ansai County, Shaanxi Province. The results showed that the relationship between S and F on biocrust slope was described as S / F =2.5×60/ T (where T was the rainfall duration). There was a negative correlation between λ and biocrust coverage ( C BSC ) described as λ =0.0791×e (-0.015× C BSC ) , R 2 =0.60. Compared with that using the standard value of λ , the efficiency coefficient of the model was increased by 338.7% and the qualified rate was increased by 16.1% after revising λ according to the biocrust coverage. The results provided a scientific basis for the calibration of λ on biocrust slopes in the Loess Plateau region, and were of great significance to accurately assess the hydrological effects of the implementation of the "Grain for Green" Program on the Loess Plateau.

Biocrusts are widely distributed throughout the world in dryland ecosystems, but their respiration rate (R s) and responses to soil temperature and mositure have not yet been fully investigated. In a semiarid region of the Loess Plateau in China, the R s of moss-dominated biocrusts was continuously (104 days) measured in summer through an automated soil respiration system, and their relationships with soil temperature and moisture at 2, 5, and 10 cm depths were intensively analyzed. The results showed the biocrusts had 115% higher (t = −3.82, P < 0.001) R s during day and 44% higher (t = −10.22, P < 0.001) R s during night as compared with the underlying soil; they averagely increased R s by 81% (2.52 vs. 1.39 μmol m−2 s−1) across day and night. More specifically, the R s of biocrusts ranged from 0.33 to 6.33 μmol m−2 s−1 in wet conditions (soil moisture ≥ 0.15 cm3 cm−3) and 0.27 to 2.83 μmol m−2 s−1 in dry conditions (soil moisture = 0.05 cm3 cm−3), with a daily mean of 2.11 μmol m−2 s−1. Particularly, in wet conditions the R s of biocrusts had positive responses to increasing temperature, and their relationship was well fitted with an exponential function (R 2 ≥ 0.44; P < 0.001), with a Q 10 value ranging between 1.73 and 2.08. Similarly, the R s of biocrusts was positively correlated with soil moisture, but their relationship was not well fitted with the quadratic function (R 2 ≤ 0.22). A model based on the soil moisture and temperature at 2 cm depth performed much better (R 2 = 0.53; P < 0.001) with R s than that based on the soil moisture and temperature at 5 or 10 cm depth or even air temperature. In conclusion, moss biocrusts made a significant contribution to soil respiration in semiarid ecosystem, firstly (and directly) due to the rich and diversified microbial communities in biocrust layer and secondly (and indirectly) due to the biocrust effects on soil temperature and moisture regimes. The temperature and moisture at 2 cm depth (biocrust layer) should be employed with a higher priority in simulating R s of biocrusted surfaces, rather than the commonly used soil depth (5 cm) in estimating R s of the ordinary soil without biocrusts. Unlabelled Image • Biocrusts increased respiration rate (R s) by 81% as compared with underlying soil. • R s of biocrusts ranged from 0.05 to 9.50 μmol m−2 s−1 with a mean of 2.11 in summer. • R s of biocrusts was highly dependent on moisture and temperature (Q 10 = 1.73–2.08). • Soil temperature and moisture at 2 cm depth are capable of estimating R s of biocrusts. [ABSTRACT FROM AUTHOR]

Biological soil crusts (biocrusts) provide key ecosystem services in dryland ecosystems, especially soil fertility and surface stability. The relationships between soil fertility and microbial communities have been demonstrated with successional stages of biocrusts, however, the effects of soil stability on microbial communities are poorly understood. Thus, we investigated the changes in microbial communities, soil stability and fertility with biocrust succession and sampled soils with biocrusts with increasing moss coverage in a semiarid climate in the Loess Plateau of China. The phospholipid fatty acid (PLFA) method was used to determine the structural diversity of the microbial groups. The results showed that soil cohesion (which represented the soil stability) increased by 40%, and that the content of organic matter, total N and available N was 93%, 8% and 1.9 times greater in the later successional moss than in the early successional cyanobacteria. Consequently, the microbial diversity according to the Shannon and Simpson indices increased by 1.80% and 0.44%, respectively, and the microbial biomass was 3.1 times higher in the moss biocrusts than in the cyanobacteria biocrusts in biocrust layer, and the underlying 0–2 cm soil layer showed the similar changes of soil properties and microbial communities with biocrust layer. These findings are important for achieving a better understanding of the functions and services of biocrusts and provide a prediction for ecosystem processes in dryland ecosystems. • Soil microbial biomass increases as the increment of moss coverage. • Soil microbial diversity increases as the increment of moss coverage. • Improved soil stability and fertility with biocrust succession drives the developed microbial communities. [ABSTRACT FROM AUTHOR]

Intensified anthropogenic activities have drastically altered many ecosystems, motivating the use of restoration to regain key ecosystem functions and services and to stem biodiversity losses. Restoration is particularly difficult when human activities have pushed an area into a new state with self‐reinforcing feedbacks. This study investigated a long‐term restoration project in a dryland ecosystem of the Tengger Desert in northwestern China, initiated in 1956. We analyzed shrub and grass cover for 49 yr after the installation of restoration infrastructure that altered external conditions (i.e., using packed straw to reduce wind erosion) and system state (by planting shrubs). After 37–40 yr, the re‐vegetation project was successful in restoring the system to a state similar to native vegetation, with high grass cover (30–50%), low shrub cover (8–10%), and a thick biological soil crust (biocrust). However, the shift to high grass cover did not begin until year 37, before which shrub cover was high (15–20%) and grasses were subdominant (usually <10%). The shift from shrub to grass dominance was abrupt, registering significant nonlinear changes over time and relative to a key driver of vegetation dynamics, estimated biocrust thickness. Biocrust thickness increased gradually over time, which reduced rainfall infiltration into deep soil and thus increased soil moisture available for the shallow‐rooted grasses. The shift from the bare soil state to the steppe state exhibited a long time lag, suggesting that it can take decades to determine whether dryland restoration efforts succeed. The results indicate that persistence might be critical to forcing desired state transitions and that dryland restoration can proceed as a series of time lags, punctuated by abrupt changes in ecosystem state. [ABSTRACT FROM AUTHOR]

We analyzed greenhouse gas fluxes at the different growth stages of algae and lichen crusts in fixed sand with mobile dune as control in the eastern Hobq Desert, China, using the spatio-temporal substitution method. We explored the correlation of these fluxes with environmental factors and with biological soil crust growth. The results showed that variation of CO 2 fluxes followed the order: lichen crust (128.5 mg·m -2 ·h -1 ) > algae crust (70.2 mg·m -2 ·h -1 ) > mobile dune (48.2 mg·m -2 ·h -1 ). CH 4 absorption rates were in the following order: lichen crust (30.4 μg·m -2 ·h -1 ) > algae crust (21.2 μg·m -2 ·h -1 ) > mobile dune (18.2 μg·m -2 ·h -1 ). The N 2 O fluxes were in the following order: lichen crust (6.6 μg·m -2 ·h -1 ) > algae crust (5.4 μg·m -2 ·h -1 ) > mobile dune (2.5 μg·m -2 ·h -1 ). CO 2 emission had obvious seasonal variation, with higher emission in the growing season. CH 4 and N 2 O fluxes had no seaonal variation. CH 4 absorption mainly occurred in the growing season and N 2 O emission mainly occurred in non-growing season. Contents of soil total nitrogen and organic carbon and the abundance of microorganisms were important factors affecting greenhouse gas fluxes. Hydrothermic factors were important for soil CO 2 emission, but not for CH 4 and N 2 O fluxes. The cumulative greenhouse gas emissions were gradually increased with vegetation restoration and the development of biological soil crust. The global warming potential increased following an order: lichen crust (1135.7 g CO 2 -e·m -2 ·a -1 ) > algae crust (626.5 g CO 2 -e·m -2 ·a -1 ) > mobile dune (422.7 g CO 2 -e·m -2 ·a -1 ).