Successional Biological Soil Crusts Research Articles

Unveiling hidden bacterial players: A closer look at the succession characteristics and co-occurrence patterns of abundant, intermediate and rare bacteria in biological soil crusts

The bacterial communities of biological soil crusts (BSCs) often comprise abundant, intermediate and rare taxa that form complex ecosystems through interspecies interactions and play essential roles in regulating the process of vegetative restoration. However, the community structure of nonabundant bacteria and the interactions among these abundance categories are still largely unknown. In light of this, we used Illumina MiSeq sequencing to investigate the succession characteristics and co-occurrence patterns of abundant bacteria (ABs), intermediate bacteria (IBs) and rare bacteria (RBs) in the three developmental stages of BSCs (cyanobacteria-, lichen- and moss-dominated crusts) on the southeastern edge of the Tengger Desert. The results showed that the changes in the Chao richness and taxonomic composition of ABs, IBs and RBs were significantly different during BSC succession. The results of the co-occurrence network analysis revealed that most of the observed associations were between RBs (more than 80 %), positive correlations were predominant (more than 77 %), and the vast majority of the keystone taxa in the BSC bacterial networks were RBs (only one belonged to IBs). Moreover, the driving effects of environmental variables on ABs, IBs and RBs were not exactly the same, and their diversity was a key driver of the co-occurrence network properties of the BSCs. These findings are crucial for understanding the establishment and maintenance of BSC bacterial diversity and provide a new perspective for utilizing microbial resources and/or regulating environmental factors to achieve the sustainable development of desert ecosystems.

Dam construction accelerated the development of biological soil crusts in degraded soil patches in the Lhasa River basin, Qinghai-Tibetan Plateau

The Lhasa River Basin currently faces severe land degradation. In recent years, several dams have been built on the main stream, whereas, their impact on land desertification remains less elucidated. Herein, we investigated variances of soil properties and microbial communities in biological soil crusts (BSCs) along the valley area of Lhasa River and the mountain slope (altitude gradient) in reservoir area after five years of dam construction. Soil nutrient and water contents, enzyme activities, and N-cycle related functional gene copies significantly increased in reservoir-affected areas and decreased with the increase of mountain slope altitude and the distance away from the reservoir, which were usually degraded soil patches; similarly, the relative abundance of bacteria, fungi and moss in BSCs significantly increased in reservoir-affected areas, whilst that of cyanobacteria was higher in degraded soil patches. The linear and structural equation models also showed that dam construction altered water distribution status of soil surface, which enhanced soil properties, N-cycle and increased the relative abundance of moss of BSCs, and successfully accelerated the development and succession of BSCs in degraded soil. This study firstly reported that dam construction could accelerate the succession process of degraded soil in the reservoir affected area and thus a reference for ecological restoration of desertification soil of the Lhasa River Basin in QTP.

The roles of rare and abundant microbial species in the primary succession of biological soil crusts are differentiated in metal tailings ponds with different states

Tailings ponds formed by long-term accumulation of mineral processing waste have become a global environmental problem. Even worse, tailings ponds are often simply abandoned or landfilled after they cease to be used. This allows pollution to persist and continue to spread in the environment. The significance of primary succession mediated by biological soil crusts for tailings pond remediation has been illustrated by previous studies. However, the process of primary succession may not be the same at different stages during the lifetime of tailings ponds. Therefore, we investigated the environmental differences and the successional characteristics of microbial communities in the primary successional stage of tailings ponds at three different states. The results showed that the primary succession process positively changed the environment of tailings ponds in any state of tailings ponds. The primary successional stage determined the environmental quality more than the state of the tailings pond. In the recently abandoned tailings ponds, abundant species were more subjected to heavy metal stress, while rare species were mainly limited by nutrient content. We found that as the succession progressed, rare species gradually acquired their own community space and became more responsive to environmental stresses. Rare species played an important role in microbial keystone species groups.

Changes in Microbial Composition During the Succession of Biological Soil Crusts in Alpine Hulun Buir Sandy Land, China

Biological soil crusts (biocrusts) are considered “desert ecosystem engineers” because they play a vital role in the restoration and stability maintenance of deserts, including those cold sandy land ecosystems at high latitudes, which are especially understudied. Microorganisms participate in the formation and succession of biocrusts, contributing to soil properties’ improvement and the stability of soil aggregates, and thus vegetation development. Accordingly, understanding the composition and successional characteristics of microorganisms is a prerequisite for analyzing the ecological functions of biocrusts and related applications. Here, the Hulun Buir Sandy Land region in northeastern China—lying at the highest latitude of any sandy land in the country—was selected for study. Through a field investigation and next-generation sequencing (Illumina MiSeq PE300 Platform), our goal was to assess the shifts in diversity and community composition of soil bacteria and fungi across different stages during the succession of biocrusts in this region, and to uncover the main factors involved in shaping their soil microbial community. The results revealed that the nutrient enrichment capacity of biocrusts for available nitrogen, total nitrogen, total phosphorus, total content of water-soluble salt, available potassium, soil organic matter, and available phosphorus was progressively enhanced by the succession of cyanobacterial crusts to lichen crusts and then to moss crusts. In tandem, soil bacterial diversity increased as biocrust succession proceeded but fungal diversity decreased. A total of 32 bacterial phyla and 11 fungal phyla were identified, these also known to occur in other desert ecosystems. Among those taxa, the relative abundance of Proteobacteria and Cyanobacteria significantly increased and decreased, respectively, along the cyanobacterial crust–lichen–moss crust successional gradient. However, for Actinobacteria, Chloroflexi, and Acidobacteria their changed relative abundance was significantly hump-shaped, increasing in the shift from cyanobacterial crust to lichen crust, and then decreasing as lichen crust shifted to moss crust. In this process, the improved soil properties effectively enhanced soil bacterial and fungal community composition. Altogether, these findings broaden our understanding about how soil microbial properties can change during the succession of biocrusts in high-latitude, cold sandy land ecosystems.

Trajectories and tipping points of piñon-juniper woodlands after fire and thinning.

Piñon-juniper (PJ) woodlands are a dominant community type across the Intermountain West, comprising over a million acres and experiencing critical effects from increasing wildfire. Large PJ mortality and regeneration failure after catastrophic wildfire have elevated concerns about the long-term viability of PJ woodlands. Thinning is increasingly used to safeguard forests from fire and in an attempt to increase climate resilience. We have only a limited understanding of how fire and thinning will affect the structure and function of PJ ecosystems. Here, we examined vegetation structure, microclimate conditions, and PJ regeneration dynamics following ~20 years post-fire and thinning treatments. We found that burned areas had undergone a state shift that did not show signs of returning to their previous state. This shift was characterized by (1) distinct plant community composition dominated by grasses; (2) a lack of PJ recruitment; (3) a decrease in the sizes of interspaces in between plants; (4) lower abundance of late successional biological soil crusts; (5) lower mean and minimum daily soil moisture values; (6) lower minimum daily vapor pressure deficit; and (7) higher photosynthetically active radiation. Thinning created distinct plant communities and served as an intermediate between intact and burned communities. More intensive thinning decreased PJ recruitment and late successional biocrust cover. Our results indicate that fire has the potential to create drier and more stressful microsite conditions, and that, in the absence of active management following fire, there may be shifts to persistent ecological states dominated by grasses. Additionally, more intensive thinning had a larger impact on community structure and recruitment than less intensive thinning, suggesting that careful consideration of goals could help avoid unintended consequences. While our results indicate the vulnerability of PJ ecosystems to fire, they also highlight management actions that could be adapted to create conditions that promote PJ re-establishment.

Dune Blowouts as Microbial Hotspots and the Changes of Overall Microbial Activity and Photosynthetic Biomass Along with Succession of Biological Soil Crusts

Biological soil crust (BSC) constitutes a consortium of cyanobacteria, algae, lichen, mosses, and heterotrophic microorganisms, forming a miniature ecosystem within the uppermost soil layer. The biomass of different organisms forming BSC and their activity changes along with succession. Previous studies focused primarily on BSC in hyper-arid/arid regions, whereas the ecophysiology of BSC in temperate climates is still not well recognized. In order to determine changes in overall microbial activity and photosynthetic biomass in BSC at different stages of the succession of inland sandy grasslands, we analyzed dehydrogenase activity and determined the content of photosynthetic pigments. We also compared these parameters between BSC developed on the dune ridges and aeolian blowouts in the initial stage of succession. Our study revealed a significant increase in both photosynthetic biomass and overall microbial activity in BSC as the succession of inland shifting sands progresses. We found that chl a concentration in BSC could be considered a useful quantitative indicator of both the presence of photoautotrophs and the degree of soil crust development in warm-summer humid continental climates. The photosynthetic biomass was closely related to increased microbial activity in BSC, which suggests that photoautotrophs constitute a major BSC component. Dune blowouts constitute environmental niches facilitating the development of BSC, compared to dune ridges. High biomass of microorganisms in the dune blowouts may be associated with a high amount of organic material and more favorable moisture conditions. We conclude that deflation fields are key places for keeping a mosaic of habitats in the area of shifting sands and can be a reservoir of microorganisms supporting further settlement of dune slopes by BSC.

Soil microbe-mediated carbon and nitrogen cycling during primary succession of biological soil crusts in tailings ponds

Tailings ponds resulting from mining operations have led to serious environmental hazards, and their bioremediation is an area of ongoing exploration. Primary succession represents the starting point of biotic community establishment and development, with soil carbon and nitrogen cycling being critical to this process. To investigate the soil microbial-mediated carbon and nitrogen cycling patterns accompanying primary succession, we selected three types of tailings ponds as study areas and set up sampling sites for different stages of primary succession. The results showed that primary succession promoted microbe-mediated carbon and nitrogen cycling. It also led to improvements in soil nutrient availability and enzyme activity. In primary succession, the main pathways of carbon cycling are 3HP and rTCA, and nitrogen cycling is nitrate assimilation. In the early stages, microbes mediated more anaerobic and microaerobic processes. As succession proceeded, the pattern of microbial contributions to the carbon and nitrogen cycles changed. As succession proceeds, the functional metabolic potential of the carbon cycle gradually rises, while the nitrogen cycle shows a dramatic increase after the accumulation of autotrophic biomass. In addition, we found a positive coupling pattern between the carbon and nitrogen cycles. These findings support the optimization of bioremediation strategies for tailings ponds.

Spectral determinants of biological soil crusts in the Gurbantungut Desert

ABSTRACT As thick crustal layers form on dryland surfaces, they affect the spectral information that is originally dominated by sand or rock. The spectral characteristics of organic matter replace the mineral elements as prominent features. In this case, the growth patterns and spectral characteristics of biological soil crusts (BSCs) can be observed. Satellite spectral data have been used for BSC spatial information extraction. However, the dynamic changes in BSCs can affect the spectra. Two aspects are involved here: moisture change and BSC growth. When these changes are superimposed with BSC succession, they lead to an increase in spectral complexity. This study explored three BSC types, including algal crust, lichen and moss, and discussed their spectra. By selecting BSC samples at different succession states and by combining coverage and simulated precipitation, the response of the spectra to BSC coverage and the spectral characteristics of BSCs under dry and wet conditions were measured and analysed. In addition, the spectral index variations caused by coverage and moisture of three types of BSCs were discussed, where the spectral indices include the normalized difference vegetation index (NDVI), brightness index (BI), crust index (CI), and biological soil crust index (BSCI). The results showed that the succession, moisture and growth of BSCs were the main factors affecting their spectra. BSC types can be distinguished in a particular climatic context to determine the degree of BSC succession. Precipitation in the monitoring areas needs to be considered to avoid the effects of dry and wet BSC variations on remote sensing monitoring. The coverage of different types of BSCs in mixed pixels can be determined by multiple indices. The results of this study will provide a basis for monitoring BSCs using satellite spectral information to guide regional ecological management.

Variations in organic carbon mineralization of the biological soil crusts following revegetation in the Tengger Desert, North China

Revegetation is an effective measure to combat desertification worldwide, which parallelly facilitates the colonization and development of biological soil crusts (BSCs), and thereby affecting C cycles of BSCs in desert regions. However, knowledge about soil organic carbon (SOC) mineralization of BSCs and the involved mechanisms during long-term revegetation process in desert is rather limited. In this study, SOC mineralization rates of the mobile sand dune (MSD) and the BSCs in revegetated sites along chronosequence in the Tengger Desert were investigated, and the relationships between the SOC mineralization and the characteristics of vegetation and BSCs, soil properties and incubation temperature (T) and soil water content (SWC) were analyzed. The results showed that SOC mineralization rate of the BSCs and the corresponding subsoils was significantly greater than those of the MSD, and it increased considerably with increasing site age, regardless of the T, SWC, soil layer and sampling time (p < 0.001). The maximum instantaneous mineralization rates (MIR), cumulative CO2-C release (CCR) and the potentially mineralizable SOC (C0) of BSCs and subsoils were 1.29–12.63, 3.67–23.16 and 3.78–23.41 times, and 0.16–2.26, 0.12–1.84 and 0.17–1.26 times greater than those of the MSD, respectively; while the proportion of C0 to total SOC (C0/SOC) decreased by 31–88 % and 24–72 % in comparison with MSD, respectively. The mineralization rate increased with T and SWC, and the temperature sensitivity of BSCs increased with site age, indicating that the loss of SOC would be accelerated with the succession of BSCs upon the future global change. The variation magnitude of SOC mineralization was largely dependent on revegetation-induced changes in the properties of BSCs and the incubation T and SWC, and their interactions. The C:N, pH, and the activities of POD and PPO and DOC, and the coverage of moss-dominated crusts, SOC, C:N, the content of clay and silt and AYL activity were the main factors affecting SOC mineralization of BSCs and the subsoils, respectively. These results indicated that multiple factors that co-varied with the succession of BSCs led to variable SOC mineralization. Our findings suggested that planting xerophytic shrubs in desert drives the development and the SOC mineralization of BSCs, but decreased SOC loss due to mineralization, leading to SOC sequestration.

Soil nutrients, enzyme activities, and microbial communities differ among biocrust types and soil layers in a degraded karst ecosystem

Being different from the well-known and extensively researched counterparts in arid and semi-arid regions, biological soil crusts (BSCs) in degraded karst ecosystems with subtropical humid climate have been paid little attention. In this study, we investigated the differences of nutrient content, enzyme activity, and microbial communities between different types of BSCs (cyanobacterial crusts, moss crusts, moss-cyanobacteria mixed crusts, and bare soils), and between the BSCs layer and underlying soil. The results showed that the soil nutrient (total nitrogen [TN], total phosphorus [TP], and NH4-N) contents and enzyme activity (urease) in the late-successional stage (moss crusts) were higher than those in early-successional stages (cyanobacterial crusts, moss-cyanobacteria mixed crusts, and bare soils). The species richness of the bacterial community increased with the succession of BSCs, but that of the fungal community did not change. The diversity and composition of both bacterial and fungal communities were not significantly different among biocrust types. The nutrient content (TN, NH4-N), enzyme activity (urease, nitrate reductase, sucrase), and the richness of bacteria and fungi were significantly higher in BSCs layer than in subsurface soils. For either bacteria or fungi, there was a significant difference in community species composition but not in diversity between BSCs layer and subsurface soils. Besides, the results of network topological properties showed that the network structures of both bacteria and fungi in the BSCs layer were more complex than that in the subsurface, and positive links dominated both networks. These results indicate the importance of the BSCs layerespecially moss crusts for nutrient accumulation and microbial genetic resources, and provide an essential basis for further understanding the ecological functions ofBSCsin degraded karst ecosystems with subtropical humid climate.

Differences of bacterial communities in different biological soil crusts around thermal power plant and their influencing factors.

To understand the effects of heavy metal pollution derived from atmospheric dust fall on bacterial community structure under different types of biological soil crusts near mining area, we measured the diversity, community composition, and relative abundance of bacteria communities in three different developmental stages of biological soil crusts (BSCs), including algae (ZB), mixed (HB), and moss (TB) crusts, and control (CK, bare soil) around a typical thermal power plant in Ningdong Energy Industrial Base, using the high-throughput sequencing technique. Environmental factors affecting the bacterial community structure were further investigated. The results showed that there were significant differences in physicochemical properties and heavy metal contents among different BSCs. The BSCs were heavily polluted due to the enrichment of heavy metals from atmospheric dust fall. Among the top ten dominant bacterial phyla, Gemmatimonadetes and Cyanobacteria were significantly distinct among different BSCs. Bacterial α diversity decreased in an order of CK&gt;TB&gt;HB&gt;ZB. The NMDS ordination plots indicated that there were significant differences in the bacteria community composition of the three kinds of BSCs and the CK. The correlation analysis showed that the succession of BSCs significantly affected bacterial community composition in BSCs. Bacterial diversity and composition were closely related to pH, nutrients, and heavy metal contents. The relative abundance of Actinomycetes and Chloroflexi was positively correlated with pH, but negatively correlated with total N, total P, and the contents of heavy metal Pb, Zn, Cd. Results of the redundancy analysis showed that organic carbon, pH, total N, and total P were the major soil factors affecting bacterial α diversity, relative abundance of some dominant phyla, whereas heavy metal contents of Zn, Cd, Pb were the major heavy metals affecting structure of bacterial community which inhibited or stimulated the abundance and diversity of bacterial communities. We concluded that pH, heavy metals, and nutrients were the key factors affecting soil bacteria community composition. The succession of BSCs would improve their physicochemical properties and significantly impacted bacterial community composition. Long term heavy metals enrichment would affect bacterial diversity and community composition of BSCs.

Changes in soil carbon sequestration and emission in different succession stages of biological soil crusts in a sand-binding area

BackgroundWe investigated the spatio-temporal dynamics of soil carbon dioxide (CO2)- and soil methane (CH4)-flux during biological soil crust (BSCs) deposition in a sand-binding area in the eastern Chinese Hobq Desert. The trends in soil organic carbon (C) content and density were analyzed during this process. The sampling sites comprised a mobile dune (control) and those with algal, lichen, and moss crust-fixed sands. The desert soil CO2- and CH4-flux, temperature, and water content were measured from May to October in 2017 and 2018. Simultaneously, organic C content and density were measured and analyzed by stratification.ResultsThe spatio-temporal variation in desert soil CO2-flux was apparent. The average CO2- fluxes in the control, algal, lichen, and moss sites were 1.67, 2.61, 5.83, and 6.84 mmol m−2 h−1, respectively, during the growing season, and the average CH4-fluxes in the four sites were − 1.13, − 1.67, − 3.66, and − 3.77 µmol m−2 h−1, respectively. Soil temperature was significantly positively correlated with CO2-flux but could not influence CH4 absorption, and C flux had minimal correlation with soil water content. The soil total organic C density at all sites was significantly different and decreased as follows: moss > lichen > algal > control; moreover, it decreased with soil depth at all sites. The accumulation of desert soil organic C could enhance soil C emissions.ConclusionIn a semi-arid desert, artificial planting could promote sand fixation and BSCs succession; therefore, increasing the C storage capacity of desert soils and decreasing soil C emissions could alter the C cycle pattern in desert ecosystems. Soil temperature is the major factor controlling desert soil CO2 flux and vegetation restoration, and BSCs development could alter the response patterns of C emissions to moisture conditions in desert soils. The results provide a scientific basis for studying the C cycle in desert ecosystems.

Null‐model analysis and changes in species interactions in biocrusts along a successional gradient in the Tengger Desert, northern China

AbstractAimsIn arid regions, the succession of biological soil crusts (biocrusts) usually accompanies substantial turnover of community composition. How interspecific interactions among biocrust‐forming mosses, lichens, algae and cyanobacteria change along the successional gradient is largely unknown. According to the stress gradient hypothesis (SGH), the frequency or intensity of facilitative and competitive interactions within the community will vary along an environmental stress gradient, with an increasing role of competition relative to facilitation under more benign environmental conditions. In the process of biocrust succession, decreasing soil stress also occurs because of slow soil amelioration, which meets the assumption of the SGH. Therefore, we hypothesize that the changes in facilitative and competitive interactions within biocrust communities will follow the prediction of the SGH and that the nature of biocrust interactions will also change.LocationSand‐binding vegetation belts for the Baotou–Lanzhou railway at the southeast fringe of the Tengger Desert in the Shapotou region of the Ningxia Hui Autonomous Region, western China.MethodsWe conducted a field evaluation at five adjacent sand‐binding vegetation belts built in chronosequence to study the successional gradient. Small‐scale patterns of co‐occurrence and null models were used to evaluate the community‐level interspecific interactions among the biocrust‐forming mosses, lichens, algae and cyanobacteria. In addition, an accessory method of species‐pair co‐occurrence based on null models was also applied to potentially identify the nature of the interspecific interactions within the biocrusts and for the causal investigation of these patterns.ResultsA strong spatial segregation of co‐occurrence patterns occurred between cyanobacteria and algae and species of mosses and lichens at early succession, and occurred among species of mosses and lichens at late succession, while the weak spatial aggregation between lichen species only occurred at middle succession.ConclusionsNegative interspecific interactions were prevalent within biocrusts throughout succession. The co‐occurrence patterns at both the community and species‐pair levels are consistent, supporting the unimodal version of the SGH that species interactions shift toward competition under extremely stressed conditions. Given the lack of research on interspecific interactions in biocrusts, our study is important for obtaining a deeper understanding of biocrust succession.

Variations in microbial functional potential associated with phosphorus and sulfur cycling in biological soil crusts of different ages at the Tengger Desert, China

Microbial communities play a very important role in soil ecological processes by regulating biogeochemical cycles during biological soil crust (BSC) succession in desert ecosystems. Although the relationships between microbial carbon or nitrogen and the BSC at different successional stages have been a source of great concern, there are few reports regarding the phosphorus (P) and sulfur (S) cycles. In this study, we used a functional gene array (GeoChip 5.0) to analyze the characteristics of microbial communities and their functional genes involved in the P and S cycles along a chronosequence of BSC succession in the Tengger Desert, China. The results showed that the functional genes associated with polyphosphate degradation and sulfite reduction were the major components involved in the P and S cycles, respectively, and the intensities of the functional genes expression involved in the P and S cycles increased significantly in 61-year-old BSCs. Compared with the fungal and archaeal communities, the bacterial community was the major contributor to the P and S cycles, and Proteobacteria (mainly Alphaproteobacteria, Gammaproteobacteria, and Betaproteobacteria) and Actinobacteria were the dominant phyla. A redundancy analysis showed that there was significant synergy between the improvement in soil properties (e.g., soil nutrient content) and the intensities of genes expression related to P and S cycles. These findings indicated that after 61 years, BSC significantly promoted the microbial metabolic potential for P and S cycling, which in turn promoted soil rehabilitation in a desertified dryland.

Increasing relative abundance of non-cyanobacterial photosynthetic organisms drives ecosystem multifunctionality during the succession of biological soil crusts

Biological soil crusts (biocrusts) are essential for ecosystem functioning, especially in drylands. However, we lack the knowledge of how ecosystem multifunctionality (EMF) responds to the development of biocrusts and the key factors mediating EMF during biocrust succession. In this study, we tested a series of essential ecosystem functions of the biocrust system and calculated a weighted EMF index, a processes-based EMFprocesses index, and a resource storage and availability based EMFpools index. Sequencing of the 16S rRNA gene and ITS gene was used to test differences in the community compositions of 16S rRNA gene-based organisms and ITS gene-based fungi in different biocrust stages. Results showed that the changing patterns and driving factors of all the three multifunctionality indices were similar. Later developed biocrust stages exhibited higher values of all three EMF indices. The 16S rRNA gene-based diversity reduced with biocrust succession. Biodiversity-EMF relationships varied when considering different biocrust stages and organisms. Across all biocrust stages, significantly negative relationships existed between the EMF indices and 16S rRNA based α-diversity, whereas positive relationships occurred between the EMF indices and both 16S rRNA and ITS gene-based β-diversity. Further analyses indicated that the increasing relative abundance of non-cyanobacterial photosynthetic organisms (represented by chloroplast sequences and lichenized fungi) was the key predictor of all three EMF indices during biocrust succession. Specifically, these organisms were Streptophyta, Chlorophyta and Bacillariophyta and lichenized fungi, e.g., Verrucaria, Caloplaca and Aspicilia. This study provided a mechanistic understanding of how biological compositions and diversity drive EMF with biocrust development.

Soil total organic carbon/total nitrogen ratio as a key driver deterministically shapes diazotrophic community assemblages during the succession of biological soil crusts

The diazotrophic community in biological soil crusts (biocrusts) is the key supplier of nitrogen in dryland. To date, there is still limited information on how biocrust development influences the succession of diazotrophic community, and what are the most important factors mediating diazotrophic communities during biocrust succession. Using the high throughput nifH amplicon sequencing, the diazotrophs in soils at different developmental stages of biocrust were comparatively studied. The results evidenced the decreases of TOC/TN ratio and pH value with biocrust development. Nostoc and Scytonema were the most dominant diazotrophic genera at all biocrust stages, while Azospirillum and Bradyrhizobium were abundant only in bare soil. Diazotrophic co-occurrence networks tended to be less complex and less connected with biocrust succession. The soil TOC/TN ratio was the most dominant factor mediating diazotrophic diversity, community composition and assembly processes, while diazotrophic-diversity and NO3−-N/NH4+-N ratio were positively correlated with the nitrogenase activity during biocrust succession. This study provided novel understandings of nitrogen fixation and succession patterns of diazotrophic community, by showing the effects of biocrust succession on diazotrophic diversity, community composition, community assembly and co-occurrence networks, and recognizing TOC/TN ratio as the most dominant factor mediating diazotrophs during biocrust succession.

Bacteria and fungi differentially contribute to carbon and nitrogen cycles during biological soil crust succession in arid ecosystems

Biological soil crusts (BSCs) are widely considered critical for soil fertility in arid ecosystems. However, how microbial communities regulate the C and N cycles during BSC succession is not well understood. We utilized GeoChip 5.0 to analyze the functional potential of bacteria and fungi involved in the C and N cycles of BSCs along a 61-year revegetation chronosequence. The normalized average signal intensities of different functional genes involved in C and N metabolism in 61-year-old BSCs were significantly different from those in younger BSCs and most functional gene subcategories and the corresponding dominant functional populations were derived from bacterial rather than fungal communities. Most C degradation genes (dominated by the starch-degrading gene amyA) were derived from Actinobacteria (mainly Streptomyces) in bacteria, but Ascomycota (mainly Aspergillus) was the key population for lignin degradation (dominated by the phenol oxidase gene) during BSC succession. N cycle genes involved in denitrification (such as narG, nirK/S, and nosZ) and N fixation (nifH) were mainly derived from Unclassified Bacteria, whereas genes involved in ammonification (ureC) were mainly derived from Streptomyces. Moreover, redundancy analysis showed that soil biogeochemical properties were closely related to bacterial and fungal functional gene structures during BSC succession. These findings indicate that bacteria play a crucial role in the regulation of C and N cycles during BSC succession in arid ecosystems, while fungi perform supplementary degradation of lignin, and these communities can successfully stimulate an increase in C and N metabolism in soil during the later successional stages of BSCs.

Biological soil crust succession in deserts through a 59-year-long case study in China: How induced biological soil crust strategy accelerates desertification reversal from decades to years

The regeneration of induced biological soil crusts (IBSCs) is regarded as an effective strategy for combating desertification. Three types of BSCs, namely, cyanobacterial, lichen and moss, are well-accepted as the main succession phases and are hypothesized to represent a continuous process. Herein, natural BSCs (NBSCs) and IBSCs with accurate ages from a 59-year-long field study were investigated to understand the entire BSC succession process. Shifts in nutrient levels, microbial composition and ecological functions suggested that cyanobacterial inoculation successfully accelerated BSC succession from decades to years by promoting the microbial multifunctions related to carbon and nitrogen fixation. The four state transitions of the BSC community accompanied by the turn-over of carbon and nitrogen fixators provide clues to the factors restricting the recovery process and climax of arid ecosystems. This study provides the first description of the continuous BSC succession, comprehensively discusses the mechanisms of BSC formation and succession and provides important guides for selection of strategies for the engineering reversals of desertification.

Development of archaeal communities in biological soil crusts along a revegetation chronosequence in the Tengger Desert, north central China

Archaea are major contributors to biogeochemical cycles and energy metabolism among soil microorganisms under extremely acidic and high-temperature conditions, however, the biodiversity and ecological function of archaea in biological soil crusts (BSCs) of desert ecosystems are not fully understood. Here, we used Illumina MiSeq sequencing and microbial functional gene array (GeoChip 5.0) to test the following hypotheses: (1) the composition and function related to biogeochemical cycles of the archaeal community would change significantly in the development process of BSCs; and (2) the key factors driving these changes may be soil biogeochemical properties. The results showed that the diversity, abundance, and functional potential of the archaeal community showed their highest levels in 5-year-old BSCs. The dominant phyla were Thaumarchaeota, Euryarchaeota, and an unclassified phylum in the archaeal community during BSC succession. The functional genes of the archaeal community were mainly involved in carbon (C) and nitrogen (N) cycles, and the functions of the three dominant phyla were complementary in these cycles. Moreover, redundancy analysis showed that soil biogeochemical properties were negatively related to the composition and function of the archaeal community during BSC succession, and the soil C:N ratio might be the major limiting factor. These results provided evidences for our hypotheses and revealed that the archaeal community played an important ecological role in the early development stage of BSCs, and might be pioneer species of soil microbial communities during BSC succession.

Population dynamics of Echinops gmelinii Turcz. at different successional stages of biological soil crusts in a temperate desert in China.

The effects of biological soil crusts (BSC) on vascular plant growth can be positive, neutral or negative, and little information is available on the impacts of different BSC successional stages on vascular plant population dynamics. We analysed seedling emergence, survival, plant growth and reproduction in response to different BSC successional stages (i.e. habitats: bare soil, cyanobacteria, lichen and moss crusts) in natural populations of Echinops gmelinii Turcz. in the Tengger Desert of northwest China. The winter annual E. gmelinii is a dominant pioneer herb after sand stabilisation. During the early stages of BSC succession, the studied populations of E. gmelinii were characterised by high density, plant growth and fecundity. As the BSC succession proceeded beyond moss crusts, the fecundity decreased sharply, which limited seedling recruitment. Differences in seedling survival among the successional stages were not evident, indicating that BSC have little effect on survival in arid desert regions. Moreover, E. gmelinii biomass allocation exhibited low plasticity, and only reproductive allocation was sensitive to the various habitats. Our results further suggest that the negative effects of BSC succession on population dynamics are primarily driven by increasing topsoil water-holding capacity and decreasing rain water infiltration into deeper soil. We conclude that BSC succession drives population dynamics of E. gmelinii, primarily via its effect on soil moisture. The primary cause for E. gmelinii population decline during the moss-dominated stage of BSC succession is decreased fecundity of individual plants, with declining seed mass possibly reducing the success of seedling establishment.