Pyrosequencing Reveals Significant Changes in Microbial Communities Along the Ecological Succession of Biological Soil Crusts in the Tengger Desert of China

Biological soil crusts (BSCs) have important ecological functions in arid and semiarid lands, but they remain poorly understood in terms of the changes in microbial communities during BSC succession under in situ field conditions. Here, 454 pyrosequencing was used to assess the microbial community composition of four BSC types in the Tengger Desert of China: alga, lichen (cyanolichen and green alga-lichen), and moss crusts, representing early, middle, and final successional stages of BSCs, respectively. The results showed the highest diversity of microbial communities inhabiting lichen crusts, whereas the lowest diversity was observed in moss crusts. Five phyla, Proteobacteria, Actinobacteria, Cyanobacteria, Bacteroidetes, and Acidobacteria, accounted for about 72% to 87% of total prokaryotic sequences in different BSCs. The most abundant eukaryotic microorganism was Ascomycota, accounting for 47% to 93% of the total eukaryotic sequences. Along the succession of BSCs, the abundance of photoautotrophic Cyanobacteria, Chlorophyta, and Bacillariophyta declined, and that of heterotrophic microorganisms such as bacteria and fungi increased. Statistical analysis showed clear divergency of microbial taxa at the class level among the different successional stages of BSCs. The clustering results at class level showed that the moss crusts were the farthest from the rest in prokaryotic composition; the alga crusts were the most different in terms of eukaryotic microorganisms and the two kinds of lichen crusts were relatively closer in both compositions. Ordination analysis showed that the main variations of community structure among BSCs could be explained best by the abundance of Cyanobacteria and Ascomycota and by physiochemical properties of BSCs, including mechanical composition, moisture, and electrical conductivity. In conclusion, our results indicate that Cyanobacteria and Ascomycota likely play an important role in the evolution of BSC structure and functions and highlight the importance of environmental factors in shaping microbial community structures of BSCs in the Tengger Desert of China.     

Ants mediate soil water in arid desert ecosystems: Mitigating rainfall interception induced by biological soil crusts?

As vital components of desert systems, the roles of ants in arid ecological processes have been well documented, while little attention has been given to their effects on soil water. We conducted a six-year investigation in sand dune systems stabilized via revegetation, to explore the hydrological role of ants through comparing the influence of ant nests on rainfall infiltration in different-aged revegetated dunes. The presence of ant nests markedly enhanced infiltration due to weakening the rainfall interception by biological soil crusts (BSCs) in revegetated dunes. The distribution of ant nest was denser in older revegetated areas, due to better developed BSCs of later successional stages, compared to younger revegetated areas. Ants prefer later to early successional BSCs because the later lichen–moss dominated crusts were thicker and their surface was more stable than the early cyanobacteria dominated crusts. Conversely, the crustal rainfall interception was positively correlated with BSC thickness. These findings suggest that the occurrence of ant nests in older revegetated areas benefited to the planted shrubs with deeper root systems and maintain a relative constant cover of shrubs in artificial sand-binding vegetation following an increase in infiltration to deeper soil layers.     

Effects of biological soil crusts on soil enzyme activities in revegetated areas of the Tengger Desert,China

Biological soil crusts (BSCs) cover up to 70% of the sparsely-vegetated areas in arid and semiarid regions throughout the world and play a vital role in dune stabilization in desert ecosystems. Soil enzyme activities could be used as significant bioindicators of soil recovery after sand burial. However, little is known about the relationship between BSCs and soil enzyme activities. The objective of this study was to determine whether BSCs could affect soil enzyme activities in revegetated areas of the Tengger Desert. The results showed that BSCs significantly promoted the activities of soil urease, invertase, catalase and dehydrogenase. The effects also varied with crust type and the elapsed time since sand dune stabilization. All the soil enzyme activities tested in this study were greater under moss crusts than under cyanobacteria–lichen crusts. The elapsed time since sand dune stabilization correlated positively with the four enzyme activities. The enzyme activities varied with soil depth and season, regardless of crust type. Cyanobacteria–lichen and moss crusts significantly enhanced all test enzyme activities in the 0–20 cm soil layer, but negatively correlated with soil depth. All four enzyme activities were greater in the summer and autumn than in spring and winter due to the vigorous growth of the crusts. Our study demonstrated that the colonization and development of BSCs could improve soil quality and promote soil recovery in degraded areas of the Tengger Desert.     

毛乌素沙地人为干扰苔藓结皮的土壤水分和风蚀效应

[目的]为了探讨在毛乌素沙地人为干扰生物结皮的必要性与可行性,并为该区生物结皮的高效利用提供实验依据。[方法]在毛乌素沙地东南缘设置裸沙、苔藓结皮、干扰苔藓结皮、沙蒿、沙蒿+苔藓结皮以及沙蒿+干扰苔藓结皮6个处理小区,通过动态监测各小区土壤水分及风蚀变化过程,分析人为干扰苔藓结皮对土壤水分及风蚀过程的影响。[结果](1)沙地苔藓结皮能够显著提高浅层土壤含水量,降低深层土壤含水量。(2)人为干扰苔藓结皮会引起浅层土壤含水量的降低和降雨入渗深度的增加。(3)与裸沙对照相比,几种处理的减蚀效率大小顺序为:沙蒿+苔藓结皮(97.01%)沙蒿+干扰苔藓结皮(90.87%)苔藓结皮(89.63%)干扰苔藓结皮(69.50%)沙蒿(64.62%)。[结论]植被覆盖度较高时,对苔藓结皮进行适当破坏,能够在不加剧土壤风蚀的前提下,一定程度上改善土壤水分状况。而在无植被或低植被覆盖的地块,要禁止对苔藓结皮的干扰破坏。     

Effects of biological soil crusts on soil nematode communities following dune stabilization in the Tengger Desert,Northern China

Biological soil crusts (BSCs) play an important role in the dune fixation and maintaining soil biota in arid desert systems. Free-living soil nematode communities could be used as significant bioindicators to reflect soil recover regime after sand burial. However, the relationship between BSCs and nematodes is rarely known. To examine the effects of BSCs on soil nematodes, 72 soil samples under cyanobacteria–lichen and moss crusts were collected to analyse nematode communities in the different aged vegetated areas at the southeastern edge of the Tengger Desert. Our results showed the colonization and development of BSCs significantly enhanced nematode diversity. Nematode abundances, generic richness, H′, MI, EI and SI were greater under crusts than those under noncrust. In particular, nematode abundances, generic richness, H′, MI, EI and SI were positively correlated with crust ages. The differences in nematode communities were also dependent upon crust types. Nematode abundances and generic richness under moss crusts were higher than those under cyanobacteria–lichen crusts. This can be contributed to the present and succession of BSCs that increased thickness of topsoil after dunes have been stabilized, namely, creating suitable habitats and providing an essential food source for nematodes.     

黄土高原干旱半干旱区生物结皮覆盖土壤水汽吸附与凝结特征

生物结皮是一种广泛分布于干旱半干旱地区土壤表层的特殊复合体,为揭示其对土壤水汽吸附与凝结过程的影响,该研究通过室内定量水汽吸附试验和野外对水汽凝结的连续观测,对黄土高原典型生物结皮(藻结皮、藻藓混生结皮、藓结皮)与裸沙的水汽吸附和凝结特征进行对比研究。结果表明:生物结皮的覆盖显著提升了表层土壤的水汽吸附能力,其平均水汽吸附量比裸沙高66.7%。不同类型生物结皮水汽吸附能力差异显著,表现为藓结皮最高,混生结皮次之,而藻结皮最低。GAB(Guggenheim-Anderson-de Boer)吸附模型能较好的描述生物结皮土壤水汽吸附与解吸附过程,模拟结果决定系数R20.99、均方根误差RMSE0.001 2 g/g及平均相对偏差百分比E16.0%;此外,生物结皮加剧了土壤水汽吸附与解吸附曲线之间的滞后效应,其滞后指数平均是裸沙的2.0～2.9倍。水汽凝结结果显示,水汽凝结过程均受气温与相对湿度等气象因子制约,且生物结皮覆盖下表层土壤的水汽凝结和蒸发过程相较于裸沙更为迅速。同时,生物结皮的日均水汽凝结量是裸沙的1.6～1.8倍。综上,干旱和半干旱地区生物结皮覆盖显著提高了表层土壤的水汽吸附能力、并增加了水汽凝结量,对区域表层土壤的水分运动过程产生了重要影响。     

黄土高原生物土壤结皮研究进展与展望

黄土高原是典型的生态脆弱敏感区和世界上水土流失最为严重的地区,也是当今我国生态恢复和生态文明建设的重点区域.生物土壤结皮是细菌、藻类、真菌和孢子植物与土壤颗粒胶结而成的有机复合体,是干旱半干旱地区地表系统的重要组成部分,它们对黄土高原的水土保护、养分积累和生态恢复具有重要的生态功能.本文论述了生物土壤结皮类型与演替过程...     

Biological soil crust development affects physicochemical characteristics of soil surface in semiarid ecosystems

Water and nutrients are scarce resources in arid and semiarid ecosystems. In these regions, biological soil crusts (BSCs) occupy a large part of the soil surface in the open spaces surrounding patches of vegetation. BSCs affect physicochemical soil properties, such as aggregate stability, water retention, organic carbon (OC) and nitrogen (N) content, associated with primary ecosystem processes like water availability and soil fertility. However, the way BSCs modify soil surface and subsurface properties greatly depends on the type of BSC. We hypothesised that physicochemical properties of soil crusts and of their underlying soils would improve with crust development stage. Physicochemical properties of various types of soil crusts (physical crusts and several BSC development stages) and of the underlying soil (soil layers 0–1 cm and 1–5 cm underneath the crusts) in two semiarid areas in SE Spain were analysed. The properties that differed significantly depending on crust development stage were aggregate stability, water content (WC) (at −33 kPa and −1500 kPa), OC and N content. Aggregate stability was higher under well-developed BSCs (cyanobacterial, lichen and moss crusts) than under physical crusts or incipient BSCs. WC, OC and N content significantly increased in the crust and its underlying soil with crust development, especially in the first centimetre of soil underneath the crust. Our results highlight the significant role of BSCs in water availability, soil stability and soil fertility in semiarid areas.     

Effects of inoculated <Emphasis Type="Italic">Microcoleus vaginatus</Emphasis> on the structure and function of biological soil crusts of desert

Microcoleus vaginatus Gom., the dominant species in biological soil crusts (BSCs) in desert regions, plays a significant role in maintaining the BSC structure and function. The BSC quality is commonly assessed by the chlorophyll a content, thickness, and compressive strength. Here, we have studied the effect of different proportions of M. vaginatus, collected from the Gurbantunggut Desert in northwestern China, on the BSC structure and function under laboratory conditions. We found that when M. vaginatus was absent in the BSC, the BSC coverage, quantified by the percentage of BSC area to total land surface area, was low with a chlorophyll a content of 4.77 × 10⁻² mg g⁻¹ dry soil, a thickness of 0.86 mm, and a compressive strength of 12.21 Pa. By increasing the percentage of M. vaginatus in the BSC, the BSC coverage, chlorophyll a content, crust thickness, and compressive strength all significantly increased (P < 0.01). The maximum chlorophyll a content (13.12 mg g⁻¹dry soil), the highest crust thickness, and the compressive strength (1.48 mm and 36.60 Pa, respectively) occurred when the percentage of inoculated M. vaginatus reached 80% with a complex network of filaments under scanning electron microscope. The BSC quality indicated by the above variables, however, declined when the BSC was composed of pure M. vaginatus (monoculture). In addition, we found that secretion of filaments and polymer, which stick sands together in the BSC, increased remarkably with the increase of the dominant species until the percentage of M. vaginatus reached 80%. Our results suggest that not only the dominant species but also the accompanying taxa are critical for maintaining the structure and functions of the BSC and thus the stability of the BSC ecosystems.     

古尔班通古特沙漠不同地貌部位生物结皮的选择性分布

新疆古尔班通古特沙漠是我国最大的固定和半固定沙漠，其间广泛发育着以地衣植物为主的生物结皮，是除种子植物以外的固定沙面的重要生物因子。研究表明，生物结皮的分布对地貌部位有较强的选择性，生物结皮的不同发育阶段种类组成亦有较大的差别。沙垄顶部为流动或半流动沙丘，主要以微生物种类分布为主。沿沙垄顶部向两侧坡间延伸，开始出现发育较弱的藻结皮，脆而极易破损。从坡上部至坡中部出现发育充分的藻结皮，具有一定的抗机械能力，并在局部微地形凸起部位有地衣植物出现。沿坡中部至坡下缘部，逐渐开始发育有良好的地衣结皮。同时，在蛇麻黄等低矮灌木群落下已经发育有苔藓结皮。丘间低地是生物结皮最为丰富的部位，苔藓植物除分布于低矮灌丛下外，还在灌丛间形成面积不等的纯群，与发育良好的地衣结皮镶嵌分布。     

毛乌素沙地三种植被下苔藓结皮的土壤理化效应

分析了沙蒿、沙柳、樟子松三种植被下有苔藓结皮、无苔藓结皮（对照）样地0—2,2—5,5—10 cm剖面土壤的理化指标,探讨毛乌素沙地苔藓结皮对土壤理化性状的影响及其与植被类型的关系。结果表明:（1）三种植被下苔藓结皮均能够明显提高土壤稳定性,增加细砂粒（0.02～0.2 mm）含量、降低粗砂粒（0.2～2 mm）含量。（2）沙柳及沙蒿植被下苔藓结皮均能够显著提高各层土壤的有机质、全氮、速效钾、全磷含量（P>0.05）,降低全钾、速效磷含量;樟子松下苔藓结皮0—2 cm剖面的理化效应同沙柳、沙蒿类似,但2—5,5—10 cm剖面内,有机质含量显著降低（P>0.05）,全氮、全钾、全磷、速效磷含量则无显著变化（P<0.05）。（3）荒漠生态恢复过程中,苔藓结皮对土壤理化性状的改善作用主要集中在表层土壤,改善程度同植被类型的关系密切。     

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

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

黄土丘陵区人工移植生物结皮集雨面的集雨效率及其抗侵蚀破损能力

[目的] 为探讨黄土丘陵区不同类型人工生物结皮集雨面的集雨效果及抗侵蚀破损能力。[方法] 以人工移植培养的藻类和藓类生物结皮为研究对象,以裸土为对照,分别对其进行野外模拟降雨和抗侵蚀试验(坡面冲刷、土壤静水崩解和单雨滴溅蚀试验),分析不同类型集雨面的集雨效率与土壤抗冲系数、土壤崩解速率和溅蚀量等抗侵蚀能力指标的差异。[结果] (1)生物结皮集雨面显著提高集雨效率。当雨强为100 mm/h时,相较于裸土,生物结皮集雨面的集雨效率显著提高33.3%(F=300.12,p<0.001)。(2)生物结皮显著降低产沙量并提高抗冲系数。与裸土相比,藻结皮和藓结皮的产沙量分别减少178.8%和364.6%,藓结皮和藻结皮的最大抗冲系数分别是裸土的4.6,2.8倍。(3)生物结皮显著降低土壤崩解速率和最大崩解率,且不同生物结皮类型差异显著。与裸土(6.46 g/min)相比,藻结皮和藓结皮的土壤崩解速率分别降低35.0%和60.2%;生物结皮平均最大崩解率较裸土降低23.8%。(4)生物结皮显著提高临界击穿雨滴动能并降低土壤溅蚀量。藓结皮和藻结皮的临界击穿雨滴动能分别是裸土(0.5 J)的3.9,21.9倍。同时,生物结皮较裸土(0.156 g)的单雨滴溅蚀量平均减少75.3%。[结论] 人工移植培养的生物结皮集雨面显著提高集雨效率和表层土壤的抗侵蚀能力,对旱区集雨措施的可持续利用和水土保持方面具有重要意义。     

Effect of vegetation manipulation of abandoned arable land on soil microbial properties

 The effect of vegetation composition on various soil microbial properties in abandoned arable land was investigated 2 years after agricultural practice had terminated. Microbial numbers and processes were determined in five replicate plots of each of the following treatments: continued agricultural practice (monoculture of buckwheat in 1997), natural colonization by the pioneer community (arable weeds), and manipulated colonization from low (four species, three functional groups: grasses, forbs and legumes) or high diversity (15 species, three functional groups) seed mixtures from plant species that are characteristic of abandoned fields in later successional stages. The results indicated that differences in above-ground plant biomass, plant species composition and plant species diversity had no significant effect on soil microbial processes (net N mineralization, short-term nitrification, respiration and Arg ammonification), microbial biomass C and N (fumigation-incubation) or colony-forming units of the major microbial groups. Hence, there were no indications that soil microbial processes responded differently within 2 years of colonization of abandoned arable land by later successional plants as compared to that by plants from the natural pioneer weed community. Therefore, it seems that during the first few years after arable field abandonment, plants are more dependent on the prevailing soil microbiological conditions than vice versa. Received: 8 April 1999     

喀斯特森林近地表层覆盖对碳酸盐岩红土蒸发过程的影响

蒸发是西南喀斯特地区薄层土壤水分损失的主要途径,浅层土壤水分的存蓄对喀斯特地区农业生产和生态恢复至关重要。以西南喀斯特森林碳酸盐岩红土为研究对象,基于室内蒸渗试验设置4个苔藓生物量(0,0.32,0.64,0.95 kg/m~2)和3个松针生物量(0,0.32,0.64 kg/m~2)共12种处理,分析森林近地表层覆盖对碳酸盐岩红土蒸发过程及表层温度时空分布的影响规律,并对比3种蒸发模型(Black、Rose、空气动力学蒸发模型)在喀斯特森林碳酸盐岩红土的适用性。结果表明:苔藓和松针覆盖显著降低累积蒸发量和蒸发速率(P0.05),接种苔藓0.95 kg/m~2和覆盖松针0.64 kg/m~2处理比裸土累积蒸发量小36.9%;苔藓和松针导致土壤含水量显著增加(P0.05);苔藓和松针增加了表层土壤的平均温度,松针对土壤温度的提升作用强于苔藓;Black、Rose和空气动力学模型均能较好地模拟碳酸盐岩红土蒸发过程,Black蒸发模型的拟合精度高于Rose和空气动力学蒸发模型。研究结果能为西南喀斯特地区的水量平衡分析提供理论支撑并加强对喀斯特森林地表水文过程的认知。     

Biocrusts resist runoff erosion through direct physical protection and indirect modification of soil properties

Purpose

Biological soil crusts (biocrusts) are ubiquitous in arid and semi-arid regions and play many critical roles in soil stabilization and erosion prevention, greatly decreasing soil loss. Although sediments may be completely controlled by well-developed biocrusts, runoff loss is observed. Consequently, it is important to study how biocrusts resist runoff erosion in different developmental stages to evaluate and manage water erosion.

Materials and methods

In the Loess Plateau Region, we sampled 32 biocrust plots representing eight stages of biocrust development and 5 slope cropland soil plots as bare soil control plots. We then used a rectangular open channel hydraulic flume to test the effects of biocrust development on runoff erosion.

Results and discussion

As expected, the establishment of biocrusts enhanced soil stability, and accordingly, soil anti-scourability significantly increased with biocrust development. Biocrusts exhibiting more than 36% or 1.22 g dm^?2 of moss coverage or biomass fully protected the soil from runoff erosion. Moreover, soil properties, such as soil organic matter, soil cohesion and soil bulk density, were also important in reducing erosion. The findings indicated that biocrusts inhibited runoff erosion through direct physical protection related to biocrust cover and biomass and through the indirect modification of soil properties. In the early biocrust development stage (when moss cover was less than 36%), cyanobacterial biocrust played a primary role in providing resistance to runoff erosion, with resistance being positively related to cyanobacterial biomass (chlorophyll a) and influenced by soil properties.

Conclusions

The relationship between soil anti-scourability and moss coverage or biomass can be divided into two stages based on a moss cover or biomass threshold. The capacity of biocrusts to resist runoff erosion was limited when moss cover was below the threshold value. Therefore, the stage corresponding to this level of moss cover should be of concern when estimating, predicting and managing water erosion.

     

Biological soil crust and surface soil properties in different vegetation types of Horqin Sand Land,China

Physical and chemical properties (including coverage, thickness, hardness, moisture, particle size distribution, organic matter and nutrient contents etc.) of biological soil crust and 0–5.0 cm 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 0–5.0 cm 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.     

Responses of soil microbial biomass and community composition to biological soil crusts in the revegetated areas of the Tengger Desert

As a key component of desert ecosystems, biological soil crusts (BSCs) play an important role in dune fixation and maintaining soil biota. Soil microbial properties associated with the colonization and development of BSCs may indicate soil quality changes, particularly following dune stabilization. However, very little is known about the influence of BSCs on soil microbes in sand dunes. We examined the influence of BSCs on soil microbial biomass and community composition in revegetated areas of the Tengger Desert. BSCs increased soil microbial biomass (biomass C and N), microbial phospholipid fatty acid (PLFA) concentrations and the ratio of fungal to bacterial PLFAs. The effects varied with crust type and crust age. Moss crusts had higher microbial biomass and microbial PLFA concentrations than cyanobacteria-lichen crusts. Crust age was positively correlated with microbial biomass C and N, microbial PLFA concentrations, bacterial PLFA concentrations, fungal PLFA concentrations and the ratio of fungal to bacterial PLFAs. BSCs significantly affected microbial biomass C and N in the 0–20 cm soil layers, showing a significant negative correlation with soil depth. The study demonstrated that the colonization and development of BSCs was beneficial for soil microbial properties and soil quality in the revegetated areas. This can be attributed to BSCs increasing topsoil thickness after dunes have been stabilized, creating suitable habitats and providing an essential food source for soil microbes.     

毛乌素沙地生物土壤结皮与沙化土地的关系研究

生物土壤结皮在荒漠地区广泛分布,是固沙和促进沙地生态恢复的重要生物因子。利用1989,2002和2006年的Landsat遥感影像,分别通过生物土壤结皮指数和NDVI阈值法,提取了毛乌素沙地生物土壤结皮和沙化土地的空间分布,并分析了二者之间联系。结果表明:(1)研究区1989,2002和2006年结皮面积所占比例分别为54.77%,55.72%和64.29%,表现为持续增加趋势,结皮斑块间连接度在增强,结皮的稳定性进一步巩固。(2)1989—2006年区内沙化土地的程度逐渐减弱,沙化程度综合指数由1989年的3.24下降到2006年的2.65。(3)1989—2006年,中度和重度沙化土地结皮分布范围最为广泛,均在70%以上;极重度沙化土地内的结皮面积表现为持续下降,1989,2002和2006年分别为19.40%,5.76%和2.80%。(4)以1989年沙化土地的格局为基础,1989,2002和2006年极重度沙化土地内生物结皮的数量比例分别为19.40%,33.18%和45.21%,处于上升趋势。     

Inferring biological soil crust successional stage using combined PLFA, DGGE, physical and biophysiological analyses

Arid areas are highly sensitive to climate change and are ideal model systems to study the potential impact of climate change on species' community structure. Biological soil crust (BSC) formation plays an ecological role in a number of key processes in the development of dry ecosystems. It was hypothesized that BSC succession and function are affected by aridity level and limited by rainfall. Furthermore, it is possible to infer the direction of the BSC succession based on aridity level, and the latter can imitate future climate change scenarios. The objectives of this study were to investigate the microbial biomass and diversity of the BSC structure in three sites differing in aridity level (semiarid, arid and hyper-arid), by combining physical and biophysiological measurements with 16S rRNA gene fragment and phospholipid fatty acid (PLFA) analyses. Physical and biophysiological parameters of the BSC were significantly influenced by aridity level. Total protein and polysaccharide contents were strongly correlated with total PLFA-based microbial biomass. Gram-positive biomarkers and microbial biomass were significantly higher in the wettest (semiarid) site than in the driest (hyper-arid) one. Multivariate-analysis based ordination of the PLFA data segregated the cluster of semiarid data from that of the hyper-arid site, while data from the arid site were dispersed between the two. The phylogenetic distribution of prominent 16S rRNA bacterial gene sequences along the aridity levels was in agreement with the PLFA analysis: the hyper-arid site was dominated by the cyanobacterium Microcoleus vaginatus, while diverse populations of cyanobacteria and soil bacteria were found in the other sites. These complementary tools allowed a simple and sensitive measurement of the influence of aridity levels on BSC successional stage. The results demonstrate that different aridity levels correspond to different BSC successional stages and those differences can be used as parameters for global change scenarios.