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Desertification is a common environmental challenge for humans,which restricts social and economic development.Biotechnologies have been used for the management of soils in areas of desertification and the use of microorganisms has shown potential benefits,but such research is still in the initial stages.Here,we have studied the microbial distribution and diversity in soil crust and in the soil under the crust along sand dunes of different fixed ages at the southeast edge of the Tengger Desert,China.The Shapotou and Yiwanquan regions were selected as sampling sites,which represented an artificial restoration region and a natural restoration region,respectively.The results showed that the number of culturable bacteria varied between 3.7×103 to 2.8×106 CFU/g,which was similar with other deserts worldwide.The number of culturable bacteria increased with the degree of sand dune fixation,and decreased with depth,which was similar to the trend of soil improvement in areas of desertification.Such characteristics did not show any significant differences between the artificial restoration region and the natural restoration region(P0.05).The genera Arthrobacter and Bacillus were the dominant bacteria at the southeast edge of the Tengger Desert.The genus Arthrobacter comprised 12.8%—69.5% and the genus Bacillus comprised 4.7%—60.7% of the total culturable bacteria,and these percentages increased and decreased with the degree of sand dune fixation,respectively.This result implied that Arthrobacter may play a significant role during preliminary soil crust formation.The results also showed that the quantity of culturable bacteria was positively correlated with the concentration of total soil carbon and total soil nitrogen(P0.05),but negatively correlated with pH values(P0.05).These results showed that microorganism number was closely related with the improvement status of desertification.Such results are likely to be caused by the interaction between soil microorganisms and soil carbon,nitrogen and organic acid.Based on 16S rDNA gene sequences and the phylogenetic tree,the culturable bacteria in the study area belonged to 18 genera: Acinetobacter,Afipia,Arthrobacter,Bacillus,Balneimonas,Brevundimonas,Chitinophaga,Chryseobacterium,Flavobacterium,Herbaspirillum,Janthinobacterium,Lysobacter,Microbacterium,Paenibacillus,Paracoccus,Phyllobacterium,Sphingomonas and Variovorax,and fell into six phylogenetic groups: high G+C Gram positive bacteria,low G+C Gram positive bacteria,α-Proteobacteria,β-Proteobacteria,γ-Proteobacteria and Cytophaga-Flavobacterium-Bacteroides(CFB) group bacteria.Such a culturable bacterial community structure was similar with previous studies of desert ecosystems.Similar results also showed that the high proportion of G+C Gram positive,low G+C Gram positive and Proteobacteria,and a low proportion of CFB group bacteria could be the main bacterial community structure in the desert ecosystem.Blast results also showed that these bacterial strains had a variety of functions.Some strains showed accelerated function for the restoration of desertified areas.For example,Afipia,Balneimonas,and Herbaspirillum had close relationships with nodule formation in plants.In contrast,some isolates in our study could also inhibit the restoration of desertified areas;some bacteria were plant pathogens,and some were identified as aerobic denitrifying bacteria.Besides these,pathogenic microorganisms and bacterial strains with potential applications were also found in desertification soil.The results of this study add to our knowledge of microorganism usage during the management of desertification.

In desert ecosystems,low soil moisture and high soil alkalinity decrease phosphorus(P) bioavailability,which results in a scarcity of soil P.Low soil P availability limits plant growth,development and reproduction,and consequently P is one of the most important limiting factors for vegetation growth and restoration in desert ecosystems.Furthermore,productivity of most plant communities increases following nutrient addition,while species diversity shows different responses to nutrient addition.Nevertheless,little attention has been directed towards identification and quantification of P pools in water-limited desert ecosystems,and results of P fertilization experiments in non-arid regions are not applicable to arid regions receiving 200 mm annual precipitation.The aim of this study was to understand the effects of artificial P addition on the vegetation structure of herbage synusia and to offer a solid basis for long-term restoration and scientific management of desert ecosystems.We selected the herbaceous vegetation in steppe desert on the southeastern margin of the temperate Tengger Desert,northern China,as a case study and collected data through field surveys after artificial addition of P fertilizer in spring.Phosphorus fertilizer was added in the first year,in which the annual precipitation was 271 mm(1.4 times the average annual precipitation).In the second year,no P was applied and the annual precipitation was 127 mm.The species,number of individuals,height and coverage were recorded monthly.Using these data,we analyzed the response in species richness,abundance,coverage,biomass and the height of frequent species of the desert herbaceous vegetation community to artificially amended P under ambient precipitation.Species richness and abundance decreased gradually with increasing level of P fertilization in the first year,and both variables did not change significantly in response to 12.5 and 25 g/m2 fertilization(P 0.05),but decreased significantly with 50 g/m2 fertilization treatment,compared with the control(P 0.05).Coverage and aboveground biomass increased gradually with the increase in P fertilization level in the same year,and in the 50 g/m2 fertilization treatment the coverage and aboveground biomass increased by 47.59% and 360.49%,respectively,compared with the control(P 0.05).In the second year,species richness,abundance,coverage and aboveground biomass all showed no significant difference in the different fertilization treatments compared with the control(P 0.05).The plant community composition changed markedly following P addition,with species number decreasing from 12 to 6 in the first year and to 5 in the second year.Addition of P decreased root biomass of the herbaceous plants.Artemisia capillaris Thunb.,Cleistogenes songorica(Roshev.) Ohwi,Allium polyrhizum Turcz.ex Regel and Tragus berteronianus Schult.showed different responses in plant height to P addition under different water-nutrient regimes.It is inferred that P addition is able to decrease species diversity and improve plant productivity of the desert herbaceous vegetation community in a moist year,whereas low annual precipitation inhibits the residual effect of P fertilizer in the subsequent drier year,which resulted in no significant effect on species diversity and population productivity.Generally,our results indicate that water is the most important abiotic factor for the growth of the desert herbaceous vegetation,and that water and nutrient are colimiting factors in the desert ecosystem.

The Ordos plateau is a relatively independent physiographic unit and a complex, quite sensitive ecotone in the semi-arid zone of north China. Shrubs are the dominant plant life-form of the region. With a progressive decrease in moisture gradient from desert steppe to steppe desert, the ecological replacement of shrub communities is obvious. This paper deals with the distribution characteristics of shrub community diversity with environmental gradient changes and analyses the influences of various environment factors on shrub community diversity in the region. The results from this study suggest that the diversity of shrub communities depends on their spatial structure, in particular the composition of the herb layer. Shrub community diversity gradually decreases from desert steppe to steppe desert. Among various factors (geographic co-ordinates, annual mean temperature, coldest monthly mean temperature, annual mean evaporation, annual precipitation, sunshine percentage, annual mean wind velocity and annual mean number of sandstorm days, etc.), moisture gradient change is the greatest factor affecting shrub community diversity in the region, but its significance is low. It is possible that the influences of overgrazing on the shrub communities far exceed the impact of moisture gradient change.

Numerous hypotheses and conceptional models dealing with the grassland deserti- fication or degradation processes recognize that the invasion of shrubs in grasslands is the most striking feature of the variation of vegetation patterns in the grassland degradation or desertifica- tion processes in arid and semiarid regions. This is because the invasion of shrubs in grasslands increases the heterogeneity of the temporal and spatial distribution of primary vegetation and soil resources. As a result, the biological processes of the soil-vegetation system are increasingly concentrated in the "fertile islands" under shrub canopies, and the soil between shrubs gradually turns into bare area or moving sand under the influences of prolonged wind and water erosion. Most of relative researches support this bio-ecological interpretation for the degraded process of grassland. However, as viewed from the other aspect, the shrub vegetation distributed in patches also serves as the "trigger spots" for the grassland restoration or desertification reversion, and this has been demonstrated by the practices of combating desertification in China. Nearly 50 years of succession of artificial sand-binding vegetation in the Shapotou area and the regional restoration of eco-environment are the theoretical verification and successful example for the desertification reversion. The establishment of artificial vegetation in the region began with the installation of sand fences and planting xerophytic shrubs relying on less than 200 mm of annual precipitation under the non-irrigation condition, this made the moving sand, an originally uni- formly distributed soil resource, occur the variation of spatial heterogeneity. Through the redis- tribution of precipitation and dustfall by the canopy of xerophytic shrubs, litter accumulation and cryptogamic crust development, soil-forming processes under shrub canopies were accelerated; in the meantime, it also created a favorable condition for the invasion and colonization of annual and perennial plant species. However, with the depletion of soil moisture in the deep layer in the sand stabilization area the coverage of shrubs in the sand-binding vegetation lowered from the highest value of 33% to 6%, the dominant position and leading effect of shrubs in the communi- ties were weakened, furthermore they were gradually taken out from the vegetation composition. This correspondingly weakened the spatial heterogeneity of soil resource distribution. The propagation of numerous cryptogams on fixed sand surface and the colonization of annual and perennial plant species promoted the succession and restoration of the vegetation towards herb-dominated vegetation, which are similar to the primary vegetation types of the adjacent steppified desert and desert steppe. This paper, taking nearly 50 years of succession of sand-binding vegetation in the Shapotou region as an example and using the geostatistical