Effects of sand burial on biomass,chlorophyll fluorescence and extracellular polysaccharides of man-made cyanobacterial crusts under experimental conditions

Soil cyanobacterial crusts occur throughout the world, especially in the semiarid and arid regions. It always encounters sand burial, which is an important feature of mobile sand dunes. A greenhouse study was conducted to determine the effects of sand burial on biomass, chlorophyll fluorescence and extracellular polysaccharides of man-made cyanobacterial crusts in six periods of time (0, 5, 10, 15, 20 and 30 d after burying) and at five depths (0, 0.2, 0.5, 1 and 2cm). The results indicated that with the increase of the burial time and burial depth extracellular polysaccharides content and Fv/Fm decreased correspondingly and there were no significant differences between 20 and 30 burial days under different burial depths. The degradation of chlorophyll a content appeared only at 20 and 30 burial days and there was also no significant difference between them under different burial depths. It was also observed a simultaneous decrease of the values of the Fv/Fm and the content of extracellular polysaccharides happened in the crusted cyanobacterium Microcoleus vaginatus Gom. It may suggest that there exists a relationship between extracellular polysaccharides and recovery of the activity of photosystem II (PS II) after rehydration.     

黄土高原北部水蚀风蚀交错区苔藓多样性及苔藓结皮发育的微生境特征

微生境因子对苔藓结皮的物种分布和发育有重要调控作用。为探明其作用途径和影响程度,以黄土高原水蚀风蚀交错区的苔藓结皮为研究对象,通过样线法调查了8种微生境中苔藓结皮的物种分布和发育特征,结合结构方程模型剖析了植物冠层覆盖、坡向和坡度对苔藓多样性和苔藓结皮发育的影响途径并量化了其影响系数。结果表明: 1)与无冠层相比,冠层覆盖使苔藓的Patrick、Shannon、Pielou和Simpson指数分别降低了63.4%、66.6%、91.0%和68.3%,但使苔藓结皮的厚度、生物量和叶绿素含量分别提高了0.5、0.2和1.3倍;2)北坡样地的4种苔藓物种多样性指数较南坡分别提高了0.6、0.9、5.6和0.9倍,而3种苔藓结皮发育指标分别提高了0.3、0.3和0.6倍;3)坡度从14°增加到34°,4种苔藓多样性指数分别降低了59.8%、84.1%、57.3%和68.0%,而3种苔藓结皮发育指标分别降低了15.2%、25.0%和16.5%;4)3种微生境因子对苔藓多样性和苔藓结皮发育的影响程度为冠层覆盖＞坡向＞坡度,其主要影响途径因微生境因子不同而存在差异。综上,植物冠层覆盖、坡度和坡向分异通过直接和间接途径显著影响了苔藓的物种分布和苔藓结皮发育,因此,在利用苔藓结皮进行荒漠化治理时应充分考虑微生境条件。     

Recovery of lichen-dominated soil crusts in a hyper-arid desert

Soil crust lichens can be the dominant vegetation in desert regions that are unsuitable for higher plants, and are vital to soil stabilization and primary production. Biological soil crusts are vulnerable to disturbance and there is little evidence of the lichen components achieving full recovery following human disturbances in semi-arid to arid environments, and no records of recovery in hyper-arid deserts. Eight sites with varying anthropogenic, mechanical disturbance regimes were assessed in the hyper-arid Namib Desert for levels of recovery and successional convergence, based on a comparative analysis of overall lichen cover and community composition in disturbed and control locations. Recovery time estimations ranged from 5 to 530 years, with no detected linear relationship to impact gradient (low to high impact). Variables that were found to most strongly influence recovery rates were the overall cover of lichen growth and total number of lichen species in the bordering undisturbed areas, followed by the extent of soil compaction in the disturbed area, altered soil surface microrelief and vitality of subsurface soil crust components. An assessment of pioneering species demonstrated a link between increased soil depressions, i.e. track ruts, and the occurrence of fragmenting, wind-dispersing species. Track ruts in hype-arid deserts are not as vulnerable to the water erosion found in less arid deserts, and may be advancing recovery by trapping fragments. However, the lichen community structure was significantly different between all of the disturbed and control areas, regardless of the recovery phase, suggesting that while the lichen community composition may not. The ecological consequences of such disturbances may be far reaching in hyper-arid deserts where lichens are primary heterotrophs soil stabilizers. Given the economic development occurring within coastal hyper-acid deserts of the world, these impacts undoubtedly call for conservation attention.     

Plant recovery techniques do not ensure biological soil‐crust recovery after gypsum quarrying: a call for active restoration

Biological soil crusts (biocrusts) are a key component of dryland ecosystems worldwide. However, large extensions of biocrusts are disturbed by human activities, gypsum quarry being an outstanding example. Restoration techniques applied have offered satisfactory results for vascular plants but they could greatly differ in promoting biocrust recovery. A basic question remains unaddressed: can measures for plant recovery accelerate or promote the recovery of biological crusts? We have examined eight different situations: undisturbed natural habitat, five treatments with no restoration measures (overgrazed area, abandoned quarry, topsoil removal from natural habitat, and two areas filled with gypsum mining spoil), and 2 areas receiving restoration measures (manual sowing and hydroseeding). We took 40 soil cores to determine cover of lichen, moss, and cyanobacteria. Biocrust richness and cover were higher in the undisturbed habitat, with remarkable differences for the different components among treatments. Cyanobacteria were well represented in all the cores (restored and non‐restored). Mosses were promoted the most by hydroseeding. Lichen cover was remarkably higher in undisturbed samples, very low in the quarry abandoned in 1992, and 0 in the rest. Complete spontaneous recovery of biocrusts was inefficient in the 25‐year period examined. Plant restoration measures could speed up its recovery comparing with non‐restored areas. Cyanobacteria and mosses can spontaneously recover fairly well. However, promoting them would accelerate the appearance of lichen. For lichen, inoculation or translocation of lichen thalli might be proposed. Therefore, our results call for the inclusion of active restoration measures of biocrust components in recovery plans, especially for lichens.     

外源输入碳在生物结皮土壤各碳组分中的分配特征

研究外源新输入碳进入生物结皮后在各碳组分间的分配特征,可以为理解生物结皮参与碳地球化学循环过程提供数据支持和理论依据。本研究针对黄土高原典型苔藓生物结皮,借助¹³C脉冲标记技术,精确示踪外源新输入碳在生物结皮碳组分中的分配特征及其与无结皮裸地的差异,揭示生物结皮对碳循环的影响。结果表明: 1)由于生物结皮养分循环速率较慢,且与维管束植物相比,其主要生物成分苔藓的生物量有限,导致生物结皮各碳组分的¹³C丰度值均随时间变化表现相对平稳。2)生物结皮的各碳组分¹³C含量均明显高于无结皮裸地,其有机碳、微生物生物量碳、可溶性有机碳中¹³C含量平均分别为0.258、0.078、0.004 mg·kg^-1,分别比裸地高3.1、18.5、2.6倍,且苔藓植株¹³C含量高达1.45 mg·kg^-1。3)生物结皮改变了有机碳各组分的分配特征,其新同化的碳主要分配于活性有机碳库和结皮生物中,表现为¹³C在微生物生物量碳中的分配率(30.6%)高于可溶性有机碳(1.7%),而苔藓植株的¹³C分配率为20.3%。4)生物结皮中微生物生物量¹³C的转移量和库容量分别是裸地的15.7和19.5倍,但其周转率(每月2.94次)略低于裸地(每月3.30次),相应周转期是裸地的1.1倍。综上,生物结皮改变了土壤有机碳组分的分配特征,提升了碳周转速率,在干旱荒漠生态系统碳循环中的作用不容忽视。     

陕北黄土高原生物结皮中7种微藻的形态与分子生物学鉴定

陕北黄土高原实施退耕还林后,生物结皮成为其典型的地表覆盖类型,含有丰富的土生藻类,对固定土壤和促进养分循环具有极其重要的作用。该研究通过平板法与水滴稀释法对陕北黄土高原生物结皮土生藻类进行分离培养,采用光学显微镜观察结皮微藻的形态特征,并对单藻种进行分子鉴定,为黄土高原生物结皮藻类的研究奠定基础。结果显示:(1)共纯化获得7种结皮藻类,经光学显微镜初步确定,其中5株为绿藻、2株为蓝藻。(2)5株绿藻SM-2-1、DB-2-1、DB-2-2、SD-1和SD-2的序列长度分别为664 bp、663 bp、662 bp、589 bp和688 bp,GC含量分别为33.43%、49.47%、50.15%、50.76%和51.01%;2株蓝藻YJ-3、YJ-2的序列长度分别为570 bp和465 bp,GC含量分别为46.31%和49.03%。(3)序列比对并构建系统树分析发现,5株结皮绿藻可分为4个分支,分别为栅藻科(Scenedesmaceae)2株(DB-2-1和SM-2-1)、衣藻目(Chlamydomonadales)、环藻科(Sphaeropleaceae)、真眼点藻科(Eustigmataceae),但5株绿藻的5.8S+ITS2序列在属内差异小、非常保守,极易确定到属;2株蓝藻YJ-3和YJ-2聚在同一大类分支的伪鱼腥藻科(Pseudanabaenaceae)中,但分别归于不同的属。研究认为:5株结皮绿藻中DB-2-1藻株是Scotiellopsis属的一种、SM-2-1归于尖带藻属(Acutodesmus)、DB-2-2可能是红球藻属(Haematococcus)的一物种、SD-1是Ankyra属的一物种、SD-2归于真眼点藻属(Eustigmatos);2株生物结皮蓝藻中YJ-3可能是伪鱼腥藻科的一新物种、YJ-2可能是细鞘丝藻属(Leptolyngbya)亲缘关系较近的一新物种。     

Warming reduces the growth and diversity of biological soil crusts in a semi-arid environment: implications for ecosystem structure and functioning

Biological soil crusts (BSCs) are key biotic components of dryland ecosystems worldwide that control many functional processes, including carbon and nitrogen cycling, soil stabilization and infiltration. Regardless of their ecological importance and prevalence in drylands, very few studies have explicitly evaluated how climate change will affect the structure and composition of BSCs, and the functioning of their constituents. Using a manipulative experiment conducted over 3 years in a semi-arid site from central Spain, we evaluated how the composition, structure and performance of lichen-dominated BSCs respond to a 2.4°C increase in temperature, and to an approximately 30 per cent reduction of total annual rainfall. In areas with well-developed BSCs, warming promoted a significant decrease in the richness and diversity of the whole BSC community. This was accompanied by important compositional changes, as the cover of lichens suffered a substantial decrease with warming (from 70 to 40% on average), while that of mosses increased slightly (from 0.3 to 7% on average). The physiological performance of the BSC community, evaluated using chlorophyll fluorescence, increased with warming during the first year of the experiment, but did not respond to rainfall reduction. Our results indicate that ongoing climate change will strongly affect the diversity and composition of BSC communities, as well as their recovery after disturbances. The expected changes in richness and composition under warming could reduce or even reverse the positive effects of BSCs on important soil processes. Thus, these changes are likely to promote an overall reduction in ecosystem processes that sustain and control nutrient cycling, soil stabilization and water dynamics.     

Impact of grazing intensity on seasonal variations in soil organic carbon and soil CO2 efflux in two semiarid grasslands in southern Botswana

Biological soil crusts (BSCs) are an important source of organic carbon, and affect a range of ecosystem functions in arid and semiarid environments. Yet the impact of grazing disturbance on crust properties and soil CO₂ efflux remain poorly studied, particularly in African ecosystems. The effects of burial under wind-blown sand, disaggregation and removal of BSCs on seasonal variations in soil CO₂ efflux, soil organic carbon, chlorophyll a and scytonemin were investigated at two sites in the Kalahari of southern Botswana. Field experiments were employed to isolate CO₂ efflux originating from BSCs in order to estimate the C exchange within the crust. Organic carbon was not evenly distributed through the soil profile but concentrated in the BSC. Soil CO₂ efflux was higher in Kalahari Sand than in calcrete soils, but rates varied significantly with seasonal changes in moisture and temperature. BSCs at both sites were a small net sink of C to the soil. Soil CO₂ efflux was significantly higher in sand soils where the BSC was removed, and on calcrete where the BSC was buried under sand. The BSC removal and burial under sand also significantly reduced chlorophyll a, organic carbon and scytonemin. Disaggregation of the soil crust, however, led to increases in chlorophyll a and organic carbon. The data confirm the importance of BSCs for C cycling in drylands and indicate intensive grazing, which destroys BSCs through trampling and burial, will adversely affect C sequestration and storage. Managed grazing, where soil surfaces are only lightly disturbed, would help maintain a positive carbon balance in African drylands.     

Experimental Studies on the Natural Restoration and the Artificial Culture of the Moss Crusts on Fixed Dunes in the Tengger Desert, China

Soil microbiotic crusts cover extensive portions of the arid and semiarid regions of the world and play an important ecological role. Moss is one of the major components in the crusts. The reproduction and establishment of the mosses are crucial to the formation of moss crusts. Bryum argenteum is the dominant species of moss crusts in the Shapotou region (104°57′E, 37°27′N) of the Tengger Desert. In search for the characteristics of natural reproduction and establishment of the mosses, 10 quadrates (10×10 cm for each) were obtained by removing the moss crusts in different positions of fixed dunes. These 10 quadrates were observed for 3 years depending on the species’ components and coverage. Meanwhile, in the third year, two quadrates (1×1 m for each) were set up in a crust-absent area and two different experiments of the asexual reproduction (broadcast planting and offshoots) were conducted, respectively. The reproductive process was observed under the microscope, and the morphological indicators of the new individuals were measured. The results were compared with the ones from indoor experiments using the same methods. All the results showed the following: (1) 70% of the quadrates (i.e., 7 of the 10 quadrates) were recovered within 3–4 years; thus, the quick recovery might be due to the dispersal and reproduction of the fragments of stems and leaves of B. argenteum; (2) as for the two quadrates in the artificial reproduction test, the new plants occupied the uncovered space of the quadrates in 1 month, and there were two main reproduction approaches, one of which was that the stems continually branched and produced young plants, and the other was that the young plants and the fragments of the stems and leaves repeatedly and extensively reproduced protonema, which finally developed into a large number of new plants; (3) the reproductive characteristics were identical, though the protonema in the field was more robust and had more branches than the ones indoors. Translated from Acta Phytoecologica Sinica, 2005, 29(1) (in Chinese)     

Small scale spatial heterogeneity of Normalized Difference Vegetation Indices (NDVIs) and hot spots of photosynthesis in biological soil crusts

Normalized Difference Vegetation Indices (NDVIs) are typically determined using satellite or airborne remote sensing, or field portable spectrometers, which give an averaged signal on centimetre to metre scale plots. Biological soil crust (BSC) patches may have smaller sizes, and ecophysiological, hydrological as well as pedological processes may be heterogeneously distributed within this level of resolution. A ground-based NDVI imaging procedure using low-cost equipment (Olympus Camedia 5000z digital camera equipped with a Hoya R72 infrared filter) was developed in this study to fill this gap at the level of field research, where carrying costly and bulky equipment to remote locations is often the limiting factor for data collection. Method principle and field data are presented, and the field experiment was deepened comparing NDVI measurements and CO₂ turnover of soil crust samples in the laboratory, backing the reliability of the approach.