Relationships between the biomass of algal crusts in fields and their compressive strength

In the desert areas of China investigated by the authors, various biological crusts were predominately associated with three blue-green algal (cyanobacterial) species, Microcoleus vaginatus Gom., Phormidium tenue (Menegh.) Gom. and Scytonema javanicum (Kütz.) Born et Flah. Their biomass and their compressive strength were measured simultaneously in the field in this study. It was also found that the compressive strength of algal crusts was enhanced with the increasing of algal biomass from an undetectable level to a value as high as 9.6 mg g⁻¹ dry soil. However, when the algal biomass decreased, the compressive strength did not descend immediately, but remained relatively steady. The higher the algal biomass became, the thicker were the algal crusts formed. Given the same biomass, the highest compressive strength of man-made algal crusts in fields was found at an algal ratio of 62.5% M. vaginatus, 31.25% P. tenue and 6.25% S. javanicum, and it reached 0.89 kg cm⁻². When the biomass of the crusts increased above the value of 8.16 mg chla g⁻¹ dry soil, the compressive strength would not ascend easily. It indicated that the compressive strength of man-made algal crusts appeared temporarily saturated in the field.     

干旱与重吸水对人工藻结皮光合特性的影响

通过接种蓝藻构建人工藻结皮促进荒漠地区生物结皮的生长发育以及整个荒漠生态系统的恢复,被认为是一种行之有效的荒漠化控制方法。在构建藻结皮过程中,接种的蓝藻以及形成的藻结皮,经常遭受干旱等环境条件的胁迫。本研究通过室内人工构建藻结皮,对形成的藻结皮进行干旱及重吸水处理,在此过程中监测结皮光合生物量、多糖含量以及结皮光合特性的变化规律。结果发现,干旱处理1d后,结皮蓝藻胞外多糖的分泌明显增加(p0.05);当结皮完全进入干燥状态后,结皮蓝藻停止所有代谢活动,结皮生物量及胞外多糖含量保持在一个相对稳定的水平。在结皮重吸水后,结皮初始荧光Fo能够迅速恢复,并在10 min内达到最大;之后Fo逐渐下降,同时结皮光合活性(Fv/Fm)按照函数y=ax/(b+x)逐渐上升并达到稳定。此外,在不同水分条件下,结皮光合活性随着水分的增加逐渐增加;然而结皮净光合速率(Pn)却随着水分的增加先增加后又下降,呈现单峰变化模式。该研究表明结皮蓝藻的代谢调节在人工藻结皮适应荒漠干旱环境中具有重要的作用,这对于进一步理解蓝藻乃至蓝藻结皮对干旱胁迫的适应,以及荒漠化防治中人工藻结皮的构建、维护、管理具有重要的理论与实践意义。     

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.     

Wind velocity perturbation of soil respiration measurements using closed dynamic chambers

Soil respiration measurements performed with closed dynamic chambers are very sensitive to pressure differences inside and outside the chamber: differences as small as 1 Pa can induce errors that are of the same magnitude as the flux itself. The problem is usually solved by adding a vent to the experimental set-up. However, although this may give acceptable results in most cases, it is not effective at sites that are very exposed to wind. At the CarboEurope–IP agricultural site of Lonzée (Belgium), on bare soil, we used a vent composed of a vertical tube whose upper end was placed between two horizontal plates. Whilst this system worked properly in low-wind conditions, it led to a significant flux over-estimation (up to 300%) under strong wind conditions. We analysed the causes of this error and attributed it to a dynamic pressure effect at the vent, leading to air aspiration from within the chamber. We suggest that this is because the wind at the vent level was not the same as that experienced in the chamber, because of the large vertical wind speed gradient close to the soil. Another vent geometry was then proposed that positioned the vent at the chamber level. This new design was tested on both manual and automatically operated chambers. It was found to be efficient in windy conditions as most of the artificial correlation between soil respiration measurements and wind speed had disappeared.     

Soil crusts and deposits as sheet erosion indicators in southern Mali

Rills are indicators of erosion, easily recognized by farmers and extension workers. However, they are rare on fields in Mali with slopes of 0–3%, even though run‐off and sheet erosion may be a problem. The suitability of three other soil surface features as erosion indicators was therefore investigated: (1) structural crusts formed by rainfall impact without lateral soil movement; (2) in situ depositional crusts, formed by sedimentation leaving the finest particles on top; and (3) run‐off deposits of fine and coarse sand, formed after the removal of finer particles. Feature (1) occurred on the elevated parts and ridges, whereas (2) and (3) occurred in lower parts and in furrows. Soil cover with a run‐off deposit of coarse sand proved to be a suitable indicator and this was well explained by erosion risk: a 2% increase in slope related to a 9% increase in deposit cover. The extent of this deposit also related well to cotton yield from the previous season: a 30% increase in deposit cover was reflected in a 23% decrease in cotton yield. Only a few farmers confirmed that the deposits were produced by erosion. It is recommended that soil and water conservation programmes pay more attention to the presence of crusts and deposits.     

Types of recent weathering crusts and their global distribution

Recent weathering crusts are defined as those formed and in process of formation under present or closely similar climatic conditions. Such crusts are classified on the basis of their composition and profile form into four types: nonleaching, weak periodic leaching, strong leaching, and freezing. Each but the last of these types is divided into a pair of subtypes according to the intensity of eluvial processes. All subtypes are divided into lithogenic groups reflecting the composition and properties of the parent materials.Recent weathering crusts of the nonleaching, weak periodic leaching, and freezing types are the most extensive and widespread over the world. Crusts of the strong leaching type, especially of the extra-humid subtype, are being formed only to a limited extent at the present time, although they were formed extensively in past geologic periods. Sedimentary parent materials with high proportions of quartz-kaolinite and goethite-gibbsite-kaolinite minerals, inherited from earlier periods, are common in many regions with arid and semi-arid climates at the present time.The potential for intensive weathering under humid and extra-humid, tropical climates has been realized in few recent weathering crusts because of the presence on the land surface of former weathering crusts that were subjected to strong hypergenic processes in the past. Where fresh parent rocks are exposed in small areas under such climates, profound weathering does now occur with the formation of new minerals.     

Soil invertebrate activity in biological crusts on tropical inselbergs

Granite inselbergs protrude from forest and savanna in the tropics. They are exposed to harsh climates (alternation of heavy rain and severe drought) and provide little nutrient for plants. Soil animals and humus components were investigated in cyanobacterial crusts close to patches of epilithic vegetation on the surface of the Nouragues inselberg (French Guiana). Three biological crust samples, corresponding to bromeliacean carpets of increasing size (supposed of increasing age), were sampled for faunal and micromorphological studies. Arthropods (mainly mites and insects) were abundant and highly diversified, the more so after enchytraeid worms ate and transformed the cyanobacterial mass. Below the superficial cyanobacterial crust, humus was made of a loose assemblage of enchytraeid faeces where these animals were present, or of a compact assemblage of cyanobacteria and amorphous organic matter where mites were the dominant animal group. Roots abounded in the humified part of the crust. We conclude that soil invertebrates, in particular enchytraeid worms, are important for the accumulation of organic matter on granite outcrops, and so therefore for the encroachment of plant succession.     

Vegetation changes and water catchment in a Dutch coastal dune area

Studies on vegetation and landscape changes resulting from water catchment in Dutch coastal dunes are briefly reviewed and discussed with special reference to Meijendel, an important catchment area near the city of The Hague. The water works mainly concern the extraction and artificial replenishment of ground water. Main ecological impacts involve lowering of the ground water table (after extraction) and (after replenishment) establishment of an unnatural ground water table and eutrophication. Main vegetation changes include drying out of dune wetlands and, after replenishment, increased nitrophilous herb cover associated with a decrease in plant species diversity and life form diversity. Management options largely aim at reducing the nutrient load of the ecosystem and regeneration of original hygrosere communities.     

Microbial aggregation of sand in an embryo dune system

The importance of microorganisms in the aggregation of sand in an embryo dune system was examined. Of three main types of aggregates formed, microbial aggregates were found to be more important at stabilizing sand than either root-microbial or debris-microbial aggregates on the beach and at the edge of the dune but roots and their associated microorganisms were more important on the hummock of the dune. The amount of microbial and root-microbial aggregated sand was higher during the winter when the vegetation was dying down and decaying. In the absence of roots, microorganisms, in particular bacteria, play a major role in aggregating sand. The bacteria are well adapted to the unfavourable habitat of the beach as they are motile and tolerant to sea water, being able to grow in a salinity of 3.5%. Bacteria may play a major role in aggregating and stabilizing sand prior to colonization by higher plants.     

Microbial aggregation of sand in a maritime dune succession

Aggregation of sand by microorganisms was assessed in a dune succession ranging from unstable foredunes to stable fixed dunes. The microorganisms isolated from aggregates were fungi, bacteria, actinomycetes and algae, including cyanobacteria. Aggregation increased as the dunes became stabilized and the higher plant succession developed. The number and weight of aggregates were interrelated and most aggregates were in the > 1 <1.4 and > 2 mm ranges. The smaller aggregates were colonized by fewer fungal species than the larger ones. The general aspects of microbial aggregation in sand dunes are discussed.     

Effects of drought and salt stresses on man-made cyanobacterial crusts

As a primary successional stage of biological soil crusts (BSCs), cyanobacterial crusts form firstly in the arid and semiarid areas. At the same time, they suffer many stress conditions, such as drought, salt, etc. In this study, we constructed man-made cyanobacterial crusts with Microcoleus vaginatus Gom. and comparatively studied the effects of drought and salt stresses on the crusts. The results showed that crust growth and photosynthetic activity was significantly inhibited by the stress conditions (P < 0.05), and inhibitory effect increased with the increasing stress intensity and treated time. Compared with salt stress, drought completely stopped crust metabolic activity, so the crust biomass was conserved at a higher level, which meant that drought itself might provide the crusts some protection, especially when the crusts simultaneously suffered drought and salt stresses. That is very important for the survival of crusts in the high-salt areas. In addition, to some extent the crusts could adapt to the stress conditions through metabolic adjustment. In our experiment, we found the accumulation of exopolysaccharides (EPS) increased under stress conditions within a certain threshold.     

Properties and spatial distribution of microbiotic crusts in the Negev Desert,Israel

Playing a cardinal role in surface stabilization and in carbon and nitrogen fixation, microbiotic crusts play a crucial role in arid regions where they may serve as useful biomarkers for wind power and wetness duration. This is especially the case on relatively unstable and infertile sand dunes in the Negev Desert where high correlations between the crust chlorophyll content and the daytime wetness duration were found. Yet, only scarce data are available as to the possible link between the chlorophyll content and other physical (color, thickness, strength, crack density, surface roughness and infiltrability) and biological (protein, carbohydrate, organic matter and species composition) factors, which determine, in turn, the crust type and its effect upon geomorphological and ecological processes. No data are available on crust type distribution. These were the aims of the current research.     

Heterotrophic N2-fixation in arid soil crusts

The cryptogamic soil crusts of the Great Basin Artemisia, Ceratoides, and Atriplex plant communities contain a significant heterotrophic N₂-fixing microbial population in addition to the predominating filamentous cyanobacteria. The bacterial association with the cyanobacteria exhibits a phycosphere-like effect. Heterotrophically fixed N gains reached 17.5 μg N· g^?1 of soil (23.1% increase above the initial soil N content) and 45.9 μg N·g^?1 of soil (57.4% increase) after 3 and 5 weeks, respectively. (NH₄)₂SO₄ and native plant material amendments to soil resulted in a 41–100% reduction in N₂-fixation. The potential input of N to soil crusts may be reduced in the presence of shrub-produced allelochemic agents and by concurrent denitrification.     

Ciliates in coastal dune soils of different stages of development

Abundance and diversity of soil ciliates were studied in the dunes of the North Sea island Norderney (D). Live counts of ciliates were carried out in spring and fall 1999. The eight examined sites show a sequence from freshly thrown sand beach to heath, including primary dunes, white dunes, gray dunes, bushes and forest. Ciliate abundance was low (5–13 ind g^–1 dm) in the young sites without humus horizon (beach, primary and white dune), with dominance of hymenostomes and hypotrichs. The dune soils in older stages showed high abundance, between 39 and 264 ind g^–1 dm. Here, the Colpodea were the dominant group. Bacterial feeders were highly dominant at all sites. While in young sites nano- and microtrophic ciliates were important, in the older sites the community was much more diverse with fungivorous and omnivorous ciliates in addition. By using ordination methods, the young soil sites separated as a group, but the older sites, which differ clearly in community structure, did not. The results show the importance of organic matter as a factor for abundance and species richness of soil ciliates.     

科尔沁沙地固定沙丘土壤氮素空间分布特征研究

为了研究固定沙丘土壤N素空间分布特征,选择以栽植小叶锦鸡儿25年后的固定沙丘为研究对象,从迎风坡、顶坡和背风坡3个位置4个层次(0～5、5～10、10～20和20～40 cm)进行取样分析.研究结果表明:全N、NO_3~--N和NH_4~+-N含量均随着土层加深而呈现出减少的趋势,0～5 cm土层显著高于其他各层.表层土壤受凋落物的影响较大,从而相对于深层土壤来说积累了更多的N素.全N、NO_3~--N和NH_4~+-N含量在不同坡位间存在显著差异(p＜0.01):全N和NO_3~--N含量在迎风坡较高,而NH_4~+-N含量在背风坡较高.丛下全N、NO_3~--N和NH_4~+-N的含量显著高于丛间(p＜0.01).土壤电导率与全N、NO_3~--N、NH_4~+-N含量呈显著正相关,而pH与NO_3~--N、NH_4~+-N含量呈显著负相关,NO_3~--N、NH_4~+-N的富集降低了土壤pH值.小叶锦鸡儿的栽植对沙土改良具有重要意义.     

Dinitrogen fixation by biological soil crusts in an Inner Mongolian steppe

Eurasian steppe ecosystems are nitrogen-limited and suffer additionally from high grazing intensities in many areas. Soil surface-bound cyanobacteria are able to fix nitrogen and can be the major source of plant available nitrogen in such ecosystems. In this study, the abundance and dinitrogen fixation capacity of the most common soil surface-bound microbial and lichen species were determined at an ungrazed, a winter-grazed, and a heavily grazed steppe site in the Xilin River catchment, Inner Mongolia, People’s Republic of China. The microorganisms were identified as Nostoc spec. and the lichen species as Xanthoparmelia camtschadalis (Ach.) Hale by a combination of classical light microscopy, confocal laser scanning microscopy and molecular analysis of the internal transcribed spacer (ITS1) region of ribosomal RNA. Both species were found exclusively at grazed steppe sites, with a clear difference in abundance depending on the grazing intensity. At the winter-grazed site, Nostoc was more abundant than Xanthoparmelia; for the heavily grazed site, the opposite was found. N₂ fixation was quantified with both the acetylene reduction method and ¹⁵N₂ incubation. Cyanobacterial colonies of Nostoc fixed N₂ vigorously, whereas X. camtschadalis did not at all. The fraction of nitrogen derived from the fixation of molecular nitrogen in Nostoc was 73%, calculated from ¹⁵N natural abundance measurements of Nostoc with X. camtschadalis as reference. The conservatively calculated N₂ uptake by Nostoc was 0.030–0.033 kg N ha⁻¹ for the heavily grazed site and 0.080–0.087 kg N ha⁻¹ for the winter-grazed site for the growing seasons of 2004 and 2005, respectively. Together with previous findings, this study demonstrates that N₂ fixation by Nostoc can potentially replace significant amounts, if not all, of the nitrogen lost in the form of N₂O and NO soil emissions in this steppe ecosystem.     

Origin and growth mechanisms of the sulfated crusts on urban limestone

Building stone surfaces exposed to the urban environment are subjected to rapid and differentiated deterioration, due to various chemical and physical attacks. Apart from a chemical deterioration below the sufated crusts, which mainly grow outwards at the unaltered gypsum/carbonatic rock interface, another physical deterioration was observed. The latter is, at times, even more important than the rapid chemical attack typical of the zones which are subjected to heavy washing by rainwater. The formation and growth of three different kinds of deterioration layers are discussed with reference to the main kinds of deterioration characterized by white, grey and black external appearance. Analyses of airborne particulate matter and of deterioration layers of urban monuments led to the conclusion that not all the gypsum of crusts which can be observed on marble monuments is derived from the transformation of the underlying rock, but it is also partially transported by aerosols.