Biological Soil Crusts in Arid Dunes

  • Moss-dominated biocrusts increase soil microbial abundance and community diversity and improve soil fertility in semi-arid climates on the Loess Plateau of China

Various ecological functions of biocrusts are mostly determined by their bacterial and fungal abundance and community diversity, which has not yet been fully investigated. To provide more insights into this issue, we collected samples of moss biocrusts, fixed sand, and mobile sand from a watershed with semi-arid climate on the Loess Plateau of China. The relative abundances and community diversities of soil bacteria and fungi of the samples were determined using high-throughput DNA sequencing. Finally, we analyzed the characteristics of bacterial and fungal community of the moss biocrusts and their relationships to the content of soil nutrients. Our results showed that the moss biocrusts had 1048 bacterial OTUs (operational taxonomic units) and 58 fungal OTUs, and their Shannon diversity indexes were 5.56 and 1.65, respectively. The bacterial community of the moss biocrusts was dominated by Acidobacteria (24.3%), Proteobacteria (23.8%), Chloroflexi (15.8%), and Actinobacteria (14.5%), and their fungal community was dominated by Ascomycota (68.0%) and Basidiomycota (23.8%). The moss biocrusts had far more bacterial OTUs (≥ 56.9%) but similar number of fungal OTUs as compared with the uncrusted soil, and their Sorenson’s similarity coefficients of bacterial and fungal communities were less than 0.768 and 0.596, respectively. Moreover, the contents of soil nutrients (C, N, P) were significantly correlated with the OTU numbers of bacteria and the relative abundances of bacteria and fungi. Our results indicated that moss biocrusts harbor a large number and high diversity of bacteria and fungi, and these diversified bacteria and fungi play important roles in ecosystem functioning through improving soil fertility.

Xiao, B., Veste, M. (2017): Moss-dominated biocrusts increase soil microbial abundance and community diversity and improve soil fertility in semi-arid climates on the Loess Plateau in Northern China. Applied Soil Ecology 117: 165-177.

  • Vegetation pattern in arid sand dunes controlled by biological soil crusts along a climatic gradient in the Northern Negev desert

Vegetation cover and biomass production in drylands are largely controlled by rainfall amounts on a regional and global scale. However, soil water availability on the small-scale is influenced by hydrological processes, soil types and surface properties. In the sand dunes of the north-western Negev biological soil crusts built up by cyanobacteria, green algae, mosses and soil lichens play an important role for the ecosystem processes. They are changing the surface properties. We investigated the vegetation in response to geo-ecological parameters and biological soil crusts along a rainfall gradient from 170 mm to 78 mm. In the interdunes the vegetation cover was 26-30% and showed no significant difference along the climatic gradient. Vegetation cover on the dune crests depends on the sand mobility and decreases towards the more arid parts. The biological crusts limit infiltration and counteract on the rainfall gradient. Therefore, no differences in the vegetation cover were detected and on the mesoscale level the biomass index was negatively correlated to the annual rainfall. Sand mobility and surface stability are important parameters determining the vegetation pattern. Surface properties like crust and fine material are key factors for the hydrological processes and control water redistribution on the micro-scale and, thus, vegetation pattern.

Veste, M., Breckle, S.-W., Eggert, K., Littmann, T., Basic and Applied Dryland Research 5, 1-16, 2011. (more…)

  • Differential hydrological response of biological topsoil crusts along a rainfall gradient in a sandy arid area: Northern Negev desert, Israel

Drylands are regarded as highly sensitive to climatic change. The putative positive relationship between average annual rainfall and runoff, assumed for areas between 100 and 300 mm ignores the fact that climatic change in drylands is not limited to climatic factors alone, but is often accompanied by a parallel change in surface properties. Data on rainfall, runoff and soil moisture regime were collected at five monitoring sites in a sandy area, along a rainfall gradient from 86 to 160 mm. Despite the uniform sandy substratum the frequency and magnitude of runoff declined with increasing annual rainfall. Under wetter conditions a thick topsoil biological crust develops. This crust is able to absorb and retain large rain amounts, limiting the depth to which water can penetrate, and therefore water availability for the perennial vegetation. In the drier area, the thin crust can absorb only limited rain amounts, resulting in surface runoff and deeper water infiltration at run-on areas. Our findings demonstrate the important role played by different types of biological soil crusts along the rainfall gradient considered, and question the generally held belief that higher rainfall necessarily leads to deeper water infiltration in sandy arid areas; and higher water availability for the perennial vegetation.

Yair, A., Almog, R., Veste, M., Catena 87 (3), 326-333, 2011. (more…)

  • A natural 15N approach to determine the biological fixation of atmospheric nitrogen by biological soil crusts of the Negev desert. 

Biological soil crusts are important cryptogamic communities covering the sand dunes of the north-western Negev. The biological crusts contain cyanobacteria and other free-living N2-fixing bacteria and are hence able to fix atmospheric nitrogen (N). This is why they are considered to be one of the main N input pathways into the desert ecosystem. However, up to now, in situ determinations of the N2 fixation in the field are not known to have been carried out. We examined the natural 15N method to determine the biological N2 fixation by these soil crusts under field conditions. This novel natural 15N method uses the lichen Squamarina with symbiotic green algae—which are unable to fix N2—as a reference in order to determine N2 fixation. Depending on the sampling location and year, the relative biological fixation of atmospheric nitrogen was estimated at 84–91% of the total N content of the biological soil crust. The cyanobacteria-containing soil lichen Collema had a fixation rate of about 88%. These fixation rates were used to derive an absolute atmospheric N input of 10–41 kg N ha−1 year−1. These values are reasonable results for the fixation of atmospheric N2 by the biological crusts and cyanolichens and are in agreement with other comparable lab investigations. As far as we are aware, the results presented are the first to have been obtained from in situ field measurements, albeit only one location of the Negev with a small number of samples was investigated.

Russow, R., Veste, M., Böhm, F., Rapid Communication in Mass Spectrometry 19 (23): 3451-3456, 2005. (more…) 

  • Using the natural 15N-abundance to assess the major nitrogen inputs into the sand dune area of the north-western Negev Desert (Israel)

The variation of the natural 15N abundance is often used to evaluate the origin of nitrogen or the pathways of N input into ecosystems. We tried to usethis approach to assess the main input pathways of nitrogen into the sand dune area of the north-western Negev Desert (Israel). The following two pathways are the main sources for nitrogen input into the system:

i. Biological fixation of atmospheric nitrogen by cyanobacteria present in biological crusts and by N2-fixing vascular plants (e.g. the shrub Retama raetam);

ii. Atmospheric input of nitrogen by wet deposition with rainfall, dry deposition of dust containing N compounds, and gaseous deposition.

Samples were taken from selected environmental compartments such as biological crusts, sand underneath these crusts (down to a depth of 90 cm), N2 fixing and non-N2-fixing plants, atmospheric bulk deposition as well as soil from arable land north of the sandy area in three field campaigns in March 1998, 1999 and 2000. The d15N values measured were in the following ranges: grass 2.5‰ to þ1.5‰; R. reatam: þ0.5‰ to þ4.5‰; non-N2-fixing shrubs þ1‰ to þ7‰; sand beneath the biological crusts þ4‰ to þ20‰ (soil depth 2–90 cm); and arable land to the north up to 10‰. Thus, the natural 15N abundance of the different N pools varies significantly. Accordingly, it should be feasible to assess different input pathways from the various 15N abundances of nitrogen. For example, the biological N fixation rates of the Fabaceae shrub R. reatam from the 15N abundances measured were calculated to be 46–86% of biomass N derived from the atmosphere. The biological crusts themselves generally show slight negative 15N values (3‰ to  0.5‰), which can be explained by biological N fixation. However, areas with a high share of lichens, which are unable to fix atmospheric nitrogen, show very negative values down to 10‰. The atmospheric N bulk deposition, which amounts to 1.9–3.8 kgN=ha yr, has a 15N abundance between 4.4‰ and 11.6‰ and is likely to be caused by dust from the arable land to the north. Thus, it cannot be responsible for the very negative values of lichens measured either. There must be an additional N input from the atmosphere with negative d15N values, e.g. gaseous N forms (NOx, NH3). To explain these conflicting findings, detailed information is still needed on the wet, particulate and gaseous atmospheric deposition of nitrogen.

Russow, R., Veste, M., Littmann, T., Isotopes in Environmental and Health Studies 40, 57-67, 2004. (more…

  • Dewfall and its geo-ecological implication for biological surface crusts in desert sand dunes (north-western Negev, Israel).

Dew is an important water source for biological soil crusts and lichens in arid and semi-arid ecosystems. These crusts influencing the ecosystem processes resulting in a patchy ecotope and vegetation distribution. Microclimatic boundary conditions for nocturnal wetting were determinate. Maximum activity of crust is reached a few hours later after dewfall starts when cumulative dewfall exceeds 0.1 mm at dew point temperature differences around 0 K. Different microclimatic approaches were applied to estimated dewfall amounts in sand dunes of the north-western Negev. The annual dewfall amounts obtained from the zeroplane model was 26 mm a-1 and from the load cell 33 mm a-1, whereas other models mostly overestimated the dew amounts. However, spatial differences in shading after sunrise could explain the crust pattern in the sand dunes.

Veste, M., Littmann, T., Journal of Arid Land Studies 16(3), 139-147. 2006. (more…)

  • Microclimatic boundary conditions for activity of soil lichen crusts in sand dunes of the north-western Negev desert, Israel 

Photosynthetic activity of soil crust lichens was thoroughly investigated. Its interrelations with microclimatic boundary conditions was measured during two field experiments in the central part of the sand dune field in the north-western Negev Desert. After nocturnal rainfall the lichens were active well until noon when they dried out finally. However, over most of the year dewfall seems to be the primary controlling factor for activation as in other lichen communities. The microclimatic conditions for activity were determined in detail. It was found that after sunset terrestrial radiation leads to a progressive development of a stable air layer above ground accompanied by decreasing temperatures and wind speed. Well before midnight dewpoint temperature differences drop below 1.0 K  and leaf wetness sensors indicate the formation of dew. It is exactly in this situation when lichen activity starts. Maximum activity, however, is reached a few hours later when cumulative dewfall exceeds 0.1 mm at dewpoint temperature differences around 0 K. In nights with advective labilization and subsequent dewfall evaporation, no lichen activity was observed. Even a heavy foggy night did not lead to any activity at the soil surface.

Veste, M., Littmann, T., Friedrich, H., Breckle, S.-W., Flora 196 (6), 465-476, 2001. (more…)

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