Biocrusts influencing ecosystem processes

The role of biological soil crusts on desert sand dunes of the north-western Negev (Israel)

Biological soil crusts are important microphytic communities and significantly infkuence both structures and processes within the ecosystems. They are built up from cyanobacteria, green algae, fungi, mosses and lichens. Various crust types could be found, depending on dune slope aspects and dewfall availaibity. In the sand dunes of the northern Negev they cover large areas and stabilize the sand surface against wind and water erosions. Free-living and symbiontic cyanobacteria are capable of nitrogen fixation and are important nitrogen source in the desert sand dunes. As biological soil crusts enhance the surface stability and soil fertilization, they are to be considered a key factor in the protection of arid and semi-arid ecosystems and, thus in comabting desertification in terms of combating desertification. 

Veste, M., Littmann, T., Breckle, S.-W., Yair, A. (2001): The role of biological soil crusts on  desert sand dunes of the north-western Negev (Israel). In: Breckle, S.-W., Veste, M., Wucherer, W. (eds.): Sustainable Land-Use in Deserts, Springer, Heidelberg-New York-Tokyo, pp. 357-367.  (read the paper…)

Biological soil crusts with Fulgensia fulgens und Collema tenax in the sand dunes, north-western Negev, Israel

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…)

Haluza sand dunes

Sensitivity of a sandy area to climate change along a rainfall gradient at a desert fringe

Global climate change has become a strongly and frequently addressed issue in the last decades. The aspect is crucial in dry-land areas, which cover approximately one third of the globe's total land area. The relationship between average annual rainfall and environmental variables has attracted the attention of many scientists. Climatologists use aridity indices to express relationships between climatic and environmental variables. These indices, based on purely climatic variables such as annual precipitation, temperature, evaporation and radiation, tend to imply that the acuteness of aridity is inversely related to annual precipitation. Although aware that soil water content depends on local soil type and precipitation regime, Walter (1939, 1960) asserted that at a larger, global scale, standing biomass is positively correlated to average annual rainfall. This approach is still followed by many researchers who assume a positive relationship between average annual rainfall and environmental variables such as water availability for plants, vegetation cover, productivity, species diversity, soil properties, human activity, and erosion rates for sub-humid to arid areas. This approach is certainly correct at the global scale, as well as for non-irrigated annual crops in dryland areas. It is, however, questionable for arid and semi-arid areas, usually regarded as highly sensitive to climate change, especially for perennial plants.

Yair, A., Veste, M., Almog, R., Breckle, S.-W., In: Breckle, S.-W, Yair, A., Veste, M. (eds.), Arid Dune Ecosystems – The Nizzana Sands in the Negev Desert, Ecological Studies 200, Springer, Berlin Heidelberg New York, pp. 425-440, 2008. (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…)

Nitrogen input pathways into the sand dunes: biological fixation and atmospheric nitrogen deposition

In arid and semiarid regions, water availability is considered to be the controlling factor for the productivity and pattern of vegetation. The total biotic and abiotic N pool size of desert ecosystems is lower than in most other ecosystems. Several studies have found that even in arid lands, nutrients are critical for plant growth and successions. After good rainy years, nitrogen can become the limiting factor,  whereas added nitrogen increased productivity in several experiments in dry areas. The main N input pathways into the ecosystems are atmospheric deposition in wet, dry and gaseous forms, and the biological fixation of atmospheric nitrogen N2. Biological fixation is carried out by free-living bacteria, Fabaceae—Rhizobium symbiosis and associative symbiontic free-living cyanobacteria, as well as by cyanobacteria in lichens. Another N source is by non-leguminous nitrogen-fixing species; particularly shrubs and trees play a major role in these ecosystems.

In this paper, we present field measurements of biological N2 fixation (BNF) obtained by the natural 15N abundance method, and use these to estimate the annual nitrogen input by the soil crusts and R. raetam. We follow a novel approach for the natural 15N abundance technique, by using the non-N2-fixing lichens Squamarina lentigeria and S. cartilaginea (=S. crassa) as reference in order to determine N2 fixation by the biological crust in situ in the Negev desert. N input by BNF of atmospheric nitrogen is compared with atmospheric nitrogen deposition.

Russow, R., Veste, M., Breckle, S.-W., Littmann, T., Böhme, F., In: Breckle, S.-W, Yair, A., Veste, M. (eds.), Arid Dune Ecosystems – The Nizzana Sands in the Negev Desert, Ecological Studies 200, Springer, Berlin Heidelberg New York, pp. 319-336, 2008 (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…) 


Biological soil crusts in the Haluza sand dunes, Negev desert

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…

North-facing slope with biological soil crusts and Retama raetam

Biological soil crusts decrease soil temperature in summer and increase soil temperature in winter in semiarid environment

In hot and wet conditions in summer, the biological soil crusts (BSCs) decreased soil temperature by up to 11.8 °C, 7.5 °C, 5.4 °C, and 3.2 °C at surface, 5 cm, 15 cm, and 30 cm, respectively; while in cold and dry conditions in winter the BSCs increased soil temperature by up to 1.2 °C, 1.2 °C, and 1.1 °C at 5 cm, 15 cm, and 30 cm, respectively. The daily mean soil temperatures of the BSCs in a whole year were averagely increased by 0.57 ± 0.04 °C, 0.31 ± 0.04 °C, and 0.22 ± 0.04 °C at 5 cm, 15 cm, and 30 cm, respectively. The effects of the BSCs on soil temperature were positively correlated with air temperature and soil moisture, and decreased with soil depth from surface to deep soil. We concluded that BSCs relieved the extreme hot and cold soil micro-environments in desert ecosystem to some extent. Therefore their effects on soil temperature are positive for improving water and nutrient availability and biological community structure, thus decreasing susceptibility to desertification. These results would be helpful for understanding the ecological and hydrological functions of BSCs in semiarid environment.

Xiao, B., Wang, H., Fan, J., Fischer, T., Veste, M., Ecological Engineering 58: 52-56, 2013. (more…)

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