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In arid and semiarid areas of the planet, biological soil crusts (BSCs) participate in functions critical to the ecosystem sustainability and are usually located in inter-vegetation spaces. These crusts are structurally and in terms of biomass dominated by mosses and lichens. Associated with them, a great variety of fungi, bacteria, and cyanobacteria have been found. Despite their ecological importance, major details of the functionalities of the microbial communities inhabiting BSCs and the underlying soil are still missing. Here, we explore the biomass, functionality, and protein-based phylogeny of the microbial communities associated with lichen-dominated BSCs under a semiarid climate. Two lichen-dominated BSCs (Fulgensia desertorum -dominated BSC, F-BSC; and Squamarina cartilaginea -dominated BSC, S-BSC) were sampled in a semiarid soil located in South-East Spain. Samples of the BSCs and the underlying soils were collected. The biomass and activity of the microbial community in both BSCs was higher than in the soil underlying them, as estimated by their PLFA content, basal respiration, and enzyme activities. The factor analysis of PLFAs and the putative phylogenetic analysis of the identified proteins indicated scarce differences in the composition of the microbial communities of the two biocrusts at the phylum level. In contrast, community-level physiological profiles (CLPPs) revealed that the microbial communities inhabiting BSCs metabolized substrates differently. The identification of ribulose-1,5 biphosphate carboxylase oxygenase (RuBisCO) peptides points to a specialization in carbon fixation which is carried out by some cyanobacterial species in F-BSC, such as Prochlorothrix hollandica and Microcoleus vaginatus, but not in S-BSC. The structure of the microbial community in the soil underlying each BSC was different, as estimated by factor analysis of PLFAs, but such differences did not correspond to changes in the functional structure, as indicated by the CLPPs. The multidisciplinary approach used in this study revealed remarkable differences between the microbial communities inhabiting F. desertorum and S. cartilaginea -dominated biological crusts, whereas the microbial communities in the soils underlying the two crusts behaved similarly. [ABSTRACT FROM AUTHOR]

To better understand the key role of biological soil crusts (BSCs) in redistribution processes of limited water resources in semi-arid sandy ecosystems, surface runoff over BSCs in a sandy area in the southern Mu Us sandy area was observed in this study. The results indicated that runoff occurred twice among six rainfall events during the experimental period. Runoff yields varied among types of BSCs, and that they increased in the following order: light algae crusts, dark algae crusts and moss crusts. These findings showed that runoff yields were increasing with the development of BSCs. The percentage of runoff in individual rainfall events increased from light algae crusts to moss crusts, which indicated that the redistribution ratios of different types of BSCs differed. Surface runoff of BSCs may be influenced by rainfall, rain intensity, degree of water saturation of the BSCs before the rain and degree of development of BSCs. Formation of BSCs on the surface of sand dunes changed the spatial distribution pattern of water in this semi-arid sandy ecosystem, which increased the heterogeneity of resources such as water and nutrients. Therefore, making appropriate disturbances of BSCs during storm season is beneficial to maintaining the balance of natural resources in the semi-arid sandy ecosystem. [ABSTRACT FROM AUTHOR]

Quantification of the relationship between surface albedo and surface temperature was done by analyzing measured diurnal variations of surface albedo and surface temperature on the biological soil crusts and sand dunes within vegetation-stabilized desert ecosystems. The surface albedos and surface temperatures of sand dunes and biological soil crusts were measured concurrently over field plots of the moving sand area and the artificially revegetated area established in 1964 in Shapotou area, on the southeastern edge of the Tengger Desert. Results indicated that the diurnal variation of surface albedo for both dry surfaces showed an upward-facing parabolic curve on clear days and was symmetrical at about local noon. Whereas, the diurnal trend of surface temperature was opposite to that of surface albedo in the daytime and asymmetrical at about local noon times. Thereby, the data were empirically divided into forenoon and afternoon data with respect to local noon to analyze the relationship between surface albedo and surface temperature. A highly correlated negative linear relationship was found in the two time periods, respectively, and we proposed an empirical model between surface albedo and surface temperature based on that relationship. The model results indicate a good potential application for using the surface temperature to specify surface albedo. [ABSTRACT FROM AUTHOR]

Abstract: Atmospheric nitrogen that is fixed by associative cyanobacteria can be released into the surrounding soil environment providing a key source of N for arctic ecosystems. Yet, little is known about nitrogen fixation by Biological Soil Crusts (BSCs) within hummock-hollow complexes that are typical of many arctic environments. In this study, we examined spatial and temporal patterns in N2-fixation, dinitrogenase reductase (nifH) gene abundance and release of N in a low arctic hummock-hollow ecosystem. The impacts of cyanobacteria on N status in soil were evaluated by assessing soil nitrogen in relation to the cyanobacterial associations found on Hummock and Hollow BSCs. In addition, potential P limitation of N2-fixation by cyanobacteria was assessed for Hummock and Hollow BSCs. The tops of hummocks and the bottoms of hollows were areas of high N2-fixation, whereas minimal N2-fixation occurred on the sides of hummock–hollow complexes. Compared with Hummock BSCs, Hollow BSCs had a higher mean growing season N2-fixation rate, a higher mean growing season nifH abundance, a higher mean total %N and δ15N values closer to that of atmospheric N2. Soil N status was linked to rates of N2-fixation by BSCs indicating that these N2-fixing associations act as important point sources of soil N in this low arctic ecosystem. Over the course of a growing season temporal variation in N2-fixation and nifH abundance were weakly linked suggesting that N2-fixation was carried out by complex communities of diazotrophic microorganisms and that factors such as nutrient availability may limit N2-fixation to a greater extent than nifH abundance. [ABSTRACT FROM AUTHOR]

Biological soil crusts (BSCs) greatly influence the N cycle of semi-arid ecosystems, as some organisms forming them are able to fix atmospheric N. However, BSCs are not always taken into account when studying biotic controls on N cycling and transformations. Our main objective was to understand how BSCs modulate the availability of N in a semi-arid Mediterranean ecosystem dominated by the tussock grass Stipa tenacissima. We selected the six most frequent soil cover types in the study area: S. tenacissima tussocks (ST), Retama sphaerocarpa shrubs (RS), and open areas with very low (BS), low (LC) medium (MC) and high (HC) cover of well developed and lichen-dominated BSCs. The temporal dynamics of available N dynamics followed changes in soil moisture. Available NH-N did not differ between microsites, while available NO-N was substantially higher in the RS than in any other microsite. No significant differences in the amount of available NO-N were found between ST and BS microsites, but these microsites had more NO-N than those dominated by BSCs (LC, MC and HC). Our results suggest that BSCs may be inhibiting nitrification, and highlight the importance of this biotic community as a modulator of the availability of N in semi-arid ecosystems. [ABSTRACT FROM AUTHOR]

Biological soil crusts composed of cyanobacteria, green algae, bryophytes, and lichens colonize soils in arid and semiarid ecosystems worldwide and are responsible for significant N input to the soils of these ecosystems. Soil crusts also colonize active sand dunes in more humid regions, but studies of structure and function of such sand dune crusts are lacking. We identified the cyanobacterial, algal, and bryophytic constituents and N production and leachates of biological soil crusts that colonize beach dunes at the Indiana Dunes National Lakeshore along southern Lake Michigan in Indiana, USA. To determine the role of these crusts in this system, we conducted a greenhouse experiment in which intact soil cores with biological crusts were subjected to artificial rainfall over a full growing season. The volume and N content of leachate from the cores were quantified in relation to degree of crust development, taxonomic composition, rainfall volume and intensity, light intensity, and the presence of plant litter. Net N throughput significantly exceeded N inputs to cores in rainwater. Net N outputs from crusts to subsurface soil ranged from 0. 01 to 0.19 g NH-N m−2 yr−1 and 0.01 to 0.61 g NON m−2 yr−1. Thus, total inorganic N inputs associated with biological soil crusts ranged from 0.02 g N m−2 yr−1 to 0.8 g N m−2 yr−1. High volume (≥2 cm) rainfall resulted in more N leaching than low volume events, and plant litter added over the surface of crusted soil cores significantly increased the amount of N in leachate. Exploratory path analysis revealed direct and indirect linkages among environmental factors, crust development, and crust composition in regulating the throughput of H2O and N from these intact soil cores. Biological soil crusts at this site, combined with other properties of the soil surface, substantially increase N inputs to this water- and nutrient-limited sand dune ecosystem. [ABSTRACT FROM AUTHOR]

Abstract: It has been suggested that the dominance of N forms should shift from dissolved organic nitrogen (DON) to nitrate along a gradient of increasing N availability. We aimed to apply this model at a local scale within a semi-arid ecosystem showing a high spatial heterogeneity in the distribution of vegetation and soil resources. By doing this, we seek a better understanding of the N cycling in spatially heterogeneous ecosystems. We took soil samples from the three major sources of spatial heterogeneity: the grass Stipa tenacíssima, the N-fixing shrub Retama sphaerocarpa, and open areas. We also sampled the biological soil crust (BSC) located in the latter areas as another source of spatial heterogeneity. BSC microsites were classified by four levels of soil coverage, ranging from high coverage (66%) to bare soil. The proportion of nitrate, ammonium and DON was determined in all microsites. DON was the dominant N form for open areas, while nitrate was dominant under the canopy of Retama; these microsites contained the lowest and highest N availability, respectively. Under BSC, DON was the dominant N form. We found high temporal variability in the dominance of N forms for all microsites. Our results suggest that the biome-derived model of explaining N form dominance across N availability gradients may be extended to local gradients. [Copyright &y& Elsevier]