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Land degradation by erosion is especially important in drylands, which are among the most vulnerable to disturbance by human activity or climate change. Biocrusts are an essential surface component of these ecosystems and one of the most important contributors to surface resistance and stability, and therefore, keeping soil fertile in these nutrient-limited-environments. Loss of biocrusts can result in increased sediment losses and subsequent loss of soil organic carbon (SOC) and other soil resources, which are vital for vegetation and general ecosystem functioning. Despite their importance, the consequences of biocrust loss on sediment and SOC losses in drylands have been poorly analysed. In this study, we examined the influence of two biocrust types (cyanobacteria and lichen-dominated biocrusts) and biocrust removal on runoff and sediment yield, from natural rainfall during one hydrological year in a semiarid badlands catchment (Tabernas, SE Spain). The influence of biocrust type and biocrust removal on SOC losses from water erosion (dissolved and sediment organic carbon losses, DOC and SdOC, respectively) was also analysed. Our results show that sediment yield significantly increased after biocrust removal, especially during the first rain after biocrust removal, when particles were left directly exposed to raindrop impact and easily washed away by runoff. Annual sediment yield was 465, 75 and 24 g m − 2 in biocrust-removed, cyanobacteria-covered and lichen-covered soil, respectively, and the first event represented 87% of annual sediment losses on biocrust-removed plots. Biocrust removal was accompanied by a significant increase in both DOC and SdOC mobilisation. Total organic carbon (TOC) mobilisation was the highest in soils where the biocrust had been removed and decreased as the biocrust was more developed. Annual TOC mobilisation was 10.2, 3.0 and 1.4 g m − 2 in biocrust-removed, cyanobacteria-covered and lichen-covered soil, respectively. TOC mobilisation was mainly driven by sediments and thus, 89% of annual TOC mobilisation occurred during the first rain after biocrust removal. The high sediment and SOC losses recorded after biocrust removal emphasize the importance of biocrust conservation for avoiding the loss of soil resources and maintaining fertility in interplant soils in drylands. [ABSTRACT FROM AUTHOR]

Abstract: Water and nutrients are scarce resources in arid and semiarid ecosystems. In these regions, biological soil crusts (BSCs) occupy a large part of the soil surface in the open spaces surrounding patches of vegetation. BSCs affect physicochemical soil properties, such as aggregate stability, water retention, organic carbon (OC) and nitrogen (N) content, associated with primary ecosystem processes like water availability and soil fertility. However, the way BSCs modify soil surface and subsurface properties greatly depends on the type of BSC. We hypothesised that physicochemical properties of soil crusts and of their underlying soils would improve with crust development stage. Physicochemical properties of various types of soil crusts (physical crusts and several BSC development stages) and of the underlying soil (soil layers 0–1 cm and 1–5 cm underneath the crusts) in two semiarid areas in SE Spain were analysed. The properties that differed significantly depending on crust development stage were aggregate stability, water content (WC) (at −33 kPa and −1500 kPa), OC and N content. Aggregate stability was higher under well-developed BSCs (cyanobacterial, lichen and moss crusts) than under physical crusts or incipient BSCs. WC, OC and N content significantly increased in the crust and its underlying soil with crust development, especially in the first centimetre of soil underneath the crust. Our results highlight the significant role of BSCs in water availability, soil stability and soil fertility in semiarid areas. [Copyright &y& Elsevier]

Cyanobacteria are widespread prokaryotic organisms that represent feasible biotechnological tools to set up valid approaches to counteract desertification. Their peculiar physiological traits, and their resilience to abiotic stresses, allow their application on abiotically constrained soils to trigger their stabilization. A successful cyanobacteria inoculation results in the formation of cyanobacterial biocrusts, complex microbial communities characterized by tangled filament meshes imbued in a matrix of self-secreted extracellular polysaccharides (EPS) that keep loose sediments and aggregates firmly in place. However, the capability to form stable cyanobacterial biocrusts is not common to all the species, and a mix of factors can hamper the process, notably inoculum amount, and substrate characteristics. The aim of this work was to assess the influence of inoculum quantity and substrate granulometry on the physical stability of cyanobacterial biocrusts induced by inoculating the strain Leptolyngbya ohadii in a microcosm experiment, under laboratory conditions. After applying three different initial inoculum amounts on two different sand granulometries (medium and coarse sand), we assayed aggregate stability, physical stability and surface hydrophobicity on the resulting biocrusts during a 30-day incubation. Also, the features and the role of the EPS synthesized by L. ohadii were studied following their isolation, characterization, and direct application on the sand. The two EPS fractions produced by the strain, one more soluble and easily released in the surrounding medium (released polysaccharides, RPS) and one solidly attached to the filaments (glycocalyx EPS, G-EPS), were separately tested. Cyanobacterial biocrusts visibly formed in all the microcosms after 15 days. However, we observed a strong effect of sand granulometry in affecting aggregate stability and tensile strength, both of which appeared weaker on coarse sand. A higher amount of initial inoculum was necessary to produce stable biocrusts on coarse sand compared to medium sand. Also, we observed how the inoculation of EPS alone did not sort most of the significant effects that we detected by inoculating the whole culture, pointing at the importance of the action of the cyanobacterial filaments in soil conglomeration. However, a significant increase in physical stability was achieved by inoculating G-EPS on medium sand, suggesting the involvement of this fraction in biocrusts structuration. This work analyzes for the first time the effects of the variable grain size and inoculum amount in the achievement of physically stable biocrusts by cyanobacteria inoculation. The results that we obtained are useful in improving and optimizing the process of biomass preparation and dispersion for future indoor and outdoor studies. • Cyanobacterization success requires an optimal initial inoculum amount. • Medium sand is more conductive to biocrust stability than coarse sand. • L. ohadii EPS alone cannot elicit sand aggregate stability and water repellency. • G-EPS appears as the fraction more involved in biocrust physical stability. [ABSTRACT FROM AUTHOR]

Atmospheric methane (CH 4) and carbon dioxide (CO 2) concentration have been increasing during the last several centuries due to changes in agricultural practices and other anthropogenic activities. Both greenhouse gases (GHGs), have a significant impact on the Earth's radiative balance. GHG effluxes of CH 4 and CO 2 were measured in a warm Mediterranean wetland in south of Spain. The dominant vegetation cover at the site was by common reed (Phragmites australis) and the measurements were done during short measurement campaign in early autumn 2015. Gas-flux measurements were carried out applying two methods, the eddy covariance (EC) technique and the chamber method (CM). These two methods representing different ecosystem subsets, with EC representing the plant/ecosystem subset and CM representing the water/soil subset. In our measurement campaigns using CM, CH 4 emissions ranged from 7.2 to 17.7 mg CH 4 -C m−2 d−1 and CO 2 emissions from 0.53 to 1.27 g CO 2 -C m−2 d−1. When using EC, the average fluxes of CH 4 and CO 2 were 31.4 mg CH 4 -C m−2 d−1 and 1.32 g CO 2 -C m−2 d−1, respectively. Relationships between gas fluxes (CO 2 and CH 4) measured by the EC method were quite closely correlated with photosynthetically active solar radiation. Our results showed higher CO 2 carbon released from the water/soil ecosystem subset in comparison to plants subset. On the other hand, the estimated CH 4 carbon balance for the plant/ecosystem subset was about twice that of the water/soil ecosystem subset. Overall, we showed that EC and CM methods cover different areas making EC advantageous for integrated measurements over larger areas, while the CM approach is suitable for local and spatially well constrained flux measurements. Hence, EC and CM methods should be seen as complementary rather than fully comparable methods. • CH 4 and CO 2 were measured in Phragmites australis wetland by EC and CM in autumn. • Average fluxes of CH 4 and CO 2 were 31.4 mg CH 4 -C m−2 d−1 and 1.32 g CO 2 -C m−2 d−1. • Higher amount of CO 2 carbon emissions from the water/soil ecosystem level • CH 4 carbon emissions at plant ecosystem level were higher than water/soil level. • EC and CM methods should be seen as supplementary rather than comparable methods. [ABSTRACT FROM AUTHOR]

In most of non-vegetated areas from drylands, soils are colonized by biocrusts, and although they represent a small fraction of the soil profile, they strongly affect several soil surface properties, such as porosity, available nitrogen and carbon content, hydrophobicity or micro-topography. The influence of BSCs on these soil properties has effects on numerous ecosystem processes, including water retention and runoff generation. Previous studies on the hydrological and erosive response of soils covered by biocrusts have highlighted the role of soil surface roughness as a key variable for understanding the influence of biocrusts on runoff and erosion, but biocrusts' effects on surface micro-topography varies depending on crust water content. Biocrusts can absorb large amounts of water in a short period of time, increasing their volume and modifying surface micro-topography, this change depending on biocruts type and development. A correct characterization of these surface variations may increase the knowledge about hydrological response of biocrusts, and for this reason, accurate ground level measurements of biologically crusted surfaces are essential. The objective of this study is to analyze the effect of wetting on surface micro-topography of biologically crusted soils. To achieve this objective, different crust types were scanned in the laboratory with high resolution laser scanner. Five samples were collected for each of the 4 different crust types (bare soil, cyanobacteria biocrust, and two different lichen biocruts). Two different scans were made in each sample, in dry and wet conditions. Random roughness (RR) was calculated for data from every scan, and the RR indexes obtained before and after wetting were compared. According with our initial hypothesis, an increase in surface height and surface roughness up to 0.24 and 0.20 mm respectively was observed in more developed lichen biocruts, under wet conditions respect to dry ones. These differences, despite being very subtle, could exert strong implications on runoff generation, and water evaporation, and show the complex interactions between biocruts, surface micro-topography and water fluxes. [ABSTRACT FROM AUTHOR]