Effect of drought stress and subsequent re-watering on the physiology and nutrition of Pistacia vera and Pistacia atlantica.

Arid and semi-arid regions are characterised by extreme conditions including drought stress and salinity. These factors profoundly affect the agricultural sector. The objective of this work is to study the effect of drought and re-watering on leaf gas exchange, chlorophyll fluorescence and mineral nutrition in Pistacia vera and Pistacia atlantica . Water stress was applied to individuals of P. vera and P. atlantica for 23days, followed by rehydration for 7days. The results showed a clear reduction in water relations, leaf gas exchange and chlorophyll content in P. vera . Compared to P. vera , P. atlantica maintained less affected water status, total chlorophyll content, leaf gas exchange and chlorophyll fluorescence, stable Zn and Fe proportion, and even elevated K and Cu. The changes in the chlorophyll fluorescence parameter were manifested particularly at the maximal fluorescence (Fm). In contrast, no change was recorded at the minimal fluorescence (F0). After re-hydration, although water status was fully recovered in both species, stomatal conductance (gs), net photosynthesis (A ) and transpiration rate (E ) remain with lower values than the well-watered seedlings. P. atlantica was better adapted to drought stress than P. vera .

Biosorption of heavy metals from phosphate-processing effluent by Serratia rubidaea NCTC12971 immobilized in Ca-alginate beads / Biosorción de metales pesados a partir de efluentes de procesamiento de fosfatos por Serratia rubidaea NCTC12971 inmovilizados en cuentas de Ca-alginato

In this study, the biosorption ability of various potentially toxic elements from phosphate-processing effluent (PPE) using the indigenous bacterium Serratia rubidaea NCTC12971 immobilized in Ca-alginate beads was investigated. The experimental data analyzed by the Langmuir isotherm revealed that the optimum dose of 2 g·100 ml−1 of immobilized S. rubidaea NCTC12971 at pH 7 and a contact time of 48 h allowed the removal of 92.07%, 98.05%, 95.57%, and 88.39% of lead (Pb (II)), cadmium (Cd (II)), copper (Cu (II)), and zinc (Zn (II)), respectively. Moreover, under the Langmuir isotherm, the maximum single-layer adsorption capacity (qmax) of the biosorbent was estimated to 32.14 mg g−1, 45.87 mg g−1, 0.06 mg g−1, and 3.01 mg g−1 for Pb (II), Cd (II), Cu (II), and Zn (II), respectively, under the stated conditions. Alternatively, the regeneration and reuse of the Ca-alginate beads was evaluated. Indeed, after four consecutive adsorption–desorption cycles, there was no significant loss in the biosorption capacity. The effectiveness of the bacterial biosorption as treatment process was evaluated by assessing the phytotoxicity of the treated effluent (TE) on Medicago sativa and Lactuca sativa seed germination and their root elongation. Results exhibited a significant toxicity removal expressed by a notable increase in the germination indices (GI), which reach 80% and 70%, respectively, for Medicago sativa and Lactuca sativa compared to the GI values of 46.6% and 16.6% of the same species in presence of the untreated effluent (PPE).(AU)

Biosorption of heavy metals from phosphate-processing effluent by Serratia rubidaea NCTC12971 immobilized in Ca-alginate beads.

In this study, the biosorption ability of various potentially toxic elements from phosphate-processing effluent (PPE) using the indigenous bacterium Serratia rubidaea NCTC12971 immobilized in Ca-alginate beads was investigated. The experimental data analyzed by the Langmuir isotherm revealed that the optimum dose of 2 g·100 ml-1 of immobilized S. rubidaea NCTC12971 at pH 7 and a contact time of 48 h allowed the removal of 92.07%, 98.05%, 95.57%, and 88.39% of lead (Pb (II)), cadmium (Cd (II)), copper (Cu (II)), and zinc (Zn (II)), respectively. Moreover, under the Langmuir isotherm, the maximum single-layer adsorption capacity (qmax) of the biosorbent was estimated to 32.14 mg g-1, 45.87 mg g-1, 0.06 mg g-1, and 3.01 mg g-1 for Pb (II), Cd (II), Cu (II), and Zn (II), respectively, under the stated conditions. Alternatively, the regeneration and reuse of the Ca-alginate beads was evaluated. Indeed, after four consecutive adsorption-desorption cycles, there was no significant loss in the biosorption capacity. The effectiveness of the bacterial biosorption as treatment process was evaluated by assessing the phytotoxicity of the treated effluent (TE) on Medicago sativa and Lactuca sativa seed germination and their root elongation. Results exhibited a significant toxicity removal expressed by a notable increase in the germination indices (GI), which reach 80% and 70%, respectively, for Medicago sativa and Lactuca sativa compared to the GI values of 46.6% and 16.6% of the same species in presence of the untreated effluent (PPE).

Potential of Seed Halopriming in the Mitigation of Salinity Stress during Germination and Seedling Establishment in Durum Wheat (Triticum durum Desf.).

The salinity of soils and irrigation water is among the main factors that limit plant productivity worldwide. Several alternatives have been proposed to get around this problem. However, these alternatives have faced difficulties in their implementation. As an alternative, the adverse effects of salinity on crop yield can be minimized by selecting species and varieties better adapted to salinity and/or by finding priming agents that give plants a certain tolerance during the vegetative and reproductive stages. The latter are strictly dependent on germination and seedling establishment. For this purpose, a laboratory experiment was conducted on three Tunisian wheat cultivars (Karim, Razeg, and Maali) subjected to moderate salinity stress (MSS, 5 g L-1 NaCl), severe salinity stress (SSS, 10 g L-1 NaCl), or control (0 NaCl) after soaking the seeds in a solution of KNO3 or ZnSO4 (0.5 g L-1). Salinity stress significantly decreased germination capacity (GC) and induced osmotic stress under MSS, which declined under SSS in favor of toxic stress. Pretreatment of seeds with KNO3 or ZnSO4 alleviated the toxic effect, and seedlings recovered initial vigor and GC even under SSS. The Karim cultivar showed better tolerance to salinity and a higher ability to react to priming agents. The calculated sensitivity tolerance index (STI) based on germination capacity, seedling growth, and initial vigor decreased in all cultivars under salt stress; however, this parameter clearly discriminated the studied cultivars. Karim was the most tolerant as compared to Razeg and Maali. We conclude that halopriming provides a benefit by alleviating the harmful effects of salt toxicity and that cultivars differ in their response to priming and extent of salt stress. KNO3 and ZnSO4 effectively alleviated the inhibitory effect of salt stress on seed germination and seedling establishment while significantly improving initial vigor.

Chemical seed priming alleviates salinity stress and improves Sulla carnosa germination in the saline depression of Tunisia.

In the saline depressions (Sebkhas) of Tunisia, Sulla carnosa expresses anarchic distribution, sometimes in association with halophytes sometimes individually. In order to explain this distribution, we investigated the effects of salinity on seed germination, the osmotic and toxic limiting factors, and the importance of some stimulating agents (priming agents) in the improvement of the tolerance to salinity at the germinating stage. A study was conducted on seeds harvested from the natural biotope Sebkha d'El Kelbia (35°50'34″N, 10°16'18″E), and an increasing concentration of NaCl (0, 5, 10, 15, and 20 g L-1) was applied. Some priming agents were used to propose efficient, rapid, and low-cost tools to improve the seed germination and tolerance of Sulla carnosa (Desf.) in saline depression. Salinity stress significantly decreased germination capacity and rate and delayed its initiation and maximum. Until 15-g L-1 NaCl, the most limiting factor of seed germination is the osmotic effect. At 20-g L-1 NaCl, the toxic effect dominates, and germination is irreversibly inhibited. Some priming agents have shown their efficiency in improving the germination capacity at 10-g L-1 NaCl and conferring a salt tolerance of up to 15-g L-1 NaCl.

Tree species as a biomonitor of metal pollution in arid Mediterranean environments: case for arid southern Tunisia.

We investigated the accumulation of Zn, Cu, Pb, and Cd in the soil and the leaves and bark of five common tree species (Eucalyptus occidentalis Endl., Acacia salicina Lindl., Cupressus sempervirens L., Casuarina equisetifolia L., and Tamarix aphylla (L.) Karst.) in the city of Gabès Tunisia to elucidate their bioaccumulation potential and determine their usefulness as biomonitors of metallic pollution in arid urban areas. Our results indicated that the bark had higher mean concentrations of Pb and Cd than leaves. In contrast, the leaves had higher mean concentrations of Zn and Cu than bark. No hyperaccumulation was detected for any of the analyzed metals in any of the studied species. E. occidentalis and T. aphylla had the highest mean concentrations of the investigated metals in leaves and bark. Based on the calculated metal accumulation index (MAI) values, these two species accumulated more metals than other studied tree species. Likewise, the concentrations of Zn, Cu, Pb, and Cd in soil had significant positive correlations with that in leaves and bark. Accordingly, E. occidentalis could be used for biomonitoring in arid areas subjected to industrial and traffic pollution. T. aphylla would be a good alternative when native species are a priority.

Plant traits and phenotypic variability effect on the phytomass production of Stipagrostis ciliata (Desf.) De Winter.

A process of continuous degradation of plant communities, due mainly to long-term overgrazing has been revealed by most ecological studies in North African arid climate. Notably, this degradation appeared across the depletion of perennial grass species exhibiting low density in the majority of range ecosystems. This study aimed to examine the phenology and the aboveground phytomass production of Stipagrostis ciliata (Desf.) De Winter accessions, a perennial grass, growing under the same environment but coming from different climates of Tunisia. Additionally, the extent of genetic variation in phenological parameters, root and shoot phytomass productivity and the correlations among these parameters were also analyzed. Significant differences in all morphological parameters of S. ciliata accessions were revealed by ANOVA test and were corroborated with significant and positive correlation indicated by Pearson's correlation analysis. Plant diameter, biovolume, root biomass with protective sleeve and spike number exhibited significant differences and high distinctiveness between S. ciliata accessions. Tukey's HDS tests indicated the presence of three groups of accessions. Principal component analysis (PCA) applied on a table with eight observations and 13 variables, and dispersion of S. ciliata accessions on the first two axes of PCA confirmed the presence of three groups of accessions. Trait variability in the field for the five accessions is more likely to be the result of phenotypic plasticity rather than of genetic differentiation between accessions. Overall, the characterization of S. ciliata accessions exhibited significant differences in terms of morphological and biomass productivity.

Study of the phytotoxic potential of olive mill wastewaters on a leguminous plant 'Vicia faba L.'

We investigated the study of the phytotoxic potential of olive mill wastewaters (OMW) on the germination and growth responses of the faba bean 'Vicia faba L.' leguminous plant cultivated under an arid Mediterranean climate. The results showed that the raw OMW blocked the germination of the seeds tested, while OMW treated soil extracts stimulate the seeds' germination rate. The monitoring of the faba bean growth parameters in the different OMW treated soils showed that the optimal growth of the faba bean plants has been recorded for the soil amended with 25 m3 ha-1 of OMW. Although the 50 m3 ha-1 dose is the most suitable for the soil studied, our results showed that the 25 m3 ha-1 dose is the most suitable for the vegetative development, as well as for the productivity of the plant tested 'Vicia faba L.'.

Effect of phosphorus concentration on the photochemical stability of PSII and CO2 assimilation in Pistacia vera L. and Pistacia atlantica Desf.

A greenhouse pot experiment was conducted at the faculty of sciences of Gafsa to evaluate the effect of phosphorus treatment on two pistachio species. The seedlings of Pistacia vera and Pistacia atlantica were subjected to six levels of phosphoric acid (P2O5) (0, 5, 15, 30, 60 and 120 ppm). Stomatal conductance, net photosynthesis, chlorophyll fluorescence (OJIP) and total chlorophyll content were measured after 1, 2, 3, 6, 8, 9 and 12 weeks of treatment. During the experiment, phosphorus application at 5 ppm increased photosynthesis and stomatal conductance, relative to the treatment 0 ppm only in P. atlantica. However, phosphorus supply at 60 and 120 ppm induced toxicity leading to an inhibition of CO2 photo-assimilation rate, an alteration of photosystem II (PSII) structure and function and reduction in leaf chlorophyll content in both species. The (OJIP) transient showed complex changes in O-J, J-I and I-P phases of fluorescence. Due to phosphorus toxicity, both donor and acceptor sides of PSII were damaged, electron transport perturbed and chlorophyll pigment reduced which resulted in the fall of CO2 photo-assimilation rate, followed by mortality in both species.

Optimization, characterization and hepatoprotective effect of polysaccharides isolated from Stipa parviflora Desf. against CCl4 induced liver injury in rats using surface response methodology (RSM).

The roots of Stipa species have been used for treatment and prevention of a various number of diseases. To the best of my knowledge, little information is regarding the antioxidant and hepatoprotective activities of polysaccharides from Stipa parviflora (SPP). Hence the polysaccharides from this plant sample have been investigated here. In addition, Box-Behnken design and response surface methodology were used to optimize the hot extraction conditions. The optimal conditions were determined as: extraction time 120 min, extraction temperature 70 °C and ratio of water to raw material 30. Crude SPP was composed of mannose, ribose, glucose, galactose, pyranose and arabinose. In addition, the SPP showed a strong antioxidant capacity in vitro. In vivo experiments showed that CCl4 treatment caused significant oxidative damage in hepatic tissues, whereas these problems were inhibited by SPP treatment. Hepatoprotective potential of SPP was evaluated by estimating the levels of serum markers like ALT and AST levels. SPP treated rat liver anti-oxidant parameters (SOD, CAT and GPx) were significantly antagonized for the pro-oxidant effect of CCl4. Histopathological studies proved that the SPP extract has significant hepatoprotective activity. In conclusion, the SPP fraction isolated from S. parviflora showed significant hepatoprotective potential which could be related to active compounds present in this fraction.

Effects of Biological Soil Crusts on Enzyme Activities and Microbial Community in Soils of an Arid Ecosystem.

Arid ecosystems constitute 41% of land's surface and play an important role in global carbon cycle. In particular, biological soil crusts (BSC) are known to be a hotspot of carbon fixation as well as mineralization in arid ecosystems. However, little information is available on carbon decomposition and microbes in BSC and key controlling variables for microbial activities in arid ecosystems. The current study, carried out in South Mediterranean arid ecosystem, aimed to evaluate the effects of intact and removed cyanobacteria/lichen crusts on soil properties, soil enzyme activities, and microbial abundances (bacteria and fungi). We compared five different treatments (bare soil, soil with intact cyanobacteria, soil with cyanobacteria removed, soil with intact lichens, and soil with lichens removed) in four different soil layers (0-5, 5-10, 10-15, and 15-20 cm). Regardless of soil treatments, activities of hydrolases and water content increased with increasing soil depth. The presence of lichens increased significantly hydrolase activities, which appeared to be associated with greater organic matter, nitrogen, and water contents. However, phenol oxidase was mainly controlled by pH and oxygen availability. Neither fungal nor bacterial abundance exhibited a significant correlation with enzyme activities suggesting that soil enzyme activities are mainly controlled by edaphic and environmental conditions rather than source microbes. Interestingly, the presence of lichens reduced the abundance of bacteria of which mechanism is still to be investigated.

Evaluation of the potential of Erodium glaucophyllum L. for phytoremediation of metal-polluted arid soils.

The present work aimed at studying pollution of traffic-related heavy metals (HMs) in roadside soils and their uptake by the Mediterranean native species Erodium glaucophyllum L., growing along Gabès-El Hamma highway, Gabès (Tunisia). Here, heavy metals were analyzed in soils and in plant roots and shoots along different distances from the highway edge. High levels of all the investigated soil trace elements were found in samples collected at 15 m distance from the highway. Overall, HM concentrations in the below- and aboveground part of E. glaucophyllum showed significant decreases with increasing distance from the highway. The lowest values were recorded at 150 m. Biological concentration factor (BCF) and mobility ratio (MR) of all investigated heavy metals were > 1 at all distances from the highway, except for Mn and Cu. High values of BCF and MR for Zn indicate that E. glaucophyllum has an excellent potential for the assimilation of this element from the soil. In addition, the higher translocation factors (TF) of Pb, Cd, Zn, and Fe in E. glaucophyllum shoots make it suitable for their phytoextraction from soil, while the lower TF for Mn and Cu make this plant convenient for their phytostabilization. Moreover, the significant positive correlations of Mn, Pb, Cu, and Zn in soil and Erodium organs may suggest its potential use as biomonitor of these trace elements. According to these results, E. glaucophyllum seems to be valued as an efficient native species for in situ phytoremediation program on traffic metal-polluted soils.

Tolerance strategies of two Mediterranean native xerophytes under fluoride pollution in Tunisia.

A field study was conducted along a fluorine gradient of soil pollution in Tunisia from Gabes, the most polluted site, to Smara, the reference site. Variations of fluoride (F) concentrations in soils were detected over 1 year in Gabes, Skhira, and Smara. F concentrations in the aerial part of two native plant species, i.e., Erodium glaucophyllum and Rhanterium suaveolens, were above the usual background concentrations. Bioaccumulation factors ranged from 0.08 to 1.3. With F concentrations in aerial parts up to 355 mg kg-1, both species may be described as F accumulators. Both species showed an earlier vegetative growth in Gabes than in Smara. However, some difference between their strategies could be observed, i.e., E. glaucophyllum shortening the period of its vegetative growth with an escape strategy and R. suaveolens decreasing its ratio of alive/dead parts potentially lowering the F toxicity by storage in dead cells. However, at a tissue level, mechanisms of tolerance were similar. Leaf section micrographs of both species showed a higher calcium accumulation in leaf midveins at Gabes than at Smara, confirming the role of calcium in plant F tolerance strategies.

Identification and prioritization of areas with high environmental risk in Mediterranean coastal areas: A flexible approach.

Interdisciplinarity and transdisciplinarity are the cornerstone for the future management of coastal ecosystems with many vulnerability and hazard indexes developed for this purpose, especially in the engineering literature, but with limited studies that considered ecological implications within a risk assessment. Similarly, the concept of prioritization of sites has been widely examined in biodiversity conservation studies, but only recently as an instrument for territory management. Considering coastal plant diversity at the species and community levels, and their vulnerability to three main potential hazards threatening coastal areas (oil spills, Hazardous and Noxious Substances pollution, fragmentation of natural habitats), the objective of this paper is to define an easy-to-use approach to locate and prioritize the areas more susceptible to those stressors, in order to have a practical instrument for risk management in the ordinary and extra-ordinary management of the coastline. The procedure has been applied at pilot areas in four Mediterranean countries (Italy, France, Lebanon and Tunisia). This approach can provide policy planners, decision makers and local communities an easy-to-use instrument able to facilitate the implementation of the ICZM (Integrated Coastal Zone Management) process in their territory.

The relative contribution of short-term versus long-term effects in shrub-understory species interactions under arid conditions.

Plant-plant interactions (competition and facilitation) in terrestrial ecosystems include: (1) short-term effects, primarily quantified with experimental removals; and (2) long-term effects, mostly due to soil weathering processes, primarily quantified with observational methods. It has been argued that these effects are likely to vary in contrasting directions with increasing drought stress in arid systems. However, few studies have used appropriate methodology to assess both types of effects and their variation across nurse species and environmental conditions, in particular in arid systems. This knowledge is crucial for predicting variation in the mediating role of facilitation with climate change and assessing the importance of nurse effects in ecological restoration. In the arid climate of central-south Tunisia, understory species' biomass, abundance and composition and soil parameters were compared in shrub-control, shrub-removed and open areas for three shrub species and in two habitats with contrasting soil moisture conditions. Long-term effects were dominant, positive for all three shrub species and associated with increasing nutrient content in shrub patches. Short-term effects, mainly related to water consumption, were weaker, mostly negative and dependent on shrub species. Additionally, long-term effects were less positive and short-term effects more negative in the dry habitat than in the wet habitat. Our study provides evidence of the primary influence of positive (facilitative) long-term effects in this arid system. However, the net effects of shrubs could be less beneficial for other species with increasing aridity under climate change, due to both a decrease in positive long-term effects and an increase in negative short-term effects.

Study of in vitro antimicrobial and antiproliferative activities of selected Saharan plants.

The aim of the present study was the evaluation of the antimicrobial and antiproliferative activities of selected Saharan species, which are applied in the traditional medicine but not studied thoroughly from chemical and pharmacological point of view. The studied plants, namely Anthyllis henoniana, Centropodia forskalii, Cornulaca monacantha, Ephedra alata var. alenda, Euphorbia guyoniana, Helianthemum confertum, Henophyton deserti, Moltkiopsis ciliata and Spartidium saharae were collected from remote areas of North Africa, especially from the Tunisian region of Sahara. After drying and applying the appropriate extraction methods, the plant extracts were tested in antimicrobial screening assay, performed on 19 Gram-positive and -negative strains of microbes. The inhibition zones produced by plant extracts were determined by disc-diffusion method. Remarkable antibacterial activities were exhibited by extracts of Ephedra alata var. alenda and Helianthemum confertum against B. subtilis, M. catarrhalis and methicillin-resistant and non-resistant S. aureus. Minimum inhibitory concentrations of these two species were also determined. Antiproliferative effects of the extracts were evaluated against 4 human adherent cell lines (HeLa, A431, A2780 and MCF7). Notable cell growth inhibition was found for extract of Helianthemum confertum and Euphorbia guyoniana. Our results provided data for selection of some plant species for further detailed pharmacological and phytochemical examinations.

Screening biological traits and fluoride contents of native vegetations in arid environments to select efficiently fluoride-tolerant native plant species for in-situ phytoremediation.

High fluoride pollution has been detected in the surrounding soils of the coastal superphosphate industries in the Gulf of Gabes (Southeast of Tunisia). A study was conducted in vicinity of factories analysing plant functional traits combined with plant fluoride accumulation and soil metal concentrations aiming to screen more efficiently native plant species tolerant to this pollution. Aerial parts of 18 plant species out of the 10 most abundant species per site were harvested on two polluted sites of Gabes and Skhira at the vicinity of the factories and on the less polluted site of Smara. Native plant species accumulated fluoride following the gradient of soil pollution. Fluoride contents of plant aerial parts ranged from 37 mg kg(-1) to 360 mg kg(-1) and five plant species were only found in the most polluted site. However these latter had low biomass and soil cover. Crossing biological traits and fluoride contents, a selection grid for potentially restorative plant species enabled the selection of three native perennials i.e. Rhanterium suaveolens, Atractylis serratuloides and, Erodium glaucophyllum as potential candidates for an in-situ phytoremediation program on arid fluoride-polluted sites. This approach may be used in other fluoride-polluted Mediterranean environments.

Changes in mesophyll element distribution and phytometabolite contents involved in fluoride tolerance of the arid gypsum-tolerant plant species Atractylis serratuloides Sieber ex Cass. (Asteraceae).

Atractylis serratuloides is an abundant native spiny species that grows in the surroundings of superphosphate factories in Tunisia. This plant species is adapted to arid environments and tolerates a high level of fluoride pollution in soils. The aim of this study was to better understand the physiological mechanisms of fluoride tolerance of this species, comparing the fluoride-contaminated sites of Gabes and Skhira with the reference site of Smara. Results demonstrated the involvement of leaf element and phytometabolite balances in the in situ response of A. serrulatoides to fluoride. Calcium, sulphur and magnesium were differently distributed between the sites of Gabes and Smara in all plant organs. No specific tissue fluorine accumulation in root, stem and leaf, even in the most contaminated site at Gabes, was detected by EDAX mapping. Lower anthocyan and flavonol levels but enhanced nitrogen balance index were found in A. serrulatoides leaves from Gabes compared to the two other sites. A. serratuloides appeared as a fluoride excluder and its tolerance involved calcium interactions with fluoride. Moreover, an occurrence of dark septate endophytes and arbuscular mycorhizal fungi in root systems of A. serratuloides was reported for the first time, and these symbioses were present but low at all sites. We suggest the use of this plant species for fluoride-polluted soil stabilization.

Climate and soil attributes determine plant species turnover in global drylands.

AIM: Geographic, climatic, and soil factors are major drivers of plant beta diversity, but their importance for dryland plant communities is poorly known. This study aims to: i) characterize patterns of beta diversity in global drylands, ii) detect common environmental drivers of beta diversity, and iii) test for thresholds in environmental conditions driving potential shifts in plant species composition. LOCATION: 224 sites in diverse dryland plant communities from 22 geographical regions in six continents. METHODS: Beta diversity was quantified with four complementary measures: the percentage of singletons (species occurring at only one site), Whittake's beta diversity (ß(W)), a directional beta diversity metric based on the correlation in species occurrences among spatially contiguous sites (ß(R2)), and a multivariate abundance-based metric (ß(MV)). We used linear modelling to quantify the relationships between these metrics of beta diversity and geographic, climatic, and soil variables. RESULTS: Soil fertility and variability in temperature and rainfall, and to a lesser extent latitude, were the most important environmental predictors of beta diversity. Metrics related to species identity (percentage of singletons and ß(W)) were most sensitive to soil fertility, whereas those metrics related to environmental gradients and abundance ((ß(R2)) and ß(MV)) were more associated with climate variability. Interactions among soil variables, climatic factors, and plant cover were not important determinants of beta diversity. Sites receiving less than 178 mm of annual rainfall differed sharply in species composition from more mesic sites (> 200 mm). MAIN CONCLUSIONS: Soil fertility and variability in temperature and rainfall are the most important environmental predictors of variation in plant beta diversity in global drylands. Our results suggest that those sites annually receiving ~ 178 mm of rainfall will be especially sensitive to future climate changes. These findings may help to define appropriate conservation strategies for mitigating effects of climate change on dryland vegetation.

Decoupling of soil nutrient cycles as a function of aridity in global drylands.

The biogeochemical cycles of carbon (C), nitrogen (N) and phosphorus (P) are interlinked by primary production, respiration and decomposition in terrestrial ecosystems. It has been suggested that the C, N and P cycles could become uncoupled under rapid climate change because of the different degrees of control exerted on the supply of these elements by biological and geochemical processes. Climatic controls on biogeochemical cycles are particularly relevant in arid, semi-arid and dry sub-humid ecosystems (drylands) because their biological activity is mainly driven by water availability. The increase in aridity predicted for the twenty-first century in many drylands worldwide may therefore threaten the balance between these cycles, differentially affecting the availability of essential nutrients. Here we evaluate how aridity affects the balance between C, N and P in soils collected from 224 dryland sites from all continents except Antarctica. We find a negative effect of aridity on the concentration of soil organic C and total N, but a positive effect on the concentration of inorganic P. Aridity is negatively related to plant cover, which may favour the dominance of physical processes such as rock weathering, a major source of P to ecosystems, over biological processes that provide more C and N, such as litter decomposition. Our findings suggest that any predicted increase in aridity with climate change will probably reduce the concentrations of N and C in global drylands, but increase that of P. These changes would uncouple the C, N and P cycles in drylands and could negatively affect the provision of key services provided by these ecosystems.