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Soils are being degraded at an alarming rate and thereby also crucial ecosystem goods and services. Nitrogen (N) enrichment is a major driver of this degradation. While the negative impacts of N enrichment on vegetation are well known globally, those on various ecological interactions, and on ecosystem functioning, remain largely unknown. Because Mediterranean ecosystems are N limited, they are good model systems for evaluating how N enrichment impacts not only vegetation but also ecological partnerships and ecosystem functioning. Using a 7-year N-manipulation (dose and form) field experiment running in a Mediterranean Basin maquis located in a region with naturally low ambient N deposition (<4 kg N ha−1 y−1), we assessed the impacts of the N additions on (i) the dominant plant species (photosynthetic N-use efficiency); (ii) plant-soil ecological partnerships with ectomycorrhiza and N-fixing bacteria; and (iii) ecosystem degradation (plant-soil cover, biological mineral weathering and soil N fixation). N additions significantly disrupted plant-soil cover, plant-soil biotic interactions, and ecosystem functioning compared with ambient N deposition conditions. However, the higher the ammonium dose (alone or with nitrate), the more drastic these disruptions were. We report a critical threshold at 20-40 kg ammonium ha−1 y−1 whereby severe ecosystem degradation can be expected. These observations are critical to help explain the mechanisms behind ecosystem degradation, to describe the collective loss of organisms and multifunction in the landscape, and to predict potential fragmentation of Mediterranean maquis under conditions of unrelieved N enrichment. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Most research on the mycorrhizal positive-negative responsiveness continuum (or 'mutualism-parasitism continuum') has focused on individual plant species growing at different levels of P availability. Here, we explore this continuum in an experimental plant community inoculated with three arbuscular mycorrhizal (AM) fungal strains (both single and mixed) growing under four resource availability scenarios. These scenarios are a factorial combination of two levels of water and N availability. Each AM fungal strain had a different origin: an arid ecosystem, a farmland, and a mine. We hypothesized that the response of the plant community to mycorrhizal inoculum would depend on the associated AM fungal strain and would be negatively related with increased nitrogen and water availability. Our results showed that mixed-strain AM fungal inoculation had more positive effects along a wider range of water and N availability scenarios than single-strain inoculation. In contrast, mycorrhizal growth response of plants inoculated with a single AM fungal strain shifted from positive to neutral and negative depending on resource availability. Adaptation of each strain to its local conditions might confer different properties to the mycorrhizal symbiosis. Therefore, we conclude that AM fungal origin and environmental limiting resources are crucial factors to predict plant community mycorrhizal growth response in changing ecosystems. [ABSTRACT FROM AUTHOR]

Nitrogen (N) deposition is a major threat to the semiarid Mediterranean ecosystems. We simulated a gradient of N deposition (0, 10, 20 and 50 kg N ha year + 6.4 kg N ha year ambient deposition) in a Mediterranean shrubland from central Spain. In autumn 2011 (after 4 years of experimental duration), soil cores were taken to extract the soil fauna. Acari (45.54%) and Collembola (44.00%) were the most represented taxonomical groups, and their abundance was negatively related to soil pH. Simulated N deposition had an impact on the total number of individuals in soil as well as on Collembola and Pauropoda abundance. Collembola abundance increased with N loads up to 20 kg N ha year and then decreased. This response was attributed to soil acidification (between 0 and 20 kg N ha year) and increased soil ammonium (between 20 and 50 kg N ha year). Pauropoda were favoured by additions of 50 kg N ha year, and it was the only taxonomical group whose abundance was exclusively related to N deposition, suggesting their potential as bioindicators. Contrary to predictions, there was a negative relationship between soil faunal abundance and plant diversity. In conclusion, soil faunal communities from semiarid Mediterranean ecosystems in central Spain seem to be primarily influenced by soil chemistry (mainly pH) but are also susceptible to increased N deposition. The main drivers of change under increased N deposition scenarios seem to be soil acidification and increased ammonium in soils where nitrate is the dominant mineral N form. [ABSTRACT FROM AUTHOR]

Abstract: Anthropogenic N deposition poses a threat to European Mediterranean ecosystems. We combined data from an extant N deposition gradient (4.3–7.3 kg N ha−1 yr−1) from semiarid areas of Spain and a field experiment in central Spain to evaluate N deposition effects on soil fertility, function and cyanobacteria community. Soil organic N did not increase along the extant gradient. Nitrogen fixation decreased along existing and experimental N deposition gradients, a result possibly related to compositional shifts in soil cyanobacteria community. Net ammonification and nitrification (which dominated N-mineralization) were reduced and increased, respectively, by N fertilization, suggesting alterations in the N cycle. Soil organic C content, C:N ratios and the activity of β-glucosidase decreased along the extant gradient in most locations. Our results suggest that semiarid soils in low-productivity sites are unable to store additional N inputs, and that are also unable to mitigate increasing C emissions when experiencing increased N deposition. [Copyright &y& Elsevier]

We review the ecological consequences of N deposition on the five Mediterranean regions of the world. Seasonality of precipitation and fires regulate the N cycle in these water-limited ecosystems, where dry N deposition dominates. Nitrogen accumulation in soils and on plant surfaces results in peaks of availability with the first winter rains. Decoupling between N flushes and plant demand promotes losses via leaching and gas emissions. Differences in P availability may control the response to N inputs and susceptibility to exotic plant invasion. Invasive grasses accumulate as fuel during the dry season, altering fire regimes. California and the Mediterranean Basin are the most threatened by N deposition; however, there is limited evidence for N deposition impacts outside of California. Consequently, more research is needed to determine critical loads for each region and vegetation type based on the most sensitive elements, such as changes in lichen species composition and N cycling. [Copyright &y& Elsevier]

Background: In the Peruvian Coastal Desert, an archipelago of fog oases, locally called lomas, are centers of biodiversity and of past human activity. Fog interception by a tree canopy, dominated by the legume tree tara (Caesalpinia spinosa), enables the occurrence in the Atiquipa lomas (southern Peru) of an environmental island with a diverse flora and high productivity. Although this forest provides essential services to the local population, it has suffered 90% anthropogenic reduction in area. Restoration efforts are now getting under way, including discussion as to the most appropriate reference ecosystem to use. Methodology/Principal Findings: Genetic diversity of tara was studied in the Atiquipa population and over a wide geographical and ecological range. Neither exclusive plastid haplotypes to loma formations nor clear geographical structuring of the genetic diversity was found. Photosynthetic performance and growth of seedlings naturally recruited in remnant patches of loma forest were compared with those of seedlings recruited or planted in the adjacent deforested area. Despite the greater water and nitrogen availability under tree canopy, growth of forest seedlings did not differ from that of those recruited into the deforested area, and was lower than that of planted seedlings. Tara seedlings exhibited tight stomatal control of photosynthesis, and a structural photoprotection by leaflet closure. These drought-avoiding mechanisms did not optimize seedling performance under the conditions produced by forest interception of fog moisture. Conclusions/Significance: Both weak geographic partitioning of genetic variation and lack of physiological specialization of seedlings to the forest water regime strongly suggest that tara was introduced to lomas by humans. Therefore, the most diverse fragment of lomas is the result of landscape management and resource use by pre-Columbian cultures. We argue that an appropriate reference ecosystem for ecological restoration of lomas should include sustainable agroforestry practices that emulate the outcomes of ancient uses. [ABSTRACT FROM AUTHOR]

We are fertilizing a thicket with 0, 10, 20 and 50kg nitrogen (N) ha−1 yr−1 in central Spain. Here we report changes in cover, pigments, pigment ratios and FvFm of the N-tolerant, terricolous, lichen Cladonia foliacea after 1–2 y adding N in order to study its potential as biomarker of atmospheric pollution. Cover tended to increase. Pigments increased with fertilization independently of the dose supplied but only significantly with soil nitrate as covariate. β-carotene/chlorophylls increased with 20–50kgNha−1 yr−1 (over the background) and neoxanthin/chlorophylls also increased with N. (Neoxanthin+lutein)/carotene decreased with N when nitrate and pH seasonalities were used as covariates. FvFm showed a critical load above 40kgNha−1 yr−1. Water-stress, iron and copper also explained variables of lichen physiology. We conclude that this tolerant lichen could be used as biomarker and that responses to N are complex in heterogeneous Mediterranean-type landscapes. [Copyright &y& Elsevier]

Several recent studies have demonstrated that the expression of a cyanobacterial flavodoxin in plants can provide tolerance to a wide range of environmental stresses. Indeed, this strategy has been proposed as a potentially powerful biotechnological tool to generate multiple-tolerant crops. To determine whether flavodoxin expression specifically increased tolerance to salt stress and whether it might also preserve legume nitrogen fixation under saline conditions, the flavodoxin gene was introduced into the model legume Medicago truncatula. Expression of flavodoxin did not confer saline tolerance to the whole plant, although the sensitive nitrogen-fixing activity was maintained under salt stress in flavodoxin-expressing plants. Our results indicate that flavodoxin induced small but significant changes in the enzymatic activities involved in the nodule redox balance that might be responsible for the positive effect on nitrogen fixation. Expression of flavodoxin can be regarded as a potential tool to improve legume symbiotic performance under salt stress, and possibly other environmental stresses. [ABSTRACT FROM AUTHOR]

Parmelia sulcata Taylor was used as a model to examine the effects of elevated CO2 and/or O3 on green algal lichens. Thalli were exposed for 30 d in duplicate controlled-environment chambers to two atmospheric concentrations of CO2 ( 'ambient' [350 μmol mol-1 and `elevated' [700 μmol mol-1 ] 24 h d-1 ) and two O3 regimes (`non-polluted' ait [CF, < 5 nmol mol -1 and polluted' air [15 nmol mol-1` overnight rising to a midday maximum of 75 nmol mcl -1). in a factorial design. Elevated (CO2, or elevated O3 depressed the light saturated rate of CO2 assimilation (Asat) measured at ambient CO2 by 30% and 18 %respectively. However, despite this effect ultrastructural studies revealed increased lipid Storage in cells of the photobiont in response to CO2-enrichment. Simultaneous exposure to elevated O3 reduced CO2-induced lipid accumulation and reduced Asat in an additive manner. Gold-antibody labelling revealed that the decline in photosynthetic capacity induced by elevated CO2 and/or O3, was accompanied by a parallel decrease in the concentration of Rubisco in the algal pyrenoid (r = 0.93). conditions inevitably imposed Some stress on the thalli, revealed as a significant decline in the efficiency of PS II photochemistry, and enhanced starch accumulation in the photobiont over the fumigation period, the study shows that the green-algal lichen symbiosis might be influenced by future changes in atmospheric composition. Photosynthetic capacity, measured at ambient CO2 was Found to be reduced after a controlled 30d exposure to elevated CO2 and/or O3, and this effect was associated with a parallel decline in the amount of Rubisco in the pyrenoid of algal chloroplasts. [ABSTRACT FROM AUTHOR]

Ozone (O3) effects on the growth and gas exchange-related parameters of two ecotypes of Quercus coccifera L. presenting different traits in the field were assessed during a 2-years exposure experiment. One-year-old seedlings grown from the acorns collected at a rock (rock ecotype) or at a well-developed soil (garrigue ecotype) were continuously exposed in open-top chambers to three different O3 treatments: charcoal filtered air, non-filtered air and non-filtered air supplemented with 40 nl l−1. Above-ground relative growth rate (RGR) and gas-exchange-related parameters were measured during each growth season. Above- and below-ground biomass were also measured at the beginning and the end of the experiment. Ozone exposure did not induce any significant effect on RGR or any biomass-related parameters. However, the two populations differed in their sensitivity to O3 regarding gas-exchange-related parameters. The observed responses were related with accumulated O3 exposure or cumulative O3 uptake. Although the re-parameterisation of the existing European Monitoring and Evaluation Programme O3 uptake model improved its performance by 30%, O3 accumulated exposure was a better predictor of the observed responses than O3 uptake. The results of the present experiment indicate the need of taking into account the intraspecific variations in plant sensitivity to O3 when assessing the potential risks or impacts of this pollutant. [Copyright &y& Elsevier]

Abstract: N deposition is currently affecting nutrient cycling in terrestrial ecosystems. We studied the effects of four years of N application (0, 10, 20 and 50kgNha−1 year−1 +background deposition) on soil chemistry and fertility in a semiarid shrubland in central Spain. Soil pH and nutrient availability fluctuated seasonally. The inorganic-N fraction in soil was dominated by nitrate, as expected in calcareous soils. N application increased inorganic N availability in soil. There was a negative correlation between N application and soil K+ availability and pH, measured as the % change after four years. Soil N and C storage (evaluated as the % change) slightly increased after four years. Our data suggest that, in the short-term, the seasonality of nutrients overwhelm any chemical alteration related to N deposition. However, the potential implication of continuous N addition on soil chemistry in the long-term is not well understood. [Copyright &y& Elsevier]

Abstract: Despite the critical role of biological soil crusts (BSCs) in arid and semi-arid ecosystem function, few studies are found concerning the most important environmental variables affecting their distribution and physiology. This study seeks to determine soil and microenvironmental factors affecting the spatial distribution and pigment production of BSC-forming lichens and mosses in open patches of a semi-arid Mediterranean kermes oak thicket. We measured late-successional BSC cover, shrub cover, distance to nearest kermes oak (to test for effects of kermes oak thicket microenvironment on BSC), and pigment concentration of one lichen (Cladonia foliacea) and one moss (Pleurochaete squarrosa) species in the Nature Reserve El Regajal-Mar de Ontígola (Central Spain). At the macroscale (>0.5m), results showed that BSC distribution and pigments were tightly coupled to a suite of soil properties, in particular soil pH, Fe, and Ca. Specifically, soil pH had a positive relationship with the cover of five individual BSC-forming lichen species and was negatively related to pigment production in C. foliacea. When pH was excluded from the analysis, Ca appeared as the main soil variable and was correlated with total BSC cover and total lichen cover. The micronutrient Fe had a significant positive relationship with the concentration of eight pigments in P. squarrosa and was also coupled with the cover of two BSC-forming lichens. Manganese, previously proposed as a key limiting micronutrient for BSCs, affected lichen diversity in a negative way. At the microscale (∼0.5m), kermes oak microenvironment, shrub cover, and moss cover were determinants of BSC distribution, and total lichen and total BSC cover were overrepresented on N and E-facing shrub microsites. Our findings suggest that soil chemical variability and microsite diversity created by neighbouring vegetation affect BSC distribution in complex and essential ways and that studies aiming to explore BSC-environment relationships should be conducted at various spatial scales. Studies based on species- or group-specific responses are, thus, inadequate to unveil the main factors determining the distribution of the diverse organisms that constitute BSCs and/or to propose potential tools aiming to restore BSC in arid and semiarid ecosystems. [Copyright &y& Elsevier]

Abstract: Interspecific introgression is a well-known phenomenon in oaks whose ecological and evolutionary consequences, although relevant, are still unclear. We investigated molecular variation and any evidence for hybridisation in nine natural populations of kermes oak (Quercus coccifera L.) from the Iberian Peninsula. Additive patterns in 59 nrDNA ITS sequences revealed that hybrid individuals showing intermediate genotypes between kermes and holm (Q. ilex L.) oaks are very frequent, although intermediate morphotypes are uncommon. Bayesian analysis of ISSR fingerprinting patterns indicated extensive gene-flow among Q. coccifera populations and neighbouring Quercus taxa. Introgression appeared to be an active mechanism in the pair Q. coccifera–Q. ilex, but no conclusive evidence supporting hybridisation between Q. coccifera and other co-occurring Quercus species was found. The role of canalisation in the maintenance of stable morphological characters in the face of extensive introgression is discussed. [Copyright &y& Elsevier]

Soil nitrogen (N) availability is a key driver of soil-atmosphere greenhouse gas (GHG) exchange, yet we are far from understanding how increases in N deposition due to human activities will influence the net soil-atmosphere fluxes of the three most important GHGs: nitrous oxide (N 2 O), methane (CH 4) and carbon dioxide (CO 2). We simulated four levels of N deposition (10, 20 and 50 kg N ha−1 yr−1, plus unfertilised control) to evaluate their effects on N 2 O, CH 4 and CO 2 soil fluxes in a semiarid shrubland in central Spain. After 8 years of experimental fertilisation, increasing N availability led to a consistent increase in N 2 O emissions, likely due to simultaneous increases in soil microbial nitrification and/or denitrification processes. However, only intermediate levels of N fertilisation reduced CH 4 uptake, while increasing N fertilisation had no effects on CO 2 fluxes, suggesting complex interactions between N deposition loads and GHG fluxes. Our study provides novel insight into the responses of GHGs to N deposition in drylands, forecasting increases in N 2 O emissions, and decreases in CH 4 uptake rates, with likely consequences to the on-going climate change. Unlabelled Image • Soil greenhouse gas fluxes are driven by N availability in a Mediterranean dryland. • N fertilisation increases soil N 2 O emissions. • Only intermediate levels of N fertilisation reduced soil CH 4 uptake. • Soil CO 2 fluxes were not affected after 8 years of N fertilisation. [ABSTRACT FROM AUTHOR]

Soil biocrusts (communities of cyanobacteria, algae, mosses, lichens, and heterotrophs living at the soil surface) are fundamental components of dryland ecosystems worldwide. There is increasing concern over the potential for increasing nitrogen (N) inputs to affect biocrusts. This is of special concern in Mediterranean Basin drylands that face the threat of increased N inputs however, the effect on biocrusts remains poorly studied. We evaluated the potential effects of increased N inputs on biocrust structure and functioning in surrounding Mediterranean shrublands in the seventh year of a N-manipulation field experiment. We tracked the N-driven changes in biotope (changes in bare soil and in the non-legume and the legume occupation areas, and the percentage of radiation intercepted by plant canopies), evaluated biocrust functional traits (based on pigments) and measured biocrust functioning in terms of C and N cycling, soil fertility (macro and micronutrients) and biodiversity, and integrated these multiple soil functions simultaneously (i.e. soil multifunctionality) Biocrust pigment concentration was significantly influenced by both plant legacy and N input. Biocrust pigments revealed a clear functional shift from: i) biocrusts dominated by photosynthetically inactive cyanobacteria that fix N 2 and are mostly committed to photoprotection at the expense of N-containing pigments under low N inputs; into ii) biocrusts more evenly composed of prokaryotes and eukaryotes, which are more photosynthetically active, but less committed to photoprotection and N 2 fixation under exposure to increased N inputs. The N-driven functional and structural changes in biocrusts resulted in trade-offs in biocrust functioning and processes (only N 2 fixation was affected) and an overall improvement in biocrust multifunctionality. By itself, biocrust pigment evenness accounted for ~50% of the observed variation in biocrust multifunctionality. The biocrust pigment functional approach we adopted to study the effects of increased N inputs from patchy developed anthropogenic landscapes provides novel and critical knowledge of biocrusts community and functioning, which may be used as a tool in biodiversity conservation strategies, ecosystem functions and ecological modelling. • Biocrust pigments were influenced by both increased N inputs and plant legacy. • The N-driven changes resulted in trade-offs in biocrust functioning. • Increased N inputs improved biocrust multifunctionality. [ABSTRACT FROM AUTHOR]