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Plant health is increasingly threatened by abiotic and biotic stressors linked to anthropogenic global change. These stressors are frequently studied in isolation. However, they might have non-additive (antagonistic or synergistic) interactive effects that affect plant communities in unexpected ways. We conducted a global meta-analysis to summarize existing evidence on the joint effects of climate change (drought and warming) and biotic attack (pathogens) on plant performance. We also investigated the effect of drought and warming on pathogen performance, as this information is crucial for a mechanistic interpretation of potential indirect effects of climate change on plant performance mediated by pathogens. The final databases included 1230 pairwise cases extracted from 117 recently published scientific articles (from 2006) on a global scale. We found that the combined negative effects of drought and pathogens on plant growth were lower than expected based on their main effects, supporting the existence of antagonistic interactions. Thus, the larger the magnitude of the drought, the lower the pathogen capacity to limit plant growth. On the other hand, the combination of warming and pathogens caused larger plant damage than expected, supporting the existence of synergistic interactions. Our results on the effects of drought and warming on pathogens revealed a limitation of their growth rates and abundance in vitro but an improvement under natural conditions, where multiple factors operate across the microbiome. Further research on the impact of climate change on traits explicitly defining the infective ability of pathogens would enhance the assessment of its indirect effects on plants. The evaluated plant and pathogen responses were conditioned by the intensity of drought or warming and by moderator categorical variables defining the pathosystems. Overall, our findings reveal the need to incorporate the joint effect of climatic and biotic components of global change, Plant health is increasingly threatened by abiotic and biotic stressors linked to anthropogenic global change. These stressors are frequently studied in isolation. However, they might have non-additive (antagonistic or synergistic) interactive effects that affect plant communities in unexpected ways. We conducted a global meta-analysis to summarize existing evidence on the joint effects of climate change (drought and warming) and biotic attack (pathogens) on plant performance. We also investigated the effect of drought and warming on pathogen performance, as this information is crucial for a mechanistic interpretation of potential indirect effects of climate change on plant performance mediated by pathogens. The final databases included 1230 pairwise cases extracted from 117 recently published scientific articles (from 2006) on a global scale. We found that the combined negative effects of drought and pathogens on plant growth were lower than expected based on their main effects, supporting the existence of antagonistic interactions. Thus, the larger the magnitude of the drought, the lower the pathogen capacity to limit plant growth. On the other hand, the combination of warming and pathogens caused larger plant damage than expected, supporting the existence of synergistic interactions. Our results on the effects of drought and warming on pathogens revealed a limitation of their growth rates and abundance in vitro but an improvement under natural conditions, where multiple factors operate across the microbiome. Further research on the impact of climate change on traits explicitly defining the infective ability of pathogens would enhance the assessment of its indirect effects on plants. The evaluated plant and pathogen responses were conditioned by the intensity of drought or warming and by moderator categorical variables defining the pathosystems. Overall, our findings reveal the need to incorporate the joint effect of climatic and biotic components of global cha

The responses of plants and their associated microorganisms to trace elements (TE) in soils present different patterns depending on the plant species and the TE concentration. In this work, the behaviours of several plant species growing in the Guadiamar Green Corridor (GGC) (Seville, Spain), contaminated by the Aznalcóllar mine spill, are discussed. Soil fungal diversity and mycorrhizal symbioses with herbaceous and tree species in the area are also summarized. TE accumulation in wild herbaceous plants (Poaceae, Asteraceae, Brassicaceae, and Fabaceae) differed among families and species but, with some exceptions, most of them tended to exclude TE from their aerial biomass. Most of the studied afforested shrubs and tree species in the GGC revealed an exclusion strategy for most of the TE, meaning that these species could be adequate for TE phytostabilization. White poplar accumulated high concentrations of Cd and Zn in its aerial tissues, thus being a good candidate for phytoextraction and biomonitoring of these elements. In contrast, other two native Mediterranean trees, holm oak and wild olive, as well as eucalyptus trees, presented a behaviour of TE exclusion (except for Mn). The study of several native thistles revealed their suitability for bioenergy production, although Cd contents in these plants were usually high compared to reference levels. Regarding fungal mycorrhization, arbuscule abundance in herbaceous roots colonized by arbuscular mycorrhizal fungi (AMF) was reduced by soil contamination. In the case of ectomycorrhizal (ECM) fungi, soil contamination caused a reduction of hyphal development in holm oak. However, our results indicate that TE in soil do not affect root fungal Shannon diversity, richness or guild abundance underneath wild olive and stone pine plants. Finally, the edible fungal sporocarps studied in the GCC presented bioaccumulation of Cu, Zn, and, especially, Cd., The responses of plants and their associated microorganisms to trace elements (TE) in soils present different patterns depending on the plant species and the TE concentration. In this work, the behaviours of several plant species growing in the Guadiamar Green Corridor (GGC) (Seville, Spain), contaminated by the Aznalcóllar mine spill, are discussed. Soil fungal diversity and mycorrhizal symbioses with herbaceous and tree species in the area are also summarized. TE accumulation in wild herbaceous plants (Poaceae, Asteraceae, Brassicaceae, and Fabaceae) differed among families and species but, with some exceptions, most of them tended to exclude TE from their aerial biomass. Most of the studied afforested shrubs and tree species in the GGC revealed an exclusion strategy for most of the TE, meaning that these species could be adequate for TE phytostabilization. White poplar accumulated high concentrations of Cd and Zn in its aerial tissues, thus being a good candidate for phytoextraction and biomonitoring of these elements. In contrast, other two native Mediterranean trees, holm oak and wild olive, as well as eucalyptus trees, presented a behaviour of TE exclusion (except for Mn). The study of several native thistles revealed their suitability for bioenergy production, although Cd contents in these plants were usually high compared to reference levels. Regarding fungal mycorrhization, arbuscule abundance in herbaceous roots colonized by arbuscular mycorrhizal fungi (AMF) was reduced by soil contamination. In the case of ectomycorrhizal (ECM) fungi, soil contamination caused a reduction of hyphal development in holm oak. However, our results indicate that TE in soil do not affect root fungal Shannon diversity, richness or guild abundance underneath wild olive and stone pine plants. Finally, the edible fungal sporocarps studied in the GCC presented bioaccumulation of Cu, Zn, and, especially, Cd.

The responses of plants and their associated microorganisms to trace elements (TE) in soils present different patterns depending on the plant species and the TE concentration. In this work, the behaviours of several plant species growing in the Guadiamar Green Corridor (GGC) (Seville, Spain), contaminated by the Aznalcóllar mine spill, are discussed. Soil fungal diversity and mycorrhizal symbioses with herbaceous and tree species in the area are also summarized. TE accumulation in wild herbaceous plants (Poaceae, Asteraceae, Brassicaceae, and Fabaceae) differed among families and species but, with some exceptions, most of them tended to exclude TE from their aerial biomass. Most of the studied afforested shrubs and tree species in the GGC revealed an exclusion strategy for most of the TE, meaning that these species could be adequate for TE phytostabilization. White poplar accumulated high concentrations of Cd and Zn in its aerial tissues, thus being a good candidate for phytoextraction and biomonitoring of these elements. In contrast, other two native Mediterranean trees, holm oak and wild olive, as well as eucalyptus trees, presented a behaviour of TE exclusion (except for Mn). The study of several native thistles revealed their suitability for bioenergy production, although Cd contents in these plants were usually high compared to reference levels. Regarding fungal mycorrhization, arbuscule abundance in herbaceous roots colonized by arbuscular mycorrhizal fungi (AMF) was reduced by soil contamination. In the case of ectomycorrhizal (ECM) fungi, soil contamination caused a reduction of hyphal development in holm oak. However, our results indicate that TE in soil do not affect root fungal Shannon diversity, richness or guild abundance underneath wild olive and stone pine plants. Finally, the edible fungal sporocarps studied in the GCC presented bioaccumulation of Cu, Zn, and, especially, Cd.

Soil microbes play a key role in the regulation of plant performance. Rapid microbial responses to climate change (facilitated by their short life cycles) might help plant species (with lower short term adaptation capacity) to survive under increasing aridity. To test this hypothesis, we conducted a greenhouse experiment where seedlings of three tree species (Quercus suber, Quercus canariensis, Olea europaea) were grown with different soil inoculums and subjected to a process of drought and re-wetting. Soil inoculums were collected in a rainfall exclusion infrastructure located in Los Alcornocales Natural Park (Cádiz), where soils had been subjected to 30% rainfall reduction during four years. Half of the inoculums were sterilized to explore the net role of the soil microbiome in the seedlings response to drought. Results showed that four years of rainfall exclusion significantly altered the diversity and composition of the fungal and bacterial soil community. However, this alteration only translated into a higher ability to resist a subsequent drought in seedlings of O. europaea. Quercus seedlings were relatively independent of the soil microbiome in terms of growth, but not of physiological performance. Whereas Quercus seedlings growing in unsterilized soils were able to down-regulate photosynthesis and stomatal conductance in response to drought, seedlings in sterilized soils did not, which might compromise their long-term survival. Overall, our results showed that soil microbes play a relevant role in the response to drought of Mediterranean tree seedlings, and that they might contribute to the adaptation of at least some species to a drier climate.

Mediterranean ecosystems are a hot-spot of climate change. Increasing warming and drought have been shown to strongly alter the diversity and composition of plant communities. However, much less is known about their interactive effects on soil microbial communities, particularly in forest systems. Here we aim to fill this gap by analysing the combined effects of rainfall exclusion (~30% reduction) and soil warming (~ 1 °C increase) on the abundance (qPCR), diversity and composition (metabarcoding) of the soil microbiome in Quercus suber forests of southern Spain. The study focused on the two dominant groups of soil microbes (bacteria and fungi), as well as on the oomycete genus Phytophthora due to its fundamental role as driver of emerging diseases in plant communities. Results showed that 4 years of climatic treatments had much larger impacts on the quantity and composition of the three microbial groups than on their diversity. For several variables, warming and rainfall exclusion had antagonistic non-additive effects where their combined effects were of lower magnitude than the sum of their individual effects. Overall, a warmer and drier climate increased the abundance of saprophytic fungi and dominant phyla of oligotrophic bacteria (Firmicutes), whereas it decreased the abundance of mutualistic and antagonistic fungi and Phytophthora as well as of copiotrophic bacteria (Proteobacteria). These changes in the soil microbiome might have important consequences for the functioning of Mediterranean forests, altering C turnover dynamics and having complex effects on tree health through the disruption of tree-mycorrhiza and tree-pathogen interactions.

Soil biodiversity loss due to pollution may affect ecosystem services negatively. This environmental problem may be solved by phytoremediation, which is an effective strategy to manage and remediate contaminated areas. During this remediation process, the establishment of plant communities may improve soil fungal community structure and, in particular, may favour mycorrhizal symbiotic associations. As a consequence, afforestation of degraded lands will have different outcomes on fungal diversity and functionality, which will depend on the selected tree and shrub species. We analysed soil fungal diversity and functional guilds by high-throughput sequencing of environmental DNA in a trace element contaminated area, part of a large scale phytoremediation project running for 20 years. We selected five habitats for comparison purposes: three under the canopy of selected tree species (wild olive, white poplar and stone pine), adjacent treeless areas (grassland) and non-remediated areas (bare soil). Soil fungal diversity and richness seemed to be enhanced by phytoremediation. White poplar soil had the highest diversity and richness compared to wild olive and stone pine. Fungal communities were especially different between stone pine, with soils rich in organic C and high C:N ratio, and grassland soils. We identified 9,428 fungal OTUs from which 1,283 were assigned to a unique functional guild; the most abundant belonging to saprotrophic, plant pathogenic and ectomycorrhizal functional guilds. Ectomycorrhizal fungi were more abundant in soils under ectomycorrhizal host trees. Saprotrophs were abundant in grassland and wild olive soils, while plant pathogens were abundant in non-remediated soils. The remediation of soils (clean-up and amendment addition) allowed the natural establishment of grassland habitats throughout the study area, increasing fungal diversity, richness, taxonomy and functionality, when compared to non-remediated soils. Tree afforestation allowed the esta

Reversion of agricultural land to heathland and acid grassland is a priority for the conservation of these rare habitats. Restoration processes require interventions to reverse the effects of fertilization and acidity amelioration undertaken during decades of agricultural production. Belowground assessments of restoration success are few, and we have examined the utility of soil indices as a rationalized tool for land managers and restoration practitioners to assess the efficacy of restoration practice. To achieve this, we assessed a large number of variables, many of which might be near redundant, that could be optimized for such indices. We used a 14-year field experiment contrasting acidified pasture (treated with elemental sulphur), control (untreated) pasture, and adjacent native heathland and acid grassland sites. Based on biotic and abiotic parameters, several ‘heathland restoration indices’ (resembling soil quality indices) were generated using a minimum dataset identified through principal component analysis and a linear scoring system. For comparison we also conducted alternative analyses of all parameters, using nonmetric multidimensional scaling plots and analyses of similarity (ANOSIM). Use of heathland restoration indices showed that elemental sulphur application had changed the soil chemical conditions, along with the vegetation assemblage, to be comparable to that of native acid grassland, but not the belowground biology. ANOSIM on full datasets confirmed this finding. An index based on key variables, rather than an analysis of all biotic and abiotic parameters, can be valuable to land managers and stakeholders in acid grassland and heathland restoration

La contaminación es uno de los principales problemas ambientales a escala global. La mayor parte de los contaminantes provienen de actividades humanas, como la industria, la agricultura y las actividades domésticas. Y entre las diferentes sustancias o elementos contaminantes encontramos a los metales pesados. El término metales pesados hace referencia a un grupo de metales y metaloides que a menudo causan toxicidad en los organismos, además de problemas ecológicos en el medio ambiente. En concentraciones altas, estos metales pueden causar problemas de toxicidad en humanos. La dosis es algo esencial para conocer el riesgo tóxico, ya que estos metales no son biodegradables y se van acumulando en nuestro organismo. Si los metales se empiezan a acumular en nuestro cuerpo, pueden comenzar a sustituir a otros elementos que sí necesitamos para vivir y alteran procesos fisiológicos básicos pudiendo desembocar en diferentes enfermedades. Los elementos más tóxicos para los humanos y animales son el mercurio, el plomo, el cadmio, el cromo y el arsénico; mientras que para las plantas son el cobre, el níquel, el cobalto y el cinc.

[EN]: The Guadiamar Green Corridor is an area contaminated by trace elements due to a mine spill in 1998. After this accident, a phytoremediation strategy was established with the aim of remediating and establishing a novel ecosystem. The first objective of this Doctoral Thesis was to evaluate the influence of the identity of the forested tree species in the Corridor on the composition and microbial activity. Soils were sampled under three tree species (wild olive, white poplar and stone pine) and in grassland soils, at two sites with different soil properties. Afforestation produced an increase in most microbial indicators. Each tree species produced specific effects on soil organic matter, pH and C:N ratio, with consequences on biomass and microbial activities. White poplar was the species that promoted a greater catabolic diversity, which could mean a greater mineralization of simple and complex carbon substrates in these soils. Furthermore, both stone pine and white poplar promoted greater enzymatic activity in these soils. The second objective of this Doctoral Thesis was to evaluate the effect of the different afforested tree species in the Corridor on the structure and composition of soil fungal communities. Afforestation of degraded soils with different plant species may promote the establishment of specific fungal diversity and functionality. Five different habitats were selected: under the canopy of three tree species (wild olive, white poplar and stone pine), on adjacent grassland soils and on non-remediated soils. We found that the phytoremediation measures promoted the fungal richness, diversity, taxonomy, and functionality along the Guadiamar Green Corridor, compared to non-remediated soils. White poplar was the tree species with the greatest diversity and richness compared to wild olive and stone pine. Ectomycorrhizal fungi were more dominant in the soils of the host species of these fungi, while saprotrophs were abundant in grassland and wild olive so, [ES]: El corredor Verde del Guadiamar se trata de una zona contaminada por elementos traza tras un vertido minero en 1998. Tras este accidente se estableció una estrategia de fitorremediación con el objetivo de remediar y establecer un ecosistema novel. El primer objetivo de esta Tesis Doctoral fue evaluar la influencia de la identidad de las especies arbóreas forestadas en el Corredor en la composición y actividad microbianas. Se estudiaron los suelos bajo tres especies leñosas (acebuche, álamo blanco y pino piñonero) y en suelos de pradera, en dos sitios con diferentes propiedades edáficas. La forestación produjo un aumento en la mayoría de los indicadores microbianos. Cada especie leñosa produjo efectos específicos en la materia orgánica del suelo, el pH y la relación C:N, con consecuencias en la biomasa y la actividad microbianas. El álamo blanco fue la especie que promovió una mayor diversidad catabólica lo que podría suponer una mayor mineralización de sustratos simples y complejos de carbono en estos suelos. Además, tanto el pino piñonero como el álamo blanco promovieron una mayor actividad enzimática en estos suelos. El segundo objetivo de esta Tesis Doctoral fue evaluar el efecto de las diferentes especies arbóreas forestadas en el Corredor en la estructura y composición de las comunidades de hongos del suelo. La forestación de suelos degradados con diferentes especies de plantas puede promover el establecimiento de una diversidad y funcionalidad fúngicas determinadas. Se seleccionaron cinco hábitats diferentes: bajo la copa de tres especies leñosas (acebuche, álamo blanco y pino piñonero), en suelos de pradera adyacentes y en suelos no remediados. Encontramos que las medidas de fitorremediación promovieron la riqueza, diversidad, taxonomía y funcionalidad de hongos del suelo en el Corredor Verde del Guadiamar, en comparación con los suelos no remediados. El álamo blanco fue la especie leñosa con la mayor diversidad y riqueza en comparación con el acebuche

In the wider context of heathland restoration, we investigated how field scale experimental acidification with sulphur (sulfur)affected soil biodiversity, fertility and function over a period of 17 years. A field experiment was conducted in the Isle of Purbeck, England, using ferrous sulphate and elemental sulphur as acidifying agents. We tested the effects of acidification on soil fertility, plant communities, litter decomposition, microbiology (including fungi bacteria and actinomycetes), arbuscular and ericoid mycorrhizal colonisation, and soil fauna (including earthworms, nematodes, rotifers and tardigrades). We found that elemental sulphur had a considerable and persistent effect on soil pH, lowering it to levels found in the surrounding reference acid grassland and heathland sites. A newly adapted heathland restoration index based on soil chemistry, found that elemental sulphur was by far the most successful treatment leading to soil conditions similar to the heathlands. Overall, acidification caused a loss of base cations and an increase in toxic aluminium compounds. Consequently the more mesotrophic components of soil biology were reduced by acidification during the course of the experiment. This transformed the soil biological system into one typical of acid grasslands and heathlands. Concomitant litter decomposition was similarly inhibited by acidification, with the microbiota more strongly hindered in acidified soil than the macroscopic fauna. Acidification resulted in a reduction in nematode and rotifer abundance and earthworm biomass. The vegetation community was also strongly modified by the elemental sulphur treatments and, where grazing was restricted, soil acidification allowed a restored heathland community to endure. Arbuscular mycorrhizal colonisation of grasses was reduced where heather plants were established, while ericoid mycorrhizas had developed sufficient populations in the acidified pastures to match the colonisation rate in the native he

Fungi play a key role in terrestrial ecosystems as organic matter decomposers and by interacting with plants as symbionts or pathogens. In trace element (e.g. heavy metal) contaminated soils, the effect of restoration measures, such as tree afforestation, in enhancing ecosystem services is of interest. In this study, we hypothesised that vegetation, through soil biotic and abiotic changes, is key in the establishment of soil fungal communities in terms of structure, diversity and function. Twenty years after the accidental spill of the Aznalcóllar mine (April 1998), where acidic waters and pyrite sludge with high concentration of trace elements were released into the Guadiamar River basin, in SW Spain, we have evaluated the recovery of the soil fungal communities. Restoration measures included sludge removal followed by amendments addition and afforestation with native tree species. We selected restored soils within a plant cover gradient consisting of bare soil, grassland to tree (the latter with a comparison between three native species: white poplar, stone pine and wild olive). We also selected two sites, North and South, located at 15 and 30 km from the mine tailing, respectively. A total of 50 soil samples were analysed. We massive sequenced the fungal community in the above-mentioned soils (ITS1F-ITS2 region using the Illumina MiSeq platform). The fungal taxonomy was inferred and the taxa classified by their ecological guild. Interaction of site and plant cover fixed factors were found significant for soil abiotic C:N and P, but not to biotic variables (Table 1). Fungal diversity indices were lower in bare and stone pine soils. Species richness was significantly lower in bare soils. The composition and structure of fungal communities were significantly explained by the interaction of site and plant cover factors (R2 = 0.405). Fungal functionality was assessed by ecological guild ordination (Figure 1). Bare soils were dominated by plant pathogen fungi, while gr

In forest stands different tree species might have contrasted signals on soil organic matter due to their patterns of leaf litter and rood exudates, thus leading to species-specific interactions with the microbial communities in the soils underneath. In trace element-contaminated soils, organic matter is also associated with the dynamics of these elements in the soil-plant system. In a trace element contaminated area in SW Spain, different native tree species were planted in order to reduce the mobility of these contaminants. After 15 years of tree growth, we evaluated whether different tree species had specific influences on: 1) trace element stabilization in soil, 2) the quality of soil organic matter and 3) the functioning of the soil microbial community. Two areas with different soil bedrock and contamination level (North and South) were selected in the Guadiamar Green Corridor. Soils and leaf litter were collected underneath three native tree species: white poplar, stone pine (Pinus pinea) and wild olive, as well as in a treeless soil covered by herbaceous plants. Main soil chemical properties were analysed, as well as soil microbial biomass and enzyme activities. The signal of different chemical groups litter and in the soil particulate organic matter (POM) was analysed by Fourier-transform infrared spectroscopy (FT-IR). Soil trace element availability was significantly higher in the North area due to the acidic soil pH and lower soil organic matter. Soils underneath pine trees in the North area presented higher acidification, which was linked to a more intense signal of those peaks characteristic of acid functional groups (pectin, resins and waxes) in the litter and the POM underneath this species. The comparison of FT-IR spectra of POM in both areas suggested a higher degree of trasnformation of the POM in the Southern area. Microbial biomass was however similar underneath the trees in both areas, but lower in treeless soil. The effects of trees on enzyme ac

Phytostabilisation is the use of plants and associated microorganisms to reduce the mobility of soil contaminants. Its efficiency is highly dependent on plant identity because of species-specific effects on soils, which also affect soil nutrient cycling and microbial growth. In a trace element (As, Cd, Cu, Pb, Zn) contaminated soil in SW Spain, a long-term phytoremediation plan was implemented to reduce the spread of contaminants in the mine-spill affected area. Soils underneath three native tree species: white poplar (Populus alba), stone pine (Pinus pinea) and wild olive (Olea europaea), were sampled 15 years after plantation to evaluate medium-term soil functionality. A significant effect of tree species on soil contamination levels was not found. In relation to soil nutrients, afforested soils presented an increasing soil fertility in comparison to treeless soils. Among species, soils underneath white poplar presented a lower acidification and a higher N content, while soils underneath stone pine were acidified and showed the highest C:N ratio. Regarding soil microbial biomass, a significant effect of stone pine was found indicating a reduction on biomass. Tree species showed different effects depending on specific soil enzyme activities, however, in general, enzyme activities were highly driven by soil acidification and contamination. In conclusion, the medium-term effect of phytostabilisation by the studied tree species was weak in terms of contamination stabilisation, however a marked tree species footprint was found in terms of soil nutrient contents as well as on microbial biomass and enzyme activities. Therefore, a species-specific effect was found with direct consequences on soil functionality.

Phytostabilisation technology in post‐closure metal‐mine industry may be a feasible reclamation solution to avoid the transfer of trace elements to adjacent systems and to provide long‐term benefits increasing the functionality of the reclaimed ecosystem. In a contaminated and remediated area in SW Spain, we determined the long‐term effects of different tree species on soil functionality. After the Aznalcóllar mine‐spill, a largescale phytostabilisation plan was launched including sludge and soil removal, amendment addition and native tree species afforestation. We selected two areas in the Guadiamar Green Corridor, the North and South fields, due to their different soil properties and contamination levels. Both fields were affected by the spill and were afforested by a mixed plantation pattern with specimens of the same age. Three native tree species were selected for this study: white poplar (Populus alba), stone pine (Pinus pinea) and wild olive (Olea europaea), and compared with adjacent treeless areas. Nineteen years after the phytostabilisation implementation, we measured soil physical‐chemical parameters and available and total trace element concentrations. For functional measurements, we analysed the microbial biomass C and N as well as a varied enzyme activities to measure the hydrolysis of assimilated products of C, N and P. A long‐term marked contamination effect was still significant along the Guadiamar Green Corridor. The North field, closer to the mine tailings, presented contamination concentrations above the Lower Guideline Values for total As, Cu, Pb and Zn. Tree afforestation was found to be positive for the increase of soil fertility and microbial biomass on trace element contaminated soils. The studied tree species were found to affect differently the soil chemistry and microbial communities. White poplar and stone pine were the tree species with the greatest effects on soil. Soils under white poplar presented less acidification and more N conten

Soil microorganisms have a huge influence in the organic matter oxidation, hydrolysis and degradation. Therefore, they are essential to constitute the optimal conditions for the recovery of the ecosystem functions and the development of a vegetation cover in contaminated soils. Restoration measures were carried out after the accidental spill of the Aznalcóllar mine in 1998, where acidic waters and pyrite sludge with high concentration of trace elements (As, Cd, Cu, Pb, S and Zn) were released into the Guadiamar River basin, in Spain. First, the sludge was removed and the soil clean-up, followed by amendments addition and afforestation with native tree species. Almost 20 years after the accident, we evaluated the recovery of the soil functionality and the ecosystem services provided. We analysed contaminated and restored soils in two sites with contrasted pH, trace elements availability and nutrient content. In these sites, different tree species, white poplar (Populus alba), stone pine (Pinus pinea) and wild olive (Olea europaea), were planted, each of them with a singular litter quality input into the soil. Adjacent soils under herbaceous cover were also analysed as non-tree control. In general, tree afforestation contributed to improve belowground activity due to a higher trace element stabilisation in the soils underneath, and enhanced soil microbial biomass and enzyme activities related to C, N and P cycles. However, significant differences among tree species were not found in relation to microbial activity. The ratios Cmic:Corg and Nmic:Ntot revealed a disturbance in the mineralization dynamic which may be explained by the effect of trace elements contamination in microbial biomass. In soils with acidic pH and, therefore, higher trace elements availability the ratios were significantly lower than in neutral pH soils.

The Guadiamar Green Corridor is an area contaminated by trace elements due to a mine spill in 1998. After this accident, a phytoremediation strategy was established with the aim of remediating and establishing a novel ecosystem. The first objective of this Doctoral Thesis was to evaluate the influence of the identity of the forested tree species in the Corridor on the composition and microbial activity. Soils were sampled under three tree species (wild olive, white poplar and stone pine) and in grassland soils, at two sites with different soil properties. Afforestation produced an increase in most microbial indicators. Each tree species produced specific effects on soil organic matter, pH and C:N ratio, with consequences on biomass and microbial activities. White poplar was the species that promoted a greater catabolic diversity, which could mean a greater mineralization of simple and complex carbon substrates in these soils. Furthermore, both stone pine and white poplar promoted greater enzymatic activity in these soils. The second objective of this Doctoral Thesis was to evaluate the effect of the different afforested tree species in the Corridor on the structure and composition of soil fungal communities. Afforestation of degraded soils with different plant species may promote the establishment of specific fungal diversity and functionality. Five different habitats were selected: under the canopy of three tree species (wild olive, white poplar and stone pine), on adjacent grassland soils and on non-remediated soils. We found that the phytoremediation measures promoted the fungal richness, diversity, taxonomy, and functionality along the Guadiamar Green Corridor, compared to non-remediated soils. White poplar was the tree species with the greatest diversity and richness compared to wild olive and stone pine. Ectomycorrhizal fungi were more dominant in the soils of the host species of these fungi, while saprotrophs were abundant in grassland and wild olive soils. T