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Understanding the combined effects of soil amendments and inoculation of mycorrhizal fungi on the response of different plant species during the phytostabilization process of trace elements contaminated soils is a challenge. This task is more difficult, but more realistic, when the study is carried out under field conditions.

A research team of the IRNAS-CSIC, in collaboration with researchers from EEZ-CSIC, has assessed the combined effects of two amendment doses (high and low doses) and mycorrhizal inoculation on the response of saplings of wild olive and stone pine, planted in a contaminated field site. The amendments were a mix of sugarbeet lime and biosolid compost. The inoculation treatments were made with a commercial inoculum of arbuscular mycorrhizal fungi for wild olive, while we used ectomycorrhizal fungi for stone pine. The experimental plot was located at the Guadiamar Green Corridor (Sevilla).

Results showed a weak or null effect of the mycorrhizal inoculation on plant growth, survival and trace element accumulation. There was only a significant increase on P nutrition for stone pine, growing on non-amended conditions. In general, the positive effect of mycorrhizal colonization on P nutrition is more evident under stress conditions and it can disappear under more favourable soil conditions.

The soil amendments were very effective reducing trace elements availability and their accumulation in both plant species, especially in roots. However, the effects on plant biomass were species-dependent and contrasted; low-dose amendments (30 T ha-1) increased the biomass of wild olive by 33.3%, but reduced by 28% that of pine. On the other hand, the high doses of amendments (60 T ha-1) produced some negative effects on plant growth and nutrition, probably related to the increase of soil salinity.

Both plant species, stone pine and wild olive, have been proved to be adequate for phytostabilization of contaminated soils under Mediterranean climate, due to their tolerance to drought and metals, and the low transfer of trace elements from root to shoot, thus reducing toxicity for the food web.

As a conclusion of this study, we highlight the importance of implementing microbial-assisted phytoremediation approaches, for which a better understanding of the diversity and ecology of plant-associated microorganisms is needed. The use of indigenous mycorrhizal fungi, locally adapted and tolerant to contamination, will favour to reach the phytostabilization purposes.

This study has been published in the Science of the Total Environment:

Madejón, P., Navarro-Fernández, C.M., Madejón, E., López-García, A., Marañón, T. (2021). Plant response to mycorrhizal inoculation and amendments on a contaminated soil, Science of the Total Environment, vol. 789, art. 147943.

The different plant strategies in the use of resources can be aligned on an economics spectrum, resulting from the trade-off between maximizing growth and resource capture (acquisitive strategy) versus the conservative strategy, with more protected organs of longer life span. Within this context, it is of great relevance to know the amount of photosynthate inverted by the plant on the construction of fine roots to explore the soil and uptake water and nutrients.

          A research team from the University of Cordoba, in collaboration with IRNAS-CSIC, CEBAS-CSIC and the University of Brandenburg-Cottbus, has studied the components of the construction costs (carbon, minerals and nitrogen concentrations) of the fine roots (< 2mm diameter) for 60 Mediterranean woody species.           The type of association with symbiotic microbes influenced on the contribution of different components in the root construction costs. Thus, the concentration of minerals was more important for the roots of plants associated with ectomycorrhizal fungi (EcM), while the organic nitrogen was dominant in roots associated with rhizobial N-fixing species. Therefore, the specific resource-use strategy associated to each type of symbiont modulate the trade-off between structural and metabolic compounds, during the building of roots.

Environmental conditions also influenced on the root construction costs. In nutrient-poor soils the energy spent per unit of root mass by plants was greater than the root cost in more fertile and favourable conditions. Plant species able to tolerate stress caused by water and nutrient scarcity often have a conservative strategy in the use of resources, with more costly roots of longer life span.   

In conclusion, the root construction cost is a key functional trait for terrestrial plants, related with their strategies in the use of resources, the type of associations with symbiotic microorganisms, and with the availability of soil water and nutrients.

This study has been published on line the 7 February 2021 in the Journal of Ecology:

de la Riva EG, Prieto I, Marañón T, Pérez‐Ramos IM, Olmo M, Villar R (2021). Root economics spectrum and construction costs in Mediterranean woody plants: The role of symbiotic associations and the environment. Journal of Ecology , 109: 1873-1885.‐2745.13612

The restoration of old mining areas, in particular if they are located near towns or villages, is essential to reduce their potential risks for human health and to minimize their visual impacts.

In this study, a research team of the IRNAS-CSIC, in collaboration with researchers from the University of Seville and the Environmental and Water Agency of the Regional Government of Andalusia (AMAYA), has assessed the effectiveness of the rehabilitation of an old waste rock pile (named Cabezo de los Gatos), derived from the mining activity, adjacent to the town of Tharsis (SW Spain).

The rehabilitation process, carried out twelve years ago, consisted in the reshape of the rock pile slope, followed by the application of limestone amendments and subsequent application of a topsoil layer. Finally, the revegetation of the area was implemented planting native shrubs and trees; in addition, a mixture of native grasses and shrubs was hydroseeded onto the remediated area.

In general, the applied measures were successful, covering with woody vegetation the upper part of the waste rock pile, and providing a greening visual landscape for the inhabitants of the town nearby. Moreover, the levels of trace elements contents in the aerial part of the vegetation growing in the upper of the slope were below the toxicity thresholds for the livestock potentially consuming them. In contrast, the lower part of the slope was almost devoid of vegetation. In addition, some plants, like the gum rose (Cistus ladanifer), growing at the base of the rock pile, had concentrations of Cd above the maximum tolerable level for animals, therefore representing a toxicity risk.

We interpret that partial failure in the rehabilitation process as due to the acid mine drainage, which caused downslope a decrease of soil pH and the consequent increased availability of trace elements, resulting in the accumulation of potentially toxic elements in the aerial parts of the plants. This contamination reduced the growth and establishment of plants, diminishing the vegetation cover.

To cope with the problem of acid mine drainage we propose an alternative approach to restore waste rock piles. We suggest to select extractive wastes and to place the non-acid generating ones around the periphery of the rock pile, as a protective cover. Thus, reducing infiltration rates, seepages and the negative effect of the acid mine drainage.

This study has been published in the Journal of Environmental Management:

Madejón, P., Caro-Moreno, D., Navarro-Fernández, C.M., Rossini-Oliva, S., Marañón, T. (2021) Rehabilitation of waste rock piles: Impact of acid drainage on potential toxicity by trace elements in plants and soil. Journal of Environmental Management, 280, art. no. 111848.

Fungi are a fundamental component of the soil microbiome, with essential ecosystem functions. They contribute to the nutrient cycling, through the organic matter decomposition; forming symbiotic associations with plants (mycorrhizas) they allow to explore a greater volume of soil to capture water and nutrients; on the other hand, pathogen fungi regulate plant populations.

A research team of the IRNAS-CSIC, in collaboration with researchers from the universities of Seville, Jaén and Copenhagen, has studied the soil fungal diversity and its relations with tree species in the Guadiamar Green Corridor (Seville, Spain). This area was remediated and afforested after a mine-spill contamination episode in 1998.   

Using high-throughput sequencing of environmental DNA allowed to identify more than 9,000 different fungal “species” (OTUs, operational taxonomic units) in 50 soil samples from two contrasted sites in the Green Corridor. In the South site (Quema), the highest fungal diversity was found under white poplar (Populus alba), with average of 1,168 species per tree, belonging to 141 different families. While under wild olives (Olea europaea var. sylvestris), the average diversity was 599 species of 95 families. In contrast, non-remediated soils, where the mine sludge was not cleaned up, and were not remediated neither afforested, had a low fungal diversity: average of only 51 species per sample and 20 families. Although these fungal species have interest due to their tolerance to extreme conditions of soil acidity and high concentration of potentially toxic elements.       

Venn diagram with fungal diversity in each habitat type.

We start to discover the magnitude of the soil biodiversity: hundreds or thousands of species in each sample, but we still ignore the functionality of all those species. The comparison with the existing database -FUNguild- allowed to assign the functional group for a 13.6% of the species recorded in the Green Corridor study. Within this representative sample (1,283 species), the dominant functional group was the saprothrophs fungi, which decompose organic matter and contribute to nutrient cycling, and were more tied to wild olive and grassland soils. The second most abundant group was the plant pathogens, mainly found in non-remediated soils. The third group, ectomycorrhizal fungi, was especially abundant in soils under trees, like pines and poplars, forming symbiotic associations with them.     

Tree planting to recover contaminated soils (phytoremediation) increases soil fungal diversity, in particular of ectomycorrhizal fungi. Restoring with different tree species, so forming a mixed forest, increases the spatial heterogeneity of environmental conditions; in consequence, the soil fungal biodiversity will be higher and the ecosystem services in the phytoremediated area will be promoted.

The study has been published in the journal Soil Biology and Biochemistry:

Gil-Martínez M, López-García Á, Domínguez MT, Kjøller R, Navarro-Fernández CM, Rosendahl S, Marañón T (2021). Soil fungal diversity and functionality are driven by plant species used in phytoremediation. Soil Biology and Biochemistry, 153: 108102.

The Aznalcóllar mine accident (Seville), in April 1998, was an ecological and socioeconomic catastrophe with important international impact. The remediation and restoration of the area affected by the mine spill transformed a devastated and contaminated area into the current Guadiamar Green Corridor Protected Landscape. Despite the successful recovery of the affected area, residual contamination is still present.

One of the main threats from the mine spill was the presence of potentially toxic trace elements, such as As, Cd, Pb and Tl, in the contaminated soil. Therefore, it exists a risk of toxicity due to their possible transfer from the soil to the food chain, since the contents of these elements at high concentrations can be harmful to livestock and wild animals, as well as to human health. A research team of the IRNAS-CSIC led by José Manuel Murillo, in collaboration with the researchers Paula Madejón, María Teresa Domínguez and Engracia Madejón, has published the book chapter entitled “Risks for the food chain in the Guadiamar Green Corridor”. In this chapter, most of the studies carried out in the Green Corridor, dealing with the transfer of trace elements at different levels within the food chain, during the 20 years since the accident are collected. In addition, the corresponding potential risks of toxicity for animals and for human consumption are evaluated.

Horse grazing in the Guadiamar Green Corridor (photo by M.T. Domínguez).

Three relevant case studies, conducted by IRNAS researchers, are described in more detail: the potential toxic effect on the horses that graze in the Green Corridor, the accumulation of trace elements in land snails, and the transfer and accumulation of these elements in mushrooms. Both snails and mushrooms are wild foods that can be collected in the Green Corridor and consumed by local residents.

Finally, it is clearly explained why the neighbours of the close villages of the Green Corridor must not collect any food product from the contaminated area, for human consumption.


Madejón P, Madejón E, Domínguez MT, Murillo JM (2020). Riesgos para la cadena trófica en el Corredor Verde del Guadiamar. In: P Madejón y T Marañón (eds.), Recuperación de suelos y provisión de servicios ecosistémicos en el Corredor Verde del Guadiamar, pp. 113-135. Editorial CSIC, Madrid.

The mine tailing accident occurring on April 1998 at Aznalcóllar (Seville, Spain) was a great ecological and socioeconomical disaster, with international impact. The remediation and restoration of that spill-affected area transformed a devastated and contaminated land into the current Protected Landscape of the Guadiamar Green Corridor.   

The Research Conference of the XX Anniversary of the Aznalcóllar Mine Accident was organized on April 2018 in the IRNAS-CSIC, Seville, as part of the transfer activities of the INTARSU project. Based on that Conference, the Editorial CSIC has published on July 2020 the book “Soil remediation and provision of ecosystem services in the Guadiamar Green Corridor” (in Spanish), edited by the INTARSU researchers Paula Madejón and Teodoro Marañón.

The final chapter of the book gathers the main topics raised during the roundtable which closed the meeting. In particular, the participants identified and discussed the main challenges and opportunities after the mine accident.    

The environmental disaster was a great challenge for the natural ecosystems within the Guadiamar Basin and for the economy of the affected area. Among the short-term challenges, it was priority the urgent cleaning and remediation of the polluted soils. At long-term, it was essential to maintain the monitoring and vigilance of the potentially toxic elements in the ecosystem. It was a relevant challenge to transmit to the local people that the toxicity risks were, although mitigated, still present and for long-term in the spill-affected area. From the conservationist perspective, the main challenge on middle- and long-term, was to consolidate the ecological corridor connecting Doñana and Sierra Morena; that ecological function should be compatible with the multiple use of the Protected Landscape. Looking at the future, a worrying challenge is the reopening of the mine; their activities must be compatible with the maintenance of environmental quality and healthy ecosystems. Finally, a pending challenge was remarked: to enforce the “polluter pays principle”.

Roundtable participants; from the left to the right, Teodoro Marañón (moderator), Miguel Ferrer, Emilio Galán, Paula Madejón, Francisco Cabrera, María Teresa Domínguez and Francisco Quirós.

At the same time, the recovery of the mine accident offered also new opportunities. The most relevant opportunity was the strong support to the environmental scientific research, in particular to fields like the remediation of soils contaminated by trace elements, and to the evaluation of potential toxicity of contaminants on plants and animals. The Guadiamar Green Corridor has been a large-scale natural laboratory where several remediation measures have been tested, and where the dynamics of trace elements have been monitored. For the conservationists, it was a real opportunity to have a privileged protected space (the Guadiamar river) able to connect the isolated populations of Doñana with the large populations of Sierra Morena. This newly protected area is also an opportunity to practice recreational activities and ecotourism. It was suggested as future opportunity for the Aznalcóllar mine, to develop new, cleaner technologies and more reliable exploitation systems.       


Marañón, T. (2020). Challenges and opportunities after the Aznalcóllar´s environmental disaster. In: P. Madejón and T. Marañón (eds.). Soil remediation and provision of ecosystem services in the Guadiamar Green Corridor, pp. 195-206. Editorial CSIC, Madrid (in Spanish).

ISBN: 978-84-00-10636-2

Root functions are multiple and essential for plant growth and survival. They include nutrient and water acquisition, resource storage, support of symbiotic soil microbes and anchorage. At the ecosystem level, they contribute to soil structure and to the carbon and nutrient cycles.

Researchers of the IRNAS-CSIC, in collaboration with the University of Seville and the University of Cordoba, have studied the variation of 27 root traits (morphological, chemical and isotopic) in seven tree species planted in the Guadiamar Green Corridor (Seville, Spain). Three of the species are deciduous: white poplar (Populus alba), narrow-leafed ash (Fraxinus angustifolia) and hackberry (Celtis autralis). While the other four species are evergreen: stone pine (Pinus pinea), holm oak (Quercus ilex), wild olive (Olea europaea) and carob (Ceratonia siliqua).

The main variation trend observed in the root traits supports the “root economics spectrum” hypothesis. There is a trade-off between tree species having lighter roots and higher length per mass unit, which maximizes soil resource acquisition and thus they grow faster on favourable conditions. On the contrary, tree species with denser roots and lower specific length tend to have resource conservation and slower growth, on adverse conditions.

Besides the main trend, there are other dimensions reflecting the root multifunctionality. 1) The root carbon concentration was not correlated with the morphological variables. 2) the ability to bind trace elements (for example, Pb and Cd) to root cells tends to be associated with the plant tolerance to soil contamination by metals. 3) The fractionation of 15N in roots is a time-integrated trait of mycorrhizal mediated nutrition.

In general, roots and leaves are functionally coordinated. The fast acquisition and processing of water and nutrients by the roots must be coupled with fast acquisition and processing of carbon by leaves. However, there was strong discordance between roots and leaves, with regards to the accumulation of several trace elements.

Soil conditions, and metal contamination in particular, affect morphological and chemical traits of tree roots. There are reciprocal interactions and feedback processes between soil and tree roots.

A remarkable physiological trait for these tree roots is the capacity to diminish the availability of trace elements into the soil, in special of those potentially toxic (Cd, Pb, As). We recommend planting trees with high “phytostabilisation” potential to remediate metal-contaminated soils.

This study has been published in the April issue of the journal Plant and Soil:

Marañón, T., Navarro-Fernández, C. M., Gil-Martínez, M., Domínguez, M. T., Madejón, P., Villar, R. (2020). Variation in morphological and chemical traits of Mediterranean tree roots: linkage with leaf traits and soil conditions. Plant and Soil, 449: 389-403.

Interactions between plants and soil are responsible for providing numerous ecosystem services from forests, including carbon sequestration, protection from erosion, improved soil fertility and stabilization of contaminants.

In the XXV World Congress of the IUFRO (International Union of Forest Research Organizations), held at Curitiba, Brasil from September 29th to October 5th, a scientific session (number E8,d,g) was devoted to the topic “Plant-soil interactions in forests”.

1) Different tree species affect differently to soil properties. For example, soil pH underneath pines (Pinus pinea) and holm oaks (Quercus ilex) was lower than under other five tree species in the same study area. These soil pH differences have relevant consequences for the mobility of trace elements and their uptake by trees (Madejón et al., 2018, Catena 166: 34-43).

2) Soil mycobiota differed in diversity and composition under wild olive (Olea europaea), stone pine (Pinus pinea) and white poplar (Populus alba), and with adjacent treeless sites. The afforestation represented 100% increase of soil fungal diversity, compared with the remediated zones (only with pastures). A total of 6535 OTUs (operational taxonomic units) were estimated as the gamma diversity of soil fungi in the study area (Gil-Martínez et al., in preparation).

The functional diversity of soil fungi (according to the FUNGuild data base) was represented by three main functional groups: saprotrophs (68% of OTUs), pathogens/parasites (18%) and mycorrhizal fungi (12%). Within the mycorrhizal fungi group, arbuscular fungi were associated to soil samples under wild olive and treeless pastures, while ectomycorrhizal fungi were abundant in soil under poplar and pine.

3) The case of mycorrhizal fungi is relevant because of their symbiotic relations with trees. Ectomycorrhizal fungi communities were studied by sampling root tips of 40 holm oaks (Q. ilex) distributed along four sites of the Guadiamar Green Corridor. A total of 55 OTUs were identified, with average of 3.8 fungal species per tree. Soil properties influencing primarily the composition of mycorrhizal fungal communities were total soil carbon, and the concentration of Ca, Cu, Ni and Zn (López-García et al., 2018, Soil Biol Biochem 121: 202-211).

In turn, the composition of mycorrhizal fungal communities affected significantly the accumulation of P in leaves, and the transfer rate of Zn from soil to root (Gil-Martínez et al., 2018, Front Plant Sci : 1682).

4) Soil fungi have a relevant role degrading organic matter and cycling nutrients, by the action of extracellular enzymes. We have found that beta-glucosidase activity (enzyme contributing to cellulose degradation) was much higher in the soil under poplar and pines than in soil under wild olive or in pastures. A feedback process can be inferred, by which tree species influence composition and abundance of soil fungi, and the rate of enzyme activity, which in turn promote organic matter decomposition and release of nutrients, which are up-taken by trees (Gil-Martínez et al., 2018, Proceed. Mine Closure 2018, Leipzig, pp. 636-647).

In summary, there are complex interactions between the aboveground and belowground subsystems of the forest ecosystem. Several examples of tree-microbiota-soil feedback processes have been studied in the Guadiamar Green Corridor. Metal uptake by trees is influenced by soil pH, which in turn can be modified by litter decomposition and root exudates. The composition and diversity of soil fungi are affected by tree species identity; on the other hand, mycorrhizal fungal communities influence some tree processes, like leaf P concentration, and Zn transfer from soil to root. Trees influence on soil enzyme activities, which in turn affect nutrient cycling and their uptake by trees.

A pdf copy of the presentation slides can be consulted in Digital CSIC.

The XXXII Reunión Nacional de Suelos organized by researchers from the field of edaphology belonging to the University of Seville and the Institute of Natural Resources and Agrobiology of Seville (IRNAS-CSIC) took place in Seville (Spain) on 10-13 September 2019. This meeting is an initiative of the Spanish Society of Soil Science (SECS) that has been taking place since 1973 as they organize excursions to get to know different Spanish soils.

The meeting began with a session of posters and presentations framed within different themes (see Abstracts Book). Within the theme of “Study and recovery of contaminated soils” Marta Gil Martínez, predoctoral researcher at IRNAS-CSIC, presented a study on the fungal functional diversity in trace element contaminated soils from the Guadiamar Green Corridor.

In this study, the soil fungal communities were analyzed with a methodology of next generation sequencing in soils with different levels of contamination by trace elements and with different plant covers. The results showed that the phytoremediation plan established after the Aznalcóllar mining accident has favored the development of fungal communities by increasing their species richness and diversity. Both the type of vegetation covers and the properties of the soil determine the structure of the fungal communities. Under the poplar and pine trees, the communities of ectomycorrhizal fungi dominate, and under the wild olive and herbaceous species dominate arbuscular mycorrhizal fungi, due to the specific symbiosis of each type of vegetation with the fungi.

Access to the poster in this link:

Gil-Martínez M, López-García Á, Navarro-Fernández CM, Domínguez MT, Marañón T (2019). Fungal functional diversity in trace element contaminated soils from the Guadiamar Green Corridor. XXXII Reunión Nacional de Suelos (Seville).

In the following days we made three diverse and representative itineraries of the main soils and landscapes of Western Andalusia: 1) a protected and unique space in Europe, Doñana National Park; 2) a global example of restoration such as the Guadiamar Green Corridor, a protected area after the Aznalcóllar mining disaster, and finally 3) the Mediterranean soils of the valley and countryside of Carmona.

In the Doñana National Park, the researcher Luis Clemente (IRNAS-CSIC) explained the types of soils and the various ecosystems that Doñana include. In the Guadiamar Green Corridor we visited the Aznalcóllar mine and the researchers María Teresa Domínguez (University of Seville) and Engracia Madejón (IRNAS-CSIC) explained the details of the accident and the studies that have been carried out since then. In Carmona, we study two soil profiles with the help of Antonio Jordán (University of Seville).


This past June, from 23-28, the 8th International Symposium on Interactions of Soil Minerals with Organic Components and Microorganisms was held in the city of Seville. This symposium was organized by Division 2.5 belonging to the International Union of Soil Sciences (IUSS). The specific theme chosen for this symposium was “Understanding Soil Interfacial Reactions for Sustainable Soil Management and Climate Change Mitigation”.

From the INTARSU Project we had the opportunity to participate in this symposium in the scientific session entitled “Dynamics of pollutants at soil interfaces – What is new and how can environmental biotechnology be beneficial for soil restoration and bioremediation?”

Marta Gil Martínez gave an oral presentation entitled “Tree species effect on soil organic matter and soil microorganisms in trace element contaminated soils” where she presented the latest results obtained from the study about the effects of phytoremediation in soils contaminated by trace elements, after the mining spill of Aznacólllar in 1998. In this study, an exhaustive study of the changes in soil organic matter was carried out under three tree species: olive, white poplar and pine, and in a treeless area dominated by herbaceous species. In addition, two zones located 15 and 30 km from the discharge point were sampled to study, as well as, how the effects of these trees vary according to the physical and chemical properties of the soils.

The objectives of this study were:

- Evaluating the effect of these species on different fractions of soil organic matter, after 15 years of tree growth, as part of the phytoremediation program.

- Analysing the influence of microbial activity in two contrasted areas with different degradation status.

The results obtained from this study showed that the North zone (closer to the mine) has greater availability of trace elements due to the acidic pH and the low content of soil organic matter. Among the species studied, the pine had an acidifying effect in this zone but not in the South zone (farther from the mine) due to the neutral pH and greater buffering power of these soils. This effect of the pine is explained by infrared analysis, which showed its litter and underneath soils some characteristic peaks of acidic functional groups (pectins, resins and waxes), which did not appear in other species infrared analysis.

The effects of phytoremediation had a positive effect on the amount of organic matter, compared to treeless areas, and this was reflected in a greater microbial biomass.

Among the conclusions of this study, we highlight the potential of pine trees to acidify the soil, however this species also generates a large amount of litter, increasing the organic matter of the soil. It has also been found that reforestation has increased microbial biomass, therefore, new functionalities are being recovered in soils.

You can access the presentation in this link:

Gil-Martínez M, Fernández Boy ME, Marañón T, Montero González JF, Navarro-Fernández CM, Domínguez MT (2019). Tree species effect on soil organic matter and soil microorganisms in trace element contaminated soils. 8th International Symposium on Interactions of Soil Minerals with Organic Components and Microorganisms (Sevilla).