Restauración de suelos contaminados por elementos traza: efecto de la vegetación arbórea en las comunidades de hongos del suelo

Abstract

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. The third objective of this Doctoral Thesis was to explore the functional changes based on the traits of the ectomycorrhizal fungal communities in symbiosis with the oak. The variation in the composition of ectomycorrhizal species was determined by C, Ca, and soil contamination; however, taxonomic diversity did not depend on the level of contamination. The values of the mean features of the ectomycorrhizal communities showed a decrease in the formation of rhizomorphs and emanating hyphae with increasing contamination, and the community converged towards species with less frequent development of rhizomorphs. This could mean a reduction in functional diversity in ectomycorrhizal fungal communities due to contamination with a potential effect on the functioning of the ecosystem. The fourth objective of this Doctoral Thesis was to evaluate the functional features of the ectomycorrhizal fungal species. In particular, how the composition of ectomycorrhizal fungi and their morphological features mediate the features of the holm oak were studied. The taxonomic and functional diversity of ectomycorrhizal fungi explained a high proportion of the variance of the functional features of the holm oak, both in roots and in leaves. The holm oaks that were dominated by the most abundant fungi Hebeloma cavipes and Thelephora terrestris showed conservative features in relation to the economic spectrum of the root, while the oaks colonized by rare ectomycorrhizal species presented an acquisitive strategy. The most conservative roots presented fungi with high formation of rhizomorphs and low melanisation. These results supported the potential of ectomycorrhizal fungi, both for their taxonomy and for their traits, in mediating the state of the host plant. The fifth and last objective of this Doctoral Thesis was to evaluate the ability of fungi to mobilize trace elements in the soil, by analysing their fruiting bodies (mushrooms). Soil-fungus relationships were explored through two wild mushrooms, the ectomycorrhizal species Laccaria laccata and the sapropharopic species Volvopluteus gloiocephalus. Isotopic analysis showed that Laccaria laccata mushrooms were 15N-enriched compared to Volvopluteus gloiocephalus mushrooms, possibly by transfer of 15N-depleted N to the host plant. Furthermore, Laccaria laccata mushrooms showed δ13C values indicating that C could come from their host, while the δ13C values of Volvopluteus gloiocephalus mushrooms were similar to those in their soil. Both species showed a high bioaccumulation of Cd and Cu in their fruiting bodies. Human consumption of these mushrooms could represent a toxicity risk due to the high concentration of Cd.