Efectos de la rizosfera y de las enmiendas orgánicas en la fitorecuperación de suelos contaminados con elementos traza

Abstract

Nowadays, soil degradation is one of the most important global environmental problems. From an anthropogenic point of view, soils act as physical support and provide different ecosystem services (e.g. the food supply). Changes on soil physical, chemical and biological conditions due to human activity cause soil degradation and, consequently, damage the associated organisms such as plants and microbiological communities. Soil degradation alters ecosystem function and affects, directly or indirectly, human health; therefore, it is crucial to maintain and improve soil quality. The experiments carried out in this Thesis are focused on the analysis of several factors affecting the remediation of trace element contaminated soils. These soils constitute an important environmental problem due the possible entrance of trace elements into the food chain. From the environmental point of view, among the different techniques for the remediation of contaminated soils, highlights phytostabilization that is a sustainable technique that involves stabilization of contaminants at root level by using plants and amendments. This Thesis evaluates the effect of three organic amendments (biosolid compost, alperujo compost and leonardite) and different plant species with potential phytostabilizer ability on trace elements dynamics in the soil-plant interface, focusing on some of the rhizosphere processes. The results are based on field and microcosms experiments. The studied soils are located in the Green Corridor of Guadiamar (Sevilla), affected by the spill of the Aznalcóllar mine, and in Tharsis (Huelva), an area presenting chronic contamination by trace elements. Firstly, the ability of three ruderal plant species (Poa annua, Medicago polymorpha and Malva sylvestris) to stabilize trace elements in Tharsis soil and to improve its quality was evaluated. Likewise, the effect of alperujo and biosolid composts on soil properties was also studied. Amendment addition increased pH, water-soluble carbon and plant biomass. In general, trace element accumulation in the studied plants was within normal ranges. Higher As and Pb concentrations were found in P. annua whereas M. sylvestris accumulated the highest concentration of Mn and Zn. With some exception, bioconcentration factor was lower than 1 (indicating the suitability of these species for phytostabilisation). Among the studied plant species, M. polymorpha showed the best characteristics to promote the trace element stabilization in the rhizosphere and improve soil quality. Moreover, the exudation of low molecular weight organic acids by the mentioned plant species was analysed under different Cd, Cu and Zn concentrations. Both amount and composition of exudates varied according to the growing media (washed sand/ contaminated soils), contamination level, organic matter content and plant species. Oxalic and malic were the main acids in the exudates and, in a lesser concentration citric and fumaric acids were also identified. In general, the increase of heavy metal concentrations in the media was related to a higher organic acid concentration in the rhizosphere. A decrease of exudates and a modification of their composition (being composed solely of oxalic acid excepting M.polymorpha) were observed in soils with the highest organic matter content. In addition, with the same plant species and experimental soil conditions of the organic acids exudation experiment, the arbuscular mycorrhizal fungi ecotypes were identified. The different organic matter content, phosphorus and trace elements in the studied soils were keys to evaluate the shift in the fungal community according to the soil conditions. The addition of both amendments (exogenous organic matter) did not affect the arbuscular mycorrhizal fungal community. However, native soil organic matter content was, besides the trace element contents, the most influential factor on fungal community. With an increase in soil quality (i.e., higher organic matter content and lower Cd, Cu and Zn concentration), a decrease of Glomeraceae abundance and an increase of Claroideoglomeraceae occurred. While simultaneously carrying out the microcosm experiments, the phylogenetic diversity of arbuscular mycorrhizal fungal communities in amended soils of “El Vicario” plot (located in the Green Corridor) was analysed. The restoration process began in 2002 allowing the evaluation of the long-term effect of biosolid compost and leonardite on these communities. In addition, the ecotypes and community shifts were related to the soil conditions. Among soil variables, trace elements concentration was the most relevant factor affecting the composition of the fungal community. Environmental filtering caused by this factor was reduced mainly by the biosolid compost addition, and in the plots amended with this compost the most diverse community was found. Finally, the effect of organic amendment applications on the increment of C sink in the Vicario plot was studied. Results showed that both amendments (biosolid compost and leonardite) modified the molecular composition of soil organic matter promoting the C sequestration. Between both, leonardite was the most suitable option for long-term C sequestration in the soil due to its molecular composition rich in aromatics and lignin-derived compounds, with a very slow turnover and low mineralization rate.