4. Conclusions
Beta vulgaris behaved as a metal indicator for Mn, Cu and Pb, indicating that it can be used for testing changes in metal availability in soils. The concentrations of metals in both plant species, B. vulgaris and B. maritima, exceeded the limits established for humans and grazing animals, which implies a health risk linked with the spread of the pollution from mining sites to agricultural areas.
For developing a bioremediation programme, cow manure can be very useful for phytostabilisation strategies, keeping metal bioavailability and plant absorption at control levels while improving plant biomass. Although manure improved soil properties and did not increase heavy metals phytotoxicity, high metal concentrations in plants still occurred.
Olive husk can be used to promote rapid accumulation of metals by accumulator species in phytoextraction, as an alternative to amendments such as EDTA. However, it should be treated before adding it to the soil or left to stabilise for several months in soil before planting, to avoid phytotoxicity. The ability of olive husk to solubilise metals is mainly a consequence of the phenolic compounds present, reducing, during their degradation, Fe and Mn oxides to soluble Fe2+ and mainly Mn2+ as well as desorbing adsorbed metals from the oxides surfaces.