First Stages of Phytostabilization

A significant increase in plant cover was observed two years into the experiment (Fig. 15.2). No plant growth was observed in the control plot due to its extremely acidic pH, which inhibits seed germination, while amended plots exhibited 60% plant cover. The addition of the amendments had a significant and positive effect on plant cover percentages. Increasing the pH, organic carbon, total nitrogen, and micronutrient contents, as well as modifying some physical properties, had a highly positive effect on plant colonization.

Regarding the plant species that grew spontaneously in the plots, Diplotaxis lagascana (Brassicaceae) was the dominant. Other species such as Atriplex hali-mus L. (Chenopodiaceae), Sisymbrium irio (Brassicaceae), Sonchus tenerrimus (Asteraceae), Malva sylvestris (Malvaceae), Sedum album (Creassulaceae), and Bromus fasciculatus (Poaceae) were also found. Different seeds were carried by the pig manure. Moreover, seeds that originated from the surroundings of the mine pond, such as those for Atriplex halimus, Dittrichia viscosa, Sonchus tenerrimus, and Sedum album, germinated and grew on the plots.

A reduction in the heavy metal concentrations in Diplotaxis lagascana plants from amended plots was observed. Thus, the addition of amendments caused a

decrease in DTPA-extractable metals, which decreased the amounts of these metals taken up by the plants (Fig. 15.3). pH and organic matter content are the main factors that influence heavy metal uptake by plants.

Other studies carried out in the same study area indicate that species such as Piptatherum miliaceum (L.) Cosson (Poaceae), Bituminaria bituminosa (L.) Stirton (Leguminosae), Lygeum spartum L. (Poaceae) or Hyparrhenia hirta (L.) Stapt (Poaceae) (Walker and Correal 2004; Lefevre et al., 2005a,b; Walker et al. 2005, 2006; Conesa et al. 2007a, b, c; Walker et al. 2007) that grow naturally in this zone could be used for phytostabilization.

Moreover, it is crucial to consider the potential of all of these plants to reduce soil erosion. Unfortunately, little information on the root systems of Mediterranean plants and their capacity to reduce soil erosion is available. De Baets et al. (2007) found that Piptatherum miliaceum and Lygeum spartum possess great potential for reducing erosion rates and constitute a good alternative for revegetation.

To continue with the phytostabilization process, the most suitable species to use may be the legume Bituminaria bituminosa, due to its ability to reduce erosion and fix nitrogen as well as its tolerance of heavy metals, and Lygeum spartum because of its ability to reduce erosion and its low rate of heavy metal accumulation. Moreover, Zygophyllum fabago, Piptatherum miliaceum and Atriplex halimus are all suitable for phytostabilization strategies, but special care should be taken with these species due to their moderate rates of heavy metal accumulation.

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