Soil contamination can result in the damage of several soil functions and the contamination of surface water and groundwater. Next to consequences for ecosystems and other natural resources, the introduction of pollutants from contaminated areas into the human food chain via plant products or drinking water is of great concern (EU commission 2002; EEA 2003). The toxic effect of heavy metals in plants include generation of reactive oxygen species and free radicals, binding to S and/or N atoms of proteins and thereby leading to disruption and inhibition of activity as well as displacement of metal cofactors (Clemens 2001; Hall 2002; Pilon-Smits and Pilon 2002; Rea et al. 2004). After decades or even centuries of human activities in industry, mining, or military as well as farming and waste practice a huge amount of sites in developed countries shows high contamination with heavy metals or organic pollutants. The official report on the environmental situation in Germany (SRU 2004) mentioned in particular three main threats to the function of soils: (a) strains of area, (b) soil erosion and (c) input of pollutants. In the EU, an estimated 52 million hectares, representing more than 16% of the total land area, are affected by some kind of soil degradation. The largest and probably most heavily areas affected by contamination are concentrated around the most industrialised regions in north-west Europe, from Nord-Pas de Calais in France to the Rhein-Ruhr region in Germany, across Belgium and the Netherlands and the south of the United Kingdom. Other areas where the probability of local soil contamination is high include the Saar region in Germany, the Po area in northern Italy, and the so-called Black Triangle region located at the corner of Poland, the Czech Republic and the Slovak Republic. However, contaminated areas exist around most major cities (EEA 2003). In Germany 362,689 potentially contaminated places are reported as to be suspected (SRU 2004). The estimates of the number of contaminated sites in the EU range from 300,000 to 1.5 million (EU commission 2002).
The costs for clean-up in the EU are estimated between € 59 billion and € 109 billion (EU commission 2002). The market for phytoremediation in USA is estimated to be actually $100-150 million per year which represents 0.5% of total remediation activities (Pilon-Smits 2005). The actual situation in Germany is that polluted soils from contaminated sites are only up to around 30% cleaned up in soil remediation facilities (SRU 2004). Most of the rest is stored on waste disposals underlining the necessity of research for alternative methods. Surprisingly, in the current report on the environmental situation in Germany (SRU 2004), the German term for remediation ("Phytosanierung") or a similar description cannot be found. Although in Europe commercial application of phytoremediation currently does not exist (Pilon-Smits 2005), it is expected to develop at the background of new members in the EU in East Europe and decreasing natural resources.
Bioavailability of a pollutant is more important than its concentration in the soil for the toxicity as well as for phytoremediation approaches (van der Lelie et al. 2001; Pilon-Smits 2005). Several factors have an impact on bioavailability, i.e. the chemical properties of the pollutant (hydrophobicity and volatility), the soil properties (particle size, organic matter content, redox conditions, pH), the environmental conditions (temperature, moisture) and the biological activities. In phytoremediation or phytostabilization projects bioavailability of pollutants can be manipulated by agronomic practices (Pilon-Smits and Pilon 2002). Plant density, species mixture and fertilization can enhance plant productivity. Simple watering will facilitate solubilization. Adding natural organic acids (malate or citrate) will have two effects: (1) lower the pH and (2) chelate metals. Adding lime increase pH and adding organic matter (humus or straw) will decrease the solubility of metals in the soil.
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