Arundo Donax L. Cultivation potential in soils contaminated with heavy metals

Among the various impacts caused by man to the environment, soil contamination deserves attention. The contamination of the soil gives rise to many environmental problems such as desertification, contamination of water resources, and contamination of food crops, which can lead to serious health problems for humans directly or indirectly. Production of energy crops in contaminated soils is a interesting option that associates the decontamination of soils (phytoremediation), with the production of a biomass that may provide an additional income to farmers. Moreover, this biomass is a low indirect land-use change-risk feedstock for bioenergy or bioproducts, that helps to reduce the dependence on fossil fuels and the greenhouse gas emissions, contributing to the decarbonisation of the economy, in line with the objectives of the European Green Deal. Therefore, this work aimed to study the effects of different heavy metals (Zn, Cr, Pb, Cd, Ni, Cu) on the productivity and biomass quality of giant reed (Arundo donax L.) during three growth cycles. The essay was performed in pots, and the soils were artificially contaminated. The established concentrations were the double of the limits established by the Decree-Law 276 of 2009 (Portuguese regulation that establishes the regime for the use of sewage sludge in agricultural soils)-Zn: 900 mg/kg; Cr: 600 mg/kg; Pb: 900 mg/kg; Cd: 8 mg/kg; Ni: 220 mg/kg and Cu: 400 mg/kg. Results indicate that there were no significant differences in the yields obtained along the three growth periods, for the control soils, and an average yield of 0.40 kg/m2 was obtained. In the first year, Ni and Cr contamination reduced the aerial productivity by 50%, and Cd and Cu contamination reduced the yields by 25%, but Zn and Pb did not affect the yields. In the 2nd and 3rd growing seasons, all contaminants, including Zn and Pb, reduced significantly the yields by 30-65%. Chromium, cadmium and nickel were the metals that reduced yields the most, and zinc the metal that reduced yields the least. Interestingly, in the contaminated pots, it was verified that between the first and the second growing season there was a decay in the yields obtained, but this effect was not observed from the 2nd to the 3rd growing season. The belowground biomass (roots and rhyzomes) was not affected by the contamination and this suggests tolerance mechanisms by the plant that allow the regrowth of giant reed after harvest. Regarding the biomass quality, the ash content was similar for all contaminations, although showing a trend to an increment due to the contamination of the soils. The calorific value was calculated, pointing that the contamination did not interfere with the energetic potential. These results showed that use of this biomass from contaminated sites for bioenergy is a promising option. Yet, more studies are needed to understand the long term effect on the plants of the contaminated soils.