Sorghum Biomass Production for Energy Purpose Using Treated Urban Wastewater and Different Fertilization in a Mediterranean Environment

With the aim at enhancing the sustainability of biomass production in the Mediterranean area, this paper analyzes, for the first time, the production of sorghum (Sorghum bicolor (L.) Moench) biomass for bioenergy production using urban treated wastewaters and bio-fertilization. For this purpose, the effects on biomass production of three different fertilizations (no-nitrogen control, biofertilizer, and mineral ammonium nitrate), four levels of constructed wetland (CW) wastewater restitutions (0%, 33%, 66% and 100%) of crop evapotranspiration (ETc) and three harvesting dates (at full plant maturity, at the initial senescence stage, and at the post-senescence stage) were evaluated in a two year trial. For bio-fertilization, a commercial product based on arbuscular mycorrhizal fungi was used. Mineral nitrogen (N) fertilization significantly increased dry biomass (+22.8% in the first year and +16.8% in the second year) compared to the control (95.9 and 188.2 g.plant−1, respectively). The lowest and highest biomass production, in 2008 and 2009, was found at 0% (67.1 and 118.2 g.plant−1) and 100% (139.2 and 297.4 g.plant−1) ETc restitutions. In both years, the first harvest gave the highest biomass yield (124.3 g.plant−1 in the first year and 321.3 g.plant−1 in the second), followed by the second and the third one. The results showed that in Mediterranean areas, constructed wetlands treated wastewaters, when complying with the European restrictions for their use in agriculture, may represent an important tool to enhance and stabilize the biomass of energy crops by recycling scarce quality water and nutrients otherwise lost in the environment.