Biomass yield and energy balance of three perennial crops for energy use in the semi-arid Mediterranean environment

Three different lignocellulosic energy crops (a local clone of Arundo donax L., Miscanthus x giganteus Greef et Deu. and Cynara cardunculus L. var. altilis D.C. cv. ''Cardo gigante inerme'') were compared over 5 years (2002-2007) for crop yield, net energy yield and energy ratio. In a hilly interior area of Sicily (Italy), two different irrigation treatments (75 and 25% of ETm restoration) and two nitrogen fertilization levels (100 and 50 kg ha1) were evaluated in a split-plot experiment. In the fourth and fifth years of the field experiment (2005-2007) no fertilizer or irrigation was used. From crop establishment to the third year, above ground dry matter yield increased over all studied factors, in A. donax from 6.1 to 38.8 t ha1 and in M. x giganteus from 2.5 to 26.9 t ha1. Fifteen months after sowing, C. cardunculus yielded 24.7 t ha1 of d.m. decreasing to 8.0 t ha1 in the third year. In the fourth and fifth years, above ground dry matter yields of all crops decreased, but A. donax and M. x giganteus still maintained high productivity levels in both years. By contrast the yield of C. cardunculus yield fell to less than 1 t ha1 of d.m. by the fourth year. Energy inputs of A. donax and M. x giganteus were higher in the year of establishment than that of C. cardunculus (34 GJ ha1 for A. donax and M. x giganteus and 12 GJ ha1 for C. cardunculus), mainly due to irrigation. Net energy yield showed low or negative values in the establishment year in A. donax and M. x giganteus. In the second and third year, net energy yield of A. donax was exceptionally high (487.2 and 611.5 GJ ha1, respectively), whilst M x giganteus had lower values (232.2 and 425.9 GJ ha1, respectively). M x giganteus attained its highest net energy yield in the fourth year (447.2 GJ ha1). Net energy yield of C. cardunculus reflected energy output of the crop, being high in the first compared to subsequent years (364.7, 277.0 and 119.2 GJ ha1, respectively for the first, second and third years). A significant effect of the different irrigation treatments was noted on all the studied parameters in all species. Conversely, only A. donax was affected by nitrogen fertilization.