Effect of irrigation and nitrogen fertilization on the production of biogas from maize and sorghum in a water limited environment

The expansion of biogas production from anaerobic digestion in the Po Valley (Northern Italy) has stimulated the cultivation of dedicated biomass crops, and maize in particular. A mid-term experiment was carried out from 2006 to 2010 on a silt loamy soil in Northern Italy to compare water use and energy efficiency of maize and sorghum cultivation under rain fed and well-watered treatments and at two rates of nitrogen fertilization. The present work hypothesis were: (i) biomass sorghurn, for its efficient use of water and nitrogen, could be a valuable alternative to maize for biogas production; (ii) reduction of irrigation level and (iii) application of low nitrogen fertilizer rate increase the efficiency of bioenergy production. Water treatments, a rain fed control (I0) and two irrigation levels (I1 and I2; only one in 2006 and 2009), were compared in a split-split plot design with four replicates. Two fertilizer rates were also tested: low (N1, 60 kg ha(-1) of nitrogen; 0 kg ha(-1) of nitrogen in 2010) and high (N2, 120 kg ha(-1) of nitrogen; 100 kg ha(-1) of nitrogen in 2010). Across treatments, sorghum produced more aboveground biomass than maize, respectively 21.6 Mg ha(-1) and 16.8 Mg ha(-1) (p < 0.01). In both species, biomass yield was lower in 10 than in I1 and I2 (p < 0.01), while I1 and I2 did differ significantly. Nitrogen level never affected biomass yield. Water use efficiency was generally higher in sorghum (52 kg ha(-1) mm(-1)) than in maize (38 kg ha(-1) mm(-1)); the significant interaction between crop and irrigation revealed that water use efficiency did not differ across water levels in sorghum, whereas it significantly increased from I0 and I1 to 12 in maize (p < 0.01). The potential methane production was similar in maize and sorghum, while it was significantly lower in 10 (16505 MJ ha(-1)) than in I1 and I2 (21700 MJ ha(-1)). The only significant effect of nitrogen fertilization was found in the calculation of energy efficiency (ratio of energy output and input) that was higher in N1 than in N2 (p < 0.01). These results support the hypothesis that (i) sorghum should be cultivated rather than maize to increase energy efficiency, (ii) irrigation level should replace up to 36% of ETr and (iii) nitrogen fertilizer rate should be minimized to maximize the efficiency in biomass production for anaerobic digestion in the Po Valley. (C) 2016 Elsevier B.V. All rights reserved.