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

tThe expansion of biogas production from anaerobic digestion in the Po Valley (Northern Italy) has stim-ulated the cultivation of dedicated biomass crops, and maize in particular. A mid-term experiment wascarried out from 2006 to 2010 on a silt loamy soil in Northern Italy to compare water use and energyefficiency of maize and sorghum cultivation under rain fed and well-watered treatments and at tworates of nitrogen fertilization. The present work hypothesis were: (i) biomass sorghum, for its efficientuse of water and nitrogen, could be a valuable alternative to maize for biogas production; (ii) reductionof irrigation level and (iii) application of low nitrogen fertilizer rate increase the efficiency of bioenergyproduction. Water treatments, a rain fed control (I0) and two irrigation levels (I1 and I2; only one in2006 and 2009), were compared in a split–split plot design with four replicates. Two fertilizer rates werealso tested: low (N1, 60 kg ha−1of nitrogen; 0 kg ha−1of nitrogen in 2010) and high (N2, 120 kg ha−1of nitrogen; 100 kg ha−1of nitrogen in 2010). Across treatments, sorghum produced more abovegroundbiomass than maize, respectively 21.6 Mg ha−1and 16.8 Mg ha−1(p < 0.01). In both species, biomass yieldwas lower in I0 than in I1 and I2 (p < 0.01), while I1 and I2 did differ significantly. Nitrogen level neveraffected biomass yield. Water use efficiency was generally higher in sorghum (52 kg ha−1mm−1) than inmaize (38 kg ha−1mm−1); the significant interaction between crop and irrigation revealed that water useefficiency did not differ across water levels in sorghum, whereas it significantly increased from I0 and I1to I2 in maize (p < 0.01). The potential methane production was similar in maize and sorghum, while itwas significantly lower in I0 (16505 MJ ha−1) than in I1 and I2 (21700 MJ ha−1). The only significant effectof nitrogen fertilization was found in the calculation of energy efficiency (ratio of energy output andinput) that was higher in N1 than in N2 (p < 0.01). These results support the hypothesis that (i) sorghumshould be cultivated rather than maize to increase energy efficiency, (ii) irrigation level should replace upto 36% of ETr and (iii) nitrogen fertilizer rate should be minimized to maximize the efficiency in biomassproduction for anaerobic digestion in the Po Valley.