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Although bioenergy sorghum has many traits that make it ideal for biofuel production, management conditions that can affect the productivity and sustainability of these systems are still poorly understood. This paper estimated the energy and CO2 balance of two bioenergy sorghum (Sorghum bicolor L. Moench.) hybrids (H128 and H133) cultivated during two growing seasons and under two different levels of crop management, high and low input. At the end of both growing season, sorghum was harvested for biomass yield determination. Calorific value and net energy production were also estimated. Crop management had important effects on sorghum CO2 and energy balance. The energy produced varied between 126 and 365 GJ ha–1 depending on crop management, hybrid and growing season. Regarding of the CO2 balance, the high level of crop management had a superior CO2 emission. However, the energy produced per kg of CO2 emitted was higher (>300%) than the energy produced with the use of fossil fuels. The use of bioenergy sorghum can contribute to better energy sustainability and reduced CO2 emission in Mediterranean ecosystems. Introduction Renewable sources are contributing in meeting energy requirements with the added advantage of greater environmental protection, especially in terms of carbon dioxide (CO2) emissions (Monti and Ventura, 2003). In this context, biomass is a promising renewable energy source. An important challenge related to the use of alternative crops for energy production is quantifying the environmental sustainability of these crops in the long term (Bonari et al., 1992). For this purpose, energy and CO2 balances represent appropriate tools for the evaluation of such sustainability. It is accepted that the energy obtained from biomass has not carbon emissions associated because the carbon emitted in its combustion is the same that plants absorbed while growing (Royal Society, 2008). Thus, the energy produced by energy crops must be higher than the energy required to produce them, to have a positive energy balance (Scholz and Ellerbrock, 2002). The energy balance shows the energy produced per unit of energy used in the process of production and transformation of biomass into electrical energy (Sartori et al., 2005). This balance has been widely used in different studies (Bonari et al., 1992; Dubuisson and Sintzoff, 1998; West and Marland, 2002; Heller et al., 2003; Sartori et al., 2005; Mead and Pimentel, 2006; Boehemel et al., 2008; Gasol et al., 2009; Nassi o di Nasso et al., 2010). On the other hand, CO2 balance has also been used (Lewandowski et al., 1995; Cannell, 2002; Kaur et al., 2002; Hoosbeek et al., 2006) for evaluating sustainability. It estimates the emissions generated in the whole crop production cycle and compared them to the CO2 fixed by the plant during its growth. CO2 balance takes into account all the issues arising from field operations and considers not only the emissions produced during the combustion of the biomass, but also the emissions generated by the production inputs (fertilisers, herbicides, etc.). There is a wide range of woody and herbaceous species used to produce energy from biomass. Bioenergy sorghum (Sorghum bicolor L. Moench) is among the most promising herbaceous species as it is considered a multifunctional crop (Lynd et al., 1991; Ding, et al., 2017) due that it can provide a wide range of products, like sugars, alcohol, syrups, biofuels, paper, and food. This crop has low input requirements, is drought tolerant, has a great yield stability under a wide range of environmental conditions (Miller and Mcbee, 1993; Buxton et al., 1999; Wight et al., 2012; Amatya et al., 2014; Ameen et al., 2017), and does not directly compete with food crops because thanks to its short growing cycle it can be cultivated in rotation with winter food crops (Garofaldo et al., 2016). Besides, the crop is used to obtain biofuel due to its high quantity of carbohydrates. Given the remarkable work of selection done with this Correspondence: Antonio María Cabrera-Ariza, Vice-rectorate for Research and Postgraduate Studies, Centro del Secano, the Catholic University of Maule, Avenida San Miguel 3605, Talca, Chile. Tel.: +56.71.2413728. E-mail: acabrera@ucm.cl

Effect of crop management intensity on energy and carbon dioxide balance of two bioenergy Sorghum bicolor hybrids