An energy balance under a conventional crop rotation system in northern Japan: Perspectives on fuel ethanol production from sugar beet

In the context of sustainable and less energy-dependent agricultural biomass production, an energy balance was developed for a conventional rotation of winter wheat (Triticum aestivum L.), sugar beet (Beta vulgaris L. subsp. vulgaris), adzuki bean (Vigna angularis (Willd.) Ohwi & Ohashi) and potato (Solanum tuberosum L.) under the highly fuel-dependent and material-intensive farming systems in the Tokachi region of Hokkaido, northern Japan. As annual energy inputs, tractor operations, truck transportations and grain drying for adzuki bean and winter wheat consumed the equivalent of 6.09 and 11.50GJha⁻¹ year⁻¹ in fossil fuels, respectively. Input-output table estimates of the energy consumption resulting from the use of materials necessary to agricultural production (chemical fertilizers, biocides and agricultural machines) ranged between 11.01±0.26GJha⁻¹ year⁻¹ for winter wheat and 24.38±0.35GJha⁻¹ year⁻¹ for sugar beet. Thus, total annual energy inputs for fuel and materials consumed in cultivation and transportation steps amounted to 22.51±0.26, 32.97±0.35, 20.71±1.58 and 24.44±0.41GJha⁻¹ year⁻¹ for winter wheat, sugar beet, adzuki bean and potato production, respectively. Chemical fertilizer consumption contributed significantly to the energy use, representing 25-43% of the total energy inputs. Based on regional crop production statistics (1999-2003), total energy outputs as yield and crop residue biomass were estimated at 151.3±18.1, 346.1±17.9, 42.0±18.1 and 163.8±11.6GJha⁻¹ year⁻¹ for winter wheat, sugar beet, adzuki bean and potato production, respectively, resulting in regional conventional cropping energy output/input ratios of 6.72, 10.50, 2.03 and 6.70. Sugar beet is the most promising biomass-derived energy feedstock crop in this region, due to its high energy output/input ratio and net energy gain (energy output-input). However, for the full sugar beet-based bioethanol production system, a much lower energy output/input ratio and net energy gain were expected, given the greater energy inputs required in the transformation process. In addition to altering agronomic practices, transformation technologies less dependent on fossil fuels are crucial to developing sustainable bioethanol production systems in northern Japan.