Energy balances and greenhouse gas-mitigation potentials of bioenergy cropping systems (Miscanthus, rapeseed, and maize) based on farming conditions in Western Germany

Biomass for bioenergy is an important option within global change mitigation policies. The present research focused on energy net production, net reduction of greenhouse gases (GHG) (considered as CO2-equivalents), and energy output:input ratio of the energy cropping systems ‘rapeseed’, ‘maize’, and ‘Miscanthus’. The system-specific main products were biodiesel (rapeseed), electricity from biogas (maize), and Miscanthus chips (loose, chopped material); the related substituted fossil resources were diesel fuel (rapeseed), electricity from the German energy mix (maize), and heating oil (Miscanthus). However, research did not aim for a direct quantitative comparison of the crops. The study followed a case study approach with averaged data from commercial farms within an enclosed agricultural area (<5 km²) in Western Germany. Cultivation techniques were considered as communicated by farmers and operation managers; the diesel fuel consumption of agricultural machinery was modeled using an online-based calculator of the German Association for Technology and Structures in Agriculture (KTBL). Overall, rounded net energy production amounted to 66 GJ ha−1 (rapeseed), 91 GJ ha−1 (maize), and 254 GJ ha−1 yr−1 (Miscanthus); the related energy output:input ratios were 4.7 (rapeseed), 5.5 (maize), and 47.3 (Miscanthus), respectively. Compared to the respective fossil fuel-related energy supply, CO2-equivalent reduction potential ranged between 30 and 76% for electrical energy from maize biomass, 29–82% for biodiesel from rapeseed, and 96–117% for Miscanthus chips, depending on whether or not the accruing by-products rapeseed cake, glycerin (rapeseed cropping system), and waste heat (maize) were considered. True ‘CO2-neutrality’ was only reached by the Miscanthus cropping system and was related to an additional credit from carbon sequestration in soil during the cultivation period; thus, this cropping system could be attributed to be a CO2-sink.The study indicated that bioenergy can be produced sustainably under commercial farming conditions in terms of a significantly reduced consumption of natural resources.

► Bioenergy cropping led to both net energy production and net reduction of CO2-eq emissions.
► CO2-eq reduction potential and energy efficiency depended closely on the use of by-products.
► General supposition of ‘CO2-neutrality’ for bioenergy is misleading.
► The Miscanthus cropping system functioned as CO2 sink.
► Regional bioenergy cropping might feature even global change mitigation policies.