Analysis of the Energy Efficiency of Production of Winter Rapeseed Fertilized with Biogas Digestate

The research hypothesis of this study assumes that nitrogen (N) from digestate has an equivalent productive effect as mineral fertilizer. Therefore, the use of digestate as a N carrier by farmers significantly reduces the energy costs of oilseed rape production. This hypothesis was verified in field experiments with rapeseed conducted in the 2015/2016, 2016/2017, and 2017/2018 growing seasons. The experiment consisted of three N fertilization systems (FSs)—mineral ammonium nitrate (AN) (AN-FS), digestate-based (D-FS), and 2/3 digestate + 1/3 AN (DAN-FS)—and five Nf doses: 0, 80, 120, 160, and 240 kg N ha−1. Maximum seed yields were 3.26, 3.32 and 3.66 t ha−1 and were obtained for optimal N doses of 170, 186 and 189 kgNn ha−1, respectively. Similar trends were observed for the following yields: straw, crude oil, and cake. The values of the Nitrogen Fertilization Replacement Value for the above yield categories slightly exceeded 100%, and for the mixed variant, they were in the range of 108–112% in relation to the mixture with ammonium nitrate. The contribution of straw, oil, and oilcake in the total energy harvested accounted for 64%, 22%, and 14%, respectively. The most favorable energy efficiency ratio, Ee = 8.15 (seeds + straw), was achieved under the mixed N fertilization variant (DAN-FS; 8.3 and 15.6 m3 ha−1, respectively) and N fertilizer doses of 40 and 80 kg ha−1. For the highest nitrogen doses, the digestate significantly stabilized the yields and energy output of winter rapeseed production. Overall, based on the results of the field experiment and calculations conducted, it is recommended that winter rapeseed biomass should be used entirely for liquid fuel (oil) and solid fuel (oilcake and/or straw) production.