Yield and energy efficiency tradeoffs in sweet sorghum under different nitrogen management strategies: Mineral vs. recycled organic fertilization

Sweet sorghum [ Sorghum bicolor (L.) Moench] biomass production is a promising component of the bioeconomy that combines high productivity with the potential use of local resources and by-products from bioenergy processes. The aim of this study was to evaluate the effects of nitrogen (N) fertilization strategy and N rate on biomass yield and the energy balance of sweet sorghum grown for energy purposes. A field experiment was conducted in northeastern Poland in 2023–2025. Nitrogen was applied at six rates (0, 40, 80, 120, 160, and 200 kg N ha −1 ) either as a mineral fertilizer (MF, ammonium nitrate) or a recycled organic fertilizer (digestate, DG). Production performance was assessed based on dry matter yield (DMY), total energy inputs (E-IN), energy output (E-OUT), energy gain (E-GAIN), and the energy efficiency ratio (E-ER). An increase in N rates led to a significant increase in DMY, although the effective response range was differentiated by the type of fertilizer. In the MF-based system, DMY increased significantly up to 160 kg N ha −1 , whereas in the DG-based production technology, DMY increased up to 120 kg N ha −1 . Mineral fertilization resulted in higher DMY as well as higher E-OUT and E-GAIN across all N rates, and the maximum E-GAIN was achieved at 160 kg N ha −1 . The replacement of MF with DG reduced DMY (by 15–33%, depending on the N rate), but also significantly decreased total E-IN (by 40–44%). Consequently, E-ER was markedly higher (by 29–34%) in the DG-based production technology than in the MF-based system. The highest E-ER values were obtained at low to moderate N rates applied as DG (40–80 kg N ha −1 ). The study demonstrated that the choice of N fertilization strategy should be determined by the production objective: maximizing biomass yield and E-GAIN or improving energy efficiency and the overall sustainability of the production system.