Can sweet sorghum and sweet sorghum-silage maize intercropping replace silage maize for sustainable biogas production in a semi-arid Mediterranean environment?

Sweet sorghum is regarded as a promising alternative to silage maize for biogas production due to its high biomass yield, drought tolerance, and low input requirements. This study compared monocrop silage maize (SM), monocrop sweet sorghum (SS), and a 2:2 sweet sorghum–silage maize intercropping system (INT) under six nitrogen (N) fertilization rates (0, 100, 150, 200, 250, and 300 kg ha⁻¹) in a semi-arid Mediterranean environment for biomass composition and biomass and biogas productivity. SM exhibited the most favourable biomass composition for anaerobic digestion, with higher contents of easily fermentable compounds and lower fiber fractions than SS and INT. However, both SS and INT achieved significantly greater methane yields due to their superior dry matter productivity. Averaged across two years, dry matter yield (DMY) was optimized at 200 kg N ha⁻¹ for SM (14.93 t ha⁻¹) and at 150 kg N ha⁻¹ for both SS (19.96 t ha⁻¹) and INT (20.34 t ha⁻¹). Likewise, methane hectare yield (MHY) was optimized at 200 kg N ha⁻¹ for SM (5361 Nm³ ha⁻¹) and INT (7000 Nm³ ha⁻¹), whereas 150 kg N ha⁻¹ was sufficient for SS (7131 Nm³ ha⁻¹). This study is the first to systematically compare nitrogen responses of sweet sorghum, silage maize, and their intercropping system for biogas production under semi-arid Mediterranean conditions. MHY was primarily driven by DMY (R² = 0.97), and both SS and INT achieved 31–33% greater MHY than SM while requiring lower nitrogen inputs, providing a practical low-carbon alternative for industrial biogas feedstock production.