Improving biomethane production from giant reed: effect of agronomic strategies and fungal pretreatment under semi-arid Mediterranean conditions

In recent years, lignocellulosic perennial grasses have attracted attention as a feedstock to produce biomethane through anaerobic digestion, considered a sustainable energy solution for reaching the RED II directive's aims. Arundo donax L., commonly known as giant reed, is a lignocellulosic perennial rhizomatous grass, widespread in the Mediterranean environment and a promising candidate for bioenergy production. Anaerobic digestion is one of the most environmentally friendly technologies to produce biomethane from lignocellulosic biomass. However, the use of lignocellulosic material as substrate requires a pretreatment to break down its recalcitrance structure and enhance biomethane yield. This study investigated the suitability of a long-term plantation of Arundo donax as a feedstock for advanced biomethane production. The effects of cropping cycle (23rd vs 24th vs 25th), harvest time (winter vs autumn), and nitrogen fertilization (non-fertilized vs 80 kg N ha-1) on biomass yield and quality were evaluated. Additionally, biomethane production through anaerobic digestion was evaluated, as influenced by pretreatment duration (30 vs 50 days) with two white rot fungi (Pleurotus ostreatus vs Irpex lacteus). Nitrogen fertilization significantly increased biomass yield, with the highest value (13.9 t ha-1 recorded for winter harvest in the 25th cropping cycle. Pleurotus ostreatus pretreatment of giant reed biomass improved anaerobic digestion, increasing the final biomethane yield compared to untreated biomass. The highest biomethane production (2122 m3 CH4 ha-1) was recorded for winter harvest on fertilised crops pretreated by P. ostreatus for 30 days, as result of the positive effect of nitrogen fertilisation on biomass yield and biomethane production. These results indicate the potential of giant reed to provide high biomass and biomethane yield combining sustainable agronomic practices and an effective pretreatment process.