Biogas Potential of Flax Straw (L. usitatissimum) in Canadian Prairies: Composition, Degradation, and Microbial Dynamics

• NaOH treated flax straw (AF) and its co‑digestion with untreated flax straw (RF) (1:1) increased biogas yields by 118.3% and 44.4%, respectively, relative to RF (p < 0.01). • NaOH pretreatment shifted the microbial community composition and was associated with improved methane production. • The bag method resulted in 163.1% (AF) and 42.9% (RF) higher biogas yields than conventional mixing. • Cellulose is the primary nutrient source in the anaerobic digestion of flax straw. • Total biogas production was similar at an adjusted C:N ratio of 25:1, while methane content varied with nitrogen source. Agricultural biomass offers significant potential to replace natural gas in Canadian prairie provinces, contributing to regional decarbonization efforts. This study evaluated anaerobic digestion (AD) of flax straw at 37.0±1.0 °C and 10.0% total solids (TS), with chemical composition changes monitored at 10 days intervals. NaOH pretreated (AF) and co-digestion (RF:AF, 1:1) enhanced biogas yields by 118.7% and 44.4%, respectively, compared to untreated flax straw (RF). Gompertz modeling confirmed strong predictive accuracy (R² = 0.9–1.0; RMSE = 7.9–10.3). The bag method demonstrated superior performance, achieving biogas yields of 245.1 and 290.1 mL/g TS for RF and AF, respectively, compared to 93.0 and 203.0 mL/g TS under normal mixing. Mass degradation increased from 38.4±2.0 to 44.1±2.5 wt.% for AF, with cellulose as the primary component degraded, while lignin remained largely recalcitrant. Nitrogen supplementation to maintain a C/N ratio of 25:1 reduced the lag phase and improved methane content. Microbial community analysis via 16S rRNA sequencing revealed dominance of Clostridium, Bacteroides, Sedimentibacter , and Methanobacterium , associated with hydrolysis, acidogenesis, and methanogenesis. AF exhibited a more balanced microbial profile than RF and RF:AF. NaOH pretreatment enhanced substrate accessibility promoted favorable microbial succession, and optimized methane yield through synergistic interactions. Future research should focus on microbial dynamics and synergistic effects during extended digestion periods to further optimize process efficiency.