Abstract
The integration of digestate in energy crop production represents a promising approach for sustainable agroenergy systems, but evidence quality and practical implementation guidelines remain inadequately developed. This PRISMA 2020-compliant systematic review synthesized 42 studies (38 % high-quality, 83 % temperate bias) to assess evidence reliability, quantify climate-dependent performance variations, and develop implementation frameworks distinguishing universal principles from region-specific adaptations. Meta-analysis demonstrated consistent positive yield responses (18.3 %, 95 % CI: 12.1–24.7 %) with statistically significant climate moderation (temperate: 23.2 % vs. subtropical: 11.4 %, p < 0.001), a pattern potentially explained by temperature-dependent mineralization kinetics (Q10 ~ 2.0–2.2) though mechanisms require further investigation. Critical methodological analysis identified a 17-point EROI differential between comprehensive life cycle assessments accounting for infrastructure embodied energy (22:1–35:1) versus operational-only studies (38:1–62:1), with implications for economic projections. The exceptional Iowa 12-year trial documented sustained soil carbon gains (0.8 Mg C ha−1 yr−1) in 86 % of monitored sites without yield decline, though long-term evidence remains limited to four trials (4–12 years) with three in temperate climates. This systematic review distinguishes universal implementation principles (standardized characterization protocols, thermophilic pathogen thresholds ≥55 °C achieving 2–5 log reduction, transport constraints <15–20 km, heavy metal monitoring based on mechanistic understanding) from climate-dependent operational parameters requiring empirical validation (incorporation timing, storage duration, application rates derived from theoretical extrapolations of temperate data). For Brazil's estimated 82–84 billion Nm3 biogas technical potential (current utilization approximately 2 %, 4.15 billion Nm3 annually), a phased research agenda addresses critical validation gaps: pathogen survival dynamics during 25–35 °C storage; ammonia volatilization kinetics at elevated temperatures (surface application documented losing 31–64 % of ammoniacal nitrogen); application rate optimization for acidic Oxisols (pH 4.7–4.9) where digestate alkalinity may reduce liming requirements; and economic modeling incorporating RenovaBio carbon credits alongside lower energy prices than European baselines. This framework enables responsible global deployment with regional adaptation while maintaining intellectual honesty about climate-specific knowledge gaps.