Performance-based criteria for safe and circular digestate use in agriculture

Anaerobic digestion converts organic waste into renewable energy (biogas) and recyclable nutrients (digestate), generating over one billion tons of digestate annually. While this represents a major resource, its safe reuse remains a bottleneck for nutrient circularity, particularly for closing global nitrogen loops. We analyzed digestates from 23 full-scale digesters in Sweden, Norway, and Denmark across whole, liquid, and solid fractions using germination index (GI) assays and chemical profiling. Three parameters predicted phytotoxicity: total ammonia nitrogen (TAN ≥ 1,122 mg N L^{- 1}), potassium (K ≥ 39.6 × 10³ mg kg^{- 1}), and boron (B ≥ 22.5 mg kg^{- 1}). When all thresholds were exceeded, germination indices dropped below 50% in every case. Based on these findings, we propose a decision-ready framework linking TAN-K-B thresholds to germination outcomes, guiding mitigation through acidification, stripping, blending, or source control. This outcome-based screening reduces monitoring complexity while maintaining compliance with EU and US pollutant ceilings. Its implementation strengthens nitrogen use efficiency, curbs NH₃ and N₂O emissions, and secures crop establishment. By shifting from origin-based restrictions to performance-based thresholds, our framework provides transparent certification, builds farmer confidence, and positions digestate reuse as a global lever for climate mitigation, nutrient circularity, and food system resilience.