Recycling fertilizers from resource-oriented sanitation systems: product-level characterization and meta-analysis of agronomic performance

Recycling fertilizers derived from source-separated human excreta offer a direct pathway to close nutrient cycles from fork to farm. However, their transition into mainstream agricultural use remains constrained by a lack of systematic product-level evidence on composition, agronomic performance, and safety. This research gap is addressed by combining a systematic review with a quantitative meta-analysis. Data from 201 references including experimental studies and 23 additional unpublished sources were analyzed to identify, classify, and characterize recycling fertilizers from resource-oriented sanitation systems (ROS). Product-level data were compiled on nutrient composition, physicochemical properties, contaminants, and hygienic quality. Agronomic performance was assessed comparing crop yields, control treatments, and cultivation systems. We identified 13 ROS-fertilizers showing a high data level for detailed characterization, including stored urine, nitrified urine fertilizers, alkaline dried urine, Mg-, Ca-, and Ca-Mg-precipitates, as well as compost, vermicompost, biochar, digestate, and dried fecal sludge derived from feces or mixed excreta. Across product types, nutrient composition was primarily determined by source material and processing pathway. Heavy metal concentrations were generally low and comparable to established fertilizers. Hygienic safety depended on source material and treatment conditions. The meta-analysis showed no systematic yield disadvantage of ROS-fertilizers compared to conventional fertilizers across field and pot experiments. While data availability remains uneven for certain product groups and parameters, the results indicate that many ROS-fertilizers can be functionally aligned with established mineral, organic, and organo-mineral fertilizer categories when properly processed and monitored. These findings move the focus from technological feasibility to implementation. ROS-fertilizers are agronomically viable and can meet core quality and safety requirements when appropriately processed and monitored. The next challenge is therefore to systematically integrate them into agricultural nutrient management through coherent product characterization, quality assurance, and enabling regulatory frameworks.