Abstract
<jats:p>The expansion of anaerobic digestion has created two linked opportunities: renewable-energy production and the return of nutrients and organic matter to land. It has also created an agronomic problem that cannot be solved by calling every residual material a biofertilizer. Digestate varies with feedstock, digestion conditions, separation, storage, post-treatment and the point at which it is sampled. Its value therefore emerges only when product composition is connected to crop demand, soil conditions, application practice and the environmental pathways that remain open after land application. This structured critical narrative review brings together evidence on crop production, nutrient replacement, soil physical and biological responses, greenhouse-gas emissions, nutrient losses, contaminants, processing and practical implementation. Greatest interpretive weight is given to inspectable original studies and replicated field evidence; reviews and meta-analyses are used to identify consistency and heterogeneity, not counted as additional experiments.The major points are the following. First, whole and processed digestates can replace part of a mineral-fertilizer programme, but the replacement value belongs to the complete product-management-soil-crop system rather than to digestate as a universal material. Trials that appear to compare equal fertilizer rates often match different quantities of total N, ammonium-N, estimated available N, P, K, S, organic N and carbon, and they frequently differ in mineral-fertilizer formulation, application timing and placement. Yield equivalence is therefore not automatically nutrient equivalence. Second, the product fraction matters. Liquid fractions commonly act as rapid N and K sources, whereas separated solids retain more particulate carbon and phosphorus and may release N more slowly. Third, long-term soil responses are real but conditional. Multi-year studies range from little detectable difference relative to slurry or mineral fertilizer to increases in soil-carbon pools under particular combinations of product, soil, crop, dose and management. Concentration changes, short incubations and degraded-soil demonstrations should not be treated as universal proof of durable stock change.Fourth, environmental performance cannot be represented by one emission factor or one safety label. Ammonia loss depends on pH, total ammoniacal N, dry matter, viscosity, storage, weather and surface exposure; nitrous oxide depends strongly on soil moisture, native mineral N, degradable carbon, placement and management history. Practices that reduce one pathway can increase another. Anaerobic digestion does not destroy metals, and evidence on pharmaceuticals, PFAS precursors, microplastics, phytotoxicity and antimicrobial resistance shows alteration of concentration, partitioning, viability or exposure rather than guaranteed removal. The central conclusion is consequently practical rather than promotional: digestate can be a valuable nutrient and, in some products and settings, carbon resource when it is characterized, matched to crop and soil requirements, applied with control of loss pathways and monitored over an appropriate timescale. It is neither intrinsically safe nor uniformly effective, but neither should it be dismissed as a waste when evidence-based management can recover useful functions.</jats:p>