Agronomic Performance of Pelleted Multi-Ingredient Organic-Mineral Recycled Nutrient Fertilizers, Enhanced with Seafood-Waste-Containing Chitosan

Recycled nutrient fertilizers (RNFs) derived from organic and industrial waste streams offer a promising approach to nutrient recovery and sustainable agricultural production. However, their agronomic performance and the potential benefits of enrichment with biologically active, waste-derived materials remain insufficiently understood. This study evaluated two multi-component RNFs, four-ingredient pellets, first with biochar (BADSS) and second with bovine shavings (SADSS), with and without enrichment using chitosan-containing seafood waste, in a four-month greenhouse experiment with ryegrass across nitrogen application rates ranging from 20 to 370 kg N ha−1. Biomass production, nitrogen uptake, agronomic effectiveness (AE), apparent nitrogen recovery (ANR), soil chemical properties, and photosystem II efficiency were assessed. The results demonstrated that fertilizer performance depended strongly on matrix composition and nitrogen application rate. BADSS induced a stronger increase in residual soil pH, whereas SADSS maintained more balanced soil conditions. Chitosan enrichment enhanced long-term nitrogen uptake and biomass production when combined with SADSS, while its effects in the BADSS matrix were less consistent, particularly at excessive nitrogen inputs. Correcting fertilizer performance for additional nitrogen introduced with the chitosan-containing amendment substantially reduced apparent AE and ANR, highlighting the importance of complete nutrient accounting when assessing enriched fertilizers. Overall, the findings indicate that seafood-waste-derived chitosan can improve the agronomic performance of recycled nutrient fertilizers, although its effectiveness depends on the composition of the fertilizer matrix and the applied nitrogen rate. Integrating seafood-processing residues into waste-derived fertilizer systems offers a promising strategy for valorizing underutilized resources, improving nutrient management, and supporting circular bioeconomy approaches to sustainable agriculture.