Yield-efficiency trade-offs for pelleted recycled nutrient fertilizers with a fermented microbial amendment: Integrating nitrogen use efficiency and agronomic effectiveness

Recycled nutrient fertilizers (RNFs) are central to circular nutrient management, yet their agronomic performance is often evaluated using yield-based metrics alone. This study evaluated yield-efficiency trade-offs of pelleted waste-derived RNFs using nitrogen use efficiency (NUE) and relative agronomic effectiveness (RAE) as complementary indicators. In a 4-month controlled glasshouse pot experiment with a forage grass mixture (Lolium perenne, Lolium multiflorum-estanzuela, Festuca rubra, and Lolium hybridum), two recycled fertilizers (BADSS [Biochar-ash-digestate-sewage-sludge pellet ("black pellet")] and SADSS [Shavings-ash-digested-sewage-sludge pellet ("yellow pellet"; chromium-free bovine shavings)]), the same pellets combined with a fermented bakery-dairy amendment containing effective microorganisms (EM), and a commercial cow manure granulate were evaluated across a pellet-N gradient of 20-370 kg N/ha over four sequential harvests. The fermented amendment supplied an additional 0.3612 g N/pot. Under these controlled conditions, RNF formulations matched or exceeded the reference fertilizer in cumulative biomass, with SADSS+EM (SADSS combined with the fermented bakery-dairy amendment containing effective microorganisms) producing the largest yield responses and RAE values. NUE declined strongly as N input increased. Accounting for N supplied by the fermented amendment substantially lowered calculated NUE for the amended treatments, showing that apparent performance gains reflected both co-supplied nutrients and treatment-specific interactions. Because the amendment also contained organic material and other constituents and no nutrient-matched carrier control was included, the experiment does not isolate a microorganism-specific effect. Non-amended SADSS showed the most balanced relationship between RAE and NUE across application rates. These results indicate a yield-efficiency trade-off under controlled conditions and highlight the need for transparent accounting of all amendment-derived nutrient inputs before field-scale inference.