Use of slow-release fertilizers labeled with 15N (tuff rich in zeolite and struvite) to evaluate nitrogen and phosphorus use efficiency in the cultivation of Lactuca sativa L.

Improving nitrogen and phosphorus fertilizer use efficiency is a key priority to reduce nutrient losses and mitigate the environmental impact of intensive agricultural systems. Excessive fertilizer inputs are a major driver of nutrient leaching and emissions, highlighting the need for alternative fertilization strategies capable of increasing nutrient retention and crop uptake. In this context, slow-release fertilizers derived from agricultural wastes represent a promising option for more sustainable crop nutrition management, particularly in high-input horticultural systems.This study presents the results of a pot experiment conducted on Lactuca sativa L. grown in a controlled soilless substrate. The experiment aimed to compare crop performance and nutrient use efficiency between innovative slow-release fertilizers and conventional synthetic fertilizers. Specifically, natural zeolites loaded with NH₄⁺ and struvite were tested as alternative nutrient sources and compared with ammonium-based synthetic fertilizers. Both recycled materials were isotopically enriched with 15N (~5 atom%), allowing a precise tracing of fertilizer-derived nitrogen within the plant–substrate system. Nitrogen-Fertilizer Use Efficiency (NFUE) was quantified using isotopic mass balance, while Phosphorus Use Efficiency (PUE) was assessed through a conventional mass balance approach under balanced nutrient application rates.The results indicate that slow-release fertilizers ensured lettuce yields comparable to or higher than those obtained with synthetic fertilizers, while achieving higher NFUE. In particular, the lettuce grows on NH₄⁺-loaded zeolite exhibited higher N derived from fertilizer. Struvite proved to be an effective combined source of nitrogen and phosphorus, supporting plant growth while providing a sustained nutrient supply throughout the cultivation cycle.Overall, this study confirms the potential of struvite and zeolites as sustainable alternatives to mineral fertilizers, demonstrating their capacity to improve nutrient use efficiency and reduce nutrient losses in intensive horticultural production systems.