Combination of nitrogen reduction and algae-based biostimulants: a sustainable strategy for processing tomato

Processing tomato (Solanum lycopersicum L.) is a key crop in the Mediterranean, where farmers are increasingly required to balance high yields with reduced inputs. In response to the EU Farm to Fork strategy, which targets a 20% reduction in fertilizer use by 2030, the aim of this study was to evaluate the potential of algae-based biostimulants to sustain crop performance under reduced nitrogen (N) supply. In this context, we carried out a two-year field trial (2023–2024) in Acerra, Southern Italy testing three N levels (100%, 90% and 80% of the optimal dose in 2023; 100%, 80% and 70% in 2024) and two commercial formulations (BIO1: Auximar + Procalcium; BIO2: Enerleaf + Pentacalcium), applied as foliar sprays. Reducing N dose led to a proportional decline in yield compared to the optimal dose (–16% in 2023, mean value of N80% and N90% and –30% in 2024 with N70%). In both years, all biostimulants increased marketable yield (+21% and +36%), mainly by raising fruit number. The N80% treatment combined with biostimulants maintained yields comparable to the full nitrogen dose, pointing to a more efficient use of fertilizer. Biostimulant treatments enhanced fruit firmness (+19% in 2023) and soluble solids total (+14% and +10% in 2023 and 2024, respectively), and boosted nutritional parameters such as ascorbic acid (12% in 2023 and 15% in 2024) and carotenoids. Lycopene was not influenced by biostimulants, but showed instead a strong dependence on seasonal conditions and N dose, with higher values recorded in 2024. Although main effects of nitrogen and biostimulants were largely independent, significant year × nitrogen × biostimulant interactions were detected for fruit nitrogen and nutraceutical compounds. The results indicate that algae-based products with a moderate (≥20%) N reduction appears to be a practical way to cut fertilizer inputs while safeguarding both yield and fruit quality in processing tomato.