Reducing Nitrogen Fertilizer Inputs in Melon Production Using NBI Plants under Arid Conditions in Southern Tunisia

Excessive use of nitrogen fertilizers in arid and semi-arid regions poses serious environmental and agronomic challenges, including nitrate leaching, soil degradation, and reduced nitrogen use efficiency. This study evaluated the potential of nitrogen fertilizer reduction combined with nitrogen biological inhibitor (NBI) plants to sustain melon (Cucumis melo L. cv.yellow canari) productivity and quality under arid conditions in southern Tunisia during the 2025 growing season. This research was carried out as part of the PRIMA-funded TELENITRO project. The experiment compared a conventional nitrogen fertilization regime (control, N1) with two reduced nitrogen levels, namely 15% (N2) and 30% (N3) reductions, integrated with four NBI plant species: Medicago sativa (alfalfa), Panicum maximum, Sorghum commun, and Brachiaria hybrida. Production parameters, yield components, fruit quality traits, and nitrogen dynamics in soil and leaves were assessed. Results showed that nitrogen reduction significantly affected fruit size and yield, although the magnitude varied according to the associated NBI species. Under the 15% N reduction (N2), treatments incorporating Alfalfa, Panicum, and Brachiaria maintained fruit length, width, and seed cavity dimensions comparable to the control, while Sorghum resulted in lower fruit weight and yield. Average total yield under N2 ranged from 24.4 to 32.3 t ha⁻¹, with Panicum and Brachiaria showing the closest performance to the control (38.5 t ha⁻¹). At 30% nitrogen reduction (N3), a general decline in yield and average fruit weight was observed; however, Brachiaria and Panicum still produced acceptable yields, indicating their higher capacity to mitigate nitrogen reduction effects. Fruit quality parameters, including soluble solid content (SSC), dry matter content (DMC), and firmness, were not significantly affected by nitrogen reduction or NBI treatments, suggesting that fruit market quality was preserved. Soil nitrate and ammonium concentrations varied significantly over time and among treatments, with reduced nitrogen treatments generally exhibiting lower soil nitrate accumulation than the control, particularly at later sampling dates. Leaf nitrate concentrations were significantly reduced under NBI treatments, especially under N3, while ammonium content and C/N ratios indicated improved nitrogen assimilation efficiency in treatments associated with Sorghum and Brachiaria. Overall, the results demonstrate that integrating selected NBI plants, particularly Brachiaria and Panicum, allows a reduction of nitrogen fertilizer inputs by up to 15%—and partially 30%—without major yield or quality losses. This approach represents a promising, environmentally friendly strategy for sustainable melon production in arid regions of Tunisia.