Effectiveness of DMPP and NBPT in Sustaining Melon production under Nitrogen Restriction in Southern Tunisia

Nitrogen fertilization is a major driver of melon productivity in arid environments, yet excessive application leads to low nitrogen use efficiency and increased environmental risks, particularly nitrate leaching. This study investigated the effectiveness of chemical inhibitors in maintaining melon (Cucumis melo L.cv yellow canari) yield, quality, and nitrogen dynamics under reduced nitrogen fertilization in southern Tunisia during the 2025 growing season. The activities were carried out within the framework of the PRIMA project TELENITRO. The experiment compared a conventional nitrogen rate (control, N1) with two reduced nitrogen levels (15% and 30% reductions, N2 and N3), combined with two chemical inhibitors: 3,4-dimethylpyrazole phosphate (DMPP) and N-(n-butyl) thiophosphoric triamide (NBPT). Production parameters, yield components, fruit quality traits, and nitrogen forms in soil and leaves were evaluated. Results showed that a 15% reduction in nitrogen fertilization combined with chemical inhibitors effectively preserved fruit size and yield. Under N2, both DMPP and NBPT treatments exhibited fruit length, width, and seed cavity dimensions comparable to the control. Average total yield reached 37.5 and 40.0 t ha⁻¹ for DMPP and NBPT, respectively, showing no significant difference from the control yield (38.5 t ha⁻¹). These results highlight the strong capacity of chemical inhibitors to compensate for moderate nitrogen reductions. In contrast, a 30% nitrogen reduction (N3) led to a significant decrease in average fruit weight and total yield, although inhibitor-treated plots still outperformed what would typically be expected under reduced nitrogen conditions. Yields under N3 ranged from 26.0 to 28.5 t ha⁻¹, with NBPT showing slightly better performance than DMPP. Fruit quality attributes, including soluble solids content (SSC), dry matter content (DMC), and firmness, were not significantly affected by nitrogen reduction or inhibitor application, indicating that marketable quality was maintained across treatments. Soil nitrate and ammonium concentrations varied significantly over time and treatments. Reduced nitrogen treatments combined with inhibitors generally showed lower soil nitrate accumulation compared to the control, particularly at later sampling dates, suggesting improved nitrogen retention in the soil. Leaf nitrate concentrations were significantly lower in inhibitor treatments, especially under N3, while ammonium concentrations and increased C/N ratios indicated enhanced nitrogen assimilation efficiency. Overall, the results demonstrate that the combined use of chemical inhibitors, particularly NBPT, enables a reduction of nitrogen fertilizer inputs by up to 15% without compromising melon yield or quality under arid conditions. Overall, the findings support the use of chemical inhibitors—especially NBPT—as a viable nitrogen management strategy to reduce fertilizer inputs while sustaining melon productivity and minimizing environmental risks under arid conditions in southern Tunisia.