Olive Mill Wastewater: A Sustainable Natural Solution for Nitrate Contamination Control in Agricultural Systems and...

The search for sustainable alternatives to synthetic nitrification inhibitors (SNIs) is critical for reducing agricultural nitrogen losses and environmental impact. This study assessed the effects of low concentrations (5, 10, and 50 mg phenolics•L^-1) of olive mill wastewater (OMW) soluble fraction on nitrogen dynamics, phytotoxicity, and quality traits of Lactuca sativa L. cultivated under controlled soil conditions simulating field conditions. Treatments included a positive control with ammonium fertilization (Control P) and Dicyandiamide (DCD) as an SNI treatment (P+DCD 25 mg/L). P+DCD 25 mg/L and P+OMW 50 mg/L reduced nitrate (NO₃ ^-) leaching by 81% and 65% and nitrite (NO₂ ^-) leaching by 93% and 54%, respectively, compared to Control P (p < 0.05). However, the former increased ammonium (NH₄ ⁺) leaching by 59%, while the latter reduced it by 34%. As a result, total inorganic nitrogen leaching was similarly reduced under both treatments61% and 58%, respectively. Lower concentrations of the OMW soluble fraction had no significant effect on nitrogen dynamics or crop performance. At 50 mg phenolics/L, the OMW soluble fraction enhanced lettuce growth and fresh biomass by over 50% compared to Control P, without phytotoxic effects, and preserved leaf morphology and coloration within the typical ranges reported for Lactuca sativa L. under optimal cultivation. Leaf NO₃ ^- contents remained below EU regulatory limits in all treatments. Also, significant matrix interferences were detected during HPLC-UV/vis quantification of NO₂ ^- in lettuce tissues, highlighting the need for complementary analytical methods in leafy vegetables. These findings support the valorization of agro-industrial byproducts within a circular economy approach to improve nitrogen use efficiency and crop quality in sustainable horticultural systems.