Citrus wastewaters increase soil nitrate and improve nutrient translocation in a copper contaminated soil-lettuce (Lactuca sativa L.) system

Citrus processing wastewaters (CWWs) have been proposed as alternative water source for crop irrigation. However, their interaction with copper in soil-plant system has not been investigated. This study investigated the combined effects of two types of CWWs, orange (OWWs) and lemon (LWWs), applied at three dilution levels (1/3, 2/3, 3/3), and three Cu concentrations (0, 200, 400 mg kg⁻¹), on soil properties and lettuce (Lactuca sativa L.) growth in a 46-day greenhouse experiment. Results showed that CWWs increased soil nitrate, particularly with OWWs due to their higher nitrogen content. High Cu concentrations inhibited nutrient translocation (Zn>K=Mg>Fe>Al>Ca=Mn) from root to leaf and increased root accumulation (Ca>Mn>Fe>Al>Mg>K>Zn), but CWWs partially alleviated this effect. Notably, LWWs more effectively reduced Cu availability than OWWs, likely due to their higher citric acid concentration, but also decreased available phosphorus. Dilution played a key role in modulating the effects of CWWs, as the application of undiluted CWWs negatively affected plant biomass. Overall, this study highlights the dual nature of CWWs as both nutrient sources and modulators of metal bioavailability. Their agronomic value depends on wastewater composition, dilution, and crop sensitivity. Properly managed, CWWs may contribute to sustainable agriculture by enhancing nutrient recycling and reducing reliance on freshwater resources, supporting their integration into circular economy systems.