Comparative Plant Responses to Copper Nanoparticles and Alginate-Encapsulated Copper Nanoparticles in Lactuca sativa L.

Copper (Cu) is an essential micronutrient, but poorly controlled inputs may increase phytotoxicity risks and alter soil–plant nutrient dynamics. Therefore, Cu formulations that regulate rhizosphere Cu availability are of agronomic interest. This study compared non-encapsulated Cu nanoparticles (CuNPs) and alginate-encapsulated Cu nanoparticles (eCuNPs) in a 42-day pot experiment with Lactuca sativa L. grown in two agricultural soils with different properties, applying 0, 10, 25, 50, and 100 mg of Cu kg−1. Soil properties, Rhizzo-extractable Cu as a proxy of available Cu, plant biomass, Cu accumulation, and nutrient concentrations were evaluated. Rhizzo-extractable Cu increased with dose under CuNPs, particularly in the soil with lower organic matter and ECEC, whereas eCuNPs maintained values closer to the control levels. In the soil with higher organic matter and ECEC, CuNPs were associated with reduced shoot and root biomass at higher doses, while eCuNPs showed a more variable response and, in some cases, higher biomass values. In contrast, biomass remained low across all treatments in the more limiting soil. Cu accumulated mainly in roots, and foliar Cu (FW) remained low and close to typical values reported for lettuce in the USDA FoodData Central database. Alginate encapsulation may reduce short-term Cu mobilization in the rhizosphere and could represent a promising strategy to improve the safety of CuNP applications, particularly in soils with higher organic matter and ECEC.