Assessment of leaf photosynthetic performances and bioaccumulation of trace metals by lettuce leaves and strawberry fruits amended with sewage sludge: Which possible re-use in agriculture?

Sewage sludge (SS) is the by-product of wastewater treatment. SS can be used to improve soil fertility thanks to its nitrogen and phosphorus contents. However, crop plants grown on SS amended soils could represent a risk for human health due to the possible high content of trace metals provided by SS. Thus, the present work aimed to investigate the possible effects of SS amendment (used at 10 and 20% of the weight of soil) on the leaf photosynthesis of two different pot-grown crop species: Lactuca sativa as a leafy vegetable and Fragaria x ananassa as a fruit species. Moreover, an assessment of the potential trace metal bioaccumulation in lettuce leaves and strawberry fruits was also performed. The results showed that SS amendments increased Pn and gs values in leaves of both species thanks to the nutrients provided by SS supplementation, also resulting in a higher leaf biomass of lettuce SS-plants compared to relative controls. SS added at 10% to lettuce plants turned out to be the best compromise between plant growth and sensitivity to trace metals contained in SS. Lettuce leaves accumulated higher amount of Cu, Zn and Hg, while strawberry fruits accumulated more Zn and Hg than relative controls. However, the reported values of trace metals were lower than the Reg. CE 1881/2006 and FAO/WHO threshold values, except for the Cu content in lettuce leaves treated with SS-20%. The results highlighted that for lettuce and strawberry plants, SS amendment could be a good agronomic practice to increase plant growth and productivity in potted plants.