Heavy Metal Uptake of Lettuce and Ryegrass from Urban Waste Composts

Interest in using urban waste composts as amendments in urban agriculture is growing nowadays. However, concerns about the potential transference of pollutants present in urban waste to the food chain are very relevant when they are recycled for food or animal feed production. Thus, for the safe use of urban waste composts, it has to be assured that no risk of metal transference to plants from compost exists. In this work, the transference of heavy metals from urban waste composts to plants has been studied in an experiment with lettuce and Italian ryegrass, grown in substrates based on five metal-rich composts and a manure vermicompost (included for comparison). A two-month pot experiment was performed under controlled light and temperature conditions, and plant growth and uptake of Cu, Pb, Cd and Zn were analyzed. For both species, the concentration of metals in plant tissue followed the sequence Zn > Cu >> Pb ≈ Cd, the same as the metal concentrations in four out of the five composts. Plant concentrations of Zn, Cu and Cd increased with their concentrations in compost, whereas this relation was not observed for Pb. The ratio between element concentration in plant and compost were much higher for Zn, Cd and Cu than for Pb, showing the lower bioavailability of Pb with respect to other metals.