Soilless system on peat reduce trace metals in urban-grown food: unexpected evidence for a soil origin of plant contamination

Urban horticulture is increasingly popular for social and economic benefits. However, edible urban crops may be contaminated by airborne pollutants, thus leading to serious health risks. Therefore, a better understanding of contamination risks of urban cultivation is needed in order to define safe practices. In particular, whereas it is commonly accepted that the contamination of urban-grown food comes from airborne pollutants, little is known on a possible contamination by soils. Here, we studied trace metal risk in horticultural crops grown in an experimental urban allotment garden in Bologna, Italy. Seven experiments were conducted between June and November 2015 on tomato, sweet basil, onion, lettuce, kale, bulb fennel and radish. Treatments included two growing systems, soil and soilless, and two fertilization managements, mineral and organic. Trace metal concentrations were measured in soils, substrates and edible plant tissues. We identified preferentially translocated metals by partitioning analysis of tomato, sweet basil and kale. Results showed that crops grown in a soilless system have a lower metal content of −70 % for Cr, −61 % for Cu, −45 % for Cd and −81 % for Ni, compared with those grown in soil. This finding demonstrates that the major contamination risk in an urban area is unexpectedly related to soil pollution.