Metal Input in Lettuce Grown in Urban Agricultural Soils

Urban agriculture plays an important role in supplying produces to big cities; however, the quality of water used for irrigation can hinder this activity. Hence, the purpose of this study was to evaluate metal inputs, as well as their transfer and translocation factors, in lettuce (Lactuca sativa L.) crops cultivated in an urban plot. The research was conducted during the dry and rainy seasons. In the former, crops were irrigated with treated wastewater, whereas during the latter, crops were maintained just with rainwater. Composite samples for soils and plants were collected from the same plot during two crop cycles in 2013. Some edaphic variables were measured. Total metal concentration was determined, for both, soils and lettuce plants (leaves and roots). Water soluble and exchangeable soil metal fractions were also analyzed. A multivariate analysis of variance was performed to test for differences between seasons, among the variables analyzed. There were significant differences in edaphic characteristics between seasons. However, there was no difference in total metal content, except for Mn. Concentration of soluble metals was lower than exchangeable metal concentration, for both seasons. There was no correlation in total metal concentration between soils and plants. Transfer factor values were higher for Cd, Mn and Zn for the dry season, while for Cu, Fe and Pb were higher during the rainy season, as well as the translocation factors for all metals. Soil characteristics, together with transfer and translocation factors, showed temporal variations, leading to different metal concentrations in the edible lettuce tissues between the two analyzed crop cycles. The incorporation of metals is particular for each site, season and crop management type. Our results indicate that the metal concentration in lettuce tissues places no harm to human health. However, management strategies for urban agriculture must consider specific studies for each site.