NITRATE CONTENT IN LETTUCE (Lactuca sativa L.) AFTER FERTILIZATION WITH SEWAGE SLUDGE AND TREATED WASTEWATER IRRIGATION
Résumé
Abstract A romaine type lettuce (Lactuca sativa L.) was cultivated during three crop seasons (spring 2005, spring 2006 and autumn-winter 2006) in the same soil in Alcázar de San Juan, Spain. Six plots of 36 m2 each, were set up in agricultural land. A drip irrigation system was used to water all plots: five plots with drinking water and one plot with wastewater from Wastewater Treatment Plant (WWTP) of Activated Sludge system. One drinking water irrigated plot was not fertilized and was considered as control. Five different treatments were applied to the soil: Three organic mixtures (sewage sludge, sewage sludge mixed with pine bark and municipal solid waste with composted sludge) and a conventional fertilizer were applied to the soil in each of four plots irrigated with drinking water. The last plot was irrigated with treated wastewater. These treatments were tested for their effects on plant growth and nitrate concentration in vegetable tissue. An increase in fresh weight in the lettuce was linked to the dosage of sewage sludge. The highest nitrate level was observed in the sewage sludge treatment in all crops and seasons, although in general, all values were below the maximum limits established by the European Commission on nitrate content in fresh romaine lettuce. In the third crop season, a significant increase in nitrate content was observed in lettuce from organic treatments. Nitrate concentration in lettuce from irrigated treated wastewater was higher than control plants, although significant differences were not found.
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