Abstract
Amendment of agricultural soils with municipal sewage sludge (SS) provides a valuable source of plant nutrients and organic matter but it may create a risk of trace-element pollution of the environment. Utilization of SS in cultivation of oil- and biofuel-producing crops such as canola (Brassica napus) may be useful for safe management of biosolids. The aim of this study was to examine the influence of SS application rate on bioavailability and phytoextraction of trace elements by canola. To achieve this goal, a greenhouse pot experiment was established in a Typic Xerfluvent from Greece amended with various rates of sewage sludge: 20 Mg ha(-1) (SS1), 50 Mg ha(-1) (SS2), and 100 Mg ha(-1) (SS3). Furthermore, a soil sample without any amendment was cultivated and used as a control (C). After a planting period of 2 months, the whole plants and soils were removed from the pots. One kg from the soils was selected for analyses and the rest of the soil was cultivated again for 2 more months with the same crop. Soil samples were analyzed for total and available forms of trace elements [iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), nickel (Ni), lead (Pb), and cadmium (Cd)] and the same elements were measured in plant tissue. The results showed that increasing SS application rate increased significantly the total amounts of all the studied trace elements compared to the control except for Fe and Ni. However, the total concentrations of all the studied elements were less than the critical concentrations in soils. Diethylenetriaminepentaacetic acid (DTPA)-extractable element concentrations increased significantly with increasing the biosolid application rate, especially in the case of SS3. However, no significant differences were recorded between control and SS1 for Fe, Mn, Pb, Ni, and Cd. Plant tissue element concentrations for all elements except for Fe increased significantly compared to the control with increasing sewage sludge application rate, especially with SS3, whereas no significant differences were found between the control and SS1 in the cases of Cu and Pb. Iron showed the opposite trend, where it decreased significantly with increasing SS rate. The ability of canola in phytoextraction of the studied elements was measured by bioconcentration ratios (BCR) (plant tissues concentration/DTPA-extractable elements). Values of BCR differed widely between the elements. Iron and Zn BCR values decreased significantly with increasing the SS application rate in both planting periods. The BCR values of Cd, Cu, Mn, Ni, and Pb increased in the SS-treated soil compared to the control with a relative increase in SS1 and SS2 treatments compared to SS3. Data from this result convinced us that there is no risk from utilization of biosolids in production of noneaten plants like canola. The recommended rate was 20 Mg SS ha(-1).