Effects of Continuous Application of Sewage Sludge Compost on Heavy Metals Accumulation and Mobility Characteristics in Soil Profile and on Heavy Metals Uptake of Wheat

Abstract

The use of sewage sludge compost(SSC)as fertilizer may cause increased leaching due to its high content of heavy metals and thus pose a threat to groundwater quality. The effect of SSC application on heavy metals leaching in calcareous soils has been studied in field trials, which provides basis for determining heavy metals environmental capacity and preventing metal pollution in farmland soil scientifically. The results indicated that the contents of Cu, Zn elevated obviously with the increase of the age and the dosage of SSC utilization in the topsoil(0-15 cm) under 4-year continuous application of SSC. Under higher levels of the compost treatment, the heavy metals Cu and Zn were found to migrate into the 15-30 cm soil and 60-90 cm soil under the experimental condition. Nevertheless, the majority of Cu and Zn from SSC accumulated in topsoil and the highest accumulation rates could reach 75.3% for Cu and 85.9% for Zn. The contents of Cd, Pb increased significantly in topsoil after 4-year continuous application of SSC, and their increases could reach 57.2%-165.2% for Cd and 13%-34% for Pb compared with CK. At 60-90 cm soil, the contents of Cr, As and Pb were also significantly higher than those in CK treatment. Application of SSC not only caused accumulation of some heavy metals in topsoil but also leached heavy metals located in the subsurface soil down in this experiment. Continuous utilization of SSC increased Zn concentration of wheat grain, and the increase could reach 13.3%-47.9%. For the concentrations of Cr and Pb in wheat grain, the values exceeded the national food and healthy standards value (GB 2762-2012) in part of compost treatments. The cumulative ratio of heavy metals carried out by wheat were all below 10% after 4-year experiment, wheat grain carried much more Cu, Zn out than wheat straw, but it was opposite for Cr, As, Cd, Pb. The cumulative ratio of heavy metals carried by wheat decreased with the increasing level of SSC utilization. The amounts of heavy metals migrated to deeper soil should be considered when determining the environmental capacity of heavy metals in farmland soil.