Availability of heavy metals for Brassica chinensis grown in an acidic loamy soil amended with a domestic and an industrial sewage sludge

The use of sewage sludge on agriculture provides an alternative for sewage sludge disposal. Therefore, it was the aim of the present study to evaluate the feasibility of using a domestic (Tai Po sludge) and an industrial (Yuen Long sludge) sewage sludge produced in Hong Kong for the growth of vegetable crops. The acidic loamy soil with or without lime treatment was amended separately with each sludge at application rates of 0, 5, 10, 25 and 50% (v/v) for the growth of a common local vegetable crop, Brassica chinensis. The plant available metal contents, as indicated by the DTPA extraction, increased with an increase in sludge amendment, but decreased with lime amendment at each sludge application rate due to the reduced metal availability at a higher pH. Sludge amendment enhanced the dry weight yield of B. chinensis and the increase was more obvious for the soil with lime treatment. The industrial sludge caused a lower yield than that of the domestic sludge amendment and a significant reduction in yield at high application rates of Yuen Long sludge was also noted. Tissue heavy metal contents, except for Fe, increased as the sludge amendment rate increased while plant grown in Yuen Long sludge amended soil contained higher Cr and Zn contents at each sludge application rate. Liming the soil reduced the heavy metal contents in the plant tissues, except for Fe, which were all below the allowable levels for vegetable crops. The present experiment demonstrates that liming was important in facilitating the growth of B. chinensis in sludge amended soil. The optimal sludge amendment rate for the soil with lime amendment was 25% Tai Po sludge and 10% Yuen Long sludge, while for the soil without lime amendment was 10% and 5%, respectively.