Sewage sludge application to corn: heavy metals uptake and soil fractionation study

A field experiment using a block design was set up from 1995-1999. Soil at this site is the well-drained Bungor Series, clayey, kaolinitic, isohyperthermic Typic Paleudults. The treatments for the first cycle are no sludge application, inorganic N (ammonium sulfate) at 140 kg N ha and sludge application rates at 150, 300, 450 and 600% of recommended fertilizer N rate of 140 kg N ha. No sludge was applied during the second cycle. During the third, fourth and fifth cycle, the treatments were similar with the exception that the rates were reduced to 100, 200, 300 and 400% of the recommended fertilizer N rate. The method of the sequential extraction procedure used in this study is the modified version of Yang and Kimura (1995) and Chlopecka et al. (1996) as put forward by Salas et al. (1998). The forms of heavy metals are water soluble, exchangeable, carbonate bound, Fe-Mn oxides bound, organically bound and the residual form. Soil samples at 0-20 cm depth from the plots with different treatments and four replications were collected after each corn harvest. The concentration of Cd, Cu, Pb, Ni and Zn in grain even in the highest sludge treatments (approximately 1%) in the five cycles were still below the maximum permitted concentration (fresh weight basis) as stipulated by the Malaysian Food Act 1985. There is however, an increase in the accumulation of Cu and Zn in the soil, and for Cd and Pb, the trend is not yet visible. As for the fractionation study, for soil Ni and Zn, the most predominant form was the residual form. Soil Cd and Pb were found mostly in the exchangeable form. Soil Cu was dominant in the organically bound form. For all these heavy metals, the water soluble fraction is less than 5% of the total heavy metal content. From the correlation study, Zn uptake by maize grain showed positive correlation with water soluble form. There were relationships between copper uptake in grain and the carbonate, organic and residual form. Meanwhile, there were relationships between Ni uptake by the grain and the Ni associated with the Fe-Mn oxides, organic and residual form. No correlation was done between Cd and Pb uptake with these metals fractions in the soil because the concentrations in the grain were very low.