Effect of Sewage Sludge on Mercury Concentration in Soil and Corn

Sewage sludge application on farmland as fertilizer is commonly practiced in many countries. However, high concentration of heavy metals like mercury (Hg) in sludge can cause pollution of soil, plant and the human food chain. In order to examine the risk of Hg transfer into plants a five year field experiment was conducted in which we investigated uptake of Hg from a sludge-amended soil by corn. Sludge application rates were 0, 25, 50 and 100 Mg ha-1. To study cumulative and residual effects of the sewage sludge, applications were repeated on 4/5 of each plot in second year, on 3/5 of plots in third year, on 2/5 of plots in fourth year and in 1/5 of plots in fifth year. After the fifth year, soil samples from the 0-20 and 20-40 cm depths were taken and analyzed for total Hg. Corn plants were harvested and roots, stems and grains were separately analyzed for Hg concentrations. Sludge application significantly increased total Hg concentration in soil. Total Hg concentration in soil ranged from 20 µg kg-1 (in control plots) to 1200 µg kg-1 (in plots with 500 Mg ha-1 sludge application). Sludge application significantly increased uptake of Hg in different plant parts. At the end of the fifth year the average Hg concentrations in root, stem, and grain were 91, 9, and 8 µg kg-1, respectively. Corn yield increased significantly with sludge application and this fertilizer effect was visible five years after a single sludge application.