REPEATED APPLICATION OF A METAL-RICH SEWAGE-SLUDGE TO GRASSLAND .1. EFFECTS ON METAL LEVELS IN SOIL

The effects of repeated applications of a metal-rich sewage sludge on metal concentrations in soil were studied over a 3-year period. Applications were made over the first 2 years of the experiment. Sludge was applied at rates of 25, 50 and 75 m3/ha on three dates each year resulting in copper additions of 18.5, 37.0 and 55.5 kg/ha, zinc 188, 374 and 564 kg/ha, and lead 2.9, 5.9 and 8.8 kg/ha, respectively, over the 2 years. At the end of the first year, increased concentrations of both total and extractable metals were observed in the 0 to 40 mm soil layer. The proportions of total copper and zinc which were extractable by EDTA were higher for the sludge-treated soil. Increased copper levels were observed in the top 52 mm of soil at the end of 2 years' sludge additions; changes at greater depths were not significant. EDTA-extractable levels showed a similar trend. Additions of zinc resulted in greatly increased soil values but changes in zinc values at soil depths greater than 156 mm did not attain significance. Topsoil (0 to 13 mm) enrichment was also observed for lead. At the end of the third year, soil copper levels were still elevated. Compared with control values, copper levels at the 13 to 39 mm depth were increased by factors of three, four and six for sludge application rates of 25, 50 and 75 m3/ha, respectively. Extractable copper values were increased to a depth of 75 mm. The effects of added zinc were evident to a depth of 152 mm at the end of Year 3; the highest application rate resulted in 10-, 6- and 2-fold increases at depths of 0 to 13, 13 to 39 and 39 to 75 mm, respectively. Other application rates had soil values that were intermediate between the control and the highest application rate. Changes in extractable zinc values exhibited a similar pattern to that for total zinc concentrations. Quantitative zinc recovery was observed in the top 75 mm soil layer for the 25 and 50 m3/ha treatments, while full recovery for the 75 m3/ha treatment was observed in the top 100 mm soil depth. Changes in soil lead levels were small.