Effect of sewage sludge on metal content of grassland soil and herbage in semiarid lands

Abstract

Utilization for agriculture is one of the main disposal methods for sewage sludge. Subirrigated pastures are close to settlements and under intensive usage. The objective of this study was to investigate the effects of sewage sludge on subirrigated pasture soil and plant metal concentrations, and the relationship between them. Activated sewage sludge was applied to surface grassland at 25, 50, 75, and 100 t ha^{-1} year^{-1} treatments for 2 years. The concentration of all metals in topsoil (0-20 cm) increased depending on application dosages, but only Zn concentration increased in subsoil (20-40 cm) consistently. While in the second year the concentration of topsoil diethylene triamine pentaacetic acid (DTPA) metals increased significantly, it decreased in the subsoil. The ratio of organic matter increased in topsoil, but decreased in subsoil compared to the control. Doses of sewage sludge of 25 and 50 t ha^{-1} did not increase the metal content of herbage. However, the herbage's Cu, Zn, Pb, and Cd content increased at 75 and 100 t ha^{-1} treatments in comparison with both lower doses and the control. Similar to the soil concentration, the Zn content of herbage increased proportionally with higher sewage sludge application doses. Topsoil DTPA-extractable Zn, Cu, and Pb concentrations were correlated to herbage Zn, Cu, and Pb content significantly and positively. In subsoil, only DTPA-extractable Cu concentration was correlated to herbage Cu content significantly and negatively (r = -0.53**). A 25 t ha^{-1} year^{-1} application of sewage sludge can be recommended as an organic fertilizer in semiarid subirrigated pastures, as it increases topsoil organic matter and results in a low concentration of DTPA-extractable and plant metal content.