Distribution of heavy metals in soil and elemental content of lettuce plants as affected by different amendments

Abstract

The accumulation of heavy metals in soil under irrigation with sewage water is a serious environmental problem. The availability of metal ions in contaminated soils can be reduced by the addition of organic and inorganic amendments. In this study, a pot experiment was conducted during the winter season of 2010/2011 to evaluate the effect of gypsum, citric acid, single superphosphate (SSP), farmyard manure (FYM) and compost on the availability and forms of some heavy metals; Zn, Cu, Ni and Cd in soil of El-Gabal El-Asfar irrigated with sewage water for many years using lettuce plant as a test crop, under irrigation with tap water or dissolved organic carbon (DOC). The forms of the concerned heavy metals in soil were determined by the sequential extraction method. Results showed that treating the soil with amendments particularly organic substances such as farmyard manure, compost and citric acid under irrigation with tap water or dissolved organic carbon (DOC) decreased heavy metals concentrations in soil. The amount of Zn was distributed among the different soil fractions following the descending order Fe-Mn oxides bound > residual fraction > organically bound > carbonate associated > exchangeable. This sequence for Cu was residual fraction > organically bound > Fe-Mn oxides bound > carbonate associated > exchangeable. Similar trends were found for Ni and Cd fractions. The dry matter weight and NPK concentrations of lettuce plants were significantly increased due to the application of these amendments especially FYM compared to the control, and lower values were obtained in the case of irrigation with tap water compared with DOC irrigation. In addition, heavy metals concentrations were significantly decreased in lettuce plants under irrigation with DOC compared to that irrigated with tap water, with the highest effect for gypsum treatment.