Plant availability of heavy metals in a soil amended with a high dose of sewage sludge under drought conditions

The objective of this research was to study the effect of water deficit on soil heavy metal availability and metal uptake by ryegrass (Lolium multiflorum Lam.) plants grown in a soil amended with a high dose of rural sewage sludge. Three fertility treatments were applied: sewage sludge (SS), mineral fertilizer (M), and control (C); unamended). The levels of irrigation were: well-watered (W) and water deficit (D). Microbial respiration decreased the total organic C (TOC) in sludge-treated soils, but this did not enhance soil DTPA-extractable heavy metal concentrations. Indeed, Zn, Cu, Mn and Ni availability decreased during the experiment. C- and M-treated soils showed either no changes or increases of some trace element concentrations during the incubation. In the plant experiment, ryegrass dry matter (DM) yield, relative water content (RWC) and leaf water potential (_w) decreased in drought conditions. Sludge addition increased metal concentrations in plants. However, in some instances, SS-treated plants showed either similar or lower transfer coefficient (Tc) values than did plants in the C and M treatments. Water deficit decreased the concentration and the Tc of some metals in roots of M and SS plants. Results indicate that sludge-borne heavy metals were maintained in chemical forms of low availability. The lower metal uptake by SS and M plants under dry conditions cannot be attributed to a lower availability of these elements in soil.