Biosolids and biosolids-ash as sources of heavy metals in a plant-soil system

Generally, the potential for biosolids (digested or composted) to contribute heavy metals to the soil-plant system has been compared with commercial fertilizers and other organic wastes but not with biosolids-ash. An column study was conducted in a greenhouse to determine the availability, extractability and leachability of metals in a degraded, non-calcareous soil amended with different biosolids (200 Mg ha(-1)). The biosolids investigated were dewatered, anaerobically digested biosolids, composted biosolids and biosolids-ash. The columns (26 cm) were planted with wheat (Triticum aestivum L. cv Mexa). The addition of digested biosolids decreased the dry matter yield of wheat. Treatments including organic biosolids increased Cu and Zn concentrations in wheat roots, straw and grain, whereas the addition of biosolids-ash did not affect the concentrations of these metals in wheat. Concentrations of Ni, Co, Pb, Cr and Cd in wheat were below reliable detection limits (0.06, 0.05, 0.1, 0.06 and 0.02 mg kg(-1), respectively). After harvesting, total and AB-DTPA extractable Cu, Zn and Pb increased in the upper layer of the soil amended with the different biosolids studied, whereas levels of AB-DTPA extractable Ni and Co were affected only when the soil was amended with digested or composted biosolids. Total chromium increased only in treatments including organic biosolids. The AB-DTPA extractable Cu, Zn and Pb in the lower layer of the soil in treatments including biosolids evidenced downward movement of these metals. However, absence of these metals in column leachates indicates that this movement was gradual.