Sequential Extraction of Soil Heavy Metals Following a Sludge Application

Abstract

Two soils treated with a composted sludge and two liquid sludges annually for 7 consecutive years and cropped to barley (Hordeum vulgare L.) each year were sampled at 4‐week intervals following planting in 1981. The soils were Greenfield sandy loam (coarse‐loamy, mixed, thermic, Typic Haloxeralf) and Domino loam (fine‐loamy, mixed, thermic, Xerollic Calciothid). Barley tissue samples were also obtained at the time of each soil sampling. Soils were fractionated with a sequential extraction procedure to determine the distribution of solid‐phase chemical forms of the sludge‐borne heavy metals. In untreated soils, essentially all of the heavy metals were present in either the sulfide‐residue form (for Cr, Cu, Ni, and Zn) or the carbonate form (for Cd and Pb). With sludge treatment, every extracted fraction showed increases in the amounts of heavy metals. However, the most significant increases occurred in the carbonate fraction and, sometimes, in organically bonded fractions as well. There was little indication that the distribution pattern of the solid‐phase heavy metals in the sludge‐treated soils changed with time from planting to harvesting. The Cd and Zn concentrations in barley grown in the sludge‐treated soil were consistently higher than those in the nonsludged control. The rate of Cd and Zn uptake was rapid during early stages of plant growth. However, the rate of Cd and Zn uptake by barley progressively decreased with plant development. As a result, the part of the plant that developed at the later stages of plant growth (e.g., the barley grain) always contained less Cd and Zn than those that developed earlier. Since the percentage of Cd and Zn in each extracted fraction did not change appreciably throughout the growing season, the differential Cd and Zn uptake by barley with plant development cannot be related to the chemical transformation of solid‐phase heavy metals in the soil during the growing season.