The purpose of this study was to determine lead (Pb) fractionation in a sandy loam soil amended with enriched (600 mg Pb kg -1 ) biosolids and cow manure before and after cultivation of sunflower ( Helianthus annuus L.) and corn ( Zea mays L. Single cross 704). In the salt treatment the Pb bounded to the oxide fraction was higher (8.7%) and the carbonate fraction was lower (12%) in the rhizosphere of sunflower compared to the rhizosphere of corn. In soils treated with biosolids and inorganic Pb salt, corn accumulated greater amounts of Pb in its roots compared to the sunflower, whereas both crops had similar root Pb concentration in the manure amended soil. A high correlation ( 92%) was found between the Pb uptake by the crops grown in the soils treated with the biosolids and cow manure to the metal concentration in the soil inorganic fractions. The phototoxic effects of Pb were clearly observed on the sunflower grown in the Pb salt treated soil while no such effect was found for the plants grown in the soils treated with the sludge and manure. Keywords: Sunflower, corn, sequential extraction, lead 1. Introduction Heavy metal bioavailability in soils from organic amendments varies widely and is affected by many soil parameters such as pH and soil minerals. Different sequential extraction procedures have been suggested to evaluate the contamination risk for soil and sediment. In all sequential extraction schemes, extractants are applied to increase reactivity, so that the successive fractions obtained correspond to metal association forms with lesser mobility and bioavailability. An extended method was developed by Tessier et al. [ 10 ], performed fractionation of metals in samples of sediments into 5 parts: (1) exchangeable fraction representing the most easily available metals, (2) carbonate fraction, (3) Fe, Mn, and Al oxides fraction, (4) organic matter fraction, and (5) residual fraction, tightly bound on silicate matrix of samples. Small changes in the experimental conditions (e.g. pH, temperature, contact time, solid to extractant volume ratio, particle size and sample pretreatment) can lead to large variations in the fractionation, making it troublesome for comparisons between results. On the other hand, crop cultivation may change the distribution of metals in different soil fractions. The relationships between the chemical fractionations of trace elements in soils and plant uptake have been estimated by simple and multiple correlation procedures [ 7 ].
