Metal uptake by different plant species was quantified in sand media amended with biosolids in a sand-culture hydroponic medium. In a previous paper (Koo et al. 2006), we concluded that total quantities of organic acids were greatest in treatments containing both plants and biosolids, with lesser amounts in treatments with plants alone, biosolids-treated media alone, and a nutrient solution-irrigated blank medium. Biosolids enhanced organic acid production in the rhizosphere. The purpose of this study was to evaluate how organic acids in root exudates affect the absorption of metals by selected plants. We found that the concentrations of metals in the plant tissue grown on biosolids-treated medium were always higher than that from the standard medium, irrespective of species and cultivar. The amount of metal transferred from the biosolids-treated medium to the plant varied with the metal element, following the order: Cd > Ni = Zn > Cu > Pb > Cr. Interspecies and cultivar differences in metal uptake were trivial compared to differences induced by the treatment. The metal uptake decreased with the growth period, and the kinetics of metal uptake, as indicated by accumulation in corn shoots, were essentially a first order during the initial 4 weeks of growth, especially for Cd and Zn. 1. Introduction The chemical and biological reactions occurring in the soil-root interface play an important role in the availability of metals to plants [1]. Metal uptake by plants depends on both edaphic and plant factors. Edaphic factors include metal concentration in soil, interactions of metal with the soil (solid) surfaces, and the pH at the root-soil interface. Plant roots may change the physical, chemical, and biological conditions of the soil immediately adjacent to the root, commonly referred to as the rhizosphere. The rhizosphere, in comparison to bulk soil, is enriched with organic substances of plant and microbial origin. They include organic acids, sugars, amino acids, lipids, coumarins, flavonoids, proteins, enzymes, aliphatics, aromatics, and carbohydrates [2–6]. Among them, the organic acids are the most abundant and highly chemically reactive with soil constituents. The commonly found organic acids in the rhizosphere are acetic, butyric, citric, fumaric, lactic, malic, malonic, oxalic, propionic, tartaric, and succinic acids [5, 7–9]. In soils, organic acids are involved in biogeochemical processes that release the not-so-readily available nutrients such as phosphorus, iron, and other micronutrients for plant nutrition [10–12]. Organic acids in the
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