| The existence of root exudates has been known for a long time. Their role and function in plant nutrition and soil chemistry have only recently begun to be understood. Studies have demonstrated that the primary constituents of the root exudates are low molecular weight organic acids that play an essential role in making the sparingly soluble soil Fe, P, Zn, and other trace metals available to growing plants. The purpose of this investigation was to define the role of root exudates in plant uptake of metals from biosolids-treated soils and to evaluate the long-term phytoavailability of biosolids-borne metals that have accumulated in soils. Corn (Zea mays L.), Durum wheat (Triticum turgidum L.), rape (Brassica napus L.), Sudan grass (Sorghum sudanense L.), chickpea (Cicer arietinum L.), and Swiss chard (Beta vulgaris L.) were grown on standard and biosolids-treated sand media to characterize the organic acids in root exudates and to evaluate the plant uptake patterns of biosolids-borne metals. 1 also determined the total root exudate-specific extractable metal concentrations in biosolids-treated soils and the rate of metal dissolution in root exudate-based organic acid mixtures, and correlated metal concentrations of plants grown on biosolids-treated soils with root exudate-specific metal dissolution rate coefficients. For field validation, the metal concentrations in biosolids-treated soils and uptake of metals by plants grown on biosolids-treated soils were evaluated and fitted to a phytoavailability model of Mt = C × (1−e −kt) where Mt was the cumulative annual metal removal from biosolids-treated soils by plants at tth year (kg ha −1), t was the time (year), and C and k were constants representing the total available metal pool (kg ha−1), and rate constant of the available metal, respectively. In this study, we developed a method to determine the quantities of root exudate extractable metals in biosolids-treated soils and to determine the kinetics of metal release from solid phases. The effects of root exudates on soil-borne metals were examined, the differences in plant uptake of metals due to differences in plant species and biosolid and soil properties were normalized with respect to chemical composition of root exudates, and data obtained from various field sites were examined to assess the phytoavailability and long-term safety of biosolids applications in cropland. |
