Modeling the Plant Uptake of Cadmium and Zinc from Soils Treated with Sewage Sludge

To evaluate models for predicting plant uptake of Zn and Cd, a growth chamber study was conducted with soils collected from research plots in Ohio, Indiana, Michigan, and Wisconsin that had been treated 4 yr previously with 100 Mg ha⁻¹ of an anaerobically digested sewage sludge from Chicago, IL. The single application of sludge supplied 19-kg Cd and 340 kg Zn ha⁻¹. Sludge application increased the concentration of Cd and Zn in the shoots and roots of corn (Zea mays L.) seedlings grown for 16 and 22 d in the four soils. The application of sludge had no effect on the growth and properties of the corn root systems. Soil characteristics and kinetic parameters for Zn and Cd uptake were combined with a nutrient uptake model to predict Cd and Zn uptake by corn seedlings. In the untreated and sludge-treated soils predicted Cd uptake (Y) was related to observed uptake (X) in nmol pot⁻¹ by: Y = 90.03 + 1.41X (r = 0.81). Overprediction of Cd uptake by 41% may have resulted from an incorrect value of the Michaelis-Menten constant, Kₘ, and/or neglecting Cd efflux from roots. Calculated Zn uptake in the untreated and sludge-treated soils was related to observed uptake by: Y = 3.29 + 0.857X (r = 0.91) where units for Y and X are µmol Zn pot⁻¹. A sensitivity analysis was conducted with the model to determine how each soil and plant factor affected predicted Cd and Zn uptake. Plant factors that had the greatest effect on Cd and Zn uptake include the root growth constants, average root radius, and water influx rate. Kinetics of Cd and Zn uptake and half-distance between roots had little effect on Cd and Zn uptake. Concentration of Cd and Zn in the soil solution (Cᵢ) was the soil parameter showing the greatest effect on predicted metal uptake. In comparison, average buffer power and diffusion coefficients for Zn and Cd had minimal impact on predicted metal uptake.