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Cadmium Uptake for Swiss Chard Grown on Composted Sewage Sludge Treated Field Plots: Plateau or Time Bomb?

Dep. of Soil and Environmental Sciences, Univ. of California, 2208 Geology Bldg., Riverside, CA 92521-0424

Hae-nam Hyun

Dep. of Agricultural Chemistry, Cheju National University, Korea

^* Corresponding author (acchang{at}ucracl.ucr.edu).

ABSTRACT

Two hypotheses have been proposed to describe the phytoavailability of potentially toxic metals in sewage sludge treated soils. The plateau theory argues that the metal adsorption capacity added with sludge will persist as long as the metals of concern persist in the soil and the metals would remain in chemical forms not readily available for plant uptake. Therefore, the metal concentrations of plant tissue will reach a plateau as sewage sludge mass loading increases and remain at this plateau after termination of sludge application. The sludge time bomb hypothesis postulates that a soil's metal adsorption capacity is augmented by soil organic matter added as sewage sludge. This capacity, however, will revert back to its original background level with time following termination of sewage sludge application as mineralization of organic matter releases metals into more soluble forms, thus a time bomb. We employed a set of experimental data obtained from a 10-yr field-based sewage sludge land application experiment to evaluate the hypotheses of the plateau and the time bomb. The evaluation involved analysis of Cd concentration of Swiss chard grown on soils that either received sewage sludge application continuously or no longer received sewage sludge application. The results indicate that necessary conditions for plateau and time bomb to take place may be found. But an actual plateau or time bomb was not evident from 10 yr of experimental data in which the sewage sludge application reached 2880 Mg ha^–1.

Contribution from Department of Soil and Environmental Sciences, Univ. of California, Riverside.

Received for publication December 11, 1995.