Long-term copper availability and adsorption in a sludge-amended Davidson clay loam
A single application of aerobically digested sewage sludge was applied by Rappaport et al. (1988) in 1984 at rates up to 210 dry Mt ha⁻¹ on a Davidson clay loam (clayey, kaolinitic, thermic, Rhodic KandiuduIts). The heavily contaminated sludge supplied up to 760 kg Cu and 620 kg Zn ha⁻¹, which are below current cumulative limits, but above annual loading limits for these metals (USEPA, 1993). Rappaport et al. (1988) reported an increase in DTPA extractable Cu and Zn with increasing sludge rate. They observed a linear increase in com yields with an increase in sludge rate, which was attributed to high levels of available N supplied by the sludge. In the present residual study, conducted at the same research site, DTPA extractable Cu and Zn followed the trend observed by Rappaport et at. (1988). However, a linear decrease in corn and sorghum yields was observed with increasing sludge rate. The sludge-related decrease in yields was attributed to phytotoxicity (probably a combination of Al, Cu, Mn, and Zn) induced by low soil pH levels. Whole plant tissue concentrations of eu and Zn at the high sludge rates were above the normal range, however grain concentrations were within the normal range. An adsorption study was conducted, in which soils were amended with Ca(OH)₂ and Al₂(S0₄)₃, to ascertain the effect of decreased pH on Cu adsorption on the soil. Copper adsorption maxima decreased with a reduction in soil pH in all treatments. The effect of ionic strength on Cu adsorption was also investigated. Regardless of pH or ionic strength, Cu adsorption increased with an increase in sludge rate. This adsorption increase was attributed to the higher organic matter content of the sludge treatments. The organic matter at the highest sludge rate was approximately double that of the control after 11 years. The adsorption study shows that sludge amendment has long-term effects on metal retention in soils and the field study reveals that careful management of sludge-amended soils is necessary to prevent phytotoxicity.