Long Term Zn Bioavailability and Phytotoxicity in Soil Treated with Metal-Contaminated Sewage Sludges.

Bioavailability and phytotoxicity of Zn caused by repeated high applications of Zn-contaminated sewage sludges can be affected by long-term transformation of Zn chemical forms in the amended soil. Sewage sludges from different wastewater treatment plants were applied annually from 1977 to 1986 to a Metea sandy loam soil (loamy, mixed, mesic Typic Hapludalfs) at four Zn loadings (0, 1.43, 2.67, and 11.3 Mg ha^-1) and replicated four times in a randomized complete block design. Soils receiving the highest Zn loading had lowest soil pH that resulted in increased plant diagnostic leaf Zn concentrations and reduced yields of crops grown between 1985 and 2007. Chemical fractionation of soil samples collected in 1998 and 2007 is consistent with 1990 samples which show the greatest concentrations of Zn ( > 40%) in chemical forms most available to plants (i.e., exchangeable and acid soluble), although water soluble Zn was minimal (< 1%). The very low Zn concentration found in water soluble form and the lack of increased Zn concentration in soils below the 30 cm depth suggest no or negligible downward movement of Zn by leaching. Increased Zn concentrations in the 15-30 cm depth is attributed to physical movement of Zn by varied tillage depth of 10-25 cm over time. While this result suggest minimal long-term environmental impacts of excessive Zn loadings on water resourses, bioavailability and phytotoxicity of Zn in the tillage depth (0-30 cm) can persist for a long time period (> 20 yr).