Extractability and bioavailability of zinc over time in three tropical soils incubated with biosolids

Phytotoxicity of heavy metal is the primary concern in applying biosolids (sewage sludge) to agricultural land. This study evaluates the changes in chemical speciation of Zn in three tropical soils of Taiwan measured with sequential extraction over a one-year period. Biosolids were applied to the soils at application rates of 10, 50 and 100 Mg ha−1, and correlated diethylene triamine pentaacetic acid (DTPA) and sequential extraction as extract for prediction of Zn bioavailability to Chinese cabbage (Brassica chinensis L.). Experimental results indicated that the exchangeable (F1) and Fe–Mn oxide (F3) fractions in the sequential extractions increased with application rate of biosolids in the soils over time. Large amounts of Zn in the soils following the cessation of biosolids application were identified as soluble and were adsorbed by Fe–Mn oxides. The organically bound Zn, which is associated with readily decomposable carbon, is in limited amounts in the biosolid-treated soils. The DTPA-extractable concentrations of Zn in all biosolid-treated soils decreased over the time. A positive and significant correlation (r2 = 0.96) was found between the Zn concentrations extracted with DTPA and sum of F1 and carbonate-bound (F2) fractions in the sequential extractions. Additionally, the concentrations of Zn extracted with DTPA were strongly correlated with the concentrations of Zn in the shoots of Chinese cabbages, indicating that F1 + F2 in the sequential extractions was reliable for predicting Zn bioavailability to Chinese cabbage in the biosolid-treated soils.