Heavy metal extractability and plant bioavailability from two sacrificial biosolids soils as influenced by intensive liming

Abstract

The influence of pH on metal extractability from two sacrificial soils that received large amounts of biosolids was studied in four separate trials. The soils (Soil 1: gravelly sandy loam; Soil 2: sandy clay loam) were collected from different water treatment plants in Gauteng. Both soils had undergone significant changes in physical (organic carbon content) and chemical characteristics (pH, metal and nutrient content) due to the prolonged disposal of biosolids. A preliminary pot trial was conducted where large quantities of lime were added to the soils. Results indicated that the soils had high pH buffer capacities due to low pH and high organic matter content. An unexpected increase in the metals extracted with NH4EDTA after intensive liming lead to the performing of three further trials to shed light on the phenomenon. In the second trial lime was added to the soils at four rates (0, 12, 24, 36 tons ha-1). An increase in most NH4EDTA extractable metals was found and this was correlated with increasing absorbance values (at 465 nm) of the extracting solutions (indicating increased extractability of organic matter). In the third trial limed (27 ton ha-1) and unlimed samples were incubated over a period of 20 weeks with regular sampling intervals. The NH4NO3 extractable metals decreased significantly in the lime treated soils but the NH4EDTA extractable metals generally increased. A fourth trial was conducted due to concern regarding the increased EDTA extractable metals after liming and a possible correlation with plant metal bioavailability. The soils were incubated in pots with four lime rates (0, 12, 24, and 36 ton ha-1) and wheat and spinach grown for two months. Although similar trends as for the previous trials were obtained in terms of metal extractability, plant metal content was best correlated with NH4NO3 extraction levels. The results indicate that liming is a safe option for sacrificial soils and that NH4NO3 extractable and plant metal levels decrease with liming. The use of EDTA in metal guidelines or in soil metal content studies is discouraged due to its increased metal extractability with liming and poor correlation with plant metal content.