The importance of different forms of Pb on diminishing biological activities in a calcareous soil

In this experimental study, we evaluated the effect of different lead (Pb) fractions on biological properties in soil spiked with 0, 600, 1200 and 1800 mg kg^?1 Pb, during a 90-day incubation period at 25?28°C. Different Pb fractions were measured by sequential extraction at days 1 and 90 after soil treatment. Diethylenetriaminepentaacetic acid (DTPA)-extractable Pb and soil biological properties at days 1, 5, 15, 45 and 90 were measured. The concentration of Pb in soluble and exchangeable fractions was very high at day 1, but it showed remarkable transfer into carbonate and residual fractions by day 90. Substrate-induced respiration, basal respiration, acid and alkaline phosphatases, microbial populations (bacterial, fungal and actinomycetes) and biomass carbon decreased significantly with soil contamination compared with the control. With Pb ageing, these biological properties increased. Metabolic quotient (qCO₂) increased significantly compared with the control with increasing Pb concentration. The toxicity of various forms of Pb for the biological status of the soil was in the following order: KNO₃ extractable>NaOH extractable>EDTA extractable>HNO ₃ extractable>total content Pb. Thus, the bioavailable fractions are better indicators of Pb pollution in soils.