Changes in enzyme activities, microbial biomass, and basal respiration of a sandy loam soil upon long-term exposure to Pb levels

In this study, Pb was added to the soil at rates of 0, 100, 200, 300, 400, and 500 mg kg(-1) as Pb (NO3)(2) and then incubated at 27 degrees C and soil moisture of 0.7 field capacity for 180 d. Soil biological characteristics including acid and alkaline phosphomonoesterases (ACP and ALP), nitrate reductase (NR), and urease (UA) activities, microbial biomass carbon (MBC), and basal respiration (BR) were measured at time points of 3, 15, 30, 90, and 180 d. NR was increased at 3 and 15 d of incubation at Pb levels of 100 and 200 mg kg(-1). After 30 d of incubation, an increment of ALP was observed from 0 to 200 mg kg(-1), while the other indices decreased by increasing Pb levels for all time points. By increasing the incubation time, we found a remarkable decrease in microbial indices even in low levels of Pb. Considering all determined biological indices, the results revealed that at time points longer than 30 d in which the added Pb levels reached the equilibrium in this soil, the Pb levels of 200-300 mg kg(-1) can be considered as the critical range of Pb concentration in the tested soil.