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Abstract
The effects of time and a labile carbon (C) source on the fates of zinc (Zn) and copper (Cu) were investigated in a slightly acidic soil in a two-factor experiment. Glucose was used as the C source to examine the effect of the expected flush of microbial activity on Zn and Cu extractability during the experimental period. The soil was amended with 500 mg kg−1 soil of Zn and Cu in the form of nitrate salts and with 4 g kg−1 glucose where appropriate, producing M (heavy metals, no glucose) and MG (heavy metals, with glucose) treatments. The treated soil samples were incubated for 3 h, 1 day (24 h), 3 days (72 h), 10 days (240 h), 30 days (720 h), and 60 days (1440 h) at 20 °C at constant moisture (≈50% of the soil's water-holding capacity). At the end of each incubation period, destructive sampling was followed by a Tessier sequential extraction procedure that yielded five metal fractions for both Zn and Cu, defined as exchangeable, acid-soluble, reducible, oxidizable, and residual. The exchangeable fraction of Zn and Cu (considered to be an availability measure) showed decreasing trends over time, while the opposite was observed for the other fractions. The presence of glucose resulted in significantly lower exchangeable fractions in the “polluted soil” for both metals up to 720 h, pointing to lower Zn and Cu availability. Redistribution from less-available forms back to exchangeable forms occurred during the second month of incubation, suggesting a sharp decline in microbial activity and a consequent remobilization and potentially increased Zn and Cu bioavailability. However, glucose did not affect Zn and Cu availability in the control soil; this indicated that its effect is mainly observed following recent pollution events. Field applications of glucose at 4 g kg−1 soil is impractical in practice; future experimentation to estimate a minimum effective application rate or an alternative form of labile C, probably derived from recycling renewable labile organic materials, is warranted.