Critical concentrations of copper, nickel, lead, and cadmium in soils based on nitrification1

Abstract

The nitrification process is sensitive to some heavy metals and, therefore, it can serve as bioindicator of their toxicity in soils. Previous work with zinc (Zn) has shown that its water extractable fraction is best related to the performance of nitrifiers. This fraction tends to unify different soils and sources of contamination with respect to thresholds of nitrification inhibition. In this study, the same approach was followed with copper (Cu), nickel (Ni), cadmium (Cd), and lead (Pb). The nitrification process at different metal loadings was assessed in three soils. The applied metal loadings were based on published critical values for these metals. Nitrification inhibition depended on metal; Pb and Cd were not inhibitors, whereas Cu and Ni impaired nitrification at loadings lower than the published critical concentrations in soil. The water-extractable fraction was used as an index of metal presence in the soil system. A common threshold based on this index, were found for Cu. More than 3.8 mg water-extractable Cu kg− 1 soil was inhibitory for nitrification, whereas less than 2 mg kg− 1 was safe. The interval between these two limits was partially inhibitory. Water-extractable Ni did not yield common thresholds and nitrification inhibition was soil dependent. The relationships between soil pH, metal input and NO3 − production showed that nitrification was not inhibited by any side effect of heavy metal application, such as lowering soil pH. *Contribution from the Department of Crop and Soil Sciences, University of Georgia.