Soil Zinc Fractions Determine Inhibition of Nitrification

The thresholds for heavy metal loading of soils that do not impair microbially mediated processes need to be determined. This study assessed the limits for Zn in relation to the inhibition of nitrification. Different Zn concentrations in soil were achieved by spiking with ZnCl₂ or from the long-term application of biosolids. Potentially available Zn was evaluated by fractionation using sequential extractions with water, 0.05 M CaCl₂, and 0.11 M CH₃COOH solutions at a soil:liquid ratio 1:40. pH (0.01 M CaCl₂) and electrical conductivity were measured in all treatments. It was not possible to obtain very clear cutoffs of Zn concentrations based on nitrification inhibition, which could apply to different soils. However, water-extractable Zn fraction was best related to nitrification inhibition limits when soils were considered together. Three approximate ranges of nitrification inhibition were identified: 0 to 0.125 mg Zn L^-1 (no inhibition); 0.125to 0.5 mg Zn L^-1 (partial inhibition); and over 0.5 mg Zn L^-1 (complete inhibition). For the same Zn loading, the water-soluble fraction was much lower in biosolid-amended soils than in the spiked ones. Consequently, biosolid-amended soils were more resilient toward Zn contamination. Weakly bounded Zn was a more reliable index than stronger bound fractions to determine common thresholds with respect to nitrification inhibition in the soils studied. Therefore, we suggest maximum Zn loadings based on the amounts in the soil solution rather than on total inputs or total content.