Combined Effects of Cadmium and Azithromycin on Soil Nitrification Process

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Heavy metals and pharmaceuticals have polluted agricultural soils mainly through wastewater irrigation, fertilizers, and soil amendment with sewage sludge. This study aims to determine the synergetic toxic effect of Cd and the selected macrolide antibiotic, azithromycin (AZI), on ammonia-oxidizing bacteria (AOB) in soil, via analyzing nitrification inhibition. A short-term acute toxicity test was used to measure the formation of Nitrite (NO2-N) to indicate the nitrification potential of the aerobic nitrosomonas bacteria in the germination period. Potential nitrification rates (PNRs) of five soil samples ranged between 3.782–17.642 mg NO2-N/kg dm PNRs of soil samples positively correlated with organic matter content and neutral pH. PNRs of the tested soils were significantly affected by Cd and AZI contamination, with interactions exhibited for their simultaneous occurrence and soil pH. A significant difference (p < 0.05) was found when soil samples with pH 6.5–pH 8.5 contaminated with environmentally relevant concentrations of Cd (1 mg/kg–21 mg/kg) and AZI (1 mg/kg–9 mg/kg). 50% PNR inhibition after 11 mg/kg Cd and 5 mg/kg AZI contamination was determined for the soil sample at pH 8.5, with 3.782 mg NO2-N/kg dm potential. From these outcomes, it was concluded that there was a risk of the soil nitrification process in case of simultaneous occurrence of Cd and AZI.