Impact of Lead Contamination on Denitrification After Nitrate Addition to Soil: An Interaction Between Nitrogen Substrates, Denitrification Genes, and Microbial Community Structures

Lead (Pb) contamination is a worldwide environmental issue that also affects microbial cycling of nitrogen, a crucial plant nutrient, and environmental health. This study performed a 4-week soil incubation experiment with four levels of Pb (0, 200, 400, and 1600 mg Pb/kg soil), with and without nitrate (NO3−) addition, to assess the impact of Pb on denitrification and microbial communities. Reductions of NO3− and nitrite (NO2−) were inhibited at 1600 mg Pb/kg soil, with nitrate reductase (narG) gene abundance decreasing with increasing Pb, however nitrite reductase (nirS) gene abundance showed no correlation to Pb. The increase in nitrous oxide (N2O) at 1600 mg Pb/kg soil may have been related to a decrease in genes encoding N2O reductase (nosZ) and the increase in genes encoding nitric oxide reductase (norB). Following exposure to Pb, 16S rRNA abundance decreased, while the Shannon diversity index increased. Proteobacteria initially dominated but decreased at 1600 mg Pb/kg soil, whereas Actinobacteria, Chloroflexi, and Nitrospirae increased with Pb at all levels. Azotobacter decreased by 50% at >400 mg Pb/kg soil, while Bacillus demonstrated high Pb resistance. The microbial communities were initially influenced by Pb but shifted toward the Control over time.