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 (<i>narG</i>) gene abundance decreasing with increasing Pb, however nitrite reductase (<i>nirS)</i> 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 (<i>nosZ</i>) and the increase in genes encoding nitric oxide reductase (<i>norB</i>). Following exposure to Pb, 16S rRNA abundance decreased, while the Shannon diversity index increased. <i>Proteobacteria</i> initially dominated but decreased at 1600 mg Pb/kg soil, whereas <i>Actinobacteria</i>, <i>Chloroflexi</i>, and <i>Nitrospirae</i> increased with Pb at all levels. <i>Azotobacter</i> decreased by 50% at &gt;400 mg Pb/kg soil, while <i>Bacillus</i> demonstrated high Pb resistance. The microbial communities were initially influenced by Pb but shifted toward the Control over time.