Abundance and Structure Composition of nirK and nosZ Genes as Well as Denitrifying Activity in Heavy Metal-polluted Paddy Soils

Denitrification is an important microbial process in soils and leads to the emission of nitrous oxide (N2O). However, studies about the microbial community involved in denitrification processes in polluted paddy fields are scarce. Here, we studied two rice paddies which had been polluted for more than three decades by metal mining and smelter activities. Abundance and community composition were determined using real-time polymerase chain reaction (PCR) assay and denaturing gradient gel electrophoresis of nitrite reductase and nitrous oxide reductase gene amplicons (nirK and nosZ), while denitrifying activities were assessed by measuring potential denitrifier enzyme activity. We found that the community structure of both nirK and nosZ containing denitrifiers shifted under pollution in the two rice paddies. All the retrieved nirK sequences did not group into either alpha- or beta-proteobacteria, while most of the nosZ species were affiliated with alpha-proteobacteria. While the abundance of both nirK and nosZ was significantly reduced in the polluted soils at Dexing (with relatively higher Cu levels), these parameters did not change significantly at Dabaoshan (polluted with Cd, Pb, Cu, and Zn). Furthermore, total denitrifying activity and N2O production and reduction rates also only decreased under pollution at Dexing. These findings suggest that nirK and nosZ containing denitrifier populations and their activities could be sensitive to considerable Cu pollution, which could potentially affect N2O release from polluted paddy soils.