Nitrogen transformation and N₂O production pathways under arsenic and cadmium stress in Chongming Island, China

Heavy metal(loid) pollution can adversely affect microbial diversity within the soil ecosystem, which are profoundly impacting soil nitrogen (N) cycling and nitrous oxide (N₂O) emissions. Here, ¹⁸O[sbnd]¹⁵N labeling techniques based on incubation experiment were used to investigate the N transformation and N₂O production pathways response to moderately contaminated, heavily contaminated and extremely contaminated of arsenic (As) and cadmium (Cd) levels in paddy soil and coastal wetland. In paddy soil, ammonification rate (PAT) and nitrification rate (PNT) decreased by 15.57 %–48.75 % and 19.62 %–30.13 %, respectively. The denitrification rate (DEN) increased by 34.07 %, 59.81 % and 74.82 % from the control (CK) treatment to the extremely contaminated treatment in paddy soil. In coastal wetland, DEN decreased by 13.60 %, 19.30 %, and 27.25 % from the CK treatment to the extremely contaminated treatment, but PAT and PNT rates increased by 0.71 %–51.05 % and 73.27 %–119.81 %, respectively. As and Cd addition changed soil properties, which altered the dynamics of DEN rate and increased N₂O production contributed by heterotrophic denitrification (HD) and nitrification-coupled denitrification (NCD) pathways. Changes of nitrifier nitrification (NN) and nitrifier denitrification (ND) pathways derived N₂O production were related to PNT rate and NO₂⁻ dynamics, respectively. Results of PAT, PNT and DEN changes affected by As and Cd addition indicated that heavy metal(loid) pollution promotes reactive N production but inhibits N removal in coastal wetland, and heavy metal(loid) in paddy soil promoted N loss but exacerbating N limitation. Therefore, heavy metal(loid) pollution regulates N transformation and intensifies N pollution and global warming.