Heavy metal pollution is a serious problem in wetland ecosystems, and the toxicity of heavy metals affects microorganisms, thus influencing the biogeochemical process of nitrogen (N). To investigate the effects of heavy metal cadmium (Cd) pollution on N mineralization in urban constructed wetland soils of the Pearl River Delta, a 40-day aerobic incubation experiment was conducted under three Cd addition treatments [no Cd addition (control), low Cd addition (LCA) and high Cd addition (HCA)]. The results showed that compared with the control, the LCA treatment enhanced the soil N mineralization rate (RM), while the HCA treatment inhibited RM, with the average RM values in the control treatment of 0.40 mg kg-1 day-1, LCA treatments (0.66 mg kg-1 day-1), and HCA treatments (0.21 mg kg-1 day-1). The average ammonification rate values in the LCA treatments (- 3.15 to 2.25 mg kg-1 day-1) were higher than those in the HCA treatments (- 2.39 to 0.74 mg kg-1 day-1) and the control treatment (- 0.68 to 0.90 mg kg-1 day-1) (P < 0.05). However, the nitrification values in the HCA treatments (- 0.37 to 3.36 mg kg-1 day-1) were higher than those in the LCA treatments (0.42-1.93 mg kg-1 day-1) and the control treatment (0.20-1.45 mg kg-1 day-1) (P < 0.05). The net N mineralization accumulation generally increased over the entire incubation time in different Cd addition treatments. The percentage of NH4+-N to total inorganic N showed a decrease, while an increase was observed for NO3--N over the incubation time. The urease activities were significantly inhibited in the LCA and HCA treatments and showed a "decreasing before increasing" trend. The abundance of ammonia oxidizing archaea (AOA) was higher in the two Cd addition treatments than the control treatment, and higher in the LCA treatments than in the HCA treatment. AOA was the dominant microorganism in the ammonia oxidation process of N mineralization in constructed wetland soils. The findings of this work indicate that Cd addition has a profound effect on the balance of N mineralization and may further impact the plant productivity and water quality of constructed wetlands.
Keywords: Ammonification; Cadmium addition; Constructed wetland soils; Nitrification; Nitrogen mineralization.