Objectives The effects of two nitrification inhibitors on nitrogen turnover in paddy soils were compared, aiming to differentiate their inhibiting mechanism and choose efficient one for the rice production in Northeast Black soil.

Methods The paddy soil, originated from meadow black soil, in Northeast China was used in a incubation experiment at 25℃. The incubation had two groups and lasted 150 days long. The treatments included no urea control (CK), adding urea 0.8 g/kg soil (Urea), and adding DCD (Urea+ DCD) and DMPP (Urea+ DMPP). In one group of experiment, the gas samples were collected everyday during the whole incubation period, and the N₂O contents were determined using gas chromatography; in the other group, soil samples were collected since the first day of incubation, the abundance of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA), and the gene copies of nirK and nirS were measured using fluorescence quantitative PCR technologies.

Results The urea application significantly increased N₂O emissions, 85% of which occurred within 2 weeks of incubation. Compared with Urea treatment, Urea + DMPP could decrease N₂O emission by 78.3%, with and emission factor of 0.05%, while Urea + DCD could decrease N₂O emission by 21.6%, with an emission factor of 0.18%, which was significantly different from Urea + DMPP. Adding urea could stimulate the abundance of AOA and decrease that of AOB. Both addition of DCD and DMPP had significant inhibition on the AOB abundance but not on the AOA. At the 3rd, 30th and 90th day of incubation, the AOB abundance in Urea + DMPP treatment was significantly lower than in Urea + DCD by 30%, 56% and 60%. For denitrifying bacteria, the copy number of nirK genes was significantly higher than that of nirS ones in all treatments. The Urea + DMPP treatment reduced the copy number of both nirK and nirS genes (3rd and 30th day), but Urea + DCD had no significant effect on them.

Conclusions In the paddy soil originated from meadow black soil in Northeast China, the nitrification inhibitor DMPP is much more effective in reducing N₂O emission and emission factor than DCD does. The main reason is that DMPP can inhibit the multiplication of ammonia oxidizing bacteria and reduce the number of denitrifying bacteria within 30 days of urea addition.