Efficiency of 3,4-Dimethylpyrazole Phosphate in Mitigating N2O Emission Varied with Irrigation Regime in Drip-Irrigated Wheat Field

Agricultural soils are major anthropogenic sources of N2O emissions. The application of nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) has been proved to be an effective management measure to mitigate N2O emissions. However, the influence mechanism of DMPP on the mitigation of soil N2O emissions under different irrigation regimes remains unclear. Therefore, a lysimeter experiment was conducted to study the effects of irrigation level (lower irrigation limits of 75%, 65%, and 55% of field capacity (FC), signed as WH, WM, and WL) and DMPP addition (0% and 1% of N application, signed as D0 and D1) on N2O emissions, soil environmental factors such as ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3−-N), water-filled pore space (WFPS), soil temperature, and the abundances of N2O-related genes (AOA amoA, AOB amoA, nirS, and nirK). The results showed that soil N2O emissions increased with the increasing of irrigation level. The efficiency of DMPP mitigating N2O emissions varies depending on irrigation regime. Compared to D0, D1 strongly decreased cumulative N2O emissions by 11.27%, 18.96%, and 15.05% in the WL, WM, and WH conditions, respectively. Meanwhile, D1 caused an obvious reduction in the AOB amoA gene by 29.73%, 47.02%, and 22.41%, respectively, but there was no significant effect on the AOA amoA gene. D1 was effective in decreasing nirS and nirK genes except in the WL condition; the percentages of reduction were 48.45%, 40.84% and 37.18%, 44.97% in the WM and WH conditions, respectively. In addition, D1 caused an increase in NH4+-N content and a decrease in NO3−-N content, WFPS, and soil temperature in all irrigation regimes. A higher significant correlation was observed between N2O emissions and NH4+-N and AOB amoA in the WL and WM conditions, while a significant correlation was observed between N2O emissions and NO3−-N, nirK, and nirS in the WH condition. It was revealed that with the increase in irrigation level, the main source of N2O emissions might change from nitrification to denitrification. Overall, our study indicated that in the WL and WM conditions, the mitigation of N2O emissions by DMPP was primarily attributable to the inhibition of the AOB amoA gene, whereas the inhibition of nirS and nirK genes was likely the dominant mechanism in the WH condition. The findings of this study will provide a theoretical basis for the application of a nitrification inhibitor for drip-irrigated winter wheat fields in the North China Plain.