Decreasing Chemical N Rate Suppressed Nitrification and Reduced N2O Emissions Under Drip Irrigation

Unreasonable nitrogen (N) management and the shortage of groundwater resources are prominent problems faced by intensive farmland in North China. The main processes to reduce N2O emissions under drip irrigation still need to be further explored. This study conducted a three-year field experiment (2021–2024) of wheat–maize cropping system in Hebei Province to study the effects of four N levels under drip irrigation (i.e., N0: no N applied; N150: 150 kg N ha−1; N210: 210 kg N ha−1; N270: 270 kg N ha−1) on yield, N2O emission, soil carbon (C) and N fractions, enzymes, and N functional genes. The results showed that N rate significantly affected N2O emissions (p < 0.05). The highest value of N2O emissions (10.64 kg N ha−1) over three years and yield-scale N2O emissions (0.25 g kg−1) were found under N270, which were 32.1% and 78.6% higher than those of N210. No significant difference in annual yields between N210 and N270 was found, but the annual yield of N150 was decreased by 14.4% as compared to N210. N rate significantly affected soil C-N fractions and enzymes, in which NO3−-N, MBC, DON, amoA-AOB, and UE enzymes, as the major factors of soil properties, N functional genes, and enzymes affecting N2O emissions, were more favorably regulated along with the increase in fertilizer N rate. Correlation analysis indicated that amoA-AOB was significantly correlated to N fractions (i.e., NH4+-N, NO3−-N, TN, DON and MBN) and UE enzymes. Structural equation modeling further proved that an increase in fertilizer N rate directly increased enzyme activities related to N transformation, which increased N fractions, thus promoting the abundance of amoA-AOB genes involved in nitrification that stimulate N2O emissions. Therefore, these findings underscore that N210 was an optimal fertigation strategy for minimizing N2O emissions without compromising yield, offering a practical pathway for sustainable intensification in water-limited regions.