How does deep-band fertilizer placement reduce N2O emissions and increase maize yields?

It is unclear how deep-band fertilizer placement influences N2O emission and maize (Zea mays L.) yield, and the production and diffusion of N2O within the soil profile under deep-band fertilizer placement treatments. Thus, we conducted a field experiment in 2019–2020 with spring maize to study the effects of different fertilization depths of 5 cm (D5), 15 cm (D15), 25 cm (D25), and 35 cm (D35) on N2O emissions. The N2O concentration in the fertilized soil profile was measured to calculate the N2O diffusion and production rates, and the surface N2O emissions were measured simultaneously. The results showed that the availability of NO3–-N in the soil was the main factor related to N2O production and emissions, and N2O emissions were strongly related to the production and diffusion of N2O in the 0–20 cm soil layer. Deep-band fertilizer placement significantly reduced surface N2O emissions, and the N2O production and diffusion rates in the 0–20 cm soil profile. The cumulative N2O diffusion in the 0–20 cm layer accounted for 79.2%, 73.7%, 58.2%, and 48.6% of those in 0–40 cm soil depth under D5, D15, D25, and D35, respectively. Compared with D5, D25 and D35 significantly reduced the N2O emissions by 30.8% and 59.3%, respectively, N2O-N emission factors by 41.6% and 80.0%, and yield-scaled N2O-N by 39.3% and 58.2%. D25 also increased the maize yield by 13.8% and obtained the highest nitrogen use efficiency (NUE, 43.6%), whereas D35 had negative effects on the maize yield and NUE. Therefore, deep-band fertilizer placement reduced the N2O production rate and diffusion flux in the near-surface soil layer by decreasing the NO3–-N content in the 0–20 cm soil layer. Thus, the location where N2O was produced transferred to the deeper soil layer to reduce surface N2O emissions. The recommended fertilization depth of 25 cm can reduce N2O emissions and significantly enhance the maize yield, N uptake, biomass, and NUE.