Deep placement and reduced application rate of nitrogen fertilizer mitigates nitrous oxide emission from rice paddy soil in South Korea

Nitrous oxide emission from agricultural soils contribute significantly to global greenhouse gas emissions. Fertilizer deep placement (FDP) and reduced application rate of nitrogen fertilizer are considered as a promising strategy to mitigate nitrous oxide emissions from arable soil and increase nitrogen use efficiency of crops. This study was conducted to determine effects of FDP and, FDP with 20% reduced application rate of nitrogen fertilizer (FDP-rf) on nitrous oxide (N₂O) emissions, rice yield, and soil properties in paddy soil. The study included three treatments: conventional (C), FDP, and FDP-rf. Rice (Oryza sativa L.) was transplanted on June 7, 2024 and harvested on October 15, 2024. Nitrous oxide flux for each treatment during the cultivation period showed similar patterns affected by the submerged period and fertilizer application. Emissions remained low across all treatments before the mid-term drainage period, and this trend continued during the mid-term drainage period (two weeks). Peaks of N₂O flux were observed in FDP-rf and C treatments right after the mid-term drainage period, while FDP maintained consistently low emissions. Subsequently, all treatments returned to low N₂O flux levels. Following the pre-harvest drainage, N₂O flux increased across all treatments, likely due to the availability of residual nitrogen. Among the treatments, FDP exhibited the most stable and minimal N₂O emissions, indicating effective nitrogen retention and consistent control of fluxes. There was a statistically significant difference in cumulative N₂O emissions depending on the fertilization method. Conventional (C) showed the highest emissions (0.8351 ± 0.0408 kg/ha), followed by FDP (0.6259 ± 0.0562 kg/ha) and FDP-rf (0.4140 ± 0.1063 kg/ha). The total and inorganic nitrogen content in the soil varied greatly depending on the fertilization method. For total nitrogen, the highest levels were observed in the conventional (C) treatment (2.11 g/kg) on harvest time, followed by FDP-rf (1.91 g/kg) and FDP (1.23 g/kg). Nitrate levels were significantly reduced in FDP (17.7 mg/kg) and FDP-rf (26.8 mg/kg) compared to C (42.2 mg/kg). Although there was no statistical difference in ammonium levels, the highest value was observed in C (56.7 mg/kg), followed by FDP-rf (54.5 mg/kg) and FDP (43.1 mg/kg). Depending on the fertilization method, the grain yield, rice straw, root, and total biomass weight varied. For grain yield, the highest was observed in FDP (6.69 Mg/ha), followed by C (5.96 Mg/ha) and FDP-rf (5.42 Mg/ha). Fertilizer deep placement reduced N₂O emissions and improved rice yields compared to C. Fertilizer deep placement with reduced nitrogen application further decreased N₂O emissions but resulted in lower yields compared to FDP. These findings suggest that FDP could be a sustainable agricultural practice that mitigates greenhouse gas emissions while maintaining crop yields.