Suppressive effect of the deep placement of lime nitrogen on N₂O emissions in a soybean field

Deep placement of slow-release nitrogen (N) fertilizers improves the growth and yield of soybean with a high N use efficiency. This study examined the effectiveness of deep placement of lime nitrogen (LN) in reducing N₂O emissions in a soybean field and compared it with conventional fertilization. Before sowing soybeans, the starter N fertilizer (16 kg-N ha^-1 ammonium sulfate) was mixed in the surface soil and the following four treatments were installed: the control with only the starter N (CT), conventional top-dressing of 60 kg-N ha^-1 coated urea (CV), deep placement (20 cm depth) of 100 kg-N ha^-1 urea (DU), and deep placement (20 cm depth) of 100 kg-N ha^-1 LN (DL). The seasonal patterns of N₂O emission rates measured using the closed chamber method differed among the treatments: in CT, N₂O emissions were relatively low; in CV, N₂O emissions derived from the top-dressed coated urea were observed from 91 days after sowing; in DU and DL, deeply-placed N was converted to N₂O in the early growth stages. The cumulative N₂O emissions in DL (1.8 kg-N ha^-1) during the soybean cultivation period were significantly lower than those in DU (3.1 kg-N ha^-1) and CV (2.8 kg-N ha^-1), and slightly higher than CT (1.2 kg-N ha^-1). The magnitude of N₂O emissions was significantly lower in DL than DU, indicating that the choice of N fertilizer is important to reduce N₂O emissions. Focusing on N₂O emissions per unit coarse grain yield of soybeans, the value in DL was 0.45 g-N kg^-1, which was significantly lower than 0.74 g-N kg^-1 in CV. In conclusion, the deep placement of LN has the potential to be a sustainable farming method that can promote yields and reduce N₂O emissions in soybean cultivation for high yield with N fertilization.