Suppressive effect of the deep placement of lime nitrogen on N2O emissions in a soybean field
2022
Ikezawa, Hiromu | Nagumo, Yoshifumi | Hattori, Makoto | Nonaka, Masanori | Ohyama, Takuji | Harada, Naoki
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.
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