Effects of mechanized deep placement of nitrogen fertilizer rate and type on rice yield and nitrogen use efficiency in Chuanxi Plain, China

This paper investigates the yield and nitrogen use efficiency (NUE) of machine-transplanted rice cultivated using mechanized deep placement of N fertilizer in the rice–wheat rotation region of Chuanxi Plain, China. It provides theoretical support for N-saving and improves quality and production efficiency of machine-transplanted rice. Using a single-factor complete randomized block design in field experiments in 2018 and 2019, seven N-fertilization treatments were applied, with the fertilizer being surface broadcast and/or mechanically placed beside the seedlings at (5.5±0.5) cm soil depth when transplanting. The treatments were: N0, no N fertilizer; U1, 180 kg N ha–1 as urea, surface broadcast manually before transplanting; U2, 108 kg N ha–1 as urea, surface broadcast manually before transplanting, and 72 kg N ha–1 as urea surface broadcast manually on the 10th d after transplanting, which is not only the local common fertilization method, but also the reference treatment; UD, 180 kg N ha–1 as urea, mechanically deep-placed when transplanting; M1, 81.6 kg N ha–1 as urea and 38.4 kg N ha–1 as controlled-release urea (CRU), mechanically deep-placed when transplanting; M2, 102 kg N ha–1 as urea and 48 kg N ha–1 as CRU, mechanically deep-placed when transplanting; M3, 122.4 kg N ha–1 as urea and 57.6 kg N ha–1 as CRU, mechanically deep-placed when transplanting. The effects of the N fertilizer treatments on rice yield and NUE were consistent in the 2 yr. With a N application rate of 180 kg ha–1, compared with U2, the N recovery efficiency (NRE), N agronomic use efficiency (NAE) and yield under the UD treatment were 20.6, 3.5 and 1.1% higher in 2018, and 4.6, 1.7 and 1.2% higher in 2019, respectively. Compared with urea alone (U1, U2 or UD), the NRE, NAE and yield achieved by M3 (combined application of urea and controlled-release urea) were higher by 9.2–73.3%, 18.6–61.5% and 6.5–16.5% (2018), and 22.2–65.2%, 25.6–75.0% and 5.9–13.9% (2019), respectively. Compared with M3, the lower-N treatments M1 and M2 significantly increased NRE by 4.0–7.8% in 2018 and 3.1–4.3% in 2019, respectively. Compared with urea surface application (U1 or U2), the yield under the M2 treatment was higher by 4.3–12.9% in 2018 and 3.6–10.1% in 2019, respectively. Compared with U2, the NRE and NAE under the M2 treatment was higher by 36.9 and 36.3% in 2018, and 33.2 and 37.4% in 2019, mainly because of higher N uptake. There was no significant difference in the concentration of nitrate in the top 0–20 cm soil under U1, U2 and M2 treatments during the full heading and maturity stages. During the full heading stage, U2 produced the highest concentration of nitrite in 0–20 cm and 20–40 cm soil among the N fertilizer treatments. In conclusion, mechanized deep placement of mixed urea and controlled-release urea (M2) at transplanting is a highly-efficient cultivation technology that enables increased yield of machine-transplanted rice and improved NUE, while reducing the amount of N-fertilization applied.