Optimizing Nitrogen Fertilization Managements Under Mechanical Deep Placement for Raising Rice Grain Yield

It is not known whether both reducing tiller-fertilizer and increasing panicle-fertilizer can significantly increase grain yield under mechanical deep placement. The two-year field experiment was conducted to access the effects of optimal nitrogen fertilization including reducing tiller-fertilizer, increasing panicle-fertilizer, with the method of mechanical deep placement on grain yield and its physiological traits of rice, in 2019 and 2020. The experimental materials were selected with hybrid rice Wufengyou615 (WFY615) and inbred rice Yuxiangyouzhan (YXYZ). There were six experiment treatments, i.e., no any fertilization (H1); traditional surface broadcast fertilization (SB) (90 kg N ha⁻¹ as basal fertilization (BF) and 60 kg N ha⁻¹ as tillering fertilizer (TF), namely, BF 90 kg N (SB) + TF 60 kg N (SB), (H2); BF 90 kg N (SB) + TF 45 kg N (DP, deep placement) + FF (flowering fertilizer) 7.5 kg N (SB), (H3); BF 90 kg (SB) + TF 45 kg N (DF) + FF 15 kg N (SB), (H4); BF 90 kg N(SB) + TF 30 kg N (DP) + FF 7.5 kg N (SB), (H5); BF 90 kg N (SB) + TF 30 kg N (DP) + FF 15 kg N (SB), (H6). The results showed that mean grain yield of WFY615 and YXYZ for H4 was 10.57 t ha⁻¹ and 10.42 t ha⁻¹, which was 14.58% and 7.49% higher than H2, respectively. The main reason was due to the increase of productive panicle per ha, spikelet per panicle and grain filling percentage. The highest total dry matter of WFY615 and YXYZ at heading (HS) and mature stages (MS) was for H4, which was 9.24, 15.97, 11.65, and 14.71 t ha⁻¹, respectively. There was 31.09, 25.96, 41.73, and 20.58% higher total dry matter material of WFY615 and YXYZ for H4 than H2 at HS and MS, respectively. The largest leaf area index of H4 was also found at HS and fifteen days after HS for two rice cultivars, which was 6.24, 8.79, 6.09, and 8.29, respectively. The H4 treatment had the largest net photosynthetic rate, followed by H3 and H2, while the least net photosynthetic rate was recorded for H1. In addition, significant improvements were also founded in chlorophyll content, glutamate synthase, and nitrate reductase activities of sword leaves at HS for H4. Therefore, the fertilizer management can be regarded as one of high-efficiency fertilization method with 90 kg N ha⁻¹ basal fertilizer by surface broadcast plus 45 kg N ha⁻¹ tillering fertilizer under mechanical deep placement and 15 kg N ha⁻¹ flowering fertilizer by surface broadcast.