Field Experimental Design and Operation

We used four treatments based on a completely randomized design with three replicates in field experiments: (1) no N fertilizer, as a control (CK); (2) conventional broadcast of N fertilizer (NBP); (3) N and (4) NPK deep placement at 12 cm below the soil surface (NDP and NPKDP, respectively), Length × width of each plot was 6 × 7 m in treatments CK and NBP and 2 × 2 m in treatments NDP and NPKDP. Planting density in each plot was equivalent to 1.8 × 10⁵ basic seedling ha^-1. The N fertilizer used was commercial granular urea and was applied at a rate of 135 kg ha^-1 (amounting to 1.6 g urea per hill) and 165 kg ha^-1 (amount to 2.0 g urea per hill) for early and late rice, respectively. Split N fertilization was performed in NBP with 40% basal, 30% tiller, and 30% boot N input for early and late rice, while urea was used as the single basal N fertilizer in NDP and NPKDP treatments. Calcium-magnesium phosphate (P₂O₅ 90 kg ha^-1) and potassium chloride (K₂O 150 kg ha^-1) were amended as single basal fertilizers for all treatments. Water in the plots of NDP and NPKDP treatments was drained as much as possible, and rice seedlings were transplanted immediately. Then, urea was placed 12 cm below the soil surface where rice seedlings were placed by self-owned stainless fertilization cylinder. Finally, the plot was flooded again and P and K fertilizers were broadcast to the flooded soil surface.