Single-time fertilization (STF) with controlled release blended fertilizer (CRBF) improves grain yield and nitrogen use efficiency (NUE) in rice production.  However, the impact of soil nitrogen (N) distribution and root growth on rice yield and NUE under STF with CRBF remains unclear.  Here, a two-year field experiment investigated the effects of two fertilizer types (normal urea (U) and CRBF) and two single-time fertilization methods (broadcast and side-deep fertilization) on the soil N distribution, plant N uptake, root characteristics, grain yield, and NUE.  The results showed that CRBF under STF increased the averages of plant dry matter accumulation, N uptake, grain yield, nitrogen recovery efficiency (NRE), and nitrogen agronomic efficiency (NAE) by 8.29, 21.85, 10.57, 79.28, and 74.8% compared to the other treatments, respectively.  Side-deep fertilization with CRBF further increased NUE by 12.78% compared to broadcast.  Moreover, CRBF under STF increased the leaf SPAD value and glutamine synthetase (GS)/glutamine oxoglutarate aminotransferase (GOGAT) activity by 5.93 and 25.58%, respectively.  CRBF under STF increased the soil inorganic N concentration and showed a “rising early and stabilizing later” pattern.  In addition, CRBF under STF improved rice root growth and increased the averages of root biomass, total root number, root average diameter, total root length, total root surface area, and total root volume by 28.30, 28.56, 18.64, 13.38, 35.26, and 37.06%, respectively, at the tillering and heading stages.  Partial least squares path modeling indicated that CRBF under STF increased the soil inorganic N concentration which improved root morphology, thereby increasing N uptake and improving the rice yield and NUE.  Taken together, our findings show that CRBF with single-time fertilization is the preferred N fertilizer strategy for achieving high yield and efficiency in rice, and that side-deep fertilization is the optimal fertilization method.

Weak seedling vigor of machine-transplanted rice during the recovery stage often limits basal-tillering nitrogen (N) uptake and yield, particularly under the split urea application with the increasing N. In this experiment, the effect of tandem long-mat seedlings (TLMS) transplanted with seedling fertilizer (SF) on yield and N use efficiency (NUE) was studied. Three-season field experiments at two sites consisting of TSF (transplanting with 7.0 kg ha^-1 SF) and T (transplanting without SF) based on five different dosages of basal-tillering N were conducted to comprehensively study the effect of SF in field. The results show that SF released rapidly after transplanting and significantly increased dry matter accumulation, N uptake and their rates during tillering stage. Consequently, TSF showed enhanced growth with early emerging tillers, significantly higher proportion of effective tillers and panicle numbers, and 7.8% higher yield than T. Consistently, basal tillering and total NUE (13.8%) of TSF was significantly higher than that of T. Notably, in some growing seasons, even with a 37.5 kg ha^-1 reduction in basal-tillering N, TSF still achieved a comparable dry weight, N uptake and yield to that of T, supporting the quantitative significance of seedling fertilizer in TLMS. Further quantification through regression analysis of yield and dosage verified that 7.0 kg ha^-1 N supplied via SF was equally effective as 24.1 kg/ha basal-tillering N, in terms of yield response. Overall, TLMS transplanting with SF is an effective strategy to enhance the early growth vigor, improve yield and NUE, and reduce basa-tillering N input in machine transplanted rice. This study successfully integrates the soilless nursery establishment with SF within mechanical rice-transplanting system and quantitatively demonstrating its contribution to post-transplantation performance of rice seedlings.