Nitrogen Fertilizer Rates Regulate Source–Sink Dynamics, Post-Anthesis N Translocation, and Yield Production in Spring Wheat on the Loess Plateau, China

One of the main factors influencing wheat productivity is nitrogen (N) management. This study examined the impact of varying N-fertilizer rates on spring wheat yield and N use efficiency by adjusting the “source-sink” relationship between assimilates and N accumulation and transport. The objective was to identify the optimal N rate for the region. The field experiment included five N-fertilizer rates: 0 kg ha−1 (N1), 52.5 kg ha−1 (N2), 105.0 kg ha−1 (N3), 157.5 kg ha−1 (N4), and 210.0 kg ha−1 (N5). Results indicated that the yield response was not proportional to N-fertilizer rates, with maximum biomass (6029 kg ha−1) and grain yield (2625 kg ha−1) achieved under N3. N fertilization primarily increased yield by regulating pre-anthesis translocation of assimilate and N. Assimilate translocation peaked at 105 kg N ha−1, increasing by 8.5–133.7% compared to other treatments. With increasing N input, N absorption efficiency and N partial factor productivity declined. The highest N agronomic use efficiency was observed under N3, which was 19.5–176.34% higher than other treatments. Overall, moderate N input (≈105 kg ha−1) optimizes yield and N-use efficiency, offering guidance for sustainable N management in dryland spring wheat production.