Post-anthesis Relationships Between Nitrogen Isotope Discrimination and Yield of Spring Wheat Under Different Nitrogen Levels

Wheat grain yield and nitrogen (N) content are influenced by the amount of N remobilized to the grain, together with pre-anthesis and post-anthesis N uptake. Isotopic techniques in farmed areas may provide insight into the mechanism underlying the N cycle. N-15-labeled urea was applied to microplots within five different fertilized treatments 0 kg ha(-1) (N1), 52.5 kg ha(-1) (N2), 105 kg ha(-1) (N3), 157.5 kg ha(-1) (N4), and 210 kg ha(-1) (N5) of a long-term field trial (2003-2021) in a rainfed wheat field in the semi-arid loess Plateau, China, to determine post-anthesis N uptake and remobilization into the grain, as well as the variability of N-15 enrichment in aboveground parts. Total N uptake was between 7.88 and 29.27 kg ha(-1) for straw and 41.85 and 95.27 kg ha(-1) for grain. In comparison to N1, N fertilization increased straw and grain N uptake by 73.1 and 56.1%, respectively. Nitrogen use efficiency (NUE) and harvest index were altered by N application rates. The average NUE at maturity was 19.9% in 2020 and 20.01% in 2021; however, it was usually higher under the control and low N conditions. The amount of N-15 excess increased as the N rate increased: N5 had the highest N-15 excess at the maturity stage in the upper (2.28 +/- 0.36%), the middle (1.77 +/- 0.28%), and the lower portion (1.68 +/- 1.01%). Compared to N1, N fertilization (N2-N5) increased N-15 excess in the various shoot portions by 50, 38, and 35% at maturity for upper, middle, and lower portions, respectively. At maturity, the N-15 excess remobilized to the grain under N1-N5 was between 5 and 8%. Our findings revealed that N had a significant impact on yield and N isotope discrimination in spring wheat that these two parameters can interact, and that future research on the relationship between yield and N isotope discrimination in spring wheat should take these factors into account.