Effects of Nitrogen-Application Rate on Forage Oat Yield, Photosynthetic Characteristics, and Nitrogen-Use Efficiency

The effect of nitrogen (N) (0, 75, 150, 225 and 300 kg N ha−1, N0-N4) on the yield, photosynthetic properties and nitrogen-use efficiency of forage oat (Avena sativa L.) was studied in the Horqin Sandy Land in northern China. Mantel tests and chord diagrams were used to investigate the correlation of traits. Plant height, stem–leaf ratio, hay production, chlorophyll content and heading stage photosynthesis indices showed a unimodal response with highest values at N3 (225 kg N ha−1; p < 0.05). The dry matter yield at filling stage, net photosynthetic rate (Pn) at heading stage and total chlorophyll content (Total Chl) of N3 were 74.52%, 101.65%, and 62.18% higher than that of N0. The nitrogen contribution ratio (NFCR) of N3 reached 42.70%, which was 60.59% greater than that of N1. Nitrogen uptake efficiency (NUpE) was achieved at N3 that was 21.50% relative to N1.The N3 treatment showed the best comprehensive performance across agronomic, photosynthetic and nitrogen-use efficiency traits. In contrast, N3 achieved a favorable balance between yield and nitrogen use efficiency and can be regarded as a suitable nitrogen application rate for high-yield forage oat production in this region.