Abstract: 【Objective】To identify the optimal nitrogen fertilization mode to maximize the yield of wide-range densely planted oats in the southern foothills of Yinshan Mountains in Inner Mongolia.【Methods】A two-factor experimental design was employed with five nitrogen levels（N1：35 kg/hm²,N2：70 kg/hm²,N3：105 kg/hm²,N4：140 kg/hm²,N5：175 kg/hm²） and two topdressing stages（T1：tillering stage,T2：booting stage）. The impact of different nitrogen application levels and topdressing stages on nitrogen balance index（NBI）,photosynthetic characteristics and yield of oats at each growing stages were analyzed.【Results】During the middle and late growth stages,both the net photosynthetic rate and NBI of oats increased first and then decreased with rising nitrogen levels. At grain-filling stage,N4 nitrogen level in the T2 treatment had best photosynthetic efficiency,but no significant difference（P>0.05）with N3 nitrogen level. At the same nitrogen level,compared with T1 treatment,T2 treatment enhanced the net photosynthetic rate and NBI of middle and late growth period stages oats by 1.40%-11.97% and 9.82%-23.26%,respectively,with N3 nitrogen level having the most significant increasing. In comparison with T1 treatment,oat yield components of T2 treatment had synergistic improvement,with grain yield increased by 3.99%-9.10%. The grain yield of N4 nitrogen level was highest at 3 901.13 kg/hm². However the difference with N3 nitrogen level was not significant（P>0.05）. Correlation analysis revealed that chlorophyll index（Chl）and NBI of oat had extremely significant positive correlations with yield during the heading and grain-filling stage,with grain filling stage having a higher correlation coefficient.【Conclusion】The optimal nitrogen application rate for wide-range densely planted oats in the southern foothills of Yinshan Mountains in Inner Mongolia was 105-140 kg/hm². Booting stage was the most effective timing for nitrogen application.

Key words: Nitrogen fertilizer, Oat, Nitrogen balance index, Photosynthetic properties, Yield