Objectives This study aimed to investigate the effects of straw returning and nitrogen application rates on the nitrogen metabolism and grain quality of dryland wheat, to screen the optimal nitrogen application rate suitable for wheat production in western Henan under straw returning conditions, and to provide a basis for wheat cultivation in this region and similar ecological areas.

Methods Field experiments were conducted at the experimental farm of Henan University of Science and Technology in 2021 and 2022. A split-plot field experiment was carried out using the wheat cultivar Luohan 22 as test materials. The main plot factor was straw return (9000 kg/hm²), namely without or with return (S0, or S1). The subplot factor was five N application rate: 0, 120, 180, 240, and 300 kg/hm², denoted as N0, N1, N2, N3, and N4, respectively. At the 0, 5, 15, 20, 25, and 30 days after anthesis, the flag leaf and grain samples were collected for analysis of free amino acids, soluble proteins, and nitrate reductase and glutamine synthetase activities. At harvest, the grain yield and its components, and the total protein content and paste characteristics were determined.

Results Under both S0 and S1 conditions, the spike number per area, grain number per spike, 1000-grain weight, and yield of wheat all generally increased and then decreased with the increase in N application rate, reaching their peak values at N3, the same trend was observed for the contents of free amino acids and soluble proteins, the activities of nitrate reductase and glutamine synthetase in wheat flag leaves and grains, as well as the contents of grain protein components, and the wet gluten content, sedimentation value, dough development time, dough stability time, and extensibility. Lower or higher than N3 reduced all the effects significantly. Compared with N0, significant effect of N rate on the nitrogen metabolism and protein composition maintained all the 30 days, while the effect of straw return on the nitrogen metabolism in flag leaves was observed merely at the first 5 and 10 days. However, all the tested indices were higher under S1 than under S0. Compared to S0N3, the average contents of soluble proteins in flag leaves and grains under the S1N3 treatment were increased by 3.80% and 4.28%, respectively, the wet gluten content in flour increased by 5.82% (P<0.05).

Conclusions Appropriate N application rate consistently and significantly influences the activities of nitrogen metabolism-related enzymes, as well as the contents of free amino acids and soluble proteins in flag leaves and grains of wheat after anthesis. This enhances protein content, optimizes protein composition, and ultimately improves the processing quality of flour. Insufficient or excessive N fertilization has a significantly lower effect on elevating nitrogen metabolism levels compared to appropriate N application. Straw incorporation has a relatively short-lasting impact on nitrogen metabolism levels in flag leaves and grains after anthesis. However, compared with no straw return, it can enhance the effectiveness of nitrogen fertilizer at the same nitrogen application level, thereby ensuring high yield and high quality of wheat. In summary, combining straw incorporation with N rate of 240 kg/hm² is a suitable nutrient management practice in the drylands of western Henan and similar ecological regions, effectively balancing wheat yield, grain nutritional quality, and flour processing quality.