Objective Wheat grain nitrogen (N) content is the basis for calculating N management indicators. Studying wheat grain N content and its influencing factors in different agro-ecological zones in China is an important basis for scientifically formulating management measures to increase wheat yield and quality.

Methods We searched the Web of Science and CNKI databases for literature published between 1980 and 2023 using the keywords of “wheat”, “yield” and “grain N content”, we adhered to the following criteria to avoid bias in screening literature: (1) the trials were conducted in the field condition in China; (2) the number of replications of the experimental treatments was at least three; (3) the experiment included at least one complete wheat growing season; (4) the grain N content must be reported, or it can be calculated by the reported grain yield and N harvest. Finally, we got 333 papers, including sample size of 3086 of grain yield and 1789 of grain N content. We analyzed the differences of wheat grain yield and N content in the four major agrecological zones of Northwest China, North China Plain, Middle and Lower Yangtze River Plain, and Southwest China, wheat N harvest and N surpluses in different agro-ecological zones were also calculated, and Pearson's correlation coefficient analysis, Meta-analysis, and Analysis of Variance (ANOVA) were used to study the effects of climate, soil, N fertilizer management, and varietal differences on wheat grain N content. Thus to analyze the reasons for the spatial differences in wheat grain N content.

Results The average N content of wheat grain in China is 2.37%, with significant differences in different agro-ecological zones. The N content of wheat grain in the North China Plain and Northwest China is relatively high, at 2.43% and 2.37%, respectively, while that in Southwest China and the Middle and Lower Yangtze River Plains is relatively low, at 2.21% and 2.18%, respectively. Wheat N harvest (N surplus) calculated based on the national average N content differed from those calculated using the regionalized grain N content by 0, -4, 10 and 8 kg/hm² in the Northwest China, North China Plain, Middle and Lower reaches of Yangtze River and Southwest China, respectively. Wheat grain N content was negatively correlated with the average annual rainfall (P≤0.001) while positively correlated with the average annual air temperature and the amount of N applied (P≤0.01). There were significant differences in the yields of different functional kinds of wheat in China, which were characterized as medium gluten (6.25 t/hm²) > strong gluten (5.99 t/hm²) > weak gluten (5.76 t/hm²). There was no direct correlation between wheat grain yield and N content, and synergy between yield and quality can be achieved at medium levels of yield. There was also a significant difference in grain N contents among different wheat varieties. Variety selection and rational N application are effective measures to synergize the improvement of wheat yield and grain N content.

Conclusions When calculate wheat N harvest, it is necessary to consider the differences in grain N content in different agro-ecological zones to accurately assess regional N surpluses and N use efficiency. The main factors affecting wheat grain N content include average annual rainfall, average annual temperature and N application. The wheat grain N content can be significantly enhanced by optimizing N application methods, such as split-application of N, reduction of the proportion of basal fertilizer and deep application of N fertilizer.