## 4 Conclusions  
The effects of nitrogen fertilizer reduction on the volatile metabolomics of Suijing 18 were investigated in this study. The results showed that a reduction in the application of nitrogen fertilizer could improve the eating quality of rice. The volatile metabolomics detected 358 volatile compounds in Suijing 18 plants from different nitrogen fertilizer treatment groups, including 16.25% esters, 15.41% terpenoids, 15.13% heterocyclic compounds, and 13.45% hydrocarbons. The types and contents of volatile compounds differed between the three treatments. There were 16 differential metabolites in Suijing 18 after the application of nitrogen fertilizer was reduced; sweetness was found to be the most important flavor characteristic of rice. The key compounds affecting the sweetness of Suijing 18 were (z)-3,7-dimethylocta-2,6-dienal, allyl benzoate, 2-methylnaphthalene, hexyl acetate, and 4-methyl-benzeneacetaldehyde, among which allyl benzoate and 4-methyl-phenylacetaldehyde had the most significant effect on sweetness. The results of our cluster analysis showed that the key sweet compounds 2-methylnaphthalene, 4-methyl-benzeneacetaldehyde, and hexyl acetate showed a trend of first increasing and then decreasing with the reduction in nitrogen fertilizer. The sweetness of rice also reached a peak when the nitrogen fertilizer was reduced by 20% (68 kg of pure nitrogen per hectare), which may be related to an increase in the starch content of Suijing 18 within a certain range of nitrogen fertilizer reduction and also reinforced the results of our eating quality evaluations. In conclusion, the sweet flavor of Suijing 18 was most obvious after a 20% reduction in nitrogen fertilizer (68 kg of pure nitrogen per hectare) relative to conventional fertilization (85 kg of pure nitrogen per hectare). Its sweetness was enhanced and its eating quality improved by this process. Therefore, through a reduction in nitrogen fertilizer, not only can the rate of nitrogen fertilizer utilization be improved, the aromatic quality of _japonica_ rice can also be enhanced. The results of this study may be used to optimize the use of nitrogen fertilizer in different rice-growing regions, serving to reduce its environmental impact and improve the quality of produce.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/foods13203310/s1](https://www.mdpi.com/article/10.3390/foods13203310/s1)]([https://www.mdpi.com/article/10.3390/foods13203310/s1](https://www.mdpi.com/article/10.3390/foods13203310/s1)), Figure S1: The total ion chromatogram of mixed samples; Table S1: Complete list of measured volatile metabolite in Suijing 18.  
Conceptualization, X.D. and H.S.; methodology, J.W., Q.W. and D.Z.; software, J.W.; investigation, J.W., D.Z. and Q.W.; resources, X.D. and H.S.; data curation, J.W.; writing--original draft preparation, J.W. and Q.W.; writing--review and editing, Q.W.; supervision, X.D. and H.S.; funding acquisition, H.S. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Key Research and Development Program, grant number 2022 YFE0117800.  
Not applicable.  
Data Availability Statement: Data are contained within the article and/or Supplementary Materials.  
This study was supported by the Academy of National Food and Strategic Reserves Administration of the People's Republic of China.  
The authors declare no conflicts of interest.