Effects of Nitrogen Fertilizer Levels on Rice Quality and Starch Properties of Common and Glutinous Japonica Rice: Implications for Sustainable Nitrogen Management

Optimizing nitrogen (N) fertilizer application within conventional rice production systems remains essential for improving grain quality while avoiding inefficient resource use. This study examined how different N application levels influence rice quality, starch structure, and physicochemical properties in two japonica rice types cultivated under cold-region conditions in Northeast China. Using two cultivars, common japonica rice ‘Putian 1498’ and glutinous japonica rice ‘Longjing 57’, four nitrogen levels were established under machine-transplanting conditions: N0 (0 kg/hm2), N1 (80 kg/hm2), N2 (135 kg/hm2), and N3 (190 kg/hm2). The results indicate that increasing nitrogen application differentially affected the milling quality of the two rice types: it reached its maximum at the N1 level for common japonica rice and at the N3 level for glutinous japonica rice. However, the taste value decreased and chalkiness increased in both types. Regarding starch properties, increased nitrogen application led to rougher starch granule surfaces, a decrease in large granules, and an increase in medium and small granules. Starch content decreased, and the amylose-to-amylopectin ratio declined. Relative crystallinity increased, while the FTIR ratio of 1045/1022 cm−1 decreased. Solubility showed an increasing trend, whereas swelling power exhibited the opposite trend. The gelatinization enthalpy and pasting temperatures were positively correlated with nitrogen rate, whereas retrogradation degree showed a negative correlation. These results demonstrate that nitrogen application regulates rice quality through changes in starch structure and physicochemical properties, with distinct responses between common and glutinous japonica rice. Moderate nitrogen input improves milling quality, but excessive application reduces eating quality, indicating a trade-off between processing performance and consumer-oriented quality. This study provides mechanistic evidence to support more precise nitrogen management in conventional rice systems, contributing to improved resource-use efficiency without overstating broader sustainability claims. In conclusion, moderate nitrogen application optimizes rice quality by balancing milling performance and eating quality through its effects on starch structure, whereas excessive nitrogen input leads to quality deterioration and inefficient resource use.