Objectives The effects of nitrogen rate and zinc fertilizer on the yield, processing and appearance quality, cooking taste, and aroma of fragrant southern japonica rice were studied.

Methods A field experiment was conducted in the experimental base of Yangzhou University, two fragrant japonica rice cultivars, Ningxiangjing 9 and Nanjing 46, were used as experiment materials. The tested N application levels were 0 (N0), 180 (N180), and 270 (N270) kg/hm² under both applying Zn fertilizer (+Zn) and no Zn fertilizer conditions. At maturing stage, the yield, yield compomemts, grain processing, tasting and fragrance quality were determined.

Results Higher N rate produced higher rice yield, N270 treatment gave the highest yield in Ningxiangjing 9 (10.48 t/hm²) and Nanjing 46 (10.40 t/hm²), +Zn did not affect rice yield. N270 elicited higher rice processing and appearance quality than N0 and N180, and +Zn improved the processing and appearance quality of rice under the same N fertilizer rate (P<0.05). The highest processing quality of Ningxiangjing 9 and Nanjing 46 rice were recorded under +ZnN270. Compared with N0, N180 and N270 increased the protein content by 9.79%−23.89%, but decreased the amylose content by 8.44%−13.16% as well as the taste value significantly. Under the same N rate, +Zn increased rice protein content and improved taste value. The setback value of rice increased, while other indicators of the rheological viscoelasticity value (RVA) decreased with the increase of N application rate. Under the same N fertilizer rate, +Zn did not affect RVA of rice flour (RVA spectrum), compared with no Zn application. The nitrogen and 2-acetyl-1-pyrrololine (2AP) content in brown rice and milled rice increased with increasing N application rate. Compared with N0, N180 increased the nitrogen content of brown rice in Ningxiangjing 9 and Nanjing 46 by 4.64% and 4.87%, increased the nitrogen content of milled rice by 9.80% and 12.88%; increased 2AP content in brown rice by 8.91% and 41,38%, and increased 2AP content in milled rice by 7.91% and 5.88%, respectively. Compared with N0, N270 increased the nitrogen content of brown rice in two cultivars by 12.17% and 11.00%, increased the nitrogen content of milled rice by 23.89% and 22.98%; increased 2AP content in brown rice by 12.16% and 91.28%, and increased 2AP content in milled rice by 16.24% and 120.78%, respectively. Under N270, compare to Zn0, +Zn treatment increased the Zn content in brown rice of Ningxiangjing 9 and Nanjing 46 by 1.00% and 3.01%, increased the 2AP content in brown rice by 0.47% and 15.27%; increased the Zn content in milled rice by 9.75% and 2.19%, and increased the 2AP content in milled rice by 5.71% and 52.04%, respectively.

Conclusions A relatively high rate of nitrogen fertilizer is beneficial for the yield, processing quality, and the aroma of milled rice (2AP value). Combined application of N with Zn fertilizer does not decrease the benefit of high N fertilizer, it rather increases the appearance quality and further improves the aroma of milled rice significantly. The recommended fertilization method for fragrant japonica rice is N 270 kg/hm² combined with zinc fertilizer, for an optimally high yield, good processing quality and cooking taste.