Combined application of nitrogen and zinc fertilizer affects the yield and quality of fragrant southern japonica rice

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.