Effects of nitrogen application rate on dry matter accumulation, nitrogenefficiency, and potassium and sodium uptake of sweet potato (Ipomoea batatas) in coastal North Jiangsu Province

Sweet potato (Ipomoea batatas) is a high energy crop with great potentials in the coastal areas in China. In order to determine optimum nitrogen (N) application rate (NAR) in sweet potato production, a field experiment was conducted in coastal North Jiangsu Province. Survival rate (SR), commodity rate (CR), dry weight vine-tuber ratio (V/T), dry matter accumulation (DMA), N accumulation (NAV), N use efficiency (NUE), N harvest index (NHI), and potassium (K) and sodium (Na) uptakes of sweet potato were investigated under six N levels (N0~N5). The N levels were 0 kg(N)·hm^-2(N0), 60 kg(N)·hm^-2 (N1), 120 kg(N)·hm^-2 (N2), 180 kg(N)·hm^-2 (N3), 240 kg(N)·hm^-2 (N4), and 300 kg(N)·hm^-2 (N5), respectively. The results were summarized as follows: (1) A sig-nificant positive correlation was noted between NAR and aboveground DMA or NAV at P < 0.01, but no significant correlation ex-isted between NAR and root NAV at P > 0.05. (2). Compared with N0, N1 treatment had no significant effects on sweet potato V/T, SR, NUE and NHI. (3) Sweet potato CR, root and tuber DMA as well as theoretical yield (TY=NAV×NUE×NHI) were highest under N1 treatment. When sweet potato NARs were 60~300 kg(N)·hm^-2 , significant positive correlation existed between NAR and V/T at P < 0.01. However, significant negative correlation was noted among SR, CR, NUE, NHI, root and tuber DMA of sweet potato at P < 0.01. (4) There was significant positive correlation between K and Na uptake at P < 0.01; both of which increased with increasing NARs. K and Na contents of aboveground tissues of sweet potato under N1 treatment were significantly higher than those under other treatments. However, K/Na ratio for the whole plant was not different among treatments. The study suggested that the optimum NAR in sweet potato fields in coastal North Jiangsu Province was 60 kg(N)·hm^-2.