Effects of different nitrogen application rates on sweet potato yield and nitrogen use efficiency under different soil fertility conditions were explored. Field experiments were carried out to investigate the effects of nitrogen fertilizer on yield

yield components

dry matter accumulation and nitrogen use efficiency of Yanshu25

under different soil fertility levels. As results showed

without nitrogen fertilizer

the number of branches

the length of the longest stem

the leaf area index

and the yield were following this order: high soil fertility＞middle soil fertility＞low soil fertility

and the differences were significant. Nitrogen fertilizer could increase the first three indices under three soil fertility conditions (p＜0.05)

but had no significant effect on the number of the largest internodes. Field plots with high

middle and low fertility levels gained the largest yields when nitrogen fertilizers were 50

100

and 150 kg/hm2

respectively

which increased by 16.06%

29.63% and 33.33%

respectively

compared to the control. Besides

nitrogen fertilizer could increase accumulation of above-ground dry matter s of sweet potato under all soil fertility levels. Lastly

with the increasing of nitrogen addition

the nitrogen accumulation were increased for middle and low fertility of plots

but first increased and then decreased for high fertility of plots. With the increasing nitrogen application rates

nitrogen use efficiency decreased under the high fertility

while increased under the low fertility

and was the highest under the middle fertility with 100 kg/hm2. The optimum nitrogen application rates for sweet potato in high

middle and low soil fertility are 50

100 and 150 kg/hm2

respectively.