Effects of nitrogen application on dry matter accumulations and qualities of sweet potato under different soil moistures conditions

To explore the effect of nitrogen application on dry matter accumulations and qualities of tuberous roots of sweet potato(Sushu 14) under different soil moistures conditions,a pot experiment was conducted in plastic-covered greenhouse with three soil moistures(40%,80% and 120%) and two nitrogen application levels(no nitrogen fertilizer as control,and 150 mg nitrogen in 1 kg air-dried soil).The results showed that increasing soil moisture was beneficial to the dry matter accumulation of sweet potato.The application of nitrogen fertilizer improved the shoot dry matter of sweet potato under all three soil moistures conditions.Nitrogen application lowered dry matter accumulation in tuberous root at low soil moisture level,and increased it at high soil moisture level,indicating that sufficient nutrients and water resource were necessary for high production of sweet potato.Increasing moisture reduced the content of vitamine C and raised soluble sugar content and related components.Hower,after nitrogen application,the down trend of vitamin C became gentle,and the contents of soluble sugar came down,resulting in the edible quality decline.The response of starchs content to soil moisture was related to nitrogen application level.Under no nitrogen application condition,the contents of amylose,amylopectin,total starch and ratio of amylase to amylopectin decreased with the rise of soil moisture.After nitrogen application,the amylose content and ratio of amylase to amylopectin decreased under low soil moisture condition,and the contents of amylase,amylopectin and total starch were remarkably increased under high soil moisture condition,resulted in the improvement of processing quality.In conclusion,both soil moisture and nitrogen application played important roles in sweet potato qualities and yield,and should be taken into consideration during potato cultivation in order to achieve high quality and yield.