Objectives Nitrogen supply levels play key roles in the formation of root tubers and their expansion in sweet potato production. The paper studied the effect of reducing conventional nitrogen application rate and topdressing half of the nitrogen on the root growth and development of sweet potato, in order to provide an efficient nitrogen management method in the cultivation of sweet potato.

Methods Pot experiments were conducted in the Experimental Base of Anhui Academy of Agricultural Sciences in 2016 and 2017. The test soil was yellow brown soil and sweet potato cultivars used were Shangshu19 and Xushu22. The conventional N rate of 100 kg/hm² in once basal application (FP) was used as control, the 20% less of N fertilizer rate with three application patterns were setup for treatments, including once basal application (JS), once topdressing at the 35th day after transplanting (KS), and 50% basal and 50% topdressing at the 35th day (FS). Root samples were collected at the stages of late tuber formation, tuber root expansion and maturing periods. The root morphological indexes, the root activity and biomass accumulation were measured.

Results Compared with FP, KS treatment increased the tuberous root number per plant, and the storage root yield was increased by 7.61% for Shangshu19 and 11.74% for Xushu22; the FS treatment increased the tuberous root number per plant and fresh weight per tuberous root, enhanced the storage root yield by 22.10% for Shangshu19 and 21.37% for Xushu22. The sweet potato in FS treatment had longer root length, higher total root surface area, root volume, root tip number in all the tested growing periods. Their root activities, aboveground and root biomass, storage root biomass and the biomass ratio of root to aboveground were all higher than other treatments as a result. The tested indices for root growth and development at the 80th day were significantly and positively related with the fresh weight per tuberous root and storage root yield.

Conclusions Reducing nitrogen rate from 100 kg/hm² to 80 kg/hm² and topdressing half of the nitrogen at the root tuber formation stage could improve root growth and optimize root morphology, which is beneficial to the formation of effective root tubers and the maintaining of higher root physiological activities during later growth stage, eventually, increase the root biomass and average tuber weight, and thus producing a higher storage root yield.