Optimizing the nitrogen fertilization rates and drip irrigation amounts to balance the root yields, nitrogen utilization, soil nitrate leaching, and economic benefits of two sweet potato cultivars

In China, following unreasonable nitrogen (N) application and irrigation, vine overgrowth of sweet potato easily occurs; this not only reduces root yield (RY) and N utilization efficiency but also leads to excessive spraying of plant growth retardants. Moreover, these influences varied depend on the cultivar. Thus, a field experiment was conducted in 2021–2022 by following a fully randomized split-split plot design with two cultivars (J26: N-susceptible; X32: N-tolerant), three N rates (N0: 0; N1: 75 kg N ha −1 ; N2: 150 kg N ha −1 ), and three drip irrigation levels (I0: CK; I1: 300 m 3 ha −1 ; I2: 600 m 3 ha −1 ). As growth continued, high N rates or irrigation levels decreased the leaf SPS activities and AGPase activities in tuberous roots in J26 but increased those in X32. The N rate and irrigation amount significantly affected tuberous root N accumulation. The cumulative impact of N rates on soil nitrate was stronger. For J26, the RY of N1I1 increased by 6.96 %–47.63 %. For X32, a higher RY was obtained under N2I1 in 2021 and under N1I2 and N2I1 in 2022. When correlation coefficients were combined, the yield difference was related mainly to the different regulatory effects of N rates on sucrose synthesis in leaves and starch accumulation in tuberous roots. For J26, N1I1 presented the highest REN, PFPN, and net income. For X32, compared with N2I1, N1I2 increased the REN by 6.94 %–12.60 %, decreased the soil NO 3 − content in the 60–80 cm layer and did not affect the total net income. 300 m 3 ha −1 irrigation and 75 kg N ha −1 as well as 600 m 3 ha −1 irrigation and 75 kg N ha −1 are more desirable for cultivating J26 and X32, respectively. It is possible to achieve higher planting benefits for sweet potatoes and reduce the risk of nitrate leaching by reducing N fertilizer inputs. • N × I showed remarkable interactive effects on the biomass of tuberous roots. • N rates or irrigation amounts had significant effects on tuberous root N accumulation. • At 150 DAP, N2I2 resulted in higher NO 3 − content in the soil layers of 60–100 cm. • N × I exerted remarkable interactive effects on RY, REN, PFPN, and net income. • Under the suitable N condition, the desirable irrigation amount for X32 was higher.