Nitrogen and potassium fertilization regulates source–sink relations, biomass allocation, and nutrient dynamics for industrial biomass production in cassava (Manihot esculenta Crantz)

Cassava fertilization primarily targets the enhancement of fresh storage-root yield, while fresh stem-and-leaf yield, source-sink allocation, and nutrient balances are often overlooked. A two-year field experiment conducted in Hainan, China, evaluated three nitrogen rates (N0, N120, and N240: 0, 120, and 240 kg N ha −1 ) and four potassium rates (K0, K75, K150, and K300: 0, 75, 150, and 300 kg K 2 O ha −1 ) using a factorial design with three replicates. Both N and K significantly affected plant height, stem diameter, and fresh stem-and-leaf yield ( p < 0.01), whereas neither nutrient affected fresh storage-root yield. Compared with N0, N120 increased fresh stem-and-leaf yield and whole-plant yield by 27.63% and 11.74%, respectively. Increasing N application to N240 reduced storage-root diameter and length by 6.88% and 9.45%, respectively. Compared with K0, K300 increased fresh stem-and-leaf yield by 22.50% and storage-root diameter by 5.93%, while reducing storage-root number by 14.75%. Both nutrients redirected fresh yield allocation toward stems and leaves. Nitrogen increased whole-plant N accumulation while reducing P accumulation, whereas potassium increased whole-plant K accumulation. The N × K interaction influenced storage-root number, P accumulation, and apparent P balance, whereas other interactions were minimal. A season with reduced rainfall during rapid bulking followed by excessive late-season rainfall resulted in decreased storage-root and whole-plant yields. Apparent N and K balances shifted from deficits to surpluses with increasing inputs, whereas P balance remained negative. Therefore, moderate N combined with adequate K can maintain storage-root production while enhancing stem-and-leaf yield.