Objective   Effects of nitrogen fertilizer reduction accompanied by a biochar addition on the yield and nutrient conversion of rice plants were analyzed to promote straw waste utilization in paddy fields.

  Method   In a field experiment, designated lots was assigned for fertilization on rice plants using either only P and K but no N fertilizer (CK), conventional 100% N fertilization (T0), 90% N fertilization with addition of 2.5 t·hm⁻² biochar (T1), 80% N fertilization with addition of 5.0 t·hm⁻² biochar (T2), 70% N fertilization with addition of 7.5 t·hm⁻² biochar (T3), or 60% N fertilization with addition of 10.0 t·hm⁻² biochar (T4). The triplicated treatments were arranged by random block group.

  Results   The rice yield increased as the N fertilizer was increasingly reduced along with the added biochar to peak at the T2 treatment. The 10.22% increase over T0 was significant (P＜0.05), but it started to decline upon further reduction on N. The greatest N accumulation in the rice shoots was found under T2 as well as in the roots under T3 and at maturation. Under T1, T2, and T3, the N accumulations in the straws of a rice plant at maturity increased by 5.05%–15.96%, in the roots by 6.76%–19.00%, and the highest in the grains by 4.65%–21.45% over T0. The straw biomass, N partial productivity, and N agronomic efficiency increased by 5.34%–41.44%, 17.43%–63.81%, and 42.50%–85.93% over T0 under T1, T2, T3, and T4, respectively, while the apparent N utilization enhanced by 77.12%–106.79% with T2 being the most. The N harvest index of T1 was lower than T0, but higher than other treatments.

  Conclusion  The 80% N fertilization with the addition of 5.0 t·hm⁻² biochar (T2) significantly increased the yield, N utilization efficiency, and N accumulation of the rice plants.