Objectives  We investigated the soil nitrogen content, rice growth and yield and nitrogen use efficiency on the first and second year of biochar application under different biochar and nitrogen fertilizer rates of application, to optimize biochar and nitrogen synergistic effect in japonica rice production.

  Methods  A 3 × 3 factorial field experiment was carried out in Liaoning Province between 2019 and 2020. The three nitrogen application levels were no nitrogen fertilizer (N₀), low nitrogen (N 180 kg/hm², N₁₈₀) and high nitrogen (N 225 kg/hm², N₂₂₅); while the three biochar rates were 0 (B₀), moderate (15 t/hm², B₁₅) and high (45 t/hm², B₄₅), composing 9 treatment combinations. The tillering dynamics, yield, yield components and N uptake of rice were determined, and soil nutrient status were analyzed at the same time.

  Results  1) Both B₁₅ and B₄₅ increased soil total and available N contents at peak tillering and booting stage of japonica rice, but decreased available N at grain filling stage (P < 0.05). 2) Compared with B₀, both B₁₅ and B₄₅ decreased the maximum tiller number, but increased the effective tiller number (P < 0.05), thus increasing the total spikelet number. Under the same N rate, the trend of yield increase was B₁₅> B₄₅> B₀. The yield of N₁₈₀B₁₅ and N₂₂₅B₁₅ was 4.4% and 3.2% higher than that of N₁₈₀B₀, while there was no significant difference between N₁₈₀B₄₅ and N₁₈₀B₀, and between N₂₂₅B₄₅ and N₂₂₅B₀. Under reduced N level (N₁₈₀), the yield was significantly decreased by combining with B₄₅, while significantly increased by combing with B₁₅, and reached the yield level under normal N level (N₂₂₅). 3) The effects of biochar on N uptake of japonica rice varied over the years. In the first year of applying biochar (2019), the N uptake of B₁₅ was significantly higher than that of B₄₅ at the N₁₈₀ level from peak tillering to filling stage. There was no significant difference between B₁₅ and B₄₅ at N₂₂₅ level. In 2020, the N uptake between B₁₅ and B₄₅ were at par regardless of N level. B₄₅ reduced the beneficial effects of biochar in the first year, and its adverse effects disappear in the second year (P < 0.05). 4) Biochar promoted N absorption and increased N use efficiency of japonica rice. The N use efficiency, agricultural use efficiency and partial productivity increased firstly and thereafter decreased with increasing biochar application amount, and attained its peak under N₁₈₀B₁₅ treatment, with the same trend in both years.

  Conclusions  Suitable biochar rate had positive interaction with N fertilizer on increasing soil fertility, rice growth and yield, while high biochar rate had negative interaction with N fertilizer during the first year of application, especially under reduced N input, but the negative effect will not continue during the following year. Therefore, the optimal combination for a synergistic effect was N 180 kg/hm² and biochar 15 t/hm².