A suitable replacement ratio of organic nitrogen for high rice yield and quality under different soil fertility levels and nitrogen application rates

Objective Organic substitution of nitrogen (N) fertilizers to reduce chemical fertilizer inputs and eliminate organic wastes is one of the key technologies promoting green rice production. However, the appropriate substitution ratio of organic N fertilizer is yet to be established, requiring urgent attention.

method In this study, we synthesized 71 published articles on the organic substitution of N fertilizer in rice production. 412 and 133 field trial data were obtained on rice yield and grain N content. We used meta-analysis to quantify the effect of the rate of organic N substitution (RS) on rice yield and grain N content to explore the appropriate RS under different soil conditions and N application levels.

Results Appropriate replacement of organic N fertilizer improved rice yield and grain N content. The soil organic matter content recorded ranged from high (>25 g/kg), medium (15–25 g/kg), to low (≤15 g/kg) levels. The suitable RS for obtaining the highest rice yield were 60%, 70%, and 30%, respectively. When the soil total N was high (>1.5 g/kg), medium (1–1.5 g/kg) or low (≤1 g/kg), the suitable RS was not more than 60%. Under high (>150 mg/kg) and low (≤90 mg/kg) levels of soil available N, the suitable RS was 10%–30%. At the medium level of soil available N (90–150 mg/kg), RS did not influence rice yield (P>0.05). Under the high N application rate (>250 kg/hm²), the RS with the highest yield was 20%, 10% for low N (≤150 kg/hm²), and 70% for medium N (150–250 kg/hm²). With the increase of RS, the nitrogen content of rice grains increased first and then decreased. The RS with the highest nitrogen content was 30%.

Conclusion The RS can be appropriately increased to 60%–70% in areas with high soil organic matter and total nitrogen content. The RS needs to be reduced to about 30% in low and medium levels. Under the high and low levels of available soil nitrogen, there was no change in rice yield when the RS was more than 30%. The optimum RS for maintaining high rice yield was 20% and 10% under high and low nitrogen application levels, respectively. The N content of rice grains could increase when RS is not more than 30%.