Objectives  This study aimed to explore the quantitative relationship between the substitution rate of organic fertilizer (SROF) and soil fertility under long-term application of fertilizers in the black soils, and to provide a theory foundation for optimal fertilization.

Methods  The dataset of a 32-years’ fertilization experiment, located on black soil in Gongzhuling City, was used for this research. Four treatments were selected from the experiment: no fertilization (CK); chemical NPK fertilizers (NPK); applying 30 t/(hm²·a) of manure (M1) and 60 t/(hm²·a) of manure (M2). The chemical fertilizer inputs were urea N 150 kg/hm², superphosphate P₂O₅ 75 kg/hm² and potassium sulfate K₂O 75 kg/hm². The data of maize yield, plant N content, soil total N, alkali-hydrolyzed N, total phosphorus, available phosphorus, total potassium, readily available potassium and organic carbon content were collected respectively. The substitution rate of chemical N with manure (SROF) was calculated based on crop removal of nitrogen (N). The decisive factor of SROF was discussed using the path analysis and multiple stepwise regression analysis, and the ultimate SROF was explored.

Results  The SROF in M2 treatment was higher than that in M1, based on the crop removal of N. A significant positive correlation existed between the SROF and fertilization years (P < 0.01). The content of soil organic carbon (SOC) was the major stationed soil fertility factors determining the variation of SROF, it decided 70% of the variances. The content of SOC was increased significantly in treatments of both M1 and M2 in black soil (P < 0.01). The SROF was significantly and nonlinearly correlated with the content of SOC (P < 0.01).

Conclusions  The substitution proportion of chemical nitrogen with organic manure is decided by 70% by the soil organic carbon content in the black soil. The long-term application of organic fertilizers is proved to be able to raise soil organic carbon content significantly, which will increase soil fertility, decrease chemical fertilizer input at the same time. When the soil carbon content is raised to 24.89 g/kg, the substitution rate of chemical fertilizer will reach its ultimate of 95%.