Objectives We quantitatively studied the relationship between nitrogen fixation (NF), nitrogen absorption and yield of soybean in China, and proposed a suitable nitrogen application rate according to their response to nitrogen fertilizer application rate, in order to provide scientific references for the simultaneous realization of high yield and high nitrogen fixation efficiency (NFE) of soybean.

Methods Literature published since 2000 were searched in CNKI and Web of Sciences, using key words “soybean, and yield, and N fixation or N fixation amount or N fixation efficiency”. Then the literature was screened by criteria of “field experiment in China, and containing whole information of soybean yield, nitrogen uptake, nitrogen fixation and nitrogen application amount”, and total of 14 papers were achieved. The data characteristics and correlation among the criteria indicators were analyzed using statistics, correlation and quadrant analysis methods.

Results Both the soybean yield and nitrogen (N) fixation amount exhibit a linear positive correlation with N uptake. For every additional 1 kg of N uptake, the N fixation amount increases by an average of 0.69 kg/hm². The N requirement per 100 kg of grain initially rises and then declines as the yield level increases. The average nitrogen fixation efficiency of soybeans is 61.1%, with an average N deficit (the difference between soybean N requirement and N fixation amount) of 76 kg/hm². Using the criterion that nitrogen fixation meets 61% of soybean N demand, both the low nitrogen fixation efficiency group with the proportion of biological nitrogen fixation in soybean N uptake (NDFA) < 61% and the high nitrogen fixation efficiency group (with NDFA > 61%) show a linear positive correlation between the N deficit and N uptake, with the N deficit increasing as N uptake rises. The average soybean N balance (the difference between soybean N fixation amount and grain N uptake amount) is −53.3 kg/hm², with only 15.38% of samples having a positive N balance. Among the data points with positive values, 83.33% belong to the high nitrogen fixation efficiency group. The yield increase rates of soybeans under N application levels of 0−30 kg/hm², 30−60 kg/hm², and >60 kg/hm² are 3.39%, 34.47%, and 14.58% respectively (with an average increase of 17.68%), while the soybean N fixation amounts decrease by 21.30%, 11.76%, and 30.29% respectively (with an average reduction of 23.23%). Based on the quadrant analysis method examining the relationship between soybean nitrogen fixation efficiency and yield, four types can be identified: high-yield and high-nitrogen-fixation (yield of 3129 kg/hm² and nitrogen fixation efficiency of 71.1%), high-yield and low-nitrogen-fixation (yield of 3152 kg/hm² and nitrogen fixation efficiency of 42.9%), low-yield and high-nitrogen-fixation (yield of 1640 kg/hm²and nitrogen fixation efficiency of 69.1%), and low-yield and low-nitrogen-fixation (yield of 1996 kg/hm² and nitrogen fixation efficiency of 53.0%).

Conclusions There is a significant positive correlation between soybean’s N uptake and yield. As the yield increase, the N requirement per 100 kg of grain decreases. Biological nitrogen fixation is hardly to fully meet the N demands of soybeans, the N gap ranges from 19.6 to 237.4 kg/hm², and the N balance is negative in most cases. Nitrogen application significantly boosts soybean yield but reduces its nitrogen fixation efficiency. However, this conflict can be mitigated to a certain extent with an appropriate amount of nitrogen fertilization. Under the current soybean yield levels in China, the suitable N application rate ranges from 30 to 60 kg/hm².