Abstract: 【Objective】 The response of N2 fixation and yield to N fertilizer, even with same N application rate, gives varying and contradicting results. The contradicting results depend on the perception of the system of soybean nitrogen fixation and root to the soil inorganic N concentration. The nodulation, fixation and yield of soybean with different soil inorganic N concentrations were studied to find the reasonable soil inorganic N concentration for promoting soybean yield, nodulation and fixation. i.e. to master the quantified relationship between soil inorganic N concentration, nodulation and fixation. Then it will have a significant theory and production meaning for adjusting N application rate, timing, and forecasting the effect of N fertilizer on the fixation and yield. 【Method】A pot experiment was conducted to study the effects of different soil inorganic nitrogen concentrations, which derived from total fertilizer N (as urea) applied at vegetative stage (V2), at early blooming stage (R1), at early podding stage (R3) and at grain filling stage (R5), on the ability of N2 fixation and yield, by measuring the nodule number, biomass and nitrogenase activity and yield components. Through the research, the response of soybean nodulation and yield to soil inorganic N concentration cleared, and thus mastering the quantity relationship between soil inorganic N concentration and N application, nodulation and yield, and then providing a theoretical basis for agriculture production and scientific research. 【Result】 The nodule number, biomass and nitrogenase activity of soybean reached peak at R4 stage, while decreased at R6 stage, and the nodule number, biomass and nitrogenase activity were in order of V2＞R5＞R3＞R1. The nodule biomass decreased by 15%, 18%, 17% and 32% compared to that of CK. The nodule number decreased by 13%, 18%, 19% and 20%. The nitrogenase activity decreased by 19%, 22%, 23% and 32%. Meanwhile, there were significant negative linear correlations between nodule number, biomass and nitrogenase activity at R6 stage and the concentration of soil inorganic nitrogen. Moreover, The trends of dry matter accumulation and yield were expressed as an order of R1＞R3＞V2＞R5. Except the treatments at R5 had no significant correlations with the treatment of CK, the other treatments had a significant promoted effect on dry matter accumulation and yield. The treatment of N3 had the highest dry matter accumulation, plant height, pod number, pod dry weight and yield than the other treatments at different growth stages. The soil inorganic N concentration at V2 stage had a significant effect on nodulation and the ability of nitrogen fixation, and that at R1 stage had a significant effect on growth and yield. There were different threshold values of soil inorganic nitrogen concentration at different growth stages. The soil inorganic N concentration should be reached 135.8 mg•kg -1 at V2 stage, 58-91 mg•kg -1 at R1 stage; 29.4-62.8 mg•kg -1 at R3 stage; 102.3 mg•kg -1 at R5 stage to promote soybean nitrogen fixation and yield. 【Conclusion】 The response of N2 fixation and yield to N fertilizer mainly depended on the soil inorganic N concentration which was affected by N fertilizer application rate and timing. The soil inorganic N concentration was adjusted by agriculture production and scientific experiment. The nodule number, biomass and nitrogenase activity were significantly affected by the soil inorganic N concentration at V2 stage than that at the other growth stages, while the biomass and yield were significantly affected by the soil inorganic N concentration at R1 stage than that at other growth stages.

Key words: soil inorganic nitrogen , nodule , N2 fixation , yield , black soil