【Objective】Soil nitrogen (N) is an important resource for N uptake by crops. The availability of soil N is dependent on soil fertility to a high extent, which in turn affects the absorption and utilization of N by crops. Understanding the various N forms in red soils with different fertility and their contribution to crop N uptake can provide a theoretical basis for clarifying the N cycle mechanism and cultivating high-yield and high-N efficiency soils.【Method】In May 2019, low-fertility, medium-fertility and high-fertility red soils were selected for the field micro-area experiments, and two treatments, no nitrogen fertilizer (N0) and conventional fertilization (N1) were set up. We determined the contents of soil mineral nitrogen (MN), fixed ammonium (FN), microbial biomass nitrogen (MBN) and soluble organic nitrogen (SON) before planting and after harvest of 2020 maize (the third crop of the experiment). The relationship model between each form of N and the quantity of N uptake was established through structural equation modeling (SEM). 【Result】The grain yield of high fertility soil under N0 treatment was about 4.6 times that of medium fertility soil. Under N1 treatment, the maize yield and biomass of high fertility soil had no significant difference with that of medium fertility soil. Meanwhile the N uptake of high fertility soil was significantly higher than that of medium fertility soil. Compared with before planting, the FN content in the medium-fertility soil after harvest was significantly increased by 63%under N0 treatment. The FN oflow and high fertility soil increased by 47% and 11%, respectively, while the contents of MN, MBN and SON in the soil were reduced. The MN content of the soils was decreased by between 0.4-4.0 mg·kg^-1. MBN was decreased by 18%-44% and there was no significant difference in soil fertility. Also, the SON was decreased by 55%-84%. Under N1 treatment, the content of MN was decreased by 22-38 mg·kg^-1and the content of MBN was reduced by 32%-72%. The SON was reduced by 99 mg·kg^-1 in high-fertility soil, which was 2.0 times higher than medium-fertility soil and 9.3 times higher than low-fertility soil. The correlation analysis results revealed a significant positive correlation between the amount of N uptake by the aboveground biomass and the decrement of MBN, SON and NH₄⁺-N. The structural equation model results further showed that the content of SON and NH₄⁺-N directly affected the amount of N uptake, while MBN indirectly affected the N uptake by maize by changing SON and MN.【Conclusion】SON and NH₄⁺-N can directly regulate the maize N uptake. The MBN and FNindirectly affect maize N uptake through the MN and SON. In the future, it is crucial to strengthen the research on the mechanism of soil N form transformation to promote red soil fertility and high-efficiency utilization of N.