The mechanism of microbial-mediated mineralization of organic phosphorus (P) under nitrogen (N) addition in farmland soil is still unclear. To determine the effects of N addition on the composition, structure, and P transformation function of microbial community and soil P fractions in croplands, we conducted a field experiment on the Central Gansu Loess Plateau in 2017. The current study analyzed a subset of 12 plots from the 48-plot factorial experiment, comprising four levels of N addition in the absence of P fertilization. The treatment included control (0 kg N ha-1 year-1, N0), low N (75 kg N ha-1 year-1, N75), medium N (115 kg N ha-1 year-1, N115), and high N (190 kg N ha-1 year-1, N190). We determined soil P fractions and microbial properties in the 0-20 cm depth from 2019 to 2023. We found that N fertilization significantly enhanced the mineralization of soil organic P, primarily by altering microbial community structure and increasing the abundance of key taxa (e.g., RB41 and Filobasidium), which in turn boosted the activities of alkaline phosphatase (ALP) and phytase (PHY). The most pronounced stimulations in microbial biomass carbon (MBC) and ALP activity were observed under the N115 treatment. Concurrently, N addition led to substantial reductions in labile inorganic and organic P pools; for instance, the content of Ca2-P decreased most markedly under N190, by 42.82% in 2023, while labile organic P forms (LOP, MLOP, MROP) also declined significantly. Structural Equation Modeling (SEM) confirmed that N addition influenced P availability through direct pathways and indirect pathways mediated by shifts in microbial community structure, ALP, and PHY. In conclusion, our study has identified the N115 treatment (115 kg N ha-1 year-1) as the optimal level for promoting microbial-mediated organic P mineralization. To maintain soil productivity in the rain-fed agricultural systems of the Loess Plateau, we recommend applying a moderate amount of N fertilizer at this optimal rate, along with strategic P supplementation. This approach can effectively mitigate soil P deficiency and enhance the availability of P.