Nitrogen deposition usually exacerbates soil phosphorus limitation, and microorganisms and plants will adopt strategies to offset this negative impact. However, it is unclear how soil microorganisms and/or plants affect soil bioavailable phosphorus under nitrogen addition. Based on field nitrogen addition experiments, this study determined physicochemical properties, microbial biomass and phosphatase activities of rhizosphere and bulk soils, and root morphology and nutrients during growing season (April) and non-growing season (October) in Castanopsis fabri forest. We quantified four forms of bioavailable phosphorus through biological methods, including calcium chloride phosphorus (CaCl₂-P), citric acid phosphorus (CA-P), hydrochloric acid phosphorus (HCl-P), and enzyme phosphorus (Enz-P). The results showed that contents of most bioavailable phosphorus in rhizosphere soils were higher than those in bulk soils during growing season, and contents of CaCl₂-P and CA-P in rhizosphere soils were significantly lower than those in bulk soils during non-growing season. Nitrogen addition significantly increased CA-P contents in bulk soils during growing season, while significantly reducing CaCl₂-P and Enz-P contents and significantly increasing HCl-P content in rhizosphere soils during non-growing season. However, the total bioavailable phosphorus content remained unchanged under nitrogen addition in the two seasons. In the two seasons, nitrogen addition resulted in varying degrees of reduction in root length, specific surface area, and tissue density of plant fine roots. However, a significant increase in mycorrhizal infection rate was observed, indicating that plants tended to intercept soil available phosphorus through cooperation with mycorrhizal fungi rather than changing root morphology under nitrogen addition. Additionally, no significant change in soil microbial biomass and phosphatase activity was found under nitrogen addition. From dynamics of bioavailable phosphorus content in non-growing season, it can be seen that plant roots and/or microorganisms activate mineral bound inorganic phosphorus by releasing protons under nitrogen addition, which is crucial for maintaining soil bioavailable phosphorus in Castanopsis fabri forest. It is worth noting that a higher response of bioavailable phosphorus to nitrogen addition was found in rhizosphere soil. Therefore, focusing solely on the dynamics of phosphorus availability in bulk soil may underestimate the impact of nitrogen addition on soil phosphorus availability. Overall, the short-term nitrogen addition maintained the potential supply of soil available phosphorus. This study helps to further understand adaptation mechanisms of microorganisms and plants to nitrogen load in low phosphorus subtropical regions, and provides a theoretical basis for the development of sustainable strategies to alleviate soil phosphorus limitation.