Mixing broad-leaved trees into coniferous plantations is one of the important measures for plantation reconstruction in the degraded red soil regions. Soil nutrient supply and transformation are important parameters for evaluating mixed effects. However, the characteristics and influencing factors of nitrogen (N) mineralization in the coniferous rhizosphere soil after mixing are not clear. In this study, we chose the Masson pine (Pinus massoniana Lamb.) plantation, slash pine (Pinus elliottii Engelmann) plantation, Pinus massoniana-Schima superba mixed plantation and Pinus elliottii-Schima superba mixed plantation as research objects. The rhizosphere soil of coniferous trees in the four plantations were collected to measure available nutrient concentrations and N mineralization rate. Enzyme activities associated with N hydrolysis were determined with microplate fluorometer and soil microbial biomass and community composition were also analyzed with phospholipid fatty acid method. The aims of this research were to study the effects of mixing on N supply and microbial characteristics in the rhizosphere soil of conifers and to analyze the correlation between soil N mineralization and microbial characteristics. The results showed that mixing with broad-leaved trees significantly increased the concentrations of ammonium N, mineral N and available phosphorus in the coniferous rhizosphere soil. However, no significant difference was found for nitrate N concentration among the four plantations. The N mineralization in rhizosphere soil of conifers was dominated by nitrification. After mixing, the ammoniation rate of coniferous rhizosphere soil significantly decreased by 27.0%, while the rhizosphere soil nitrification rate of conifers increased by 55.4%, which induced an increase in the soil net N mineralization rate by 24.1% of coniferous trees. Among the two tree species, the mineral N concentration and net N mineralization rate in rhizosphere soil of Masson pine were significantly higher than those of slash pine. Mixing significantly increased the biomass of fungi, arbuscular mycorrhizal fungi and total microbes, and the fungi/bacteria ratio in coniferous rhizosphere soil. The total microbial and fungal biomass in the rhizosphere soil of Masson pine were 18.9% and 27.0% higher than that of slash pine, respectively. The activity of rhizosphere soil β-N-acetylglucosaminidase increased significantly after mixing, and the rates of nitrification and net N mineralization positively correlated with soil microbial parameters and enzyme activity. Generally speaking, mixing significantly enhances rhizosphere soil nitrogen supply, which helps coniferous trees to cope with the pressure of nutrient competition caused by the mixture of broad-leaved trees. Masson pine tends to adopt positive strategies by increasing soil N mineralization to cope with external environment changes.