Background and aims <p>In karst rocky desertification areas, vegetation restoration is frequently limited by nitrogen availability. Although afforestation modifies soil nitrogen dynamics and nitrogen‑fixing microbial communities, the relationships between soil nitrogen fractions and these microbial shifts remain unclear. This study investigated <i>Biancaea sappan</i> plantations across different age classes to examine how stand age influences nitrogen-fixing microorganisms in the rhizosphere soil and their links to soil nitrogen fractions.</p> Methods <p>We analyzed soil chemical properties and nitrogen-fixing microbial characteristics across forest age classes in rocky desertification areas, examining changes in microbial diversity and soil nitrogen dynamics with stand age.</p> Results <p>Soil nitrogen fractions (i.e. alkali-hydrolyzable nitrogen, total nitrogen, and ammonium nitrogen) all increased in content with <i>B. sappan</i> stand age, whereas the diversity of nitrogen-fixing microorganisms showed a decline. Notably, the assembly process of the nitrogen-fixing bacterial community shifted from stochastic to deterministic dominance as stand age increased. Partial Least Squares Path Modeling indicated that stand age was associated with changes in the composition of the nitrogen-fixing bacterial community through increases in soil nitrogen fractions.</p> Conclusion <p>The artificial cultivation of <i>B. sappan</i> significantly enhances the physical properties and nutrient content of rocky desertification soil and influences the assembly of soil nitrogen-fixing bacterial communities. Our results suggest that increases in soil pH and nitrogen saturation are strongly linked to the shift in these communities from stochastic to deterministic assembly processes. These findings provide correlational evidence that may inform ecological restoration strategies in karst regions. Further experimental studies are needed to establish causal relationships.</p> Graphical Abstract <p></p>