Abstract: Bacterial and fungal communities are critical drivers of soil nitrogen cycling. However, it remains unclear how encroachment of Moso bamboo into subtropical evergreen broadleaved forest alters soil nitrogen minerali-zation and the contributions of bacterial and fungal communities. We used a paired experimental design combined with ¹⁵N isotope tracing and the acetylene inhibition method to investigate the effects of Moso bamboo expansion on soil nitrogen transformation and the relative contributions of microbial groups. The results showed that fungi community played a major role in driving soil gross nitrogen mineralization in broadleaved forest, whereas both bacterial and fungal communities contributed to gross nitrogen mineralization in bamboo forest, with the relative contribution of bacteria (82.9%) being higher than fungi (49.7%). After Moso bamboo expansion into broadleaved forest, gross nitrogen mineralization shifted from fungal dominance to bacterial dominance. Following the Moso bamboo expansion, the microbial community structure changed and soil gross nitrification rate decreased by 21.5%, mainly due to a reduction in bacterially dominated autotrophic nitrification. Before and after Moso bamboo expansion, soil heterotrophic nitrification rates were 0.76 and 0.68 mg·kg^-1·d^-1, respectively. Both accounted for more than 79% of the gross nitrification rate, indicating that heterotrophic nitrification dominated the soil gross nitrification process. After Moso bamboo expansion, the contribution of fungi to soil heterotrophic nitrification decreased from 89% to 41.5%, whereas the contribution of bacteria increased from 49.3% to 79%, indicating that fungi dominated heterotrophic nitrification in broadleaved forest before Moso bamboo expansion and bacteria became dominant after expansion. This study would provide theoretical basis for understanding the effects of Moso bamboo expansion on soil nitrogen mineralization and the microbial mechanisms.

Key words: gross nitrogen mineralization rate, heterotrophic nitrification rate, autotrophic nitrification rate, microbial community, Moso bamboo encroachment