Diazotrophs, nitrogen (N)-fixing microbes, play a pivotal role in N cycling in tropical/subtropical forests. However, little is known about the dynamics of diazotroph communities and the factors that drive their abundance during forest succession. Bulk soils were sampled across two chronosequences in two subtropical forests: a long-term natural succession (LS) and a short-term artificial-intervened succession (SS); both include an early-, mid- and late-successional stage. The results show that the diazotrophic diversity increased with forest succession for the SS, but did not change for the LS. The relative abundance of the dominant genus Bradyrhizobium was significantly greater in the early-successional stage than in the late-successional stage, and some occasional genera appeared in the late-successional stage, both for the LS and SS. Variation partitioning analyses showed that the diazotroph community composition was mainly correlated with soil phosphorus (P) concentration, especially the plant-available soil P concentration, which explained 33.3% of the diazotroph community variation. Our findings revealed the patterns of diazotroph community across forest succession and highlight the importance of soil P availability in mediating diazotroph community during succession in subtropical forests, which is valuable for guiding restoration practices in terms of nutrient management in subtropical forests.