Ecological restoration is an important intervention strategy to halt biodiversity loss and land degradation. Under natural conditions, diazotrophs regulate terrestrial productivity by alleviating nitrogen limitation. However, little is known on the effects of revegetation on diazotroph community (composition, diversity and functioning). Here, by using nifH gene amplicon sequencing, community assembly models, and ¹⁵N₂-labeling technique, we investigated the dynamics of soil diazotroph communities along a revegetation chronosequence of 0 (cropland), 5, 10, 20 and 30 years of Chinese prickly ash (CP) in a karst area of Southwest China. Diazotroph diversity decreased and its composition was significantly changed after revegetation. Soil pH was the edaphic factor which correlated the most with diazotroph community composition and diversity. The most abundant diazotrophs belonged to the Proteobacteria phylum and changed along revegetation chronosequence: the genus Azohydromonas dominated at early stages (5-y CP), while at later stages (20-y CP) the genera Azospirillum and Bradyrhizobium became more abundant. The N-fixation rates were highest in the 5-y CP plantation, and relatively lower N-fixation rates were observed in the cropland and 10-y CP plantation. The Sloan neutral and null models showed: i) a phylogenetic turnover in the diazotrophic taxa assembly, which was largely deterministic at 5- and 10-y, and stochastic at 20- and 30-y; and ii) a bacterial community which was assembled largely by stochastic processes along the revegetation chronosequence. Taken together, our results highlight that knowledge on dynamics of diazotroph community (composition, diversity and functioning) is important to understand ecological mechanisms regulating belowground community along revegetation process.