Biological nitrogen fixation by diazotrophs plays a pivotal role in nitrogen input to natural grassland ecosystems. Although animal grazing is the predominant land-use practice in these systems, its influence on diazotrophic communities are poorly understood. In this study, transcript-based quantitative PCR and nifH amplicon sequencing were employed to assess the effects of grazing on the diversity, composition, and abundance of active soil diazotrophic communities across three major steppe types along a climatic gradient in Mongolia. The abundance of nifH transcripts in meadow steppe soils (1.17 x 105-2.54 x 105 copies/g dry soil) was significantly higher than that in typical (2.66 x 104-5.10 x 104) and desert steppes (1.93 x 104-2.66 x 104), whereas no significant differences were observed among grazing intensities within the same steppe type. RNA-based sequencing revealed that the diversity of active diazotrophs was the highest in the meadow steppe. Steppe type exerted a stronger influence on diazotrophic community structure than on grazing management. Active diazotrophic communities were dominated by Gammaproteobacteria (31.22%) in typical and desert steppes and by Alphaproteobacteria (58.32%) in the meadow steppe. Transcript nifH gene abundance was significantly and positively correlated with soil physicochemical properties, climatic variables, and vegetation characteristics. Soil and climate factors were the primary drivers of diazotrophic community patterns. This study provides the first characterization of active diazotrophic community structure and abundance in Mongolian steppe soils subjected to centuries of nomadic grazing.