Old fields are spreading in the world because of agriculture abandonment, and they show a persistence of exotic plant species with little recovery towards the original vegetation composition. Soil biota may also differ between old fields and native grasslands, but were comparatively less studied than plant communities, despite their importance in biogeochemical processes. Here we compared soil bacterial communities of exotic-dominated old fields with those of remnants of native grasslands in the Inland Pampa, Argentina, using the 16S rRNA gene amplicon sequencing approach. We also characterized plant communities, soil physico-chemical properties, and soil respiration. We expected more diverse soil bacterial communities, with higher heterogeneity, in remnant grasslands than in old fields because of a more diverse and more heterogeneous plant community. However, our results showed that soil bacterial communities had higher Shannon diversity in old fields than in remnant grasslands, but richness was not significantly different. Also we found different bacterial community compositions between grasslands even at a low taxonomic level. On the other hand, old fields harbored less heterogeneous bacterial communities than remnants, and bacteria and plant beta diversity were correlated. Despite contrasting plant and bacterial composition between old fields and remnant grasslands, soil physico-chemical properties were quite similar between grasslands. Overall, our results showed that bacterial communities in grassland soils were associated with changes in plant communities after agricultural abandonment. Plant-microbial feedbacks might regulate plant and soil bacterial community assemblage in old fields, yet further research is needed to demonstrate this potential feedback mechanism.