Restoration of tallgrass prairie on former agricultural land is often impeded by failure to establish a diverse native species assemblage and by difficulties with nonprairie, exotic species. High levels of available soil nitrogen (N) on such sites may favor fast-growing exotics at the expense of more slowly growing prairie species characteristic of low-N soils. We tested whether reducing N availability through soil carbon (C) amendments could be a useful tool in facilitating successful tallgrass prairie restoration. We added 6 kg/m2 hardwood sawdust to experimental plots on an abandoned agricultural field in the Sandusky Plains of central Ohio, United States, increasing soil C by 67% in the upper 15 cm. This C amendment caused a 94% reduction in annual net N mineralization and a 27% increase in soil moisture but had no effect on total N or pH. Overall, plant mass after one growing season was reduced by 64% on amended compared with unamended soil, but this effect was less for prairie forbs (−34%) than for prairie grasses (−67%) or exotics (−62%). After the second growing season, only exotics responded significantly to the soil C amendment, with a 40% reduction in mass. The N concentration of green-leaf tissue and of senescent leaf litter was also reduced by the soil C treatment in most cases. We conclude that soil C amendment imparts several immediate benefits for tallgrass prairie restoration––notably reduced N availability, slower plant growth, and lower competition from exotic species.