Soil carbon addition controls weeds and facilitates prairie restoration

Soil nitrogen enrichment and consequent vigorous weed growth are thought to hinder the restoration of tallgrass prairie. Adding carbon to the soil may facilitate prairie restoration by inducing immobilization of plant-available nitrogen. Early attempts to use this method, however, have had mixed results. Success of C addition depends on three conditions: weeds must suppress prairie-species in the absence of C addition, weeds must be nitrophilic relative to prairie species, and C addition must result in a large enough decrease in N to alter the balance of competition among weeds and prairie species. We examined these conditions by comparing productivity of 10 weeds and 11 tallgrass prairie species under 14 levels of C addition, ranging from 84 to 3346 g C/m². Carbon was tilled into the soil prior to planting. To control for non-N effects of C addition, N was added to a subset of plots. Relative to untreated plots, the highest level of C addition resulted in an 86% decrease in available NO₃-N, a 14X increase in early season light availability, a 54% decrease in weed biomass, and a sevenfold increase in prairie biomass. Nitrogen addition significantly reduced or reversed all of these effects. Significant species-specific responses to C addition included decreased biomass for six annual weeds and increased biomass for six prairie species, one annual weed, and three perennial weeds. These results suggest that C addition may be a useful tool for restoring N-limited plant communities.