Soil has been identified as a possible carbon (C) sink to mitigate increasing atmospheric CO 2 concentration. However, several recent studies have suggested that the potential of soil to sequester C is limited and that soil may become saturated with C under increasing CO 2 levels. To test this concept of soil C saturation, we studied a gley and organic soil at a grassland site near a natural CO 2 spring. Total and aggregate‐as...