Abstract
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‐associated soil organic C (SOC) concentration showed a significant increase with atmospheric CO 2 concentration. An asymptotic function showed a better fit of SOC and aggregation with CO 2 level than a linear model. There was a shift in allocation of total C from smaller size fractions to the largest aggregate fraction with increasing CO 2 concentration. Litter inputs appeared to be positively related to CO 2 concentration. Based on modeled function parameters and the observed shift in the allocation of the soil C from small to large aggregate‐size classes, we postulate that there is a hierarchy in C saturation across different SOC pools. We conclude that the asymptotic response of SOC concentration at higher CO 2 levels indicates saturation of soil C pools, likely because of a limit to physical protection of SOC.