Soil carbon saturation limits soil organic carbon responses to global change

Many soils, especially those that have been significantly depleted in soil organic carbon (SOC) through cultivation show a linear response between annual C input rates and steady-state SOC levels. However, some mineral soils with relatively high SOC contents show only a weak or no response to increasing rates of C input, suggesting that there is a dependency between SOC levels and the basic kinetics governing SOC stabilization. It is postulated that these soils exhibit the phenomenon of soil C saturation. We tested the postulation of soil C saturation in three experiments representing gradients in C input due to changes in land use, management and elevated CO₂. We fractionated the soil in different SOC pools governed by different SOC stabilization mechanisms. At all sites, the light fraction C increased linearly with increasing C input, but the mineral-associated SOC showed an asymptotic response, suggesting soil C saturation of the mineral soil fraction. A further fractionation of the mineral-associated SOC indicated that the particulate organic matter (POM) fractions also linearly responded to increasing C input. However, the microaggregate-associated and free silt and clay particles responded asymptotically to increased C input. These results indicate that as C input increases, the mineral fraction of the soil saturates, which induces an accumulation of additional C input in labile soil C pools (LF and POM). Hence, with a decreasing saturation deficit of the stable C pools, the labile C pools gain more importance and account for an increasing proportion of total soil C. Thus, C saturation not only puts a limit on the amount of SOC that can be stabilized in the long term, but also determines the accumulation rates and dynamics of labile C pools in a changing global environment.