• When soil at time 2 a later date show a higher carbon value than the same soil at time 1 an earlier date the difference or increase of carbon and nitrogen would represent sequestration. When soils are initially very depleted in soil carbon and nitrogen the improvement or increase of these levels show a curve of plateau or steady state called by economic a curve of diminishing return. Soil scientist sees the state of not continuing to show soil carbon and nitrogen as a type of saturation. It can be argued that the soil is really not saturated as soil profile layers grow in depth. While much work concentrates on the 15 cm top soil depth anthrosol can accumulate layers of more than 300 cm deep.  The use of compost biochar cover cropping can lead to enhanced soil sequestration. These will contribute as long as they are employed. The use of fallow and plow tillage will lead to loss of carbon and nitrogen from the soil and rather than contribute to reducing greenhouse gases will become major contributors to them. While most current greenhouse gas elevation comes for fossil fuel use. The enrichment of greenhouse gases has also been seen as initiated by an agricultural revolution which causes major greenhouse gas generation from burning clearing and plowing for agricultural field expansion. The agricultural revolution was initiated a little over 10000 years ago which coincides with the ending of the last ice age.  

The concentration will stabilise depending on a number of properties of the system, not just the soil.   Sequestered SOM contains not just C, but N, P S etc

For those interested in the potential role of agriculture kickstarting climate change check out Ruddimann Hypothesis as a search term. 

Thanks so much Dr Happerly and Dr Milham for sharing your answer

Soils become "saturated" with carbon when the sequestration capacity is met. Soil texture, mineralogy, climate, etc all affect this capacity. Carbon can be stored in macro- and microaggregates in soil as well as being adsorbed onto surfaces of silt and clay particles. Thus, soils with more silt and clay that have greater capacity to aggregate and adsorb and protect or slow organic carbon-containing materials from further decomposition generally will have more absolute capacity to sequester carbon. Once this capacity is met, additional added organic carbon tends to be less protected from microbial mineralization..  

Soil C sequestration is the process of converting plant C fixed through photosynthesis to soil organic C as plant residue is returned to the soil, C sequestration rate is the rate of increase (positive) or decrease in soil organic C per year. As C is continuously sequestered in the soil over time, there comes a time when there will be no more sequestration due to high C level. This is called C saturation.