Most of the soil organic matter models assume a linear relationship between soil C content and C input rate. However, in some soils, soil C may not increase even when the C input rate increases. This indicates that there may be an upper limit to how much C can be sequestered in soil. Previous studies on soil C dynamics have suggested soil C saturation based on the silt and clay content of the soil, but other mechanisms of soil C protection that leads to C saturation are yet to be explored. Soil can act as a sink of C, and storing C in soil can be an effective option to mitigate CO₂ emission to the atmosphere. Moreover, higher organic matter content can improve the fertility of the soil. However, we are uncertain about limits in soil C sequestration, and through which mechanism this limit is imposed. We defined C saturation as the maximum capacity of soil to store C when C input rate at steady-state increases and when soil disturbance is minimized. The objective of our study was to test the C saturation concept in a temperate agroecosystem where there is gradient of C input to soil. We tested the C saturation concept in a long-term corn cultivation experiment established in Lexington, Kentucky. Because this site has been maintained for 35 years, we assumed that soil C content approximates equilibrium. This experiment is a randomized complete block design, and the treatment factors are tillage (no tillage and conventional tillage) and N fertilization. Nitrogen fertilizer was applied at the rate of 0 kg N ha^-1, 84 kg N ha^-1, 168 kg N ha^-1, and 336 kg N ha^-1. In this experiment, corn productivity increased with higher fertilization rate under both tillage systems, producing a range of C inputs over which to examine the C saturation concept. If C sequestration is governed by the saturation processes, C content in whole-soil and each soil fraction would reach an asymptote as C input increases. We used soil size fractionation through wet-sieving and C analysis to test our hypothesis. Analysis of the relationship between soil C input and soil C content showed that as cumulative C input to soil increases, the C content in whole-soil, macroaggregates and microaggregates increases. Because both linear and curvilinear regression models explained the relationships between the soil C input and C content, it remains to be determined whether C sequestration dynamics are influenced by the saturation processes in these soil fractions. However, regression analysis showed that the C content in silt + clay fraction does not increase (soil C content (g C/ kg soil fraction) = 0.01(cumulative C input (Mg C ha^-1) + 7.6, r² = 0.15) even when cumulative C input increased from 75 to 175 Mg C ha^-1, indicating that C may be saturated in this soil pool. Therefore, it is likely that C saturation determined by the physicochemical properties of soil is more obvious in soil pools with slower C turnover rates.