The silt and clay particles play a key role as stabilizing agents of soil organic carbon (SOC). Several lines of evidence indicate a theoretical maximum or C saturation in individual particles. In the present study, we hypothesized that a C fraction displaying linear accumulation relative to the SOC is not influenced by C saturation, while a fraction displaying an asymptotic relationship is regarded as saturated (Stewart et al., 2008). The aim of the present study was to compare the amount of C in the silt and clay sized fractions in temperate and subtropical cropping soils across a range of textures with different mineralogy. Twenty-one and 18 soil samples containing 1:1 and 2:1 clay of temperate soil from Chile under monoculture of maize (Zea maiz L.) for at least 30 years and 9 subtropical soils from Mexico under maize and bean (Phaseolus vulgaris L.) cropping for 9 years having mixed clay were collected at 0-0.1 m. The SOC of 2:1 soils was significantly higher (14±0.5 g kg-1 dry soil) than 1:1 soils (10±0.7 g kg-1). However, subtropical soils showed the highest values (59±0.5 g kg-1). A positive (P < 0.01) relationship was observed between the SOC and the C in the silt fraction (R2 0.80-0.97, P < 0.01). In contrast, the clay fraction remained constant or showed asymptotic behavior. We conclude that the silt fraction, unlike clay, showed no evidence of C saturation, while clay accumulates C to a maximum. On average, the 2:1 clay was saturated at 1-2 g C kg-1 and 1:1 at 1 g C kg-1, and subtropical soils at 14 g C kg-1
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