Many soils show a linear relationship between C input and soil organic matter (SOM) content, but according to the soil C saturation concept, the amount of SOM that can be stabilized in soils is limited. While several studies have found a saturating relationship between C inputs and SOM concentration, the concept still needs further corroboration.

A key requirement for testing soil C saturation behavior is to use soils that are near steady-state in terms of organic C concentration, and have been subjected to differing amounts of organic C inputs over the long-term. SOM stabilization by fine soil particles may be limited by finite mineral surface area, and thus mineral-associated C pool is the most likely to exhibit saturation. Our hypothesis was that as organic C inputs increase, SOM concentrations in fine soil particles will rise to a maximal level. We tested this hypothesis by measuring the amount and relative stability of SOM in the fine soil fractions isolated from soils receiving differing amounts of organic C inputs.

Soils were collected from several long-term experimental sites, each consisting of a non-amended treatment and at least two organic C amendment rates. Soils were fractionated by size and density to isolate organo-mineral complexes whose organic C loading (mg C m^-2 mineral surface area) was determined as a proxy for C saturation. The mineral phase of the complexes was characterized by XRD, and determination of CEC and Fe-oxide concentrations. The relative stability of SOM in the organo-mineral complexes was characterized by laboratory incubation, thermal analysis and further chemical fractionation.

Preliminary results indicate that the mineral-associated C pool does saturate and leads to a mixture of relatively stable and relatively unstable C associated with the minerals at saturation.