Stewart, C. E. 2006. Soil carbon saturation: A new model of soil organic matter stabilization and turnover. Ph.D. Dissertation, Colorado State University, Ft. Collins, Colorado, USA.

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The soil C saturation concept suggests an ultimate capacity of the soil to store C, dictating the rate and duration that soil may be effective in mitigating increasing atmospheric CO 2 . This places a physicochemical limit on soil that is associated with textural, mineralogical and structural soil properties. This concept has been articulated in terms of four theoretical pools capable of C saturation: non-protected, physically-(microaggregate), chemically- (silt + clay), and biochemically-protected pools. My dissertation represents a multifaceted approach to examine C saturation in both whole soil and measurable soil fractions representing the four conceptual C pools. I evaluate the soil C saturation concept theoretically by modeling these relationships using published whole soil data, primary field data and through laboratory experiments.

Analyses using published long-term soil C data from agroecosystem experiments suggested that within a given site, there was little support for models including C saturation, but when all sites were combined; there was strong support for the C saturation model. In general, published data were too sparse to adequately test individual sites.

To evaluate the concept of C saturation for the four C pools, I used a three-part density, chemical, and physical fractionation scheme combined with modeling, using new data collected from eight agroecosystems in the US and Canada. I found that the chemically- and biochemically-protected pools showed strong evidence for C saturation, while the non-protected and physically-protected pools were non-saturating.

In a 2.5 year laboratory experiment, I tested C stabilization rates and limits at two C addition rate to soils differing in soil C content and physicochemical characteristics. I found C saturation dynamics were most evident in the chemically-, biochemically- and some microaggregate protected C pools. I found greater C accumulation in the non-protected pool of the high C soil, suggesting C saturation of other pools.

I conclude that SOC sequestration in many soils may be influenced by C saturation dynamics, impacting both decomposition kinetics and C stabilization. Soil C sequestration may be overestimated in models that do not account for C saturation dynamics.

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