The main objective of this study is to identify and characterize functional SOM fractions that correspond to the "intermediate" turnover pool in SOM models, i.e. the portion of organic matter which is mostly affected by management. We hypothesized that this organic matter is largely comprised of plant-derived and microbial-derived products which are partially protected from decomposition through their association with soil aggregates. We isolated, characterized and quantified soil organic matter (SOM) fractions that accounted for much of the difference in soil C and N observed between native ecosystems (grassland and forest) and no-till and conventionally tilled agroecosystems, at four sites across the US. Our investigation of soil organic matter fractions, using physically-based fractionation procedures and stable isotope methods, suggested that decreased soil aggregate turnover (due to the absence of soil disturbance by tillage) led to an increased stabilization of C within microaggregate structures under no-till compared to conventionally tilled systems. Results from one site having a more highly weathered soil, suggested that the interrelationship between aggregate and SOM dynamics varies as a function of clay mineralogy. We developed a model linking aggregate and SOM dynamics, soil disturbance and soil mineralogy - deduced from the organic matter analyses for the different sites and management treatments. Funded by the National Science Foundation.

Macroaggregate structure (center): fungi (top right), bacteria (bottom right), microaggregates (top left) and particulate organic matter (bottom left)