Forest and grass riparian buffer systems provide year-round soil cover, limiting erosion, and favoring soil development processes by potentially increasing soil C sequestration. Plant-soil interactions influence patterns of soil aggregation and organic matter storage. And have a major positive impact on the soil ecological functions that maintain and enhance both water and environmental quality. In this dissertation a new theoretical and experimental framework is presented that introduces the concept and determination of aggregate size-stability distribution. In addition, two new indexes, the soil stability index and the total soil stability index, both based on aggregate, are proposed for studying soil stability. Finally, the soil aggregates dynamics model, that integrates the aggregation, disruption, stabilization, and destabilization processes of soil aggregates, is developed for studying soil aggregate dynamics.;The size-stability distribution and the soil aggregate dynamics model were used to assess aggrading and degrading processes that occur in riparian soils. Pooled data from 1997 and 1998 showed that the major soil process following conversion of cool-season grass to agricultural row crops is disruption, with 19% of the large and small macroaggregates being disrupted. This disruption of macroaggregates exposes previously protected labile organic carbon to decomposers, resulting in a loss of 11.3 mg C g-1 soil and further destabilizing the macroaggregates. The amount of total particulate organic C was three times greater under cool-season grass than under cropped system and accounted for 16% of the total organic carbon under and 7% under cropped system. The results indicate that macroaggregates under cool-season grass are more stable and provide an important mechanism for C sequestration supporting higher amounts of both light and heavy particulate organic matter than cropped system. Additional results indicate that the "net" soil aggregate process in a 7-year old switchgrass stand that was converted from a cropped system is aggregation, which yields 3% new unstable macroaggregates. The storage of soil organic C under switchgrass occurs at a rate of ~43 g m-2 y-1.