Soil organic C is an important soil property having implications for nutrient cycling, soil water dynamics, and soil physical structure. A field study was initiated in 2002 to determine if quality and placement of C affected soil organic C and soil aggregation. Sources of C included none, sugar, flour (starch), paper (high C:N), wood chips (lignified cellulose), alfalfa pellets (low C:N), wheat, and switchgrass. Wheat and switchgrass treatments consisted of three plots each with C inputs either from aboveground residue (fallow until plant harvest), roots (aboveground residue removed at harvest and placed on fallow plot), or aboveground residue and roots. During 2005 soil respiration, soil microbial biomass, and labile C were measured four times. In 2002 and 2005 aggregate size distribution and aggregate stability were measured. In plots having no vegetation there was a decrease in 2- to 8-mm diameter aggregates and in increase in aggregates in smaller size fractions. Where wheat or swithgrass was present there was no change in aggregate size distribution. Soil respiration was related to soil microbial biomass and labile C in wheat and swithgrass plots but not in plots receiving other C sources. Preliminary conclusions are that physical protection of the soil surface from physical degradation and erosion is essential for maintaining SOC and aggregation. Physical protection is more important than C quality.