||Switchgrass (Panicum virgatum L.), a potential biofuel crop, can sequester soil organic carbon (SOC) and improve soil quality. However, its influence on soil aggregate mechanical properties controlling the macro-scale behavior of the whole soil needs to be assessed to understand processes that affect soil quality. This study assessed the impact of long-term (> 10 years) switchgrass, row crop, cool season grass pasture, and forest management on properties of soil aggregates for five ecosystems in the southeastern United States, including Blacksburg and Orange (VA), Knoxville (TN), Morgantown (WV), and Raleigh (NC). Relationships among aggregate properties were also determined. Tensile strength (TS), bulk density (p,g,), soil moisture retention (SMR), and SOC concentration of 1- to 8-mm aggregates were determined at the 0- to 10-cm and 10- to 20-cm soil depths. Management significantly affected the aggregate properties (P < 0.05), but the magnitude of the effects was site-dependent. The TS for switchgrass was the lowest (similar to 271 kPa) at all but the Blacksburg site for the 0- to 10-cm depth. The rho(agg) for switchgrass was 10% lower at Blacksburg and 20% lower at Orange than that for row crop at the 0- to 10-cm depth. The SOC concentration for switchgrass was 2.5 times higher than that for row crop at Orange but not at Blacksburg. The TS increased with increasing rho(agg) at Morgantown and Raleigh, but it decreased with increasing aggregate size at all sites. Aggregate size, rho(agg), and SOC were significant predictors of TS. Longterm switchgrass systems in the southeastern United States improve the aggregate strength properties, unlike row crop and cool season grass pastures, but their impact on SOC concentration is variable.