Carbon isotope natural abundance (δ¹³C) of soil organic matter (SOM) is used to elucidate plant source (C₃ versus C₄ plant metabolism) and relative age of soil carbon, assuming that soil carbon is largely derived from fixed plant carbon. We determined the (δ¹³C signature of SOM to investigate the proportion of recently deposited C (<20 yr) in water-stable aggregate fractions of a Maury silt loam (Typic Paleudalfs) in three long-term field experiments. The experiments contained tillage, nitrogen, manure and crop rotation treatments. Crops include both C₄ and C₃ species, including corn (Zea mays L.), soybean (Glycine max L. Merrill) and wheat (Triticum aestivum L.). All plot areas had previously been in bluegrass (C₃) (Poa pratensis L.) sod. Field-sieved (8 mm) air-dry soil samples from the 0 to 10 cm depth increment were wet sieved to obtain a range of aggregate sizes. Measurements of ¹²C and ¹³C were performed on each water-stable aggregate size fraction, allowing calculation of the proportion of aggregate C derived from C₃ and C₄ plants. The mass distribution of soil among water-stable aggregate size fractions and the proportion of recently deposited C were strongly affected by management, especially tillage and crop rotation. No-tillage and continuous corn enhanced abundance of larger aggregate sizes and the proportion of recently deposited C. Across the three experiments, the small macro-aggregate fraction (0.25 to 2.00 mm) was the most abundant size class. Nevertheless, no aggregate size fraction was uniquely sensitive in terms of changes in SOM due to management. The proportion of C₄-derived C was generally positively related to aggregate size in two experiments. The persistence of SOM was negatively related to aggregate size.