The characteristics of soil organic matter (SOM) such as stocks, sources and stability, are important for understanding soil functioning and C cycling in an eroding field. In this study we investigated the characteristics of SOM in bulk soil and aggregates collected from three positions (shoulder, back, and foot) on a sloping field in Northeast China using stable and radioactive isotope (¹³C and ¹⁴C) and thermogravimetry (TG) techniques. The field had originally been populated with C₃ grassland plants before conversion to C₄ corn dominated arable cropping approximately 80 years ago. Soil samples were collected from 0–120 cm soil profiles in 20 cm depth intervals. Erosion rates were estimated by ¹³⁷Cs analysis to be 0.21 cm yr^–1 and 0.08 cm yr^–1 at the shoulder and back positions, respectively. A greater proportion of small macroaggregates (2–0.25 mm) and microaggregates (0.25–0.053 mm) were found in the topsoil and subsoil layers at the foot position compared with the shoulder position. The concentration and storage of soil organic C (SOC) in the bulk soil was significantly lower at the shoulder and back positions compared with the foot position. Greater thermal-labile SOM concentration and storage were observed at the foot position compared with the shoulder and back positions. Positive relationships between C₄-derived SOC and the proportion of thermal-labile SOM suggested that most thermal-labile SOM was C₄-derived from the recent corn (young SOC), while most stable SOM fractions were C₃-derived from previous grassland vegetation (old SOC). However, labile SOM from old SOC was also an important part of total thermal-labile SOM, particularly in deep soil horizons (below 40 cm) at all sampling positions. Small macroaggregates had a greater SOC content, δ¹³C, and thermal-labile SOM proportion, and younger SOC age, compared with microaggregates and silt + clay (<0.053 mm) fractions, except at the 40–60 cm depth at the foot position. As the mass of aggregates accounted for 32–50% of the bulk soil, about half of thermal-labile and total SOC were stored as associations with silt + clay fractions at erosional and depositional points. Our results indicated that: (1) more thermal-labile and total SOM were stored at depositional position, (2) compared with new carbon sources, old SOM contributed to a large proportion of thermal-labile SOM on an eroding slope, particularly in deep soil, and, (3) soil aggregates and silt + clay fractions had equivalent roles in SOC sequestration in both erosional and depositional locations.