Soil organic carbon storage in four irrigated cotton cropping systems sown on permanent beds in a cracking clay with subsoil sodicity was determined over a 9-year period in a trial near Narrabri, NSW. The treatments were: cotton-cotton (CC), cotton-vetch (CV), cotton-wheat where wheat stubble was incorporated (CW), and cotton-wheat -vetch where wheat stubble was retained as standing stubble (CVW). Vetch was terminated during or just prior to flowering by a combination of mowing and contact herbicides, and the residues retained as mulch. Average carbon storage in the 0-0.3 m and 0-1.2 m depths was higher when vetch was part of the rotation with similar values occurring in CWV and CV. On average, cropping systems that included vetch stored 2.3 t C/ha more in the 0-0.3 m depth and 5.3 t C/ha more in the 0- 1.2 m depth than those that did not. These differences correspond to inputs of nutrient-rich biomass. Net carbon sequestration rates did not differ among cropping systems and did not change significantly with time in the 0-0.3 m depth but net losses occurred in the 0-1.2 m depth.