Increasing soil organic matter (SOM) is a major factor in overcoming soil degradation. An incubation experiment using 2 soil types (Red Clay and Black Earth) and 2 different rotations, a clover (Trifolium subterraneum)/cereal rotation and a long fallow/cereal rotation, from a long-term crop rotation trial located at Tamworth, NSW, Australia was conducted to investigate the decomposition of 3 different plant materials, medic (Medicago truncatula) (C : N = 13), rice straw (Oryza sativa) (C : N = 25) and flemingia leaf (Flemingia macrophylla) (C : N = 13), labelled with 13 C and 15 N. A control treatment with no added residue was also included. The impact of the residue decomposition on total organic carbon, labile carbon, total nitrogen, aggregate stability and the formation of large macro-aggregates from smaller macro-aggregates were studied. Total C (C T), stable carbon isotope composition (δ 13 C), total N (N T), and % 15 N excess were measured by catalytic combustion and an isotope ratio mass spectrophotometer, while labile C (C L)was determined by oxidation with KMnO 4. Aggregate stability [mean weight diameter (MWD)] was determined by immersion wet sieving. Correlations of C fractions with MWD were also investigated. The location of the newly added plant residue materials within soil aggregates was studied using a soil aggregate eroding machine. Loss of C from the added plant residues was highest for the medic and lowest for the flemingia, while the rice straw initially lost C at a slower rate but by 200 days was equal to the medic. The medic treatment was the only residue to lose N by gaseous loss during the experiment and it was all lost during the first 10 days. In both soils, the addition of residues increased C T and C L compared with the control treatment, with flemingia showing the greatest increase. Factors other than their C : N ratio were clearly determining C turnover.