The addition of organic wastes is being used increasingly as a suitable method for the recovery of degraded semiarid soils. This, in turn, can contribute to the disposal of considerable amounts of these wastes. The aim of this work was to study how the application rate and maturity of the added organic waste influence the dynamics of the organic carbon (Corg) and its fixation and partitioning in different soil particle‐size fractions. For this, a semiarid degraded soil (S) was amended, at rates of 1.1 (d1) and 3.3 (d2) g Corg 100 g soil−1, with a mixture of organic domestic waste and aerobic sewage sludge at two different maturities (fresh, FR; composted, MC) and incubated for 9 months under controlled conditions. The association of Corg with different soil particle‐size fractions was determined at the start and end of the incubation period. The organic C mineralization rates were greatest in soils amended at the larger rate. Even so, after 9 months of incubation, the soils amended at the greater rate had significantly (P org than the soils amended at the smaller rate. The greater rate of addition of composted material resulted in the greatest C fixation, although the efficiency of fixation was greatest with the smaller rate; composted material was better than fresh material and the C was associated increasingly with the finer fractions as incubation progressed.