The input of organic fertilizers into soils is an interesting option as a substitution for mineral fertilization, but how their interaction with crop residues affects the fate of added carbon (C) and nitrogen (N) in the soil is still poorly known. Therefore, we analysed the effect of adding together organic fertilizer and straw on subsequent C and N mineralization. We incubated sugarcane straw (S), pig slurry (PS) and solid sewage sludge (DS) separately and in mixtures (PS-S and DS-S) at 28 degrees C during 182 days. To discuss interactions, we used a simple additivity model based on measurements and a mechanistic model for C and N transformations in soil (CANTIS). Both models overestimated the C mineralization and did not correctly predict N mineralization of the two mixtures. The differences between observed and expected values calculated with the models were negative for C mineralization, indicating an antagonistic interaction in mixtures. The limitations for C decomposition might be the result of many factors, such as negative priming effect or limitation in N accessibility, which are not considered by CANTIS. We assumed that the priming effect induced by the mineralization of a mixture was not significantly different from the priming effect induced by the mineralization of the organic matters incubated alone. The use of a contact factor in CANTIS allowed the predicted C and N kinetics for the mixtures to be correctly fitted to measured data. It reflects the effect of fine-scale C and N distribution heterogeneities on the intensity of microbial decomposition. A better integration of the interactions between different N and C sources should be addressed to develop modelling as an accurate tool for agroecosystem management. Highlights Experimental data and modelling allowed assessment of effects of mixtures on C and N mineralization. Mineralization of the organic material mixture was not the sum of the mineralization of each material. A contact factor reflecting the N-rich substrate accessibility was needed to predict C and N data. Pig slurry and sewage sludge mixed with straw showed antagonistic effects on mineralization.