Composts are often applied on arable soils to sustain the content in soil organic matter (SOM). The mechanisms leading to an increase of SOM content are still poorly understood. The changes of OM during the thermophilic phase might be different from the changes naturally occurring in soils, which might influence the dynamics of composted OM compared to non composted OM in soils. Our objectives were to better understand the effect of composting on the dynamics of C and N in soils and to investigate the relationships with the chemical composition. To this aim, we used 13C and 15N labelled plant materials composted (12 weeks) or not. Composted (COM) and noncomposted (NCOM) plant materials were then incubated with arable soils during 3 years under controlled condition (20°C, 80%WHC). Throughout the experiment, 13C and 15N concentrations were analysed in the mineralised fraction (CO2 or nitrates) and the microbial biomass (fumigation extraction technique) using GC- or EA-IRMS. The composting processes reduce the C/N ratio from 11 to 7 (loss of 69% of C and 21% of N). Molecular analysis of plant residues performed by GC-MS showed that COM was depleted in non-cellulosic carbohydrates and enriched in lignins compared to NCOM. The C mineralisation of NCOM was higher than COM and 3 years after the amendments in soil, the loss of C due to composting process were compensated. The N mineralisation of NCOM was lower than COM during the first year, due to higher amount of N assimilated to the microbial biomass.