Drainage and land-use are key factors that trigger CO2 emissions from cultivated organic soils. Drainage enhances aeration, and land-use might affect peat decomposition due to the input of fresh organic matter (FOM). The effect of FOM addition on peat decomposition of agriculturally used organic soils has seldom been quantified experimentally. In this study, we i) incubated soil samples taken from three adjacent former peatland sites that were drained and managed as cropland, perennial grassland and forest, respectively, and measured CO2 release over 16 months at 20 °C. In a second experiment, ii) we incubated soil samples from the same three sites with and without adding corn straw as FOM (2% w/w) at 20 °C over three weeks. The 13C and 14C signatures of soil organic carbon (SOC) and emitted CO2 enabled us to apportion the amount of decomposed corn, as well as to estimate relative effects of corn addition on the decomposition of SOC from old peat (SOCold) and from young soil organic carbon (SOCyoung). In the first experiment, samples lost between 0.03 and 0.09 mg CO2-C mg SOC−1 in the order forest > grassland > cropland. The higher 14C values of the emitted CO2 vs. that of SOC indicated that SOCyoung was more easily decomposable. FOM addition induced negative (−14.8 ± 5.2%), positive (+14.9 ± 4.6%) and neutral priming (−14.2 ± 30.0%) of SOC decomposition in the forest, grassland and cropland samples, respectively. Most importantly, we find that the relative contribution of SOCold to the overall CO2 release consistently decreased after FOM application, whereas decomposition of SOCyoung was rather stimulated. The latter finding is in line with previous studies on intact peat. Our results show that young and old C pools in managed organic soils respond differently to the addition of fresh plant residues, and FOM addition can effectively reduce the decomposition of old peat. Hitherto negative priming was never reported for agriculturally used organic soils and it might be caused by the overall poor decomposability of old peat.