Hydrothermal carbonization and dry pyrolysis transform sewage sludge (SS) into nitrogen‐ (N) and phosphorus‐ (P) rich hydrochars (Hyd) and pyrochars (Py), respectively, which may act as slow‐release fertilizers with carbon (C) sequestration potential. Whereas this has been mostly studied with short‐term experiments, this study focused on the cycling of char‐derived N, P and C after ageing during a second grass cropping cycle. Lolium perenne was grown for 3 months in pots on soil mixed with 13C and 15N‐enriched SS, Hyd or Py and allowed to age during a first cropping period of 10‐month incubation. The δ15N of the plants confirmed that even during the second cropping, N derived from the amendments was plant accessible. Higher uptake of N from Hyd than from Py is explained by the lower biodegradability of the latter. Plant growth during the second cropping period was associated with a decrease of total P in all treatments, but only the soils with Hyd and Py evidenced an increase of Olsen P. Thus, during the second cropping, more insoluble P was mobilized from the carbonized residues than P needed for plant growth. Compared to control soils prepared with and without KNO3, higher biomass production was yielded with the amended soils. Hyd proved to have the highest longer‐term N mobilization potential. Following the change in δ13C of the soil, we observed that during the second incubation, independently of their aromaticity, all amendments and the native soil organic matter had comparable turnover rates, although the amount of organic matter with slower turnover added with the amendment increased with aromaticity. A rough estimation of the impact of thermal treatment of SS on its C‐sequestration potential revealed no major differences between char types. The higher fertilization capacity of Hyd, however, indicates that it is a good candidate for soil amendment as long‐term fertilization is combined with a long‐term increase of the SOC pool. Highlights Medium‐term fertilization and C‐sequestration potential of chars from sewage sludge were tested. Hydrochar (Hyd) and pyrochar (Py) provide N and P for plant growth during a second cropping period. Compared to soils with and without KNO3, Hyd and Py increase plant productivity of the second crop. On a long‐term scale, native SOM, amended SS, its aged Hyd and Py show comparable turnover rates.