The biodegradability of chars derived from pyrolysis and hydrothermal carbonisation (HTC) was studied in short-term dynamic incubation experiments under controlled conditions. Carbon dioxide C (CO2) emissions from soil-char mixtures in combination with solid digestate or mineral nitrogen (N) fertiliser were measured in dynamic chambers for 10 d. Compared to the original material (maize straw), pyrolysis and HTC chars showed significantly lower CO2 emissions and slower decay dynamics; and compared to the soil control, HTC char increased soil respiration to a significant extent, while pyrolysis char did not. The addition of mineral N resulted in a delayed respiration dynamics for HTC char, while the addition of digestate resulted in an increase in the respired CO2 for pyrolysis char and a decrease for HTC char. For the first time, a peculiar two-stage decay kinetics was observed for HTC char, indicating a highly inhomogeneous substrate consisting at least of two C pools.