The object of the present work was to analyse the degradation dynamics of four chars and a digestate applied to a sandy soil as well as possible initial priming effects on the mineralisation of soil organic carbon in a two-year field experiment. For that purpose, soil carbon content, soil respiration and the corresponding carbon isotopic abundances were repeatedly measured throughout two consecutive vegetation periods. In order to quantify and separate the amount and the degradation of the substrate-derived carbon and to assess soil priming effects, isotopic mixing models were applied to soil-derived and substrate-derived carbon, and to the respired CO₂.
Pyrolysis char was degraded with decreasing intensity over time with an estimated half-life of about 80 years. HTC (HydroThermal Carbonisation) char showed a high degradation during the first year but, during the second year, the remaining recalcitrant pool was degraded much slower with a half-life between 49 and 61 years. Digestate was degraded at a constant intensity with a half-life of about 14 years. When the chars were fermented before being applied to the soil, the initial degradation of HTC char was reduced, but on the two-year scale, the degradation of both chars was higher than for untreated chars, yielding a half-life between 11 and 15 years, comparable to digestate. The results showed considerable stability of the untreated pyrolysis and HTC chars under field conditions, and moreover, no net influence of chars as well as of digestate on the degradation of soil organic carbon after two years.