An incubation study on the stability and biological effects of pyrogenic and hydrothermal biochar in two soils

The success and feasibility of CO2-sequestration through incorporation of biochar into soils depends strongly on the long-term biochar stability and on the improvements of physical and microbial soil properties. In this study, the stability of two maize-derived biochars (from pyrolysis and hydrothermal carbonization) and of a compost-biochar mixture and their effects on microbial biomass and enzyme activity were determined in two soils during a 57-day incubation. Soil samples amended with biochar increased soil organic carbon (SOC) content by 20 or 40%. Samples amended with hydrothermal biochar showed the largest respiration rates and the largest increase in microbial and enzymatic activity compared with the untreated controls. Carbon and C-13 mass balances showed that between 13 and 16% of the added hydrochar was mineralized within 8 weeks. In the arable soil, hydrochar additions greatly stimulated the degradation of SOC, thus inducing positive priming effects. The mineralization of pyrogenic biochar (pyrochar and a pyrochar-compost mixture) was significantly less (1.4-3%) and comparable to the SOC mineralization in the control soils.