Stability of Biochar Series in Soils and Induced Priming Effects.

Persistence in soil is a key quality of biochar products when considering carbon sequestration application. Feedstock and pyrolysis conditions are known to strongly influence biochar properties.

Here we aimed at assessing the stability in soil of a biochar series as a function of feedstock and pyrolysis conditions.  Further we aimed at assessing effects of biochar history and application rate on mineralization of biochar and priming of SOC.

We tested a series of corncob and miscanthus biochars produced with hydrothermal carbonization (HTC) and 250-800°C slow pyrolysis. The evolution of CO₂ and its ^₁₃C signature was monitored in three incubation studies.  A one-year incubation was conducted with non-pyrolized biomass, HTC products, and eleven slow pyrolysis biochars added to soil at a rate of 30 t ha^-1.  A shorter incubation was performed using low and high temperature biochars with application rates between 6 and 150 t ha^-1.  Biochar was also added to soils already containing low and high-temperature biochars for one year. 

It was found that after 350 days, only 0.3 – 0.6% of the total biochar carbon was mineralized from slow pyrolysis biochars prepared above 300^oC.  In comparison, very low-temperature slow pyrolysis biochar and HTC products lost 10 to 15% of the total C4-carbon, and non-pyrolized biomass lost 30 – 40% of its carbon.  Approximately 3% of SOC was mineralized in control soils while addition of high-temperature biochars increased this mineralization up to 5% of the total SOC. 

In conclusion, biochar mineralization is much slower than that of native soil carbon, with the exception of low-temperature biochars.  In some cases, the mineralization rate of native soil carbon is increased with the addition of pyrolyzed biomass, it some cases it was not. Conditions for triggering priming effects will be discussed.