Effect of biochar addition to compost on biological stability of the mixture

Application of biochar, a solid product produced from biomass pyrolysis under low oxygen conditions, has been suggested as a low emission technology capable of increasing soil C sequestration to mitigate climate change. Its combined application with compost may be a promising avenue to ameliorate soil quality while increasing C sequestration. We hypothesized that biochar addition to compost reduces the mineralization of the mixture compared to compost alone. The study aimed to compare the mineralization rate of six biochar-compost mixtures differentiated by biochar feedstocks. Biochars were produced at temperatures ranging from 450 to 650°C for 10 minutes. Our conceptual approach included incubation of fresh and artificially aged biochar-compost mixtures. Physical ageing of the mixtures was performed with successive cycles of humidification/drying and freezing/thawing. We evaluated elemental composition and biological stability of the fresh and aged mixtures after incubation with a soil inoculum for 1 year. We monitored components of biochar-compost mixtures decomposition when biochar were produced from C4 feedstock by determination of the ¹³C signature of emitted CO₂.

Combination of compost with biochar induced synergistic effects in terms of the mixtures stability. Isotopic analyses showed that carbon mineralization from compost was greatly reduced, while biochar mineralization was increased. Physical ageing induced a strong leaching of water-soluble compounds of both substrates. Carbon mineralization of aged material was however not reduced as much as expected when comparing with mineralization rates of single compounds of the mixture. Furthermore, isotopic signatures showed that compost, when amended with biochar, mineralized better after ageing. We thus suggest that the water-soluble fraction of biochar may induce an inhibitive effect on the mineralization of compost. The intensity of this effect seems to be dependent upon the feedstock of the biochar in the mixture.

We conclude that biochar addition to compost may reduce the mineralization of the mixture depending on biochar feedstock and that this effect may be alleviated after ageing.