Biochar as an additive in composting: impact on process performance and on the agronomical quality of the end product

There is growing interest on the use of biochar in composting and also on its effect as growing media constituent. This study addresses the main effects of the use of biochar as an additive in organic waste composting in terms of the enhancement of the process and the quality and agronomical value of the end product. Biochar presents favorable physicochemical properties (such as large porosity, surface area and cation exchange capacity) that can enhance the environmental conditions for microbial growth in the composting piles and, consequently, affect key microbially-mediated processes such as organic matter degradation and nutrient cycling. Key impacts of biochar on composting performance, compost maturity and stability are reviewed, with special attention to those agronomical aspects related to organic matter humification, reduction of N losses and nutrient bioavailability. The interaction of biochar and compost as growing media constituents is also evaluated.