The impact of charcoal and soil mixtures on decomposition and soil microbial communities in boreal forest

• Six humus and 9 charcoal types were incubated singly and in all two way mixtures.
• More mass was lost when humus and charcoal were in mixture than when separate.
• Enhanced mass loss in mixture was due to humus rather than charcoal mass loss.
• Magnitude of increased mass loss depended on humus types but not on charcoal type.

Fire is a natural disturbance that operates as a major ecological driver in many ecosystems worldwide, and it produces charcoal which is incorporated into soil in significant quantities. Charcoal can serve as a long-term carbon (C) sink, but it is not inert, and could potentially impact native soil organic matter and decomposer micro-organisms. However, studies have shown contrasting results for how charcoal impacts the belowground subsystem, and the mechanisms involved are poorly understood, especially in pyrogenic ecosystems. We performed a laboratory experiment in which six contrasting boreal forest soil types and nine charcoal types (each from different woody plant species) were incubated for 9.5 months, both by themselves and in 50:50 mixtures for all possible soil–charcoal combinations. At harvest we measured mass loss, and for several charcoal-soil combinations, we measured microbial properties, and composition of C compounds using 13C CP-MAS nuclear magnetic resonance (NMR) spectroscopy. Overall, mixtures of charcoal and soil lost more mass than expected based on when the components were incubated separately. The magnitude of increased soil mass loss in mixtures did not differ among charcoal types, but varied among soil types, because greater mass loss occurred when soil from a site dominated with herbaceous vegetation was used, relative to other soil types. The use of NMR spectroscopy showed that enhanced mass loss in mixtures was due mainly to mass loss of soil organic matter rather than charcoal. However, mixing of charcoal and soil did not influence key decomposer microbial groups compared with expected values derived from when components were incubated alone, irrespective of charcoal and soil type. This study shows that when charcoal is incorporated into boreal forest soil (e.g., after wildfire), there is enhanced loss of total C (arising primarily from mass loss of soil organic matter), with this effect being relatively consistent across contrasting charcoal and soil types. This effect, in combination with recently documented impacts of charcoal on aboveground processes, reveals important but largely overlooked legacy effects of charcoal on forest processes that contribute to ecosystem C balance and ecosystem functioning.