Reclamation of intensively managed soils in temperate regions by addition of wood bottom ash containing charcoal: SOM composition and microbial functional diversity

•Wood bottom ash contains charcoal with a low degree of condensation.•Microbial biomass and functional diversity were increased by ash addition.•Wood ash was useful for restoring soil fertility and the C-sequestration capacity.•Soil microorganisms utilized the charcoal, showing that it was not stable.