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Wood biochar (6%, 12% and 18% of fresh sludge weight) adding to a sludge-and-straw composting system was investigated to assess the potential of biochar as a composting amendment. Organic degradation efficiency, temporal humification profile of the water-extractable organic fraction and solid organic matter, through spectroscopic, microscopic and elementary analysis were monitored. Fluorescent excitation and emission matrix indicated that concentrations of aqueous fulvic-acid-like and humic-acid-like compounds were, respectively, 13–26% and 15–30% higher in the biochar-amended treatments, than those in the control without biochar-amended. On the first day of sludge aerobic incubation, the presence of biochar resulted in increased oxygen uptake rates of 21–37% due to its higher nano-porosity and surface area. SEM indicated that, in the biochar-amended sludge, the dense microstructure on the …