The edible white button mushroom (also known as ‘champignon’; Agaricus bisporus) is one of the most produced mushrooms in the world. The industrial substrate for these ‘champignons’ is based on low-value starting material, such as wheat straw mixed with horse manure, which undergo a series of bio-transformation phases. During a short microbial composting phase, a dark film develops that is layering the wheat straw, which was isolated and analysed. This dark film is rich in humic substances and partly composed of lignin-nitrogen complexes, and disappears when the fungus starts to grow. Furthermore, substantial lignin (i.e. from the wheat straw) degradation was revealed, both during the microbial composting stage, and during fungal growth. Excitingly, we found that A. bisporus not only degrades lignin, but has also the ability to utilize (13C-labelled) lignin as carbon and nutrient source.