When a fungus dies, other organisms decompose it. The assumption is that dead fungal tissue presents a single challenge — a set of chemical structures that are either easy or hard to break down. Recalcitrance, the resistance of organic matter to decomposition, is typically treated as a property of the substrate.
Narayanan and colleagues tested 52 bacterial and 83 fungal strains on fungal necromass and found that recalcitrance is not a property of the substrate. It is a property of the interaction. Fungal decomposers are blocked primarily by melanin — the dark pigment that reinforces fungal cell walls. Bacterial decomposers are blocked primarily by cell wall polysaccharides — the structural sugars that melanin is embedded in. Both groups converge on the same resource. Each is stopped by a different component of it.
The finding reframes carbon cycling models. If recalcitrance were universal, the rate of decomposition would depend only on the chemistry of the dead material. But if different decomposers face different barriers, then the rate depends on who shows up. A melanin-rich necromass that resists fungal decomposition might decompose quickly if the right bacteria are present. A polysaccharide-heavy necromass that bacteria struggle with might be cleared efficiently by fungi. The same material decomposes at different rates depending on the composition of the decomposer community — not just the composition of the dead.
This is a measurement problem disguised as a chemistry problem. Recalcitrance measured with one class of decomposer tells you about the interaction, not the substrate. Generalize from that measurement and you get the wrong carbon budget.