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The decaying mosses release humic acids and sphagnan, a complex sugar, that make life difficult for the microorganisms that would normally cause rotting and decay.

All of it has darkened somewhat because of the development of humic acids.

Organic-rich soils seem particularly problematic; they are ripe with molecules like humic acid, which behaves like DNA and can foul later sequencing.

Other ‘humic’ organics are released from soil and coal.

When a plant dies, decay brings organic material, a component of which is large carbon-based molecules called humic acids, into the soil and binds them to the soil’s molecules.