How is Humic substance destroyed in nature?
Sunlight is aggressive, as is microbiology.  Ozone, hydroxyl radicals (including Fenton chemistries with peroxides), air pollution in general eventually degrade and destroy near anything organic.  Examine sediments downstream of rivers, including outflows, especially for heavily colored waters.  Sampling vs. linear distance and depth will give you histories of degradative chemistries and biologies.  Note that phenolics are overall good microbiocides re lignin.

Terra preta soils demonstrate the survivability of charcoal.  Humic substances are more functionalized and more reactive.  One can start at the other end, too - weathering of coal.  Coal contains dispersed iron pyrite and other sulfide minerals that speed degradation.  Powdered coal heaps can self-ignite from cumulative oxidation heat.  Carbon dioxide, water, and elemental nitrogen are deep thermodynamic holes.