Some gasses will be consumed by plants and fungi. Granted, methane won't be one of them. But CO2 will. Now, when compost is dumped on barren soil, there is nothing to consume the gasses.
A modern landfill where the gas is burned and used as energy source is way way better at reducing green house gases than fungi and microorganisms. The C02 wont go away but most of the rest way worse greenhouse gases.