This depends on the landfill conditions. Effective landfill management requires the waste to be stored as solids or else the landfill will collapse, so the correct sealing of landfills is key to this. Keeping moisture, air and microbes out of a landfill is a challenge. If air is present, and the temperatures and moisture levels are right, natural waste in a landfill is likely to be consumed by microbes, and this includes PLA.
As part of this natural consumption process, methane will be produced and this will increase if more air is added. This is normal, it is the decomposition process at work. The rate at which this happens depends on the conditions within the landfill and how well conditions are controlled. The number and activity levels of these microbes are affected by these conditions.
Scientific tests have concluded that extremely low levels of methane are generated during the digestion of PLA molecules. Methane gas is being captured by some landfills as an energy source. Methane released to the atmosphere adds to the natural generation of methane, as we need methane in our atmosphere to form clouds and produce water. However, an excess of methane is not good as this raises global temperatures.