i recall some archeologists doing some core sampling in garbage dumps. i think they brought up corn kernels in a layer with 50 year old newspaper (which is how they dated that depth). i don’t have a cite or more specifics that i recall.
to decompose rapidly takes oxygen. if it is sealed in a landfill there is no oxygen, it will decompose over a geological length time period.
landfills don’t allow yard waste or large quantities of organic matter. they will often have composting programs for this. cost of landfill and transport are too high to allow for compostables now.