As a Mulch or to dig in?

Over the years we have an area off our farm where we dump all of our garden grass clippings, I mean years and there is a huge quantity just naturally composting away. It also has our neighbours grass and donkey/pony dung mixed around. I am growing potatoes this year, would it be better added as a mulch or dug into the trench aswell? A lot of its is that sticky gooey rotted down sort of stuff, but thats nearer the bottom. Should i stick with more fresh clippings?

Thanks