What's done is done, but in future years, as others suggest, collect up the leaves and compost them separately, either in a contained heap or in binliner bags with holes in the bottom. Leaves should be wet, crammed in a binliner, the tops tied shut, holes poked in the bottom, then left in a corner somewhere to rot down over a year or so, by which time they will have shrunken right down to something commonly known as 'black gold'. This can then be added to your soil with, if applied in spring, a handful or two of nitrogen in some form or other. In the UK, that would be Growmore or other granular nitrogenous, but balanced, feed.
The reason you wouldn't normally allow leaves to degrade slowly on the ground in your garden, or dig in shredded or whole leaves, is because the fungi and bacteria which break down the leaves require nitrogen to function. Therefore, if you add chopped leaves directly to your soil, the activity of those organisms will increase, and remove more nitrogen from the soil in which your plants are growing, leaving less available nitrogen for plants to take up.