I have been shredding leaves with a worx leaf shredder for a few yrs now, and they will need to be lightly covered with soil, or covered with something to prevent them from blowing away, or stored by tarping over a pile. Keeping the leaves/area moist will facilitate decomposition, as moisture (and temp) is needed to jump start the life cycle of microbes that break down organic material. When I piled/stored the shreds over winter, and then topped my beds with them, they would be gone in about 2 months, most likely from earth worms that love dead organic plant matter - especially the invasive jumping worm. While yes, there is a nitrogen sink when tilled into the soil, the nitrogen becomes available again as the leaves break down some more - hence the shredding to speed things up, plus unshredded leaves as a mulch can cause a sheet mulching affect (impenetrable layer of water resistant/gas blocking - cause anaerobic situation) - which is why leaves are used as mulch with caution, often to help insulate perennials trees shrubs over winter, and removed when spring arrives. Wood chips (not shreds) does not form a sheet mulch effect, and the fungal growth with moistened can be very quick. All organic mulch will create a thin contact layer of nitrogen sink with the soil it is in contact with, but as the organic mulch breaks down quickest at the contact level, it releases that sunk nitrogen dependent on decomposition time, so leaves will definitely release its original nitrogen plus sunk nitrogen in a few weeks if moisture is present - more so than wood chips. Both are awesome organic method to suppress weeds and slowly (but surely) condition the soil as it breaks down.
Pine needles does not acidify the soil in fast, large amounts, but rather, slowly breaks down... plus most plants prefer a slightly acidity environment, with blueberries and rhodes and others 'acid-lovers' requiring a low pH medium. I love pine needles, and my source, a fellow about 25 miles away from me who has a giant pine, that I fill my SUV to capacity, sometimes using the attachment trailer too, for the last decade. They are a beautiful mulch, esp for ornamentals, and easy to pull back to plant. And they are wonderful for slopes. They do break down much slower than leaves, and sometimes, slower than wood chips. As I don't have enough needles for my extensively cultivated 1/3 acre, I use the leaf shredder, and wood chips as well.
