I think there are two answers to this.
The first is how much biomass is actually contained in leaves/clippings.
Answer: Not. Very. Much. A giant bag of crass clippings might give once decomposed, dried out, and with some loss to wind/digestion/etc, I dunno, maybe a single handful of organic matter if you're lucky? Try throwing a cup of
coffee grounds on the ground and see how much higher it is... not at all.
The second answer though is that said handful of organic matter
should ideally get mixed into the top of your 'regular' soil by worms, blending the line between the two, and dual a bit of dual duty in total "good soil" creation.
I find the best answer is to collect metric crap-tons of leaves during the fall, and inter-layer those with a bit of manure (I have to get mine bagged as I have no free source, and composted bagged manure from Home Depot is less expensive and easier to move around than bulk deliveries (which is strange, but true). The manure (maybe use 1ft3 per every 4-5 full bags) results in a) weighing the leaves down a bit for the winter and b) much, much faster decomposition and saves me having to chop them, turn them, etc. They don't break down all the way in a year, but good
enough to plant through.
