Answer: Mulching with leaves is a great idea. Collecting them with a bagging mower is also very smart and partially shreds them as it picks them up. Recent studies at an eastern university indicated that even pine needles which are often accused of being "too acidic" do not appreciably lower soil pH. Over two or three years of the study, only a slight dip in pH was detected.
If you want, you can have a pH test done every 5 years or so, and can add a sprinkling of lime when the results indicate pH moving a bit lower than you want it to be. But I'd wager that would seldom be necessary. So, mulch away! It is the way the forest was designed to work and no one limes the forest.
Regarding nitrogen, if you are mulching with leaves (placing them on top of the soil) there is very little tieup of nitrogen. The area where leaves and soil contact is where breakdown will be primarily occuring. In time, there will even be some nitrogen released from the decomposing microorganisms at work on the leaves. Therefore adding more leaves to the surface of the mulch does not necessitate the addition of nitrogen.
Thanks for the question!
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