Answer: As the leaves decompose, there is a temporary period where the microbial activity uses the available nitrogen to decompose the leaves. Then, as the microbes begin to die and decompose, the nitrogen is released for other microbes and plant life. Some gardeners will add a little nitrogen with the fresh leaves to "jump start" the biological activity and prevent the temporary nitrogen deficit.
But even without added nitrogen, by next spring there should be no lack of nitrogen due to the fresh leaves from this fall. However, nitrogen tends to be very volatile and may need to be added for next spring's planting anyway.
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