When you mix a bunch of carboniferous material into the soil, the microbes start throwing a party. They scrape up every bit of nitrogen they can find and go to town. The woody material will break down after a while... and the nitrogen will hit the soil again... but it can take some time.
In really tough soil... when I don't feel like double-digging, I put down cardboard, then stack leaves, wood chips, compost, etc. on top of it, then wait a year. That's basically the "Ruth Stout" or "Lasagna gardening" method. The material crushes weeds and attracts worms like you wouldn't believe. They in turn become your tillers. I turned a patch of rocky clay into rich, loamy soil that way... but it took one year to become nice ground... and another year to reach its peak.
