The bacterias that process ammonia and nitrite are aerobic, so when you bury them, they die. And since you only have as many bacteria as are necessary to process the available ammonia and nitrite (it's not like you have any extras just hanging around - when they don't have enough food, they die), when bacteria are killed off, there are suddenly not enough to get rid of all the natural byproduct of decaying organic matter. However, the change is often insignificant, plants can take up some slack as they will use ammonia as a nutrient, and bacteria reproduce quickly, so you might not notice.
But, you might get a mini-cycle. It depends on things like how much filter media (including prefilters) and random equipment and hardscape you have in the tank, as well as total plant mass. The more total surface area there is, the less bacteria there are per given two-dimensional area. Just test daily for ammonia for a week or so, and if you see any, do a big water change.