Short answer: no with a "but" Long Answer: yes with an "if"Nystina said:
Actually, there are bacteria but you need to understand a little chemistry first... urea is oxidized by oxygen to nitrate, so urea is the "fuel" if you will in this case, nitrate is the low energy product. This is why to try to aerate our biofilters as much as possible, to speed this up. In order to reduce nitrate to N2 (or back to NH3 in some cases) you need an environment where nitrate is once again the "fuel", in order for this to occur there can't be any oxygen around. One place this might happen naturally is a deep substrate that has gone anaerobic... you might have heard of deep sands beds the saltwater people use, this is the idea behind it. There are also de-nitrifying coils, I have no idea how these work in the aquarium, industrially you usually add a little sugar to deplete the stream of oxygen then let bacteria do its work.
But in a well aerated biofilter and tank, this won't normally happen unless you provide conditions for it, as most of our tanks don't have an oxygen-poor area. The easiest way to remove nitrates is fast growing plants, if you're looking to cut back on tank maintainence with plants read Diana Walstad's Book Ecology of the Planted Tank, she does a water change once every 6 months, very healthy tanks... they're not Amano aquascapes but they are easy and keep things green... For freshwater too many things can go wrong if you attempt to de-nitrifiy, like H2S gas production... (stinks and can kill your fish)
Jeff
