Sheep urine as a pollution reducing agent...

ABC News Online writes:

Bus operator Stagecoach has fitted a tank to a bus in Winchester, southern England, which sprays urine into the exhaust fumes to reduce nitrous oxide emissions.
...
The urine, from farmyard waste, is refined into pure urea. Ammonia in the urea converts nitrous oxides into nitrogen gas and water.

I can't help wondering about the side-effects, but wow, a simple (and presumably cheap) way to eliminate NOx at its source.

Is there enough urine in the world to make a dent on the NOx emissions from domestic vehicles?