Not sure if the Urea injection has been further explained, but here's the main principle being used MOST diesels will have to meet the FINAL Tier IV emission standards. It's also called SCR
http://www.agcocorp.com/E3/technology/howitworks.html
"An onboard diagnostic system monitors the Diesel Exhaust Fluid (DEF)/AdBlue level and indicates when to refill the tank.
DEF/AdBlue is a non-hazardous, high-purity, colorless solution containing 32.5% urea. The rest is deionized water.
Urea is made up of ammonia and carbon dioxide. It's used as an artificial fertilizer and it's found in cosmetics and other products."
DEF/Adblue will sell for approx. $3/gallon, and off AGCO's website, a 5 gallon tank will last for over 300gallons of fuel. So i'd expect the similar results in a onroad vehicle as well.
Just hope this "great" idea wont interfere with farmers paying for fertilizer or being put on the back burner or a waiting list for getting the fertilizer we need to feed America.