what causes exhaust soot in gasoline engine?

Your exhaust outlet would show black sooty deposits if it's running rich, Burning oil it would be have an oily feel to it. Having said all that, if you only do short journeys the exhaust will be black. once you have done a decent run 10-20 miles look again, if it is grey or light brown/grey then there is nothing wrong.

Check you plugs. If they are clean, it is normal build up. My 2011 Civic has black sooty residue in the tail pipe tip, and it runs perfectly.

Black carbon deposits in the absence of smoke is typical of too-rich fuel mixture. If you were burning that much oil, you would also have blue smoke from the tailpipe.

Its in MOST exhaust systems, some more,,...some less

it is caused from running to rich. you need to have a tune up and possibly have your fuel injectors cleaned or replaced. bot , it definenately running to rich.