burning oil?

I have a Mazda 6 2 litre diesol engine, 5 years old with 45k miles on the clock, over the last 5 weeks I have had to top up the engine with 2 litres of engine oil. when I put my foot down on the throttle I can see black smoke from the exhaust. Are these cars prone to burning oil,? could it be the oil rings on the pistons/. which would be strange with such low milage, anyone any ideas?

I NA cars I know are not prone to leaking oil...Yours could be different

sounds like the rings are blown. i am guessing that since it is diesel, it is turbo? what happens is the motor is idling and leaking oil onto the top of the pistons and the compression of the takeoff burns off all of the oil. not really a mazda problem, just a diesel problem.