Why is black smoke coming from underneath riding lawnmower?

The three things that could cause black smoke which come to mind are: 1) You have a belt or pulley problem which is causing the belt to slip and burn. 2) You have an oil leak that is getting on to the exhaust and burning. 3) The mower is running rich. Which means it is getting too much gas. The most common of these is choice 1. Oil leaks don't normally get onto the exhaust since most exhausts aren't under the engine. The running rich problem also does not normally come from under the mower. It would typically come from the exhaust unless it has an exhaust leak.