Cats like something nice and loose to scratch in, which mulch provides. Also the smell of ammonia (from fertilizers, or previous cat offerings) is a "this is a litter box" cue for cats.
I'd suggest a motion detector sprinkler like:
http://www.mountainvalleygrowers.com/scarecrow.htm
http://www.shop.com/Havahart_Spray_Away_Motion_Sprinkler_151_Protects_1000_Sq_Ft_-5629738-7974964-p+.xhtml?sourceid=298