Burning the grass does release the metal nutrients to the lawn, but the trade off in loss gas nutrients is not worth the trade off. When you burn the grass, stubble, or crop residues, you loose all the stored-up Nitrogen, Hydrogen, Oxgen and Carbon. You cannot build up humis levels if you are releaseing the stored Carbon, in the forms of Carbon Dioxide, Carbon Monoxide and other such forms, to the atomosphere. You would be much better off applying a biostimulant to increase microbial activity that would eatup the residues and reduce thatch buildup. This would be better for the enviroment (not to mention not having to worry about a fire getting out of control), and would release the gas nutrients in the forms the plants can use and help build humis levels at the same time. Large crop producers do often burn off the stubble and crop residues in their fields, but the reason isnt to return nutrients to the soil, its so that they can work their fields without plant residues clogging up their plows and planters.