That simply can’t be true in the wild. If that much carbon was released from plants in the wild, we’d never have coal or oil, right? Composting the grass might be the same as cows, but that process doesn’t happen everywhere there’s grass. In the wild, the grass would be trampled and new grass would sprout up to cover it, layers of soil would be generated. But honestly I’m just guessing, but your claim seems so far off the mark.