What if your body applies force to the ground as weight. If you weigh 450 newtons and carry a backpack that weighs W newtons what is the net force applied downward to the ground?

The net force applied downward to the ground is 450 newtons + W newtons. When standing still (not accelerating), the net force must be zero according to Newton's first law. This means the force pushing down must be balanced by the force pushing up (from the ground).