It is air that that holds the weight. There are actual formulas to calculate how much air you should have in the tires to support the weight, keeping the boundaries of not to exceed maximum tire pressure, and making sure you are above the minimum. If the tire is under inflated, heat builds up in the tire, leading to catastrophic failure. I have enclosed an informative document on this subject.
Not sure about Nitrogen and tires. Air is 80 percent or so Nitrogen, and the very first time one adds air to the tires, any marginal gains by pure Nitrogen are lost. You are better off making sure you have properly inflated tires than worrying about the Nitrogen content.