(1) Ammonium nitrate contains nitrogen and is a nitrogen fertilizer;

The mass of this element in each bag of fertilizer is at least: 50kg×34.3=17.15kg.

So the answer is: nitrogen; 17.15;

(2) The relative molecular mass of ammonium nitrate is 14×2 1×4 16×3=80;

The mass fraction of nitrogen in NH4NO3 is 2880×100=35>34.3; therefore, this advertisement is a real advertisement.

So the answer is: 80; true.