(1) In ammonium bicarbonate (NH4HCO3), the mass ratio of each element is N:H:C:O=14: (1×5): (16×3): 12=14: 5:48:12.

(2) From the meaning of the question, the mass fraction of NH4HCO3 in this bag of fertilizer is 98%, and the net content of each bag of fertilizer is 50kg, then the actual mass of nitrogen-containing elements in the fertilizer is 50kg×98% ×1414+1×5+12+16×3×100%≈8.7kg．

So the answer is: (1) 14:5:48:12; (2) 8.7.