From my Oxford Concise Dictionary:
barren * adj. 1 (of land) too poor to produce much or any vegetation > (of a tree or plant) not producing fruit or seed. 2 (of a female animal) unable to bear young. 3 bleak and lifeless. 4 (barren of) devoid of. * n. (barrens) chiefly N. Amer. barren tracts of land.
- DERIVATIVES barrenly adv. barrenness n.
- ORIGIN ME: from OFr. barhaine, of unknown origin.
Given that, both Acid and Fire could work. I would suggest negative energy, but where talking about a metallic dragon here, so it doesn't really fit.