Subject: What's source of the energy in nitrates and nitrites mined from caves?

Mammoth cave was extensively mined for nitrates and nitrites during 
the War of 1812. The materials were chemically processed to produce 
potassium nitrate, saltpetre, for American production of gunpowder 
during the British blockade of our ports. How did these nitrogenous 
compounds originate? Are they from some microbial processes? If so, 
what was the energy source for N fixation? did the fixed N come down 
from above in percholating water? What fixed the compounds in the 
cave dust?

I visited Mammoth Cave, and no one knew the answer to these 
questions, and I've been unable to find any answers elsewhere, ether. 
Where does the energy in these cave soil compounds come from, and how 
did they get there? It's a mystery to me. I have no explanation. 
Please help.

Re: What's source of the energy in nitrates and nitrites mined from caves?