I just looked up the Nitrogen cycle, on Wikipedia, and found this.
"Ammonification
When a plant or animal dies, or an animal expels waste, the initial form of nitrogen is organic. Bacteria, or fungi in some cases, convert the organic nitrogen within the remains back into ammonium (NH4+), a process called ammonification or mineralization. Enzymes Involved:
GS: Gln Synthetase (Cytosolic & PLastid)
GOGAT: Glu 2-oxoglutarate aminotransferase (Ferredoxin & NADH dependent)
GDH: Glu Dehydrogenase:
Minor Role in ammonium assimilation.
Important in amino acid catabolism."
"Nitrification
Main article: Nitrification
The conversion of ammonium to nitrate is performed primarily by soil-living bacteria and other nitrifying bacteria. The primary stage of nitrification, the oxidation of ammonium (NH4+) is performed by bacteria such as the Nitrosomonas species, which converts ammonia to nitrites (NO2-). Other bacterial species, such as the Nitrobacter, are responsible for the oxidation of the nitrites into nitrates (NO3-).[2] It is important for the nitrites to be converted to nitrates because accumulated nitrites are toxic to plant life. "
hxxp://en.wikipedia.org/wiki/Nitrogen_cycle
What it appears to be is that your biomass in your soil mix is in the process of ammonification. I would add some more soil to the mix, and maybe an innoculation of microbeasties. Another alternative, is to let the soil cook for awhile, until the nitrogen cycle has done it's thing. But it would be a good Idea to turn your mix a couple of times a week to allow all of your mix to be exposed to oxygen.
I hope this helps.
HomieHogleg