denitrification in substrates

Does anyone know the redox potential or range of redox at which
conversion of ammonia/nitrate to nitrogen occurs? Are bacteria
involved in this process? Is there any way to prevent this?

To confirm my own understanding, H2S would not form until redox
potential reaches 0 or negative, true? My understanding is that
the principle problem of substrates with redox ranging from
200 - 0 mV in a fertile soil substrate will be loss of available
nitrogen, not H2S. Unfortunately, this is also the redox most
favourable for Fe2+ formation.

Anybody know how to build a DIYS redox measurement device?
What does one retail for?

Steve in Vancouver BC   raining and ~10C daytime temps <brrr>