When the amount of oxygen in the environment declines the redox potential falls and reduction reactions become more likely. This potential is measured in millivolts (mV).
At +350mV the redox potential is considered "neutral". This is the point at which oxygen is just as likely to be "reduced" to water (H2O) as it is to oxidized to gaseous oxygen (O2).
At lower redox potentials the following reactions important to the biochemistry of plants tend to occur:
|below +350mV||O2 --> H2O|
|below +220mV||NO3- --> NO2-, N2, and NH4+|
|below +200mV||Mn+4 --> Mn+2|
|below +120mV||Fe+3 --> Fe+2|
|below -150mV||SO4-2 --> H2S|
|below -250mV||CO2 --> CH4|