Biochemical relationships and inorganic nitrogen equilibrium in semi-enclosed basins1,2

In anoxic basins the oxidation of organic matter has so far been represented taking only the reduction of sulphate into consideration. Theoretical studies, however, show that anaerobic bacteria reduces nitrate prior to the reduction of sulphate. Hence, in representing the total redox reactants, the inclusion of ammonia gives a better representation of its relation to the total phosphate regeneration in stagnant basins. An attempt is made to study the state of equilibrium of the inorganic nitrogen compounds from measurements of the concentrations of nitrate-, nitrite-, and ammonia-nitrogen, using the parameter pE. The systems NO₂^--NO₃^-, NH₄⁺-NO₃^-, and NH₄⁺-NO₂^- are applied for calculations. Studies are made of the variations of the three redox pairs in oxidising and in reducing environments. In narrow basins, under oxidising conditions, the system NO₃^--NO₃^- is found to approach equilibrium values. From the proximity of the calculated values to the equilibrium values the system NO₃^--NO₂^--NH₄⁺ appears to exist at least in partial equilibrium, if not in total equilibrium conditions, in marine environments independent of the general equilibrium condition controlled by the system H₂-H⁺-H₂O.