Thermodynamics of Inorganic Nitrogen Transformations1

Abstract

Standard free energies of formation for various inorganic N species were selected from the literature and used to calculate equilibrium constants for reactions involving various inorganic N species. These reactions were systematized into stability diagrams and used to discuss possible N transformations that may occur in soils. The developments show that N2(g) is the only stable N species in the redox range of pe + pH of 4 to 18. At lower redox NH4+ is stable, whereas at higher redox NO3- is stable. Stability diagrams were developed on the basis that first N2, then N2O, and finally N3- do not attain equilibrium with other inorganic N species. These diagrams provide insight into possible reaction sequences that may be involved in chemical denitrification. The hypothesis proposed herein is that NO0 (nitric oxide) and possible H2N2O2 (hyponitrous acid) may be rate-limiting intermediates in chemical denitrification. These N species are metastable but reach maximum stability in the narrow redox range of pe + pH 13.5 to 14.0. The possibility that these species may be the kinetically feasible intermediates in chemical denitrification are discussed.