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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.

¹ Contribution from the Dep. of Agronomy, Colorado State Univ., and the USDA-SEA-AR, Fort Collins, CO 80523. Published on Scientific Series no. 2509.

² Professor, Research Scholar, and Soil Scientist, respectively. The second author is now Research Scientist, Research Institute, Univ. of Petroleum and Minerals, Dhahran, Saudi Arabia.

Received for publication December 14, 1979. Accepted for publication August 25, 1980.