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Transient Microsite Models of Denitrification: III. Comparison of Experimental and Model Results¹

ABSTRACT

Experiments were designed to investigate the validity of proposed reaction-diffusion models of denitrification. Brass respirometers were used to measure fluxes of oxygen (O₂), carbon dioxide (CO₂), and nitrous oxide (N₂O) from a 5 x 10^–3-m layer of saturated soil incubated at 25°C. Gaseous fluxes were measured as a function of time after wetting, with and without 0.42 mol m^–3 of acetylene (C₂H₂) present in the headspace. The presence of C₂H₂ in the headspace doubled O₂ fluxes 35 to 60 h after incubations commenced. When C₂H₂ was not present, N₂O evolution was negligible after 25 h of incubation. Nitrous oxide fluxes predicted by the models were compared to those observed experimentally. When C₂H₂ was present, the models predicted N₂O fluxes reasonably well (r² = 0.90 and 0.78) for initial nitrate (NO^–₃) N levels of 7.1 and 15.9 mg N kg^–1 soil, respectively. When C₂H₂ was not present, the models predicted the experimentally observed initial flux of N₂O if an 8 to 10 h lag in N₂O reductase activity was included in the models. The best model for describing N₂O fluxes in these experiments used differential rates of Michaelis-Menten reduction without NO^–₃ and nitrite (NO^–₂) inhibition of N₂O reduction. Experimental techniques and parameter characterization, however, need considerable refinement. Independent measurements are needed to quantify relationships between reaction order and kinetic parameters. Still, given the general agreement between observed and predicted fluxes, there can be little doubt that the reaction-diffusion models are essential to a better understanding of denitrification.

¹ Contribution from Dep. of Agronomy, Cornell Univ., Ithaca, NY 14853.

² Formerly Graduate Research Asst., Professor of Soil Science, and Assoc. Professor of Soil Chemistry, respectively. Cornell Univ., Ithaca, NY 14853. Senior Author is currently Asst. Prof. of Soil Science, Dep. of Agronomy, Mississippi State Univ., Mississippi State, MS 39762.

Received for publication August 20, 1984. Accepted for publication February 5, 1985.