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Transient Microsite Models of Denitrification: II. Model Results¹

ABSTRACT

Solutions to four models of denitrification are presented. The models are based on four coupled reaction-diffusion equations, each model describing a different type of reaction term. The equations were solved by standard finite-difference methods and a boundary tracking technique when needed. For a chosen test problem, the zero-order model predicted sequential reduction of nitrogenous species, a N₂/N₂O ratio of 0.17, and relatively high transient NO^-₂ levels. The Michaelis-Menten model with threshold and concentration-dependent NO^-₃ and NO^-₂ inhibition of N₂O reduction predicted N₂/N₂O ratios of 19.15 and 6.69, respectively. The competitive inhibition model predicted a N₂/N₂O ratio of 0.23 and N₂ evolution followed N₂O. A sensitivity analysis of the Michaelis-Menten model with concentration-dependent inhibition showed that the species of gaseous N evolved depended strongly on NO^-₃ levels in the soil. N₂/N₂O ratios were also influenced by O₂ fluxes, diffusion coefficients, and diffusion distances that affect anaerobic volume and solute transport. The predicted N₂/N₂O ratios were less sensitive to the Michaelis-Menten kinetic parameters Vⁱ_max and Kⁱ_M where i = nitrate, nitrite, or nitrous oxide. The qualitative behavior of gaseous N evolution as a function of nitrate levels, gaseous oxygen concentration, and depth of saturation predicted by the models agrees with data in the literature.

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

² Formerly Graduate Research Asst, and Professor of Soil Science, 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.