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Nitrification-Denitrification Reactions in Flooded Soils and Water Bottoms: Dependence on Oxygen Supply and Ammonium Diffusion¹

ABSTRACT

Ammonium nitrogen in a flooded soil or water bottom exposed to oxygen from the water column undergoes sequential nitrification and denitrification. Oxygen moving through the overlying water column causes the development of an aerobic surface layer of soil or sediment. Ammonium in this aerobic surface layer is nitrified and the resulting ammonium concentration gradient across the aerobic layer and the underlying anaerobic layer causes ammonium in the anaerobic layer to diffuse upward into the aerobic layer where it also undergoes nitrification. Nitrate produced in the aerobic layer then diffuses downward into the anaerobic layer where it is denitrified to N₂ and N₂O. Nitrate derived from ammonium nitrogen in the aerobic layer appears as an intermediate product in the nitrification-denitrification reaction.

A laboratory experiment utilizing ¹⁵N as a tracer showed that approximately one-half of the nitrogen involved in the nitrification-denitrification process was ammonium originally present in the surface aerobic soil or water bottom layer with the remainder diffusing up from the underlying anaerobic layer. Where oxygen was absent or limiting, nitrification either did not occur or occurred at a lower rate, resulting in a reduced amount of nitrate available for the denitrification process.

Key Words: N loss • ammonium oxidation • nitrate reduction • aerobic layer • anaerobic layer • submerged soil

¹ Contribution from the Lab. of Flooded Soils and Sediments, Dep. of Agron., Louisiana Agric. Exp. Stn., Louisiana State Univ., Baton Rouge, LA 70803. Supported in part by U. S. Environ. Prot. Agency Proj. R 800428.

² Professor and Research Associate, respectively.

Received for publication December 29, 1975.

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