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The Role of Nitrate Diffusion in Determining the Order and Rate of Denitrification in Flooded Soil: I. Experimental Results¹

ABSTRACT

The effect of NO₃^--N diffusion on the order and rate of denitrification reaction was studied in 15 soils from various locations in the U.S. The soils were amended with 0.5% rice straw and incubated under saturated conditions without O₂, either with no excess floodwater or with a 3-cm overlying layer of floodwater. The disappearance of added and native NO₃^--N and its conversion to N₂ and N₂O followed apparent zero-order reaction kinetics when all the NO₃^--N was present in the active soil layer (no excess floodwater). When added NO₃^--N was present in both the floodwater and the soil layer, NO₃^--N disappearance followed apparent first-order reaction kinetics. A decrease in NO₃^--N concentration in the soil layer caused NO₃^--N in the floodwater to diffuse into the soil layer (a first-order reaction with respect to NO₃^--N concentration) where it was denitrified. Thus where NO₃^--N loss involved both diffusion from the overlying floodwater and denitrification, NO₃^--N disappearance appeared to be a first-order rather than a zero-order reaction.

The rate of denitrification was faster in soils incubated with no excess floodwater compared to soils incubated under 3 cm of floodwater. One soil (Crowley silt loam), when incubated with no added carbon source and no excess floodwater, showed NO₃^--N disappearance to follow first-order kinetics. When this soil was incubated with an additional carbon source, NO₃^--N disappearance was zero order.

¹ Contribution from the Lab. of Flooded Soils & Sediments, Dep. of Agronomy, Louisiana Agric. Exp. Stn., Louisiana State Univ., Baton Rouge, LA 70803, and Dep. of Agronomy, Univ. of Kentucky, Lexington KY 40506.

² Post-Doctoral Research Associate, Professor of Agronomy, Louisiana State Univ., and Professor of Agronomy, Univ. of Kentucky. The first author is currently with Biol. & Agric. Eng. Dep., North Carolina State Univ., Raleigh, N.C. and the second author is currently with the Center for Wetland Resources, Louisiana State Univ., Baton Rouge, LA 70803.

Received for publication February 28, 1977. Accepted for publication November 7, 1977.