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

Abstract

Abstract The effect of NO 3 ‐ ‐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 O 2 , either with no excess floodwater or with a 3‐cm overlying layer of floodwater. The disappearance of added and native NO 3 ‐ ‐N and its conversion to N 2 and N 2 O followed apparent zero‐order reaction kinetics when all the NO 3 ‐ ‐N was present in the active soil layer (no excess floodwater). When added NO 3 ‐ ‐N was present in both the floodwater and the soil layer, NO 3 ‐ ‐N disappearance followed apparent first‐order reaction kinetics. A decrease in NO 3 ‐ ‐N concentration in the soil layer caused NO 3 ‐ ‐N in the floodwater to diffuse into the soil layer (a first‐order reaction with respect to NO 3 ‐ ‐N concentration) where it was denitrified. Thus where NO 3 ‐ ‐N loss involved both diffusion from the overlying floodwater and denitrification, NO 3 ‐ ‐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 NO 3 ‐ ‐N disappearance to follow first‐order kinetics. When this soil was incubated with an additional carbon source, NO 3 ‐ ‐N disappearance was zero order.