Denitrification Kinetics in Soil Systems: The Significance of Good Fits of Data to Mathematical Forms

The loss of NO 3 ‐ added to two central Illinois soils was determined in experiments in which the soils were incubated under waterlogged conditions. The loss was measured as a function of substrate concentration in one experiment in which samples were incubated for a single time (24 hours) and as a function of time in a second experiment in which the concentration of the added NO 3 ‐ was held constant (200 ppm NO 3 ‐ ‐N). The rate of loss of NO 3 ‐ was about 5.5 times higher in one of the soils than in the other. This difference was largely overcome by the addition of glucose (1% with respect to the soil) which also greatly enhanced the rate of denitrification in both soils. The experimental points representing the rate of NO 3 ‐ loss plotted as a function of the concentration of added NO 3 ‐ , were equally well fit by Michaelis‐Menten and exponential equations as well as by the solution to a pair of nonlinear differential equations representing a system in which the product of one reaction (e.g., the reductant generated by the oxidation of carbon compounds) is a substrate in a second sequence (e.g., the denitrification of NO 3 ‐ ). The significance of such fits is discussed. The authors point out that while such fits have certain uses, it is not possible to infer from them the mechanism of the reaction.