The Nitrogen Isotope Effect Associated with Nitrate and Nitrite Loss from Waterlogged Soils

Abstract Changes in the isotopic composition of added NO 3 ‐ , during the course of its disappearance from two central Illinois soils are reported. The only known difference in the two soils is the cultural history of the fields from which they were taken. A large (approximately three‐fold) difference between the two soils was observed in the isotopic fractionation factor associated with NO 3 ‐ loss. Large differences in the two soils with respect to both the rate of NO 3 ‐ loss and the isotopic fractionation associated with it were apparently related to differences in the two fields rather than to temporal or spatial variation within the fields. Addition of glucose (1% with respect to dry soil weight) caused the difference between the two soils to disappear. It also appreciably reduced the apparent isotopic fraction factor in both soils. When NO 2 ‐ , rather than NO 3 ‐ , was added as substrate, the isotopic fractionation factor associated with its loss was the same in the two soils. The effect of glucose was similar whether NO 3 ‐ or NO 2 ‐ was the substrate; i.e., the isotopic fractionation factor was substantially reduced. Possible explanations for the observed variations in isotope effect associated with substrate loss are discussed. In addition, the implications of these variations for the study of denitrification in natural systems are discussed.